The Origin of Bipedalism: hypotheses

Except for the reference to monogamy, this is one of the better hypotheses for the origin of bipedalism. The citations are rather dated, however.

The Origin of Bipedalism

The origin of bipedalism, a defining feature of hominids, has been
attributed to several competing hypothesis. The postural feeding
hypothesis (Hunt 1996) is an ecological model. The behavioral model
(Lovejoy 1981) attributes bipedality to the social, sexual and
reproductive conduct of early hominids. The thermoregulatory model
(Wheeler 1991) views the increased heat loss, increased cooling,
reduced heat gain and reduced water requirements conferred by a
bipedal stance in a hot, tropical climate as the selective pressure
leading to bipedalism.

Hunt’s postural feeding hypothesis asserts that the arboreal food
gathering postures of arm-hanging and vertical climbing, a shared
adaptation and postural specialization of apes, are sufficiently
common to influence anatomy. Both chimpanzee behavior and
australopithecine anatomy inform the model. Eighty percent of
chimpanzee bipedalism is during feeding with arm-hanging stabilizing
the posture 93% of the time in terminal branches and 52% in the
central parts of trees. Torso form in australopithecines features
adaptations to arm-hanging, inferring australopithecine adaptation to
arboreal bipedal fruit gathering. According to Hunt, this early and
specialized origin of bipedalism only later evolved into habitual
bipedal locomotion.

The behavioral model, as presented by Lovejoy, focuses on social
behavioral mechanisms that influence survivorship and birthrate.
Human sexual behavior and anatomy are hypothesized as implying a
monogamous mating structure, a social form seen as prerequisite to
male provisioning. Provisioning behavior with the upper limbs used to
transport food to a mate and offspring is seen as a strong selection
factor for bipedality by directly improving offspring survivorship
and increasing reproductive rate.

Wheeler’s thermoregulatory model proposes, as the selective pressure,
bipedalism conferring reduction in heat gain and facilitation of heat
dissipation. Bipedalism raises the mean body surface higher above the
ground, where more favorable wind speeds and temperatures prevail.
Greater wind flow translates to higher convective heat loss.
Bipedalism reduces evaporative cooling requirements and conserves
body water. Vertical orientation also minimizes direct solar exposure
during the time of day when the solar radiation is most intense.

The timing of the appearance of bipedalism is of critical importance
in assessing these competing hypothesis. The models all present
plausible selective pressures needed for evolutionary change: food
access, provisioning, survivorship assurance, increase in offspring,
predator and injury avoidance and energy and water conservation.
Under different conditions the individual importance of these
pressures will change. The important question is what conditions
prevailed at the time that bipedalism appears in the fossil record.

Early footprints evidencing a convergent toe and well-developed
arches were found at Laetoli, on a paleosurface tuff dated to 3.56
±0.2 mya (Klein 1999:170). Paleoecological reconstructions for that
time include bushland and aquatic fauna at Laetoli and closed
woodland at Hadar. Direct evidence of bipedality in Australopithecus
anamensis dates from between about 3.9 and 4.2 mya (Leakey, et. al.
1995). An A. anamensis tibia from Kenya has bipedal derived Homo-like
characteristics. Bipedality is also inferred for the 4.4 mya
Ardipithecus ramidis by the anterior foramen magnum (White, et. al.
1994). The pelvis and lower body of A. afarensis, dating after 3.4
mya., provides even more extensive evidence of bipedalism.

Vrba’s “turn-over pulse” hypothesis supports a major climate change,
with onset of drier conditions and diminution of wooded habitats,
beginning in the Pliocene around 2.5 mya. Faunal ratios evidence a
change from frugivores to grazers spanning from 2.3 to 1.8 mya,
inferring a mosaic of ecological conditions during that span. Habitat
reconstruction based on faunal associations with hominid fossils
demonstrate that Australopithecus species lived in wooded and well
watered environments (Reed 1997). Homo is the first hominid known to
have adapted to open, savannah-like habitats, well after the
evolution of bipedality. Arm-hanging anatomy persists long after the
onset of bipedal characteristics, indicating occupation of wooded
niches for the early bipedal hominids.

Thus, from an paleoecological perspective, the thermoregulatory model
does not fit the evidence. Perhaps thermoregulatory selective
pressure influenced the direction of Homo evolution, favoring greater
height, but bipedalism is well established millions of years before
the dramatic change in African ecology and before the change in leg
length seen in early Homo. The short lower limbs and broad pelvis of
early bipedal Australopithecines are arboreal adaptations, not
terrestrial, as are long forelimbs enabling greater access to food
and facilitating arm-hanging. These features converge in support of
the ecological evidence.

A. anamensis had primitive large canines. Both A. afarensis and A.
africanus have moderately large canines. With the behavioral model,
which hypothesizes monogamous pair bonding and reduction in mate
competition as changing social factors antecedent to bipedalism, one
would expect an earlier reduction in canine size, in parallel with
the evolution of bipedalism. The social behavioral changes suggested
by this model are far likelier to have occurred, if at all in early
hominid evolution, in tandem with a greater social capability
conferred by the larger cranial capacity seen in early Homo. As with
the paleoecology evidence insinuating habitat change, the hominid
encephalization is far later in the sequence than bipedalism. Fossil
morphology, paleoecology and the fossil chronology converge in
support of the postural feeding hypothesis.

Works Cited.

Hunt, Kevin D., 1996. The postural feeding hypothesis: an ecological
model for the evolution of bipedalism. South African Journal of
Science 92:77-90.

Kline, Richard G., 1999. The Human Career. The University of Chicago.
Chicago.

Leakey, Meave G., Craig S. Feibel, Ian McDougall and Alan Walker,
1995. New four-million- year-old hominid species from Kanapoi and
Allia Bay, Kenya. Nature 376:565-571.

Lovejoy, C. Owen, 1981. The Origins of Man. Science 211:341-348.

Reed, Kaye E., 1997. Early hominid evolution and ecological change
through the African Plio-Pleistocene. Journal of Human Evolution
32:289-322.

Wheeler, P. E., 1991. The thermoregulatory advantages of hominid
bipedalism in open equatorial environments: the contribution of
increased convective heat loss and cutaneous evaporative cooling.
Journal of Human Evolution 21:107-115.

White, Tim D., Gen Suwa and Berhabe Asfaw, 1994. Australopithecus
ramidis, a new species of early hominid from Aramis, Ethiopia. Nature
371:306-312.