THE JOURNEY OF MAN: A GENETIC ODYSSEY
June 6, 2008 on 9:59 pm | Friedrich Braun | Genetics & Human Bio-Diversity , Immigration | | Email This Post | Print this PostSpencer Wells, Genetic Anthropologist, on the first Great Migrations
Spencer Wells is a Genetic Anthropologist. He is Director of the
Genographic Project, operated by National Geographic to capture a
“genetic snapshot” of humanity, before Globalization blurs our genetic
trails. Geneticists in many countries - India, China etc - are
participating.
Wells shows that Europe’s ancestry derives mainly from people in that
continent around 30,000 years ago; not from early agriculturalists in
the Middle East. But there was an invasion of Aryans from the steppes,
which imposed the Indo-European languages on Europe and northern India.
Before that, Basque might have been more typical of European languages.
UNESCO is holding a conference on the “First Great Migrations of
Peoples” in Paris on June 19th 2008:
http://www.firstgre atmigrations. org/
The Journey of Man: A Genetic Odyssey
Spencer Wells.
ALLEN LANE an imprint of PENGUIN BOOKS
London 2002
{See the associated map at
http://users. cyberone. com.au/myers/ wells-Y-marker- map.jpg
or at http://mailstar. net/wells- Y-marker- map.jpg}
{p. 27} Inside each of our cells, what we think of as our genome - the
complete DNA sequence that encodes all of the proteins made in our
bodies, in addition to a lot of other DNA that has no known function -
is really present in two copies. The DNA is packaged into neat, linear
components known as chromosomes - we have twenty-three pairs of them.
Chromosomes are found inside a cellular structure known as the nucleus.
One of the main features of our genome is the astounding
compartmentalizatio n - like computer folders within folders within
folders. In all there are 3,000,000,000 (3 billion) building blocks,
known as nucleotides (which come in four flavours: A, C, G and T), in
the human genome, and we need some way to get at all of the information
it contains in a straightforward way.
{p. 28} The reason we have two copies of each chromosome is more
compli- cated, but it comes down to sex. When a sperm fertilizes an egg,
one of the main things that happens is that part of the father’s genome
and part of the mother’s genome combine in a 50:50 ratio to form the new
genome of the baby. Biologically speaking, one of the reasons for sex is
that it generates new genomes every generation. The new combinations
arise, not only at the moment of conception with the 50:50 mixing of the
maternal and paternal genomes, but also prior to that, when the sperm
and egg themselves are being formed. This pre-sexual mixing, known as
genetic recombination, is possible because of the linear nature of the
chromosomes - it is relatively easy to break both chromosomes in the
middle and reattach them to their partners, forming new, chimeric
chromosomes in the process. The reason why this occurs, as with the
mixing of Mum’s and Dad’s DNA, is that it is probably a good thing,
evolutionarily speaking, to generate diversity in each generation. If
the environment changes, you’ll be ready to react.
But wait, you might say, why are these broken and reattached chromosomes
any different from the ones that existed before? They were supposed to
be duplicates! The reason, quite simply, is that they aren’t exact
copies of each other - they differ from each other at many locations
along their length. They are like duplicates of duplicates of duplicates
of duplicates, made with a dodgy copying machine that introduces a small
number of random errors every time the chromo- somes are copied. These
errors are the mutations mentioned above, and the differences between
each chromosome in a pair are the polymorphisms. Polymorphisms are found
roughly every 1,000 nucleotides along the chromosome, and serve to
distinguish the chromosomes from each other. So, when recombination
occurs, the new chromosomes are different from the parental types.
The evolutionary effect of recombination is to break up sets of
polymorphisms that are linked together on the same piece of DNA. Again,
this diversity-generatin g mechanism is a good thing evolutionarily
speaking, but it makes life very difficult for molecular biologists who
want to read the history book in the human genome.
{p. 29} Recombination allows each polymorphism on a chromosome to behave
independently from the others. Over time the polymorphisms are
recombined many, many times, and after hundreds or thousands of
generations, the pattern of polymorphisms that existed in the common
ancestor of the chromosomes has been entirely lost. The descendant
chromosomes have been completely shuffled, and no trace of the original
deck remains. The reason this is bad for evolutionary studies is that,
without being able to say something about the ancestor, we cannot apply
Ockham’s razor to the pattern of polymorphisms, and we therefore have no
idea how many changes really distinguish the shuffled chromosomes. At
the moment, all of our estimates of molecular clocks are based on the
rate at which new polymorphisms appear through mutation. Recombination
makes it look like there have been mutations when there haven’t, and
because of this it causes us to overestimate the time that has elapsed
since the common ancestor.
One of the insights that Wilson and several other geneticists had in the
early 1980s was that if we looked outside of the genome, at a small
structure found elsewhere in the cell known as the mitochondrion, we
might have a way of cheating the shuffle. Interestingly, the
mitochondrion has its own genome - it is the only cellular structure
other than the nucleus that does. This is because it is actually an
evolutionary remnant from the days of the first complex cells, billions
of years ago - the mitochondrion is what remains of an ancient bacterium
which was swallowed by one of our single-celled ancestors. It later
proved useful for generating energy inside the cell, and now serves as a
streamlined sub-cellular power plant, albeit one that started life as a
parasite. Fortunately, the mitochondrial genome is present in only one
copy (like a bacterial genome), which means that it can’t recombine.
Bingo. It also turns out that, instead of having one polymorphism
roughly every 1,000 nucleotides, it has one every 100 or so. To make
evolutionary comparisons we want to have as many polymorphisms as
possible, since each polymorphism increases our ability to distinguish
between individuals. Think of it this way: if we were to look at only
one polymorphism, with two different forms A and B, we would sort
everyone into two groups, defined only by variant A or variant B. On the
other hand, if we looked at ten polymorphisms with two variants each, we
would have much better resolution, since the likelihood of
{p. 30} multiple individuals having exactly the same set of variants is
much lower. In other words, the more polymorphisms we have, the better
our chances of inferring a useful pattern of relationships among the
people in the study. Since polymorphisms in mitochondrial DNA (mtDNA)
are ten times more common than in the rest of our genome, it was a good
place to look.
Rebecca Cann, as part of her PhD work in Wilson’s laboratory, began to
study the pattern of mtDNA variation in humans from around the world.
The Berkeley group went to great lengths to collect samples of human
placentas (an abundant source of mtDNA) from many different populations
- Europeans, New Guineans, Native Americans and so on. The goal was to
assess the pattern of variation for the entire human species, with the
aim of inferring something about human origins. What they found was
extraordinary.
Cann and her colleagues published their initial study of human
mitochondrial diversity in 1987. It was the first time that human DNA
polymorphism data had been analysed using parsimony methods to infer a
common ancestor and estimate a date. In the abstract to the paper they
state the main finding clearly and succinctly: ‘All these mitochondrial
DNAs stem from one woman who is postulated to have lived about 200,000
years ago, probably in Africa.’ The discovery was big news, and this
woman became known in the tabloids as Mitochondrial Eve - the mother of
us all. In a rather surprising twist, though, she wasn’t the only Eve in
the garden - only the luckiest.
The analysis performed by Cann and her colleagues involved asking how
the mtDNA sequences were related to each other. In their paper they
assumed that if two mtDNA sequences shared a sequence variant at a
polymorphic site (say, a C at a position where the sequences had either
a C or a T), then they shared a common ancestor. By building up a
network of the mtDNA sequences - 147 in all - they were able to infer
the relationships between the individuals who had donated the samples.
It was a tedious process, and involved a significant amount of time
analysing the data on a computer. What their results showed were that
the greatest divergence between mtDNA sequences was actually found among
the Africans - showing that they had been diverging for longer. In other
words, Africans are the oldest group on the planet - meaning that our
species had originated there.
{p. 31} One of the features of the parsimony analysis used by Cann,
Stoneking and Wilson to analyse their mtDNA sequence data is that it
inevitably leads back to a single common ancestor at some point in the
past. For any region of the genome that does not recombine - in this
case, the mitochondrion - we can define a single ancestral mitochondrion
from which all present-day mitochondria are descended. It is like
looking at an expanding circle of ripples in a pond and inferring where
the stone must have dropped - in the dead centre of the circle. The
evolving mtDNA sequences, accumulating polymorphisms as they are passed
from mother to daughter, are the expanding waves, and the ancestor is
the point where the stone entered the water. By applying Zuckerkandl and
Pauling’s methods of analysis, we can ’see’ the single ancestor that
lived thousands of years ago, and which has mutated over time to produce
all of the diverse forms that exist
{p. 32} today. Furthermore, if we know the rate at which mutations
occur, and we know how many polymorphisms there are by taking a sample
of human diversity from around the globe, then we can calculate how many
years have elapsed from the point when the stone dropped - in other
words, to the ancestor from whom all of the mutated descendants must
have descended.
Crucially, though, the fact that a single ancestor gave rise to all of
the diversity present today does not mean that this was the only person
alive at the time - only that the descendant lineages of the other
people alive at the same time died out. Imagine a Provencal village in
the eighteenth century, with ten families living there. Each has its own
special recipe for bouillabaisse, but it can only be passed on orally
from mother to daughter. If the family has only sons, then the recipe is
lost. Over time, we gradually reduce the number of starting recipes,
because some families aren’t lucky enough to have had girls. By the time
we reach the present century we are left with only one surviving recipe
- la bouillabaisse profonde. Why did this one survive? By chance - the
other families simply didn’t have girls at some point in the past, and
their recipes blew away with the mistral. Looking at the village today,
we might be a little disappointed at its lack of culinary diversity. How
can they all eat the same fish soup?
Of course, in the real world, no one transmits a recipe from one
generation to the next without modifying it slightly to fit her own
tastes. An extra clove of garlic here, a bit more thyme there, and
voila! - a bespoke variation on the matrimoine. Over time, these
variations on a theme will produce their own diversity in the soup bowls
- but the recipe extinction continues none the less. If we look at the
bespoke village today we see a remarkable diversity of recipes - but
they can still be traced back to a single common ancestor in the
eighteenth century, thanks to Ock the Knife. This is the secret of
Mitochondrial Eve.
The results from the 1987 study by Cann and her colleagues were followed
up by a more detailed analysis a few years later, and both studies
pointed out two important facts: that human mitochondrial diversity had
been generated within the past 200,000 years, and that the stone had
dropped in Africa. So, in a very short period of time - at least in
evolutionary terms - humans had spread out of Africa to
{p. 33} populate the rest of the world. There were some technical
objections to the statistical analysis in the papers, but more extensive
recent studies of mitochondrial DNA have confirmed and extended the
conclusions of the original analysis. We all have an African great-great
… grandmother who lived approximately 150,000 years ago.
{p. 42} The further apart the polymorphisms are, the more likely it is
that they have been shuffled. And because shuffling obscures the
historical signal, this means that most of our genome isn’t terribly
useful for tracing migrations.
There is one piece of DNA, though, that has recently proven to be an
invaluable tool for inferring details about human history - providing us
with far greater resolution than we ever thought possible about the
paths followed by our ancestors during their wanderings. It is the male
equivalent of mtDNA, in that it is only passed from father to son. For
this reason, it defines a uniquely male lineage - a counterpart to the
female line illuminated by studying mtDNA. It is the patrimoine in our
Provencal village, and the details of lineage extinction and
diversification that went on with the soup recipes also apply to this
piece of DNA. It is known as the Y-chromosome.
Now wait a minute, you might be saying - what’s going on with all of
this maternal and paternal lineage gibberish? I thought that the whole
idea of sex was to mix the mother’s and father’s genomes in a 50: 50
ratio to produce the child? Why do we have these oddities that break the
rules? For the mitochondrial DNA the answer is easy - it is actually
outside of what we think of as the human genome, an evolutionary remnant
of a time when it was a parasitic bacterium living inside the earliest
cells. The story for the Y is a bit more complicated.
One of the quirky features of sexual reproduction is that the chromo-
somes that actually determine our sex - the so-called sex chromosomes -
are exceptions to the 50: 50 sexual mixing rule. The double layout of
our genomes, with two copies of each chromosome, fails us when we get to
these chromosomes. This is because of the way in which sex is determined
in most animals, through the presence of a mismatched sex chromosome. In
the case of mammals, it is the male that is mismatched, with one X and
one Y-chromosome. In females, the X-chromosome is present in two copies,
like the other chromosomes, allowing normal recombination. In males,
however, the Y only matches with the X in short regions at either end,
which serve to align the sex chromosomes properly during cell division.
The rest of the Y-chromosome, known as the non-recombining portion of
the Y, is pretty much completely unrelated to the X. Thus it has no
paired chromosome with
{p. 43} which it can recombine, and so it doesn’t. It is passed
unshuffled from one generation to the next, for ever - exactly like the
mitochondrial genome.
The Y turns out to provide population geneticists with the most useful
tool available for studying human diversity. Part of the reason for this
is that, unlike mtDNA, a molecule roughly 16,000 nucleotide units long,
the Y is huge - around 50 million nucleotides. It therefore has many,
many sites at which mutations may have occurred in the past. As we saw
in the last chapter, more polymorphic sites give us better resolution -
if we only had Landsteiner’ s blood types to work with, everyone would be
sorted into four categories: A, B, AB and O. To put it another way, the
landscape of possible polymorphisms is simply much larger for the Y. And
critically, because of its lack of recombination, we are able to infer
the order in which the mutations occurred on the Y - just like mtDNA.
Without this feature, we can’t use Zuckerkandl and Pauling’s methods to
define lineages, and Ock the Knife can’t help us with the ancestors.
{p. 44} As we have seen, we can only study human diversity by looking at
differences - the language of population genetics is written in the
polymorphisms that we all carry around with us. These differences define
all of us as unique individuals - unless we have a twin, no other person
in the world has an identical pattern of genetic polymorphisms. This is
the insight behind a DNA ‘fingerprint’ , used to identify criminals.
Applied to the Y-chromosome, it allows us to trace a unique male lineage
back in time, from son to father to grandfather, and so on. Taken to the
extreme, it allows us to travel back in time from the DNA of any man
alive today to our first male ancestor - Adam. But how does it link
unrelated men to each other in regional patterns? Surely each man must
trace his own unique Y-chromosome line back to Adam?
The answer is no, but the reason is a bit complicated. It’s because
we’re not as unrelated as we think. Imagine the situation for the
majority of our genome - the parts that don’t come uniquely from our
mother or our father. Since we inherit half of this DNA from each of our
parents, the pattern of polymorphisms it contains can be used to infer
paternity, since it connects us to both our mother and our father. If my
DNA is shown in court to have a 50 per cent match with that of a child
I’ve never met, it is likely that I will be paying for the support of
that child for many years to come - the probability of a match
{p. 45} occurring by chance is infinitesimally small. So polymorphisms
define us, and our parents, as part of a unique genealogical branch. No
other group of people on earth has exactly the same story written in its
DNA.
If we extend this further, and begin to think about our grandparents,
and their grandparents, and so on, we lose some of the signal in each
generation. I have a 50 per cent match with my father, but only a 25 per
cent match with my grandfather, and only a 6 per cent match with his
grandfather. This is because we acquire new ancestors in each generation
as we go back in time, and they start to pile up pretty quickly. Each of
my parents had two parents, and each of them had tvo parents, and so on.
{p. 37} The earliest Homo Erectus fossils yet discovered date from
around 1.8 million years ago, and they were all found in east Africa
(the African)
{p. 38} variant of Homo erectus is sometimes given the name Homo
ergaster). Recent discoveries in the medieval city of Dmanisi, in the
former Soviet Republic of Georgia, show that they left Africa soon
thereafter - perhaps reaching east Asia within 100,000 years. From this
we can infer that all Homo erectus around the world last shared a common
ancestor in Africa nearly million years ago. But according to the
Berkeley mitochondrial data, Eve lived in Africa less than 200,000 years
ago. How can we reconcile the two results?
It’s all about timing
Let’s step back for a moment and consider the case objectively. The
evidence for an African Genesis of Homo erectus is circumstantial - we
see evolutionary ‘missing links’ in Africa, either uniquely or first.
These include an unbroken chain of ancestral hominids stretching back
more than 5 million years to the recently discovered chimpanzee- like
apes Ardipithecus. But is this evidence sufficient to conclude that
Africa was also the birthplace of our species? …
Rather, the conclusion from the mitochondrial data is that modern humans
evolved very recently in Africa, and subsequently spread to populate the
rest of the
{p. 39} globe, replacing our hominid cousins in the process. It’s a
ruthless business, and only the winners leave a genetic trace.
Unfortunately, Homo erectus appears to have lost.
As we’ll see, other genetic data corroborates the mitochondrial results,
placing the root of the human family tree - our most recent common
ancestor- in Africa within the past few hundred thousand years.
Consistent with this result, all of the genetic data shows the greatest
number of polymorphisms in Africa - there is simply far more variation
in that continent than anywhere else. You are more likely to sample
extremely divergent genetic lineages within a single African village
than you are in whole of the rest of the world. The majority of the
genetic polymorphisms found in our species are found uniquely in
Africans - Europeans, Asians and Native Americans carry only a small
sample of the extraordinary diversity that can be found in any African
village.
Why does diversity indicate greater age? Thinking back to our
hypothetical Provencal village, why do the bouillabaisse recipes change?
Because in each generation, a daughter decides to modify her soup in a
minor way. Over time, these small variations add up to an extraordinary
amount of diversity in the village’s kitchens. And - critically - the
longer the village has been accumulating these changes, the more diverse
it is. It is like a clock, ticking away in units of rosemary and thyme -
the longer it has been ticking, the more differences we see. It is the
same phenomenon Emile Zuckerkandl noted in his proteins - more time
equals more change. So, when we see greater genetic diversity in a
particular population, we can infer that the population is older - and
this makes Africa the oldest of all.
{p. 56} While all African populations contain deeper evolutionary
lineages than those found outside the continent, some populations retain
traces of very ancient lineages indeed. These groups are found today in
Ethiopia, Sudan and parts of eastern and southern Africa, and the
genetic signal they contain is very good evidence that they are the
remnants of one of the oldest human populations. The signals have been
lost in other groups, but today these eastern and southern African
groups still show a direct link back to the coalescence point - Adam.
The populations involved encompass the African Rift Valley, extending
into south-western Africa, where people known as the San - formerly
called Bushmen - have a very strong signal of the diversity that
characterized the earliest human populations. They also speak one of the
strangest languages on the planet, notable for its use of clicks as
integrated parts of words - like the clicking sound we might make when
we guide a horse, or imitate a dripping tap. …
The pattern of deep genetic lineages within the San is also seen for
mitochondrial DNA, and the convergence of these three independent
{p. 57} lines of evidence - Y, mtDNA and linguistic - strongly suggests
that the San represent a direct link back to our earliest human
ancestors. …
One of the distinguishing features of the San people is their ‘non-
African’ physical appearance. … When most of us think of Africans, we
tend to picture the typically Bantu features of central Africans … The
San are a much smaller people, with lighter skin, more tightly curled
hair and a thicker layer of skin over the eyes - the so-called
epicanthic fold that also characterizes people from east Asia. It is
this latter feature which has led some researchers to suggest that the
epicanthic fold is an ancestral characteristic of our species, and was
simply lost in western Eurasian and Bantu populations.
{p. 58} It is unlikely that our African ancestors were the hairy,
brutish troglodytes portrayed in museums - these are probably overly
influenced by our perception of Neanderthals, who may have been pretty
hairy and brutish. Rather, they are likely to have been fairly gracile
and elegant, at least in comparison to Neanderthals. The simple reason
is that the great mass of a Neanderthal, and the likely hairy exterior,
is thought to have been an adaptation to the cold Eurasian climate.
Because our earliest ancestors lived in the relatively warm climes of
southern and eastern Africa, they would not have needed the warmth
provided by a furry exterior….
Early humans probably had fairly dark skin. This is because of the
nature of the environment where they lived - a sunny African savannah -
where the protection against solar radiation afforded by dark skin would
have been a distinct advantage. It is also because at least some of the
mutations that produce light skin colour in Europeans and north-east
Asians are derived from the ancestral, darker form of the gene (known as
MCI R, or melanocortin receptor), which is virtually
{p. 59} the only form found in Africa today. Thus, it seems likely that
Africans have retained a darker colour, rather than evolving it from a
lighter form.
Our ancestors of 60,000 years ago were probably about the same height as
you and I …
So, the picture that emerges is of a dark-skinned (although perhaps not
as dark as some Africans today), reasonably tall, thin person - perhaps
with an epicanthic fold. Someone who wouldn’t look that out of place
today dressed in a suit and sitting opposite you on the train. Not
surprising, I suppose, given that he only lived about 2,500 generations ago.
Out of the nest
Accepting the evidence at face value, the implication is that Adam lived
in population groups directly ancestral to the modern San, in eastern
and/or southern Africa, around 60,000 years ago. The date of the
earliest modern human populations - the first of our species - remains
to be assessed, and could be anywhere between 60,000 and several hundred
thousand years ago. We simply lose the signal from our genes at that
stage, as all of the genetic diversity present today coalesces to a
single ancestor. What is clearly implied by the data, however, is that
all modern human genetic diversity found around the world was in Africa
around 60,000 years ago. The mtDNA and Y-chromosome give us the same
dates for the earliest non-African genetic lineages, and it is now
agreed by most geneticists that humans began to leave Africa around this
time. There may have been occasional
{p. 60} forays into the Middle East prior to this, as suggested by
100,000-year- old human remains at sites such as Qafzeh and Skuhl in
present-day Israel, but the Levant of 100,000-15O, 000 years ago was
essentially an extension of north-eastern Africa, and was probably part
of the original range of early Homo sapiens. The real expansion was
beyond the Mediterranean world, into the uncharted territory of Asia proper.
Here we run headlong into what the Australians might call ‘a curly one’.
According to the dated remains in Australia, humans were there, 15,000
km east of Africa by the shortest land route, at the same time we are
all supposed to have been in Africa, 50-60,000 years ago.
{p. 66} Africa is the most equatorial continent on earth. The entirety
of its landmass is found between latitudes of 38°N and 34°S, and 85 per
cent of its land area is in the tropical zone between Cancer and
Capricorn. Sea-level freezing temperatures are rare in Africa - uniquely
among all the continents.
{p. 68} Recent research by Robert Walter, an American geophysicist based
in Mexico, suggests that a large-scale drying up of the African
continent at the onset of the last ice age resulted in modern humans
favouring coastal environments. This is because savannahs are unusual
places. They are closely related to tropical forests in the chain of
climatic relationships, and the two zones are interchangeable depending
on the level of rainfall. In general, the areas of tropical Africa with
more than three months of low rainfall are savannah, while those with
fewer than three are forest. If there are substantially longer dry
periods, the environment grades into steppe, and ultimately into desert
as moisture becomes extremely scarce. While these regions are all found
in particular locations in present-day Africa, their past extent has
fluctuated. What Walter’s research suggests is that as Africa began to
dry up, the savannahs of eastern Africa were replaced by steppe and
desert, except in a narrow zone near the coast. It was in these coastal
savannah environments that early humans would have congregated,
exploiting food sources from the sea as well as those of the land
animals living near by.
While the universality of this theory is uncertain, and it may turn out
to be a minor sideline of human evolution, one thing is clear: there is
incontrovertible proof that early humans were able to live off of the
sea. Large middens, or garbage dumps, of shells from clams and oysters
have been found in Eritrea, on the eastern Horn of Africa, dating from
around 125,000 years ago. These middens also have human stone tools
interspersed among them, showing that humans were living in the region
and exploiting coastal resources. The presence of butchered remains of
rhinoceros, elephant and other large mammals conjures up a prehistoric
’surf ‘n’ turf’ feast reminiscent of the massive platters of steak and
shellfish served in American restaurants. It seems that our
{p. 69} distant ancestors had quite well-developed palates, even in
those days of apparent hardship.
One of the most exciting details to emerge from Walter’s work is the
fact that there appears to have been exchange with coastal dwellers
thousands of kilometres away, who were exploiting the same types of
resources in southern Africa. This is suggested by the similarities in
tools found at the sites, coupled with their roughly contemporary dates.
It seems that humans were able to migrate over long distances,
relatively rapidly, by following the coast of eastern Africa.
Now for the big leap: if humans could migrate over long distances within
a continent, using the same technologies and exploiting the same
resources, why couldn’t they do the same between continents? The coastal
route would be a sort of prehistoric superhighway, allowing a high
degree of mobility without requiring the complex adaptations to new
environments that would be necessary on an inland route. The resources
exploited in Eritrea would be pretty much the same as those in coastal
Arabia, or western India, or south-east Asia, or - wait for it -
Australia. And because of the ease of movement afforded by the coast,
the line of sandy highway circumnavigating the continents, this would
allow relatively rapid migration. No mountain ranges or great deserts to
cross, no need to develop new toolkits or protective clothing, and no
drastic fluctuations in food availability. Overall, the coastal route
seems infinitely preferable to anything further inland. There were only
a couple of sections of open water that would have required a boat to
cross. Most likely these boats would have been rather simple - probably
a few logs lashed together - but we have no direct evidence, because
wood disintegrates very quickly. Nevertheless, they did make it across.
It is clearly plausible that the early presence of humans in Australia,
almost immediately after they left Africa, can be accounted for by
migration along this coastal route - beachcombing along the southern
coast of Asia. There are two remaining pieces of the puzzle to be
evaluated, though - rather critical ones, in fact. If one of the early
wavcs of migration out of Africa followed a coastal route, is there a
telltale genetic pattern? It depends on the way in which the migration
occurred, and what the migrants did along the way. We might expect to
see a band of particular genetic markers along the coast,
{p. 70} differentiated from the populations living further inland. Or
perhaps the signals have been homogenized among descendants of the
coastal dwellers and the land migrants. The only way to find out is to
examine populations from along the route and see what the genetic
pattern is. The second critical piece of evidence is to be found in the
pattern of archaeological remains along the route - are they consistent
with such a journey?…
The beauty of the genetic data is that it gives us a clear, stepwise
progression out of Africa into Eurasia and the Americas. The diversity
we find around the world is divided into discrete, although related,
units, defined by markers - the descendants of ancient mutation events.
By mapping these markers on to the map of the world, we can infer
details of past migrations. Following the order in which the mutations
occurred, and estimating the date and any demographic details (such as
population crashes or expansions), we can gain an insight into the
details of the journey. And the first piece of evidence comes from one
man in particular, who had a rather important, random mutation on his
Y-chromosome between 31,000 and 79,000 years ago. He has
{p. 71} been named, rather prosaically, M168. More evocatively, he could
be seen as the Eurasian Adam - the great … great-grandfather of every
non-African man alive today. The journeys taken by his sons and
grandsons defined the subsequent course of human history.
It is perhaps surprising that the clearest evidence for the route
followed by our ancestors on their journey out of Africa comes from the
Y-chromosome - surely men tend to ’sow their oats’, causing the
widespread dispersal of regional genetic signals? Oddly enough, no - and
the rapid loss of ancient soup recipes on the male lineage (which we
used to explain Adam’s recent date) means that men living in a
particular area tend to share a recent common ancestor, providing us
with clear ‘fingerprints’ of particular geographic regions. What this
means is that the Y gives us the clearest evidence for the journeys
followed by early humans. It is literally a ‘journey of man’, but it is
the best tool we have for inferring the details of the trip. It is
obviously important to examine the female lineage to see if it follows
the same pattern - to make sure the fish stays with the bicycle, so to
speak - but the Y-chromosome does provide us with the cleanest
distillation of human migrational history.
As we look more carefully at the arrangement of branches on the
mitochondrial tree, we find that there is a similar pattern - all of the
non-African mitochondrial branches descend from a particular branch of
the tree trunk, implying that our M168 Adam was paired with an Eve.
Thankfully, this Eurasian Eve lived around 50-60,000 years ago,
suggesting that she and Eurasian Adam could have met. She is called by
the (again) rather mundane name L3, and her daughters accompanied the
sons of M168 on their journey to populate the world.
Based on the distribution of the descendants of M168 and L3 in Africa
today, it is likely that they both lived in north-east Africa, in the
region of present-day Ethiopia and Sudan. Like all men alive today, M168
shared deeper roots with his African cousins. His lineage is a major
branch leading off the human family tree, with his descendant ‘terminal
branches’ found in the DNA of all of today’s Eurasians, but he connects
them back through M168 to our species’ African root. In our tree
metaphor, each marker that we study defines a node on the tree - a point
where a branch splits into two smaller branches. If we had no markers
apart from Ml68 and L3, our trees would be fairly sparse,
{p. 72} comprising a root (Adam and Eve) and one split on the tree,
defined by M168 or L3, on the branch leading out of Africa, and another
branch remaining in Africa. Luckily, the tree is packed with dense
foliage, defining a pattern of growth that traces the map of our journey.
Intriguingly, on both the mitochondrial and Y branches, there is another
split, immediately after M168 and L3, dividing the Eurasian branch
structure into distinct clusters - two in the case of mtDNA, and three
in the case of the Y. For both the Y-chromosome and mtDNA, one cluster
is more common than the other(s), accounting for around 60 per cent of
the non-African branches (or lineages) in the case of mitochondrial DNA,
and more than go per cent in the case of the Y. In other words, the
majority of non-Africans alive today have mtDNA and Y-chromosomes
belonging to the more numerous clusters - people living all over the
world, in places as disparate as Europe, India and South America. The
rarer lineages, though, are found only in Asia, Australasia and the
Americas. It is these rare lineages that constitute the majority of the
mitochondrial and Y types in the Australian Aborigines.
Our rare mitochondrial cluster is given the name M - like the head of
MI6 in James Bond movies. In biblical terms, Eve begat L3, and L3 begat
M. According to recent research by Lluis Quintana-Murci, a Catalan
researcher working in Paris, the distribution of the M cluster is
indicative of an early migration out of Africa, which proceeded along
the coast of south Asia, ultimately reaching south-east Asia and
Australia. M is virtually absent from the Middle East, and is not found
at all in Europe, but it constitutes 20 per cent or more of the
mitochondrial types in India, and close to 100 per cent of those in
Australia. Quintana-Murci estimates its age to be 50-60,000 years, and
from its distribution it seems that people who carried the M lineage
never made it into the interior parts of the Middle East. The most
[p. 73} likely explanation is that the ‘M people’ left Africa very early
on, carrying their distinctive genetic signature across the south of the
continent along the coastal highway.
And what about the Y? Is there a male counterpart to our M mitochondrial
lineage? Luckily, the answer is yes. Again assuming biblical style, Adam
begat M168, and M168 begat M130. M130 appears to have accompanied
mitochondrial M on her coastal journey, and the present-day distribution
of his descendants provides us with an insight into the nature of the
trip. Like the M mitochondrial lineages, M130 Y-chromosomes are limited
to Asia and America, but the dynamics of lineage extinction that we see
for the Y have left a much more striking pattern than the one seen for
their mitochondrial counterparts. M130’s descendants are virtually
unknown west of the Caspian Sea, but they comprise a substantial
proportion of the men living in Australasia. M130 is only found at low
frequency in the Indian subcontinent - 5 per cent or less. But as we
move further east,
{p. 74} the frequency increases: 10 per cent of Malaysian, 15 per cent
of New Guinean and 60 per cent of Australian aboriginal men trace their
ancestry directly to M130. There is a quirkily high frequency of M130 in
north-east Asia, particularly in Mongolia and eastern Siberia, which
suggests a later migration that we will revisit in Chapter 7. For the
purposes of our Australian story, though, M130 provides us with a clear
fingerprint of the coastal migration out of Africa.
One other piece of evidence suggests a direct link between Africa and
Australasia - physical appearance. The dark skin of the Australians is
reminiscent of that found in Africa - something that begs an
explanation. Most of the people living in south-east Asia today would be
classified as ‘Mongoloid’ peoples, implying a shared history with those
living further north in China and Siberia. There are, however, isolated
populations of so-called Negritos living throughout south-east Asia who
closely resemble Africans. The most obvious examples are found in the
Andaman Islands, under the jurisdiction of India but
{p. 75} actually 400 km off the west coast of Thailand. The largest
tribal groups, known as the Onge and Jarawa, have many features that
link them with the Bushmen and Pygmies of Africa, including short
stature, dark skin, tightly curled hair and epicanthic folds. Other
Negrito groups, such as the Semang of Malaysia and the Aeta of the
Philippines, have mixed substantially with Mongoloid groups and have a
more ‘Asian’ appearance. The Andamanese, probably because of their
island home, have escaped much of the admixture seen on the mainland.
Because of this they are thought to represent a relic of the
pre-Mongoloid population of south-east Asia - ‘living fossils’, if you
will. The suggestion made by many anthropologists, particularly Peter
Bellwood of the Australian National University, is that the population
of south-east Asia prior to 6,000 years ago was composed largely of
groups of hunter-gatherers very similar to modern Negritos. Migrations
from north-east Asia over the past few millennia have erased the
evidence of these early south-east Asians, except in the case of small
groups living deep in the jungles or - in the case of the Andamanese -
on remote islands.
So, both the Y-chromosome and the mtDNA paint a clear picture of a
coastal leap from Africa to south-east Asia, and onward to Australia.
Taking the genetic dates as a guide, modern humans could have made this
journey around the same time as the earliest archaeological evidence
pointing to human occupation in Australia. DNA has given us a glimpse of
the voyage, which almost certainly followed a coastal route via India.
But is there any archaeological trace of this journey along the route?
A swim in Ceylon
This brings us back to the issue of the dating, particularly as applied
to the Australian remains. No evidence for other hominids has ever been
found in Australia - Homo erectus did not make it across the long
stretches of open ocean that separated it from south-east Asia, despite
living only a few hundred kilometres away in Java. Because Homo sapiens
is the only hominid species that has ever been found in Australia, any
evidence of human occupation sticks out like a
{p. 76} proverbial sore thumb. Stone tools unearthed in Arnhem Land
could only have come from one source - us. And if the radiometric dates
say that stone tools were present in Australia 50-6,0000 years ago,
almost immediately after the genetic dates show us that our ancestors
were still in Africa, this means that modern humans must have made use
of a route that afforded extremely rapid movement. The coastal
superhighway seems to be the most likely one.
As we have seen, though, there were other hominids living along the
route followed by these beach dwellers. They also made stone tools, and
these have been found throughout Eurasia. The easternmost extension of
the range of Homo erectus was Java, and it is possible that they even
survived until around 40-5,0000 years ago - long enough for the coastal
migrants to have encountered them as they moved through the Indonesian
archipelago. It is clear, though, that they must have become extinct
almost immediately after the arrival of the Moderns, if not before. What
is uncertain is whether we actively forced them out of - the picture - a
genocidal scenario that we will explore in greater detail when we get to
Europe later in the book.
{p. 77} Even before the current era of globalization, the world had its
‘killer apps’ that dominated everything else. In the case of the period
we are talking about, 50-6,0000 years ago, the killer apps are grouped
into a common cultural phenomenon known as the Late Stone Age, or more
technically, the Upper Palaeolithic. The tools of the Upper Palaeolithic
mark a radical departure from those that pre-date them, and are clear
evidence for the presence of anatomically modern humans, as opposed to
Homo erectus or Neanderthals, who remained trapped in a Middle
Palaeolithic time-warp.
The details of the Middle to Upper Palaeolithic transition will be
examined in the next chapter, but for the purpose of the story of our
Australian coastal dwellers it is sufficient to say that the earliest
Upper Palaeolithic tools mark the initial migration of modern humans
into any geographic region. And that is why India is unusual, since
there is actually very little evidence of the Upper Palaeolithic there.
There is a general dearth of human remains from all periods leading up
to the Upper Palaeolithic, but at least there are abundant tools from
the earlier periods. The Upper Palaeolithic provides no telltale signs
until very late in the day, and even then they show up in an unexpected
place.
Fa Hien cave in Sri Lanka provides us with the earliest sign of the
Upper Palaeolithic in the Indian subcontinent. The date, however, is a
problem - the earliest clearly modern artefacts date from no earlier
than 31,000 years ago. Nearby Batadomba Lena cave contains the earliest
skeletal material from anatomically modern humans, also dating from
around 30,000 years ago. The combination of age and location gives us
two clues in our search for traces of the coastal migration. First, the
Sri Lankan caves suggest that the earliest modern humans arrived in
India from the south, rather than from the north via the more obvious
inland route. This implies that they were living on the coast,
consistent with the theory of an early coastal migration.
The second clue, which comes from the date, is that the Batadomba people
could not have been the ancestors of the Australians, since they
actually lived over 20,000 years after the earliest evidence for human
settlement in Arnhem Land. Another curly one. It may turn out that
archaeological layers below those already excavated will yield earlier
{p. 78} evidence for modern human presence, but for the time being it
appears that Batadomba is too late to help us along on our voyage. In
fact, late dates are found along the entirety of our coastal route to
Oz. In Thailand, for instance, there is evidence for modern human
occupation from about 37,000 years ago at Lang Rongrien cave - but not
before. As we move closer to the scene of the crime, the dates get older
- advanced, Upper Palaeolithic stone tools dating from 40,000 years ago
have been found at Bobongara, on the Huon Peninsula of eastern New
Guinea. This would have been the final stepping-stone on the journey,
but there is still nothing approaching the 50-60,000-year- old dates in
Australia. Thus, in spite of the genetic pattern tracing an early
coastal route out of Africa, the archaeology appears to have failed us.
Where is the evidence for our coastal route?
Unfortunately we don’t know, but there is a likely hypothesis. Since
almost all archaeological work today is carried out on land, we are
probably missing the artefacts that are hidden underwater. ‘Rubbish -
surely Atlantis is a myth!’ you might be saying. Well, yes and no. While
the evidence for an entire civilization falling catastrophically into
the sea is fairly sparse, what is unequivocal is that sea levels have
indeed fluctuated substantially - if somewhat more gradually - over the
past 100,000 years. Those of 50,000 years ago were around 100 metres
lower than they are today, as large amounts of moisture were tied up in
the expanding ice sheets of the northern hemisphere. This may not sound
like much of a difference, but remember that we are not as interested in
the depth as we are in the extent of the land that would have been
exposed by these fluctuations. Since the continents typically have very
shallow slopes as they fall off into the sea, a difference of 100 metres
can make a huge difference in the amount of land exposed. For example, a
drop in sea level of this magnitude would expose as much as 2oo km of
land off the west coast of India. Sri Lanka and India would have been
connected by a land bridge, the Persian Gulf and the Gulf of Thailand
would have been fertile river deltas, and Australia and New Guinea would
have been two bulbous extremities of a single landmass. All in all, our
entire coastal route would have been much different 50,000 years ago.
What the recent sea-level rise means is that, if our coastal voyagers
were living primarily off resources provided by the sea, the places
{p. 79}where they chose to live would have been those that are now
under- water. The Y-chromosome pattern in Eurasia shows that our M130
coastal marker is found predominantly in the southern and eastern parts
of the continent. Furthermore, M130 chromosomes in the south appear to
be older than those found further north, suggesting a later migration
originating in the tropics. These results, coupled with a lack of
archaeological evidence for modern human occupation until after 40,000
years ago, suggest that the early coastal migrants did not stray far
from the sea. Adapted to a coastal lifestyle, the surfers do not appear
to have made significant colonial forays into the turf. Knowing this, it
would seem more appropriate for archaeologists in search of the first
Indians to be wearing scuba gear rather than pith helmets. It is likely
that the earliest Upper Palaeolithic tools in the subcontinent will be
found underneath thousands of years’ worth of sand and coral growth.
{p. 108} The first Upper Palaeolithic humans may have reached the Middle
East during the relatively warm and moist conditions around 50,000 years
ago, when the eastern Sahara was in retreat and a gateway opened along
the Red Sea. Perhaps they migrated down the Nile to the Mediterranean,
then spread eastward across the Sinai peninsula. Alternatively, early
human populations may have moved across the strait of Bab al Mandab into
southern Arabia, a short hop of 20 km or so. Once there, the relatively
moist conditions along the coastal mountain range of western Arabia -
which served to scoop moisture from the prevailing westerly winds coming
off the Red Sea - may have created savannah-like hunting conditions for
these Upper Palaeolithic people. Even today there is a narrow strip of
steppe extending as far north as the city of Medina in Saudi Arabia,
unique in the harsh environment that defines most of the Arabian
peninsula. In the past, this tenuous steppe environment may have been
joined with its ecological equivalent extending southward from the Gulf
of Aqaba in Jordan, effectively opening a door to the interior of Eurasia.
William Calvin, a neurobiologist who has written extensively on climate
and early human evolution, has compared the Sahara to a kind of hominid
‘pump’. During wetter periods, the Sahara would have sustained human
populations, perhaps focused around oases or rivers, or limited to zones
that received moisture from prevailing winds. As the conditions turned
drier, the Sahara would have returned to uninhabitable desert, forcing
human emigration. Calvin suggests that the climatological downturn after
50,000 years ago may have been the impetus for the migration of Upper
Palaeolithic humans out of northern Africa and into the Middle East.
However the earliest Upper Palaeolithic moderns reached the Levant, it
is clear that the deteriorating climate after 45,000 years ago
effectively locked them into their new home. The Sahara would have been
at its driest between 40,000 and 20,000 years ago, and it is likely that
any previously inhabitable areas there would have been engulfed by
desert during this time. Modern humans were trapped in a new continent.
{p. 109} The genetic pattern bears this out, and provides the next clue
on our journey. M89, the marker that occurred immediately after M168 on
our main line into Eurasia, has been dated using the absolute method
detailed above to around 40,000 years ago. Due to possible errors in the
assumptions that go into the calculation, particularly in determining
the rate at which new mutations occur, this estimate actually
encompasses a range between 30,000 and 50,000 years, and it is likely
(given the climatic data) that it appeared at the earlier end of this
range, perhaps 45,000-50,000 years ago. This is because it serves to
unite populations living in north-eastern Africa - Ethiopia and Sudan in
particular - with the populations of the Levant. The shutting of the
Saharan gate after these M89-bearing populations were allowed through is
suggested by the low frequency in north-eastern Africa of Eurasian
markers that occurred later on the M89 lineage. If Africa and the Levant
had been part of a continuous range occupied by humans throughout the
Upper Palaeolithic, we would expect to see a relatively homogeneous
distribution of markers throughout. In fact, it seems that the
emigration of populations bearing M89, which we can call a Middle
Eastern marker, signified the last substantial Upper Palaeolithic
exchange between sub-Saharan Africa and Eurasia. The world had been
divided into African and Eurasian, and it was to be tens of thousands of
years until significant exchange was to take place again.
The presence of M89 in both north-eastern Africa and the Middle East,
and the age of the Upper Palaeolithic archaeological sites in the
Levant, helps us to answer the question of whether Eurasia was settled
in a single southern coastal emigration from Africa. M130 chromosomes
are not found in Africa, suggesting that this coastal marker arose on an
M168 chromosome en route to Australia. Conversely, M89 Y-chromosomes are
not found in Australia or south-east Asia - but they appear at fairly
high frequency in north-eastern Africa. The implication is that M89
appeared slightly later than M130 in a population that stayed behind in
Africa after the coastal migrants left for Australia. It was these
people, sans M130 chromosomes, who first colonized the Middle East.
There is archaeological evidence for a modern human presence in the
Levant from around 45,000 years ago, consistent with the arrival of
modern humans from somewhere else. North-eastern Africa is the only
nearby location with archaeological
{p. 110} sites dating from around the same time - and, crucially, the
same genetlc markers we see in the Levant. Thus, the genetic and
archaeological patterns tell us that there was a second migration from
Africa into the Middle East.
Once our Upper Palaeolithic migrants had arrived in the Levant, the road
into the heart of Eurasia was open. There was a continuous highway of
steppe - not unlike African savannah in terms of its species composition
- that stretched from the Gulf of Aqaba to northern Iran, and beyond
into central Asia and Mongolia. The hurdle of the Sahara having been
overcome, the subsequent dispersal of these fully modern humans would
have been limited only by their own wanderlust. They had all of the
intellectual building blocks that would enable them to conquer the
continent, and the process began with gradual migrations along this
Steppe Highway, the continental equivalent of the southern Coastal Highway.
At this time, game would have been plentiful. The large, grazing mammals
of the steppe zone - particularly antelope and bovids, the ancestors of
the domestic cow - would have been easy prey for early humans, and they
gradually expanded their range as their numbers
{p. 111} grew. Moving northward and westward, some may have entered the
Balkans early on - the first modern humans in Europe. The numbers would
not have been great, though, since it was far easier to stay within the
bounds of the steppe zone to which they had become so well adapted. The
mountains and temperate forests of the Balkan peninsula would have
seemed rather alien to early Upper Palaeolithic people, and the genetic
data bears this out. Very few Europeans trace their ancestry directly to
the Levant of 45,000 years ago, as attested to by the Y-chromosome
results. Our canonical Levantine Upper Palaeolithic lineage, M89, is
found at frequencies of only a few per cent in western Europe. It may
have been these few Middle Eastern immigrants who introduced the
earliest signs of the Upper Palaeolithic to Europe, a culture known as
the Chattelperronian, but they did not leave a lasting trace. The true
conquest of Europe, and the demise of the Mousterian, would have to wait
for a later wave of immigration - people with a few more ingredients in
their genetic soup.
Eastward ho!
The main body of Upper Palaeolithic people began to disperse eastward.
As with other early human migrations, it almost certainly wasn’t a
conscious effort to move from one place to another. Rather, it seems
that the continuous belt of steppe stretching across Eurasia provided an
easy means of dispersal, gradually following game further and further
afield. It was during this time that another marker appeared on the M89
lineage, given the name M9. It was the descendants of M9, a man born
perhaps 40,000 years ago on the plains of Iran or southern central Asia,
who were to expand their range to the ends of the earth over the next
30,000 years. We will call the people carrying M9 the Eurasian clan.
As the steppe hunters migrated eastward, carrying Eurasian lineages into
the interior of the continent, they encountered the most significant
geographical bollards so far. These were the great mountain ranges that
define the southern central Asian highlands - the Hindu Kush running
west to east, the Himalayas running north-west to south-east and the
Tien Shan running south-west to north-east. The three ranges
{p. 112} meet in the centre, at the so-called Pamir Knot in present-day
Tajikistan, and each radiates off like a spoke in a wheel.
The first humans to see them must have been absolutely awe-inspired.
Although they had encountered the Zagros range in western Iran, it was a
permeable barrier, with numerous valleys and low passes that would have
allowed easy movement. The Zagros themselves actually would have been
part of the geographic range of the prey species hunted by Upper
Palaeolithic people, with the herds migrating into higher pastures
during the summer and descending to the surrounding plains in the
winter. The high mountains of central Asia were a different beast
altogether. Each of the ranges has peaks that soar to 5,000 metres or
higher (in the case of the Tien Shan and Himalayas, over 7,000 metres),
and the radiating high-altitude ridges would have been formidable
barriers to movement. Remember that the world was in the grip of the
last ice age, and temperatures would have been even more extreme than
today. It was because of these mountains that our Eurasian migrants
would have been split into two groups - one moving to the north of the
Hindu Kush, the other to the south, into Pakistan {p. 113} and the
Indian subcontinent. How do we know this? The Y-chromoome again traces
the route.
Those who headed north, toward central Asia, had additional mutations on
their Eurasian lineage that we will trace below. The Upper Palaeolithic
people who headed south, though, had an unrelated mutation on their
Y-chromosome known as M20. It is not found a appreciable frequencies
outside of India - perhaps 1-2 per cent in some Middle Eastern
populations. In the subcontinent, though, around 50 per cent of the men
in southern India have M20. This suggests that it marks the earliest
significant settlement of India, forming a uniquely Indian genetic
substratum - which we can call the Indian clan - that pre-dates later
migrations from the north. The ancestors of the Indian clan, who moved
into southern India around 30,000 years ago, would have encountered the
earlier coastal migrants still living there. From the genetic pattern,
it seems likely that any admixture with them was not reciprocal: as we
saw in Chapter 4, mitochondrial DNA retains strong evidence of the
coastal migrants in the form of haplogroup M, while the Y-chromosome
primarily shows evidence of later migrants from the north. Thinking back
to the scenario we imagined for the birth of the Upper Palaeolithic in
Africa, this is the pattern we woul expect to see if the invaders took
wives from the coastal population, but the coastal men were largely
driven away, killed, or simply not given the chance to reproduce. The
result would be the widespread introduction of M mtDNA lineages into the
Indian population, while the Coastal Y-chromosome lineages would not be
nearly as common - precisely the pattern we see. Today, the frequency of
the Coastal marker is only around 5 per cent in southern India, and it
falls in frequency as we move northward. This pattern suggests that the
contribution from the coastal populations was minimal, at least on the
male side. The contrast between the two types of data gives us a glimpse
the behaviour of these first Indians, and hints at a cultural pattern we
will explore in more detail in Chapter 8.
The migrating Eurasian masses were not only shunted down in India, of
course - some of them also migrated to the north of the Hindu Kush, into
the heart of central Asia. The Tien Shan would have been an even more
formidable barrier than the Hindu Kush, keeping the Upper Palaeolithic
hunters out of western China. It is around this
{p. 114} time that another mutation occurred on the Eurasian lineage. It
was known as M45, and it will help us to trace two very important later
migrations. Using absolute dating methods, we can infer that the M45
mutation occurred approximately 35,000 years ago in central Asia. Today,
M45 is found only in central Asians and those who trace their ancestry
to this region - thus, it defines a central Asian clan. Descendants of
the central Asian clan occur only sporadically in the Middle East and
East Asia, and at somewhat higher frequency in India, where the clan
appears to have migrated much later (as revealed by the presence of
additional mutations). The ‘ancestral’ form - the deepest split in the
genealogy of Y-chromosomes from the central Asian clan - is found only
in central Asia. This allows us to pinpoint the location of what is
effectively a ‘regional Adam’, in much the same way that we identified
our African Adam as being an ancestor of the San Bushmen. The deepest
branches in the M45 genealogy are found today only in central Asia - not
India, or Europe, or east Asia. Thus, M45 arose in central Asia.
The limited distribution of the oldest descendants of the central Asian
clan suggests that the population where it arose was isolated from
people living in the surrounding parts of the continent. While the Hindu
Kush provides a ready explanation for why there was no easy migratory
path to India, it is not clear why this population had no contact with
groups living in the Middle East. After all, our Eurasian clan had
migrated into central Asia along this route - why couldn’t the central
Asian clan make the return trip? The inference is that another bollard
had entered the story, and given that it hadn’t been an insuperable
barrier several thousand years before when the central Asia clan’s
ancestors first migrated to the heart of the continent, it was likely to
have appeared after that first migration.
Today, the Dasht-e Kavir and Dasht-e Lut deserts of central Iran are
scorched, parched wastelands. The tiny population living there ekes out
a meagre living using a highly developed system of agriculture, complete
with miles of underground irrigation channels known as ghanats that have
been in use for thousands of years. During the heat of the day the
residents of cities such as Yazd retire to subterranean chambers cooled
by wind channelled down long pipes, creating a haunting wail that can be
heard from miles away. It is inconceivable
{p. 115} that anyone could survive for long in this harsh climate
without such a well-adapted lifestyle. Hunting and gathering would be
impossible - at least today. Similarly the Karakum and Kyzylkum deserts
of central Asia are harsh, desolate places with very few inhabitants
apart fro few nomadic shepherds.
There are, however, two belts of continuous steppe across the deserts of
central Iran, one to the north of the deserts, near the Caspian, one to
the south, near the Arabian Gulf. When the world was in midst of its
climatic schizophrenia around 40,000 years ago, it is likely that the
steppelands and deserts of Iran and central Asia went through periods
when the amount of moisture in the atmosphere would have been similar
to, or perhaps greater than, today. This could have been aided by
changes in the prevailing winds, bringing moisture in off the Arabian
Sea. During these relatively wet periods, which may have been brief,
humans would have been able to migrate fairly easily across the Iranian
plateau and into central Asia - again, the prey and hunting methods
would be virtually identical throughout the entire journey. We know that
they did so because of the genetic trail they left in their descendants,
which traces a direct path from the Levant to central Asia.
Once the ice age reached a threshold temperature, though, there was a
significant decrease in precipitation and humidity as evaporation
stalled and water became frozen into the expanding ice sheets of the far
north. This seems to have happened between 40,000 and 20,000 years ago,
and it resulted in the creation of a new desert bollard on our route.
The continent was now split into northern, southern and western
populations, all headed into the coldest part of the ice age. The people
living in India and the Levant had the benefit of the sea which served
to mitigate the effects of the increasingly cold and arid conditions.
Those trapped north of the Hindu Kush, however, had to adapt to the
increasingly harsh lifestyle of the Eurasian steppes - or die.
It is likely that these early central Asians would have stayed in the
relatively warm environs of the southern steppes had encroaching
desertification not forced them on. Some stayed behind, retreating into
the foothills of the Hindu Kush where the water supply from glacial
melting, and the number of animals, were sufficient for survival. Most
{p. 116} though, appear to have followed the migrating herds of game to
the north - into the face of the storm, as it were. It is likely that
they first reached Siberia during the early part of this period, around
40,000 years ago, when Upper Palaeolithic tools make their appearance in
the Altai Mountains. The conditions would have been unimaginably
different from those their ancestors had left behind in Africa 10,000
years before. Winter temperatures dropped to -40°C or lower, and much of
their time would have been spent hunting for food and keeping warm. But
the animals they hunted would have made the difficulties worthwhile.
We saw earlier that one of the defining features of species living at
high latitudes is their great size - Bergmann’s rule. The reason is that
large animals have less surface area relative to their volume than small
ones, and heat is lost through the surface. Shrews must eat constantly
to maintain their hyperactive metabolism, in part because their tiny
size makes it extremely difficult for them to retain heat. In cold
environments, then, there is selection for large animals with slower
metabolisms (since the food resources are not as plentiful as they are
in warmer regions) - big, lumbering beasts that aren’t particularly
clever. This is how natural selection created animals such as the woolly
mammoth.
{p. 117} The Eurasian interior was a fairly brutal school for our
ancestors. Advanced problem-solving skills would have been critical to
their survival, which helps us to understand why it was only after the
Great Leap Forward in intellectual capacity that humans were ready to
colonize most of the world. During their sojourn on the steppes, modern
humans developed highly specialized toolkits, including bone needles
that allowed them to sew together animal skins into clothing that
provided warmth at temperatures not unlike those on the moon, but still
allowed the mobility necessary to hunt game such as reindeer and mammoth
successfully. They had to venture beyond sheltering hills and caves, out
on to the icy open steppe and tundra, necessitating the development of
portable shelters. Their migrations would have taken them far beyond
ready sources of the fine-grained stone they used to make weapons, so
they had to become more economical in their tool-making. This led them
to develop microliths, small stone points (such as arrowheads) that were
hafted on to wooden shafts and used as weapons.
The problem-solving intelligence that would have allowed Upper
Palaeolithic people to live in the harsh northern Eurasian steppes and
hunt enormous game illustrates something that could be called the ‘will
to kill’. Survival depended on finding sufficient food resources,
whatever the obstacles - and the steppes were a veritable meat locker.
It was the necessity of obtaining food that led them into the freezer,
but it would take them well beyond central Asia. The Steppe Highway gave
them a straight shot to the extreme ends of the continent, and once they
had adapted to the harsh conditions a new world lay open to them. {p.
118} The genetic composition of these first Siberians was a mixture of
both central Asian and ancestral Eurasian clan lineages. While M45 is
the marker that we use to infer the migrations of the early central
Asian steppe hunters, there were still many men alive who did not have
Y-chromosomes marked with M45 - they would have had unmarked Eurasian M9
Y-chromosomes. This is because new markers do not immediately increase
in frequency to the point where all other markers - such as the
ancestral M9 lineage - are lost. All of the Y-chromosome markers we
study originated in a single man at some point in the past, so their
original frequency was one (that individual) divided by the total number
of men in the population - a very low frequency in all but the smallest
groups. Over time, they become more common primarily due to the effect
of genetic drift - the random changes in frequency that characterize all
human populations. Thus the earliest people to colonize southern Siberia
would have had members of both the central Asian M45 and the older
Eurasian M9 clans, although drift appears to have caused them to lose
most of their ancestral Middle Eastern chromosomes by this point.
As with the Eurasians who entered India on the other side of the Hindu
Kush, some of these Eurasian clan members would have migrated to the
north and east, guided in their journey by the Tien Shan mountains. Some
of them, perhaps taking advantage of the so-called ‘Dzhungarian Gap’
used thousands of years later by Genghis Khan to invade central Asia,
made it into present-day China. It is likely that the majority were
migrants along the Steppe Highway further to the north, avoiding the
harsh deserts of western China by detouring through southern Siberia.
However, make it they did. We know this because they left descendants
from another Y-chromosome marker that is almost completely limited to
east Asia, and is entirely absent from western Asia and Europe - M175.
Today, M175, which arose on a Eurasian M9 chromosome, is found at
highest frequency, around 30 per cent, in Korean populations. Based on
absolute dating methods, it appears to be roughly 35,000 years old,
coinciding very closely with the appearance of the Upper Palaeolithic
{p. 119} in Korea and Japan. There are several more recently derived
markers that have M175 as an ancestor (particularly M122, which will
play a significant role in Chapter 8), and together these related
lineages account for 60-90 per cent of the Y-chromosomes in east Asia
today. Like a collection of soup recipes that all have a common
ingredient, M175 unites most Asian men living east of the Hindu Kush and
Himalayas, defining an east Asian clan.
When these modern humans reached east Asia, they found themselves in an
area that had been inhabited by their distant relatives Homo erectus for
nearly a million years. Dubois’ missing link had relatives in China,
called (before being united with their Javanese cousins to the south)
Peking Man. But mysteriously, no erectus remains from Chinese sites are
found after 100,000 years ago - there is a gap in the record until fully
modern Homo sapiens make their appearance around 40,000 years ago. What
caused this hominid gap is unclear, although the likely culprit is -
once again - the steadily deteriorating climate. For example, the cave
at Zhoukoudian, where many erectus remains have been found, is located
in north-eastern China, near Beijing - a region that experiences
extremely cold winters even today. …
We know that erectus didn’t change substantially for I million years in
east Asia, perhaps the result of stable selection pressures. Isolation
from other hominids and a penchant for relatively uniform climatic
conditions would have favoured continuity rather than change, and there
is no evidence for an erectus Great Leap Forward. While some Chinese
scientists argue for an evolutionary model known as ‘regional
continuity’, in which east Asian erectus evolved into a local variant of
Homo sapiens independently of what was happening in Africa, there is
absolutely no genetic evidence for this. Moreover, the genetic results
show that there was not even any interbreeding between modern human
immigrants to east Asia and erectus - if in fact any populations still
existed 40,000 years ago that are invisible to today’s archaeologists.
In a recent analysis of over 1,000 men from throughout east
{p. 120} Asia, geneticist Li Jin and his colleagues found that every
single one traces his ancestry to Africa within the past 50,000 years -
because every man has our old friend M168 on his Y-chromosome. Everyone….
If the story ended there, it would be very tidy and self-contained. But
unfortunately, life is never that simple. In this case, the spanner in
the works comes in the form of the presence of our Coastal lineage at
high frequency in some east Asian populations. The Coastal lineage is
found at a frequency of 50 per cent in Mongolia, and it is common
throughout north-east Asia. How it reached this location remains a
mystery, but it is likely that the early coastal migrants to south-east
Asia gradually moved inland, migrating northward over thousands of
years. The M130 chromosomes in the south are older than those in the
north, consistent with such a migration. At some point, perhaps 35,000
years ago, they would have met the descendants of the other, main line
of migrants - our incoming Eurasians. The presence of both Eurasian and
Coastal lineages in east Asian populations attests to the extensive
admixture that occurred between them.
The picture that emerges is that east Asia was settled by modern humans
from both north and south, like migrational pincers or ‘chop- sticks’.
The northern route, which was characterized by Eurasian clan members,
probably entered around 35,000 years ago from the steppes of southern
Siberia. The southern route, which was composed primarily of members of
the Coastal clan, was probably in place before this - perhaps as early
as 50,000 years ago. The present composition of east Asia still shows
evidence of this ancient north-south divide. Luca Cavalli-Sforza,
working with Chinese colleagues, examined several dozen non-Y-chromosome
polymorphisms in east Asian populations. In their analysis, they saw a
clear distinction between the northern and
{p. 121} southern Chinese. Even members of the same ethnic group, such
northern and southern Han, are most closely related to their geographic
rather than their ethnic neighbours; northern Han group with other,
non-Han northern populations, and the southerners form separate group.
It seems that the ancient evidence of a two-pronged settlement is still
visible in the blood of today’s Chinese.
So, our Middle Eastern clan had made it to the eastern extreme the
continent. Along the way it had acquired additional marker producing the
widespread Eurasian clan, the Indian clan, and the central Asian clan.
The mountain ranges of central Asia served; effective barriers to
migration 40,000 years ago, as they continue to do today. The effect of
this was to produce an isolated east Asia Y-chromosome clan that only
occasionally pops up in the west. But while the route to eastern Asia
was clear, that to Europe required more circuitous tour. As we saw,
modern Europeans contain rather too many ingredients in their soup to
have been the direct descendants of the Middle Eastern clan. The search
for the ancestors of the first Europeans is where we are headed next.
{p. 122} One evening in the autumn of 1997 as I was driving up 25th
Avenue, I heard a news announcement that almost caused me to swerve into
an oncoming bus. I pulled over and listened, hanging on every word.
The announcer was reporting that a team of scientists led by Professor
Svante Paabo of the University of Munich had just published the first
DNA sequence from a Neanderthal.
{p. 123}The field of ancient DNA research was pioneered in the Ig80S by
Svante Paabo and his colleagues (including Allan Wilson, of
mitochondrial Eve fame) in Berkeley and Munich. The impetus behind this
work was to do the impossible - to go back in time by examining the DNA
that existed in a long-dead individual. It was, in effect, an attempt to
develop a genetic time machine that would allow us to answer questions
about our ancestors directly. One of the first applications was in the
analysis of DNA from Egyptian mummies, but soon people were trying it on
fossils that were millions of years old. Michael Crichton’s novel
Jurassic Park was based on the heady early days of the field, when it
seemed that anything would be possible - even getting intact dinosaur
DNA from bloodsucking insects embedded in amber!
While the claims for successful retrieval of DNA from sources that were
tens of millions of years old eventually proved unfounded, usually
resulting from minute amounts of contamination by modern DNA, it was
sometimes possible to retrieve DNA from more recent samples, or those
that had been preserved in ideal conditions for tens of thousands of
years. The frozen bodies of mammoths and ancient alpine
{p. 124} travellers yielded analysable DNA, as did the dried remains
from mummies and other desert-dwellers. Even then, the analysis was
almost always limited to mitochondrial DNA, present in huge numbers of
copies in every cell - making it more likely that one copy would have
survived the Russian roulette of molecular degradation over the
centuries. It was still extremely difficult to do this sort of analysis,
though, because in most cases the molecules had completely disinte-
grated after death. This meant that negative results were far more
common than positives - but the stories revealed by the tiny fraction of
cases where DNA could be successfully extracted made the effort
worthwhile. It was with this in mind that Paabo’s group had developed
reliable ways of evaluating and extracting DNA from ancient samples, and
his laboratory represented the state of the art in the early 199OS -
they were the undisputed experts in the field.
The scientific coup that led to my near-death experience in San
Francisco actually began with the very first Neanderthal bones to be
unearthed. Comprising the so-called type specimen - the one against
which all the others were judged by palaeoanthropologis ts - these bones
had sat in a museum in Bonn for nearly 140 years when the Munich group
was approached to do the analysis on them. Paabo jumped at the chance,
and his graduate student Matthias Krings performed the DNA analysis as
part of his PhD thesis work. In over a year of tedious trial-and-error
work, Krings gradually managed to extract enough intact mitochondrial
DNA to create a 105-base-pair sequence. What he saw when he pieced it
together was extraordinary. …
After painstakingly reproducing the result from a separate bone
fragment, and duplicating the experiment in a laboratory on a different
continent (to be certain that a contaminant in the Munich laboratory was
not producing an experimental artefact), he accepted the validity
{p. 125} of the sequence. By repeating the procedure several times, he
eventually managed to obtain 37 base pairs of mitochondrial DNA sequence
from the remains - enough to generate a statistically significant
estimate of its evolutionary divergence. The sequence was clearly not
from modern human mtDNA, but it didn’t belong to an ape either. Rather,
it came from a hominid that last shared a common ancestor with modern
humans around 500,000 years ago. This date was consistent with what was
predicted by palaeoanthropologis ts who had studied the dispersal of
so-called ‘archaic humans’ from Africa into Europe, and it proved that
Neanderthal was not the direct ancestor of modern humans. Rather,
Neanderthals represented a local population of archaic hominids who were
later replaced by modern Homo sapiens - with no detectable admixture. Of
the thousands of human mitochondrial sequences that have been obtained
from people all over the world, not one is anywhere near as divergent as
Krings’ Neanderthal sequence. Neanderthals fall well outside the range
of genetic variation found in the human species - and therefore they
represent a separate species. This early result has been confirmed by
two additional genetic studies of Neanderthal remains from different
parts of Europe, showing that the Neanderthals were closely related to
each other, but very distantly related to us. The genetic data is
incontrovertible - modern Europeans trace their recent ancestry to
Africa, in common with everyone else in the world.
Along with the study of 1,000 Asian Y-chromosomes discussed in the last
chapter, the Neanderthal results placed the final nail in the coffin of
multiregionalism. Our hominid relatives were clearly replaced by modern
humans who spread out of Africa within the past 50,000 years.
{p. 126} We saw earlier that the most obvious location from which to enter
{p. 127}Europe, the Middle East, appears to have contributed little to
the gene pool of modern Europeans. The Y-chromosome lineage defined
solely by M89, which would have characterized the earliest Middle
Eastern populations around 45,000 years ago, is simply not very common
in western Europe. It is such a tiny hop across the Bosporus from the
Middle East to Europe that we might ask why it took so long - perhaps
10,000 years - for modern humans to make a significant foray into
western Europe. To solve this riddle of where the majority of Upper
Palaeolithic Europeans came from - we need to examine the genetic
markers in western Europe and ask which Eurasian lineage they could have
come from, and when.
I said at the beginning of Chapter 5 that my Y-chromosome is defined by
a marker known as M173. It turns out that this marker is not unique to
me - in fact, it is found at high frequency throughout western Europe.
Intriguingly, the highest frequencies are found in the far west, in
Spain and Ireland, where M173 is present in over 90 per cent of men. It
is, then, the dominant marker in western Europe, since most men belong
to the lineage that it defines. The high frequency tells us two things.
First, that the vast majority of western Europeans share a single male
ancestor at some point in the past. And second, that something happened
to cause the other lineages to be lost.
Desperate for a date
The first thing most of us want to know when we hear that almost all
western Europeans trace their family line back to one man is ‘when did
he live?’ This is where our absolute dating methods come in. If we
examine genetic variation - polymorphisms - on the M173 chromosomes, we
can estimate how long it would have taken for our mutational clock to
create it. But if all of the chromosomes are M173, how can we study
variation? Surely they are all identical?
Fortunately for us, they are not. While all of them are very closely
related, and thus share the M173 marker, there are other markers that
help to distinguish them. Unlike the stable markers we have studied to
define the order - or relative dates - of the Y-chromosome lineages,
these other markers do not involve simple one-letter changes in the
{p. 128} genetic code. Rather, they exist because of a biochemical
speech impediment. When we replicate our DNA, the double strands of the
molecule open up and tiny machines known as polymerases actually do the
hard work of assembling the complementary copy. Remember that if we know
the sequence of one strand of the double-stranded DNA molecule, then we
also know the other, because of the inviolable rules of molecular
biology. A always pairs with T, and C always pairs with G. This works
very well for more than 99 per cent of the genome, where the letters
occur in a unique order and it is easy to tell how the pairing should
work. Unfortunately, a small fraction of our genome is not so simple. It
consists of what are known as tandem repeats - short sections of the
same sequence, repeated several times in a row in the DNA strand. These
often take the form of a couple of letters, such as CACACACACA …, but
there can be three, four or more letters in the motif that is repeated.
As you might expect, the polymerase can become confused when it
encounters these parts of the genome. After all, if there are a dozen or
more repeats, how can you tell where you are in the sequence - is it
repeat number ten or eleven? So, in a reasonable number of cases (about
one in every thousand), the polymerase makes a mistake when it is
assembling the complementary strand, and adds or subtracts a repeat. If
the original strand had twelve repeats, the copy may have eleven or
thirteen - simply by chance, due to an error at the molecular level. It
is a process that Luca Cavalli-Sforza has called genetic ’stuttering’ .
One in a thousand may not seem like a very common event, but it is when
we are talking about the work of DNA copying. If that was the rate at
which polymerases made single-letter copying mistakes, then we would
introduce an average of over a million mistakes, or mutations, into our
DNA every time it was copied. Since genetic copying takes place when we
are having offspring, this means that each child would be born with over
a million new mutations. Biology takes a dim view of this level of
mutation, and it is likely that the child would die of a horrible
inherited disease - if it were born at all. Thus, the usual rate at
which new mutations appear is much more sedate, perhaps twenty or thirty
per generation. This is around 100,000 times lower than the mutation
rate we see for repeats, which means that new mutations in ‘regular’
sequences are much less common than those in
{p. 129} repeats. The repeats are on an evolutionary speedway,
accumulating diversity at an extraordinary pace.
While this has very little effect on the health of the child, since
repeats are usually found in regions of the genome that do not affect
well-being, it does give us a tool for studying diversity. These
repeats, known as microsatellites, are particularly valuable when we
want to ask questions about variation on lineages where we do not have
much single-letter variation - such as the M173 chromosomes. They give
us a way to determine absolute dates that we can use to test our
hypotheses about the timing of human migrations. The rate at which
mutations occur is roughly constant, so the level of variation tells us
how long they have been mutating. This tells us how old the chromosome
is, because all of the chromosomes descend from a single chromosome at
some point in the past. By definition, the level of variation at this
point was zero, since there was only one copy.
When several microsatellites from M173 chromosomes are examined, the
level of variation is consistent with an age of around 30,000 years. Of
course, as with any estimate of time, this has a substantial range of
error, but the most likely date for the origin of M173 is around 30,000
years ago. This date means that the man who gave rise to the vast
majority of western Europeans lived around 30,000 years ago - consistent
with a recent African diaspora, and again showing that Neanderthals
could not have been direct ancestors of modern Europeans.
Significantly, it is around this time that the Upper Palaeolithic
becomes firmly established in Europe - and the Neanderthals disappear.
… By 30,000 years ago the Neanderthals had been nearly eradicated, or
perhaps 25,000 years ago they had disappeared entirely. The coincidence
of the genetic and archaeological dates, as well as the increase in
population size implied by the large number of Upper Palaeolithic sites
from around 30,000 years ago, suggests that the invading moderns actually
{p. 130} displaced the Neanderthals.
{p. 132} As we saw, lineages belonging to the Middle Eastern clan -
which we would expect to find if there had been a straight shot out of
Africa to Europe, via the Middle East - are hardly found at all in
Europe. M173, our 30,000-year- old marker, has the advantage of being
present at very high frequency in the most isolated European populations
(including the Celts and the Basques), and its age corresponds roughly
to the inferred date of modern human settlement based on archaeology.
Other major Y lineages present in Europe are younger than M173, and thus
arrived later, or descended from M173 itself. Thus M173 is the likely
marker of the first modern Europeans, defining the European clan. Of
course, it is simply the terminal marker in a long line of genealogical
descent that traces back to M168 and our African Adam The penultimate
marker, though, actually solves the mystery of where the earliest
Europeans came from. This marker, a stepping-stone on the way to M173,
is M45 - making Europeans a subset of the central Asian clan.
As we discussed earlier, the steppelands of 30-40,000 years ago
stretched across a vast swathe of the Eurasian landmass. To Upper
Palaeolithic hunters, this ecosystem would have been a land of plenty
and migration along it would have allowed modern humans to disperse well
to the west, into Europe proper, as well as to the east into Korea and
China. During this period, the steppe zone extended well into
present-day Germany, and may have reached France. We know from bones
that have been found in French caves of 30,000 years ago that reindeer -
a species adapted to the cold steppe and tundra of norther Eurasia -
were common in France around this time. The climate had opened a window
into Europe that allowed these central Asian steppe hunters to enter. As
we have seen, they soon took over, dominating the region within a few
thousand years. …
{p. 149}Middle Eastern archaeologists have found that the end of the
last ice age was a period of intense climatic variation in the eastern
Mediterranean, with a general pattern of change from a continental to a
Mediterranean climate. As archaeologist Brian Fagan summarizes it, this
had the effect of producing an ecological zone with long, dry summers
and short, wet winters. The effect of this climatic change was to favour
grasses, which produce seeds in the spring and then lie dormant over the
summer. Early humans would have exploited the relative plenty of food
during the spring by harvesting large quantities of seed, then storing
it for the rest of the year. This concentrated gathering behaviour would
have favoured a settled lifestyle, which set the stage for the
revolution that was to follow.
After 9000 BC the eastern Mediterranean summer was becoming increasingly
drier as the full effects of rising global temperatures kicked in. This
reduced the yield of cereals, and (as with arid periods in the distant
past) would have favoured mobility. However, the necessity of storing
their gathered grain would have tied the Natufians to one
{p. 150} location. The pull of these two forces - reduced yields and a
relatively immobile lifestyle - would, within a few hundred years, lead
some Natufian settlements such as Jericho to try a new innovation:
planting some of the gathered cereals (which are really seeds) in order
to simplify the gathering process. Kenyon’s work at Jericho traced the
development of the Neolithic, or New Stone Age, following this early
innovation. Archaeologists and anthropologists continue to debate what
happened after the first crops were cultivated - whether the need for a
reliable water supply in order to grow crops led to the development of
irrigation, which may have fostered water-rights issues and social
hierarchies, and so on. What is clear is that the end of the ice age
appears to have set in motion a series of events that were to culminate
with the development of settled, agrarian communities within a thousand
years. What archaeologist V. Gordon Childe called the ‘Neolithic
Revolution’ had arrived.
The second Big Bang
The Neolithic marked a turning point for the human species. It was at
this point that we stopped being entirely controlled by climate - as we
were during our Palaeolithic wanderings - and began to assume control of
our own destinies. …
The second development was that of greatly increased population density.
One of the consequences of cultivating food and settling in
{p. 151} one place is that the necessity of not over-exploiting limited
resources is relaxed - after all, if you want to have more children, you
can simply plant and harvest more crops. While this does oversimplify
the situation, it is true that settled, agrarian societies are more
densely populated than those of hunter-gatherers. Coupled with the
freedom to choose where to live, this can lead to very rapid population
expansions, with agriculturalists spreading throughout a region. It is
estimated by palaeodemographers, who study past population sizes using
archaeological and anthropological methods, that the entire population
of the globe was around 10 million at the time agriculture originated;
by the dawn of the Industrial Age, around 1750, world population had
risen to over 500 million. If Palaeolithic hunter-gatherer populations
had taken over 50,000 years to increase from a few thousand individuals
living in sub-Saharan Africa to a few million scattered around the
globe, clearly the agriculturalists of the last ten millennia were
making up for lost time.
The final new feature of the Neolithic revolution is that it
demonstrates the importance of new technology to human migration. …
{p. 152} While agriculture played a critical role in the development of
modern society, the genetic effects of agriculture were equally
pronounced. While Upper Palaeolithic hunter-gatherers tended to maintain
a relatively stable population size, except via the settlement of new
territory, agrarian societies were able to expand massively without
leaving home. As the first farming communities increased in size, their
inhabitants gradually moved further afield in search of cultivable land.
When they did so, they carried with them their genetic markers. One of
the consequences of this is that we see the expansion of certain genetic
lineages, giving us a glimpse of the origin and spread of agriculture.
In the case of the Middle East, the genomes of today’s western Eurasians
still retain a signal of those events at Jericho 1,000O years ago.
Archaeologists had long known that agriculture spread from its origin in
the Middle Eastern ‘Fertile Crescent’ to Europe over the course of
several thousand years. The earliest evidence is found in the Balkans,
and it appears later and later as you move to the north-west. It is only
relatively recently that ancient Britons left behind their
hunter-gatherer lifestyle, several thousand years after their cousins in
Jericho had done the same. Crucially, it is exactly those plant species
initially cultivated in the Fertile Crescent that make their appearance
in the advancing wave of agriculture as it moved into Europe. It seems
that the European hunter-gatherer lifestyle was replaced by the new
Middle Eastern invention.
In the 1970S Luca Cavalli-Sforza, along with fellow geneticists Alberto
Piazza and Paolo Menozzi, initiated a study of the genetic effects of
agriculture. The question that they asked was about the way in which
agriculture had spread. In particular, they wanted to know if the
migration of agriculture into Europe marked the migration of people, or
simply the spread of a sexy new cultural development - the MTV of its
era. In effect, they were asking a question about the genetic
composition of modern Europeans. Was there evidence for an expansion of
certain genetic markers out of the Middle East, or did modern Europeans
appear to be relatively free of Neolithic markers?
At the time the study was done, the only data available was that on
{p. 153} the ‘classical’ markers we learned about in Chapter - blood
groups and other cell-surface protein markers that served as convenient
polymorphisms but gave little information about their underlying DNA
sequence changes. The analysis of these markers led Cavalli-Sforza and
his colleagues to conclude that there had been a mass migration of genes
out of the Middle East, and the genetic pattern was very similar to that
observed for the first appearance of agriculture: the genetic signal
decreased regularly as you moved from south-east to north-west across
Europe. The methods of analysis used in this study limited what the
researchers were able to infer, since it wasn’t possible to obtain an
accurate date for this migration, but their findings did corroborate the
theory that agriculture had spread with farmers as their population had
expanded, rather than as a purely cultural phenomenon that ‘migrated’ as
Palaeolithic Europeans learned farming skills.
Cavalli-Sforza’ s results became accepted wisdom, leading to what they
called the ‘Wave of Advance’ model for the diffusion of agriculture. The
assumption made by many (although not Cavalli-Sforza and his colleagues)
was that the majority of the European gene pool was Neolithic in origin,
since it was the most pronounced genetic pattern in Europe (although
Cavalli-Sforza’ s later work showed that it still accounted for less than
a third of the genetic variation). Many anthropologists remained
sceptical, but it was to be over twenty years before the model received
a serious re-evaluation. This came in the late I99Os with the detailed
analysis of mtDNA lineages in Europe and south-west Asia by Martin
Richards and his colleagues at Oxford University. In a series of
scientific papers they analysed mtDNA lineages from a selection of
populations across Europe and the Middle East, carefully dating the
lineages using the absolute methods similar to those we learned about
earlier. This allowed them to evaluate the relative contributions of
various migrations to the European gene pool. Their results suggested
that, rather than having a significant genetic effect on the population
of Europe, the expansion of agriculture involved very few Middle Eastern
migrants. Most of the lineages in Europeans seem to have been present
since the time of the Upper Palaeolithic, between 20,000 and 40,000
years ago.
One of the objections to the Richards study, raised by Cavalli-Sforza
and others, was that mtDNA actually provided very little resolution
{p. 154} between European populations. It was difficult, for instance,
to distinguish between eastern and western Europeans with mtDNA data
alone - they have very similar patterns of mtDNA markers. Nonetheless,
the mtDNA result was suggestive. What was needed was to look at the male
lineage, with its greater inherent resolution, in order to see if it
showed the same pattern.
This was finally done in2,000, when Ornella Semino and her colleagues
(among them Cavalli-Sforza) analysed the Y-chromosomes of over 1,000 men
from Europe and the Middle East, looking specifically for evidence of
the agricultural expansion. What they found was that lineages defined by
Neolithic Middle Eastern markers are found in a minority of modern
Europeans. In fact, the results from the Y agree almost perfectly with
the mtDNA data, suggesting that 80 per cent of the European gene pool
traces back to other waves of migration, primarily during the
Palaeolithic. In western Europeans, this Palaeolithic component is
largely defined by our friend from the last chapter M173, which links
Europe back to central Asia. Only 20 per cent of European Y-chromosomes
- defined by more recent markers, particularly one known as M17 - derive
from Neolithic Middle Eastern immigrants. In effect, modern Europeans
are largely genetically Cro-Magnon on both their maternal and paternal
sides.
This is not to say that the advent of agriculture had no effect on
Europe - far from it. There is clear genetic evidence for a significant
European population expansion after the end of the last ice age, almost
certainly aided by the onset of food production. Evidence for this comes
in the form of a recent analysis by David Reich and his colleagues at
Massachusetts Institute of Technology. They studied markers from many
independent regions of the genome and found a pattern of variation
suggesting that the European population underwent a substantial decrease
in population size between 30,000 and 15,000 years ago, as Europe was
moving into the depths of the last ice age. This was then followed by a
population expansion from the few survivors after the end of the last
ice, producing the relative dearth of variation seen in Europe today. In
other words, the human population had been through what is known as a
bottleneck - a reduction in size followed by a period of growth.
Patterns of mtDNA variation also support this model of postglacial
population growth. Archaeological evidence
{p. 155} suggests that the Palaeolithic population of Europe was
confined to Iberia, southern Italy and the Balkans during the period of
most extensive glaciation around I6,000 years ago, and that human
populations then expanded northward during the postglacial period.
Agriculture almost certainly contributed to the end of this population
expansion, because it allowed much higher population densities.
How do we reconcile the pattern seen for the Y-chromosome and mtDNA, of
a Palaeolithic European population relatively unaffected by Neolithic
immigration, with the Wave of Advance? The pattern seen by
Cavalli-Sforza and his colleagues clearly exists, but they were studying
large-scale patterns across the entirety of Europe and the Middle East.
The agricultural expansion was simply one population movement into
Europe - there is clear archaeological evidence for several others. As
their later analysis showed, it still accounted for a minority of the
genetic variation in Europe. Furthermore, because the Wave of Advance
had no estimate of age, the Neolithic component could have been
confounded with Palaeolithic immigration from the Middle East. Finally,
since central Asian populations were not included in their analysis
(there was no data available at the time their study was conducted), it
is possible that the pattern reflects a general trend of migration from
Asia to Europe during the Upper Palaeolithic. After all, if we simply
had Y-chromosome data from the Middle East and Europe, we would infer
that M89-bearing populations had migrated into Europe via the Balkans,
giving rise to M173 in Europe. It is only because we know that M173
arose on an M45-containing lineage that we trace the Upper Palaeolithic
settlement of Europe back to central Asia.
The Y data actually provides a partial solution to the apparent
conundrum. It seems that southern European populations experienced a
greater influx of Neolithic farmers from the Middle East, carrying
lineages such as M17, than did northern Europeans. One possible scenario
is that farming spread first around the Mediterranean, with Neolithic
Middle Eastern immigrants favouring its climate, similar to that of the
Levant. Only later did indigenous Palaeolithic Europeans take up
agriculture in the interior, gradually spreading the culture - but only
a small percentage of the genes - of the Neolithic throughout. The
Cro-Magnons of northern Europe appear to have made a
{p. 156} conscious decision to leave behind the Palaeolithic for a new
Middle Eastern lifestyle with a small minority of Middle Eastern immigrants.
… The pattern of settlement and intense exploitation of a few plant
species that characterized the Middle East was seen at around the same
time in China. Northern Chinese sites such as Banpo and Zhangzhai in
Shaanxi province show early evidence of millet agriculture, around 7,000
BC. Millet, a cereal crop like wheat, seems to have been domesticated
around the Yellow River, spreading from there to the rest of northern
China. The remains at Pengtoushan, on the Yangtze River in central
China, indicate that rice was being cultivated there independently
around the same time. At both sites pottery was used for storing grain,
and the people lived in carefully constructed clay houses, suggesting
that the Neolithic lifestyle was well developed, even at this early
date. Agriculture soon spread throughout China, with rice dominating in
the south, where the wet, humid conditions favoured this grain. Rice
agriculture spread down the Yangtze, and was widespread in southern
China by 5000 BC, perhaps helped by a second, independent domestication
along the south coast. By 3500 BC it was being cultivated in Taiwan, and
by 2000 BC in Borneo and Sumatra. It reached the rest of the Indonesian
archipelago by 1500 BC. Overall, the archaeological evidence suggests
that rice agriculture spread from an origin in central-southern China to
the islands of south-east Asia in the space of roughly 3,000 years -
timing similar to that of the agricultural expansion into Europe.
However, unlike in Europe, there is a very strong genetic signal of this
expansion, suggesting that it was people, and not merely the culture,
that moved.
In Chapter 6 we learned that one descendant lineage of M9, defined by a
marker known as M175, is widespread in east Asia. Based on its present
distribution, this marker probably arose initially in northern China or
Korea. Looking at the pattern of Y variation in modern
{p. 157} Chinese populations, it is now clear that the first
agriculturalists in China were descendants of M175. In fact over half of
the entire male population of China have Y-chromosomes defined by a
marker that shows evidence of a massive expansion in the past 1O,000
years. M122, which first appeared on an M175 chromosome, is now
widespread throughout east Asia. It is hardly found west of the great
central Asian mountain ranges, and does not occur at all in the Middle
East or Europe. This is the pattern we expect to see with a recent
expansion, rather than an ancient event that typically leaves a more
widespread trail.
The genetic data shows that the development of rice agriculture in east
Asia created a Wave of Advance. However, while the wave leaving the
Fertile Crescent for Europe seems largely to have dissipated after
inundating the Mediterrapean, the one leaving China was to saturate the
entirety of east Asia. Today, M122 - marking the descendants of the
first Chinese rice agriculturalists - is found from Japan to Tahiti. A
recent study by David Goldstein and his colleagues at University College
London shows that microsatellite diversity on M122 chromosomes is very
high in China and Taiwan, but drops significantly moving southward into
peninsular Malaysia and Indonesia. This is precisely what would be
expected from a population expansion originating in China within the
past 10,000 years - and exactly parallels the archaeological evidence
for the spread of rice agriculture. Together, MI122 and a related
Chinese haplotype (also a descendant of M175) defined by marker M119,
account for nearly half of the Y-chromosomes in south-east Asia. In
Europe, on the other hand, Neolithic immigrants account for only o per
cent of the present Y diversity. In comparison to Europe, the Wave of
Advance in east Asia appears to have been more of a tsunami.
{p. 158} Early agriculturalists were taking on a new set of risks when
they committed themselves to a settled existence. The most important was
a decrease in the breadth of their resource base. By focusing
cultivation on a few species, they were reducing their choices in the
event of a climatic shift. Droughts, intense periods of cooing (such as
the Dryas periods at the end of the last ice age) and shifts in
watercourses were all very easy to deal with for Palaeolithic
hunter-gatherers. Their response to any of these changes was to move
into another area with better resources. …
The second main worry for our Neolithic agriculturalists was the
increase in disease. While hunter-gatherers may appear to have had a
difficult life, relying as they did on apparently ‘primitive’ technology
and the necessity of killing or gathering enough food to survive, in
fact they were surprisingly healthy. …
Infectious diseases do not arise spontaneously as a by-product of a
settled lifestyle, but rather from exposure to disease-causing organisms
in such a way that transmission occurs from one infected individual to
another. Most diseases can exist only in large populations, where a
threshold number of people remain infected, allowing the disease to
remain in the population. These are so-called endemic diseases, such as
smallpox or typhoid. A population of several hundred thousand is
{p. 159} necessary to maintain the disease - otherwise it is lost
because not enough people remain susceptible to infection. Populations
of this size only arose after the development of agriculture. Other
diseases can be introduced from an outside source, such as an animal.
While human had contact with animals as hunter-gatherers, the sort of
prolonged. close contact that encourages the spread of disease occurred
only aftel the domestication of animals in the Neolithic. …
The final negative aspect of a sedentary lifestyle was the growing
stratification of society. In general, hunter-gatherers are remarkably
egalitarian, having few social divisions.
{p. 160} The onset of the Neolithic established many of the regional
patterns of cultural diversity we see in the modern world. Expanding
waves of agricultural migrants in east Asia spread rice cultivation to
Indonesia and beyond, and today their descendants still carry the
genetic traces of this event. As we saw earlier, the first inhabitants
of south-east Asia may have been more similar to today’s Andamanese or
Semang Negritos. It is likely that most of these groups were engulfed by
the wave of expanding rice-growers, their culture subsumed into the
agricultural mainstream. Similarly, hunter-gatherer groups in Europe,
the Americas and Africa all gave up their Palaeolithic lifestyle in
favour of the new way of feeding themselves. But culture is defined by
far more than eating - it encompasses social traditions, clothing and
tool-making styles, means of transport and thousands of other things.
And one the most important aspects of culture is language.
{p. 161} Language similarities had been recognized since Classical
times, particularly among such well-studied European examples as Latin,
French, Spanish and Greek. By the eighteenth century scholars had begun
to take a broader view, focusing on the languages of Asia, Africa and
the Americas. For instance Janos Sajnovics, in his obscure I770 treatise
‘Demonstration that the language of the Hungarians and Lapps is the
same’, arrived at the conclusion given in the title. We now know that
both Hungarian and Lapp belong to something known as the Ural