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Re: Bipedalism and theorizing... was Re: Morgan and creationists
John Hawks (hawksj@umich.edu)
Mon, 19 Aug 1996 09:38:50 -0400
John Waters wrote:
JW: Me too. It fits my hypothesis that bipedalism was caused
> by the extension in the period of infantile helplessness from
> 24 hours (as in Apes today), to three months (as in Hss today).
>
> . . . Once the period of infantile helplessness
> extended beyond a week, the nursing female would have been
> forced to adopt a bipedal form of locomotion while carrying
> her baby in her arms. This could be expected to occur while
> the specie was still in its Ape habitat.
>
> The mere carriage of the baby in the arms of the nursing
> female would lead to thermoregulation problems for the
> baby. Extended carriage could lead to death by heatstroke
> under extreme conditions. While a reduction in the growth
> rate of body hair would help the infant, the moulting of
> the fetal hair on its head would probably precede such a
> development. This is a characteristic of some human babies
> today. However, it has no affect on the individual as an
> adult.
>
> Does this seem a reasonble proposition?
>
> John
This is a good hypothesis, in terms of its ability to explain several
aspects of human biology. However, I'm fairly sure it is incompatable
with the evidence that we have. I would point out that your hypothesis
makes at least one testable prediction, as follows:
If the shift to bipedalism is the result of the extended period of
infantile helplessness, then evidence for the cause of this helplessness
should appear before or simultaneous to the evidence for bipedalism in
the fossil record. The explanation for infantile helplessness, as I
understand it, is that the larger and more advanced brain of human-like
creatures requires more time to develop than it is possible to maintain
the fetus within the womb and carry out a safe delivery. It is, in other
words, an adaptation to increased brain size operating in the limits on
safe childbirth. Therefore, if your hypothesis is correct, then we
should see evidence for expanded brain size in hominids contemporary
with or even predating the evidence for bipedalism.
This, however, is not the case. The first bipedal australopithecines, A.
anamensis and A. afarensis, do not have brains that are greatly expanded
over the African apes. While a case could be made for delayed childhood
development in these animals (and, I will point out, a case could and
has been made against it), no case has or can be made for greatly
increased difficulty in childbirth. Particularly, there is no evidence
that an ape-shaped pelvis could not deliver an australopithecine-shaped
baby head. The evidence for expanded brain size follows the evidence for
bipedal locomotion by over 2 million years. When brain size expansion
does occur in the fossil record, it is rapid and unmistakable.
On the basis of this evidence, I would consider the hypothesis
falsified. If there is evidence that you can draw on in its favor, I
would be interested to hear it, as the hypothesis is an interesting one,
and it took me a while to evaluate it.
John Hawks
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