Human, chimp lineages interbred after splitting, study suggests [they still do; for ex., Heidi Klum and chimp “Seal”]

December 30, 2006 on 2:50 pm | Friedrich Braun | Evolution, Origin of Man , Science & Technology | | Email This Post | Print this Post

gast4.jpg
Heidi Klum’s ugly little hybrid with chimp “Seal”.

Human, chimp lineages interbred after splitting, study suggests

May 17, 2006
Courtesy The Broad Institute of MIT and Harvard
and World Science staff

Probably the most shocking aspect of Darwin’s theory of evolution has
always been its implication that we descend from ape-like ancestors. But
that idea may be easy to stomach compared with new findings.

Skull of “Toumaï” or Sahelanthropus tchadensis, thought to be the earliest
fossil from the human family tree. If the results of a new study are
correct, it could have come from a time when the chimp and human lineages
had begun to split, but were still interbreeding. (courtesy M.P.F.T.)
A study has concluded that human and chimp ancestors may have interbred for
a long time after their two lineages began to split apart evolutionarily.

The research also found the final separation was more recent than previous
research suggested.

“The study gave unexpected results about how we separated from our closest
relatives, the chimpanzees,” said David Reich of the Broad Institute of
Harvard University and the Massachusetts Institute of Technology in
Cambridge, Mass.

Reich is senior author of a paper detailing the findings, published in the
May 17 online edition of the research journal Nature.

“Something very unusual happened at the time of speciation,” he added.

Speciation, the evolutionary branching off of new species from existing
ones, is the key mechanism that creates new species, according to
evolutionary theory. Since chimps are our closest relatives, our speciation
from them would be the pivotal event that put us on the road to humanhood.

Previous genetic studies have focused on the average genetic difference
between human and chimpanzee across their genomes.

By contrast, the new study scrutinized the variation in evolutionary
history across the whole human genome. In theory, some regions of the
genome should be “older” than others, the researchers explained. That is,
different regions should have characteristics traceable to different times
in the evolutionary history of the common ancestors of humans and chimps.

This analysis led to three surprising conclusions, the scientists said:

* The time from the beginning to the end of the splitup ranges over
more than 4 million years across of the genome. In other words, the date of
the divergence seems different depending on where in the genome you
look—suggesting the process may have been gradual, and marked by interbreeding.
* The youngest genomic regions are surprisingly recent, no more than
6.3 million and probably no more than 5.4 million years old. This would
suggest the final speciation itself occurred on the same time frame, more
recently than scientists previously thought.
* The X chromosome, which contributes to sexual characteristics, falls
almost completely at the more recent end of the time frame.

Chromosome X’s young age is a “smoking gun” for interbreeding, said Eric
Lander, a co-author of the paper and director of the Broad Institute.

Interbreeding is known to produce strong pressure for evolutionary
change—called selective pressure—in sexual characteristics, the scientists
said. That, they added, could explain the chromosome’s young age.

The researchers said their estimate for the time of the final splitup is
more recent than previous figures based on studies of the famous Toumaï
fossils, widely thought to be the oldest from the human family. Those
previous estimates put the divergence time at between 6.5 million and 7.4
million years.

The Toumaï fossil may itself be “more recent than previously thought,” said
the institute’s Nick Patterson, one of the authors of the new study. “But
if the dating is correct, the Toumaï fossil would precede the human-chimp
split. The fact that it has human-like features suggests that human-chimp
speciation may have occurred over a long period with episodes of
hybridization between the emerging species.”

Hybridization, or interbreeding, is thought to play a common role in plant
speciation, but not usually in animals. However, the apparent lack of such
events among animal species, Reich said, “may simply be due to the fact
that we have not been looking for them.”

http://www.world- science.net/ othernews/ 060517_hybridfrm .htm

3 Comments »

RSS feed for comments on this post. TrackBack URI

  1. The same phenomemon, that is, newly-emerging species continuing to breed with progenitor species before the completion of speciation or even after completion, was seen millions of years later in the process in which our species, Homo sapiens sapiens, emerged from the pre-human progenitor species:

    ______

    “The speciation event that produced Homo sapiens sapiens could not have occurred contemporaneously in more than a very few individuals. It follows that those few s. sapiens would have possessed a very restricted sample of the progenitor species’ genetic diversity. However, the diversity observed in current populations implies that there were never less than several thousand breeding pairs in the human ancestry (Harpending et al., 1998). Accordingly, the founding s. sapiens and their descendants must have interbred with the progenitor species (and perhaps other pre-human populations) in order to preserve the diversity which exists today. While some changes in the genome must have occurred after the speciation event, the ‘lifetimes’ of the genetic elements considered (in the works cited here) are far longer than new estimates of s. sapiens’ age (Mountain et al., 1994). As a consequence, most of the current diversity must be the result of interbreeding with pre-human populations.”

    ______

    The author of the linked article, R. Fonda, thinks there’s evidence H. sapiens sapiens bred with pre-human species possibly including H. erectus, producing certain modern populations which are in fact interspecies hybrids. I see every reason he might be onto something. Certain of his ideas are, needless to add, shunned by the politically correct among the scientific community. But truth always outs in the end: the days of lies are always numbered.

    Comment by Fred Scrooby — January 1, 2007 #

  2. Here’s what Fonda’s article says about that last issue I referred to (this article is something like five or six years old but I don’t think he’s changed his mind):

    ______

    “The radiation of low-diversity s. sapiens from Eurasia is also the best explanation for the discoveries, dates, morphology and genetic data in S. E. Asia. There, s. sapiens and H. erectus lived in proximity for as long as 20,000 years (Swisher et al., 1996), evidently interbreeding to produce extant population types. [Emphasis added, here and below.] Many students of fossil morphology have long contended that there is continuity between S. E. Asian Hominid fossils and extant indigenous peoples.8 Genetic data show these populations are distinct from northern Asian populations and of comparable diversity to Africans (Chang et al. 1996).9

    -

    “The Ngandong specimens, in particular, have occasioned much debate on account of their mixture of s. sapiens and erectus traits and their affinities with extant Australian populations.10 We would expect that the skulls of such hybrids would show affinities to both species, and that is why these fossils are so hard to classify. Some authorities say they are clearly erectus, while others point to modern traits, and especially that very similar skulls (from overlapping dates) are found in Australia. Moreover, the traits in question occur in the modern population. This is not merely consistent with, but constitutes strong evidence for, the view that radiating, low-diversity s. sapiens interbred with relic erectus populations to the extent that they acquired near-African diversity. Primitive morphological traits are manifest in the Asian fossil record and in living people.

    -

    “The hypothesis presented here uniquely explains one particular aspect of the Australian fossil record. The oldest fossils from Australia are the most modern in morphology. On this view, this is explained by the fact that the first humans that passed through S. E. Asia on their way to Australia were less hybridized with resident erectus populations because they spent less time living among them. Populations that settled Australia later (leaving the Kow Swamp-type skulls) had been living in S. E. Asia for as much as 20,000 years and were far more hybridized in consequence.

    -

    “Wolpoff accepts that the Ngandong skulls are representative of the population which produced the Kow Swamp-type specimens, and left descendants in the modern population. But he explicitly rejects the view, as set forth here, that there was inter-species gene flow, and calls it ‘unacceptable.’ This, however, is a socio-political rather than a scientific statement. He does not contend that it is an unreasonable construction of the data, but rejects it on grounds of dogma, because of its implication that some modern populations express a more primitive genome. Wolpoff considers that the hypothesis of hybridization is ‘unacceptable’ because it ‘raises the specter that some human populations can be interpreted to differ from others because they have more genes from an extinct, primitive human species.’ Thus, according to Wolpoff and other adherents of this doctrine, scientific truths which conflict with their politically-correct ‘just so’ paradigm are outside the bounds of contemplation.”

    Comment by Fred Scrooby — January 1, 2007 #

  3. I neglected to close the “bold” tag above, sorry about that!

    Comment by Fred Scrooby — January 1, 2007 #

Leave a comment

XHTML: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <code> <em> <i> <strike> <strong>

Powered by WordPress.
Entries and comments feeds. Valid XHTML and CSS. ^Top^

Donate towards my web hosting bill!