Re: Bipedalism and other factors
Pat Dooley (patdooley@aol.com)
2 Jul 1995 21:02:45 -0400
I wrote:
>>Which leads back to the $64k question. If bipedalism wasn't a savannah
>>adaptation, what was it?
>>
>>Arboreal? The arms would be more orang like, and the legs much shorter.
>
Alex Duncan responded:
>How about this: hominids evolved from an ancestor that was so specialized
>for an arboreal existence that its only effective mean of terrestrial
>locomotion was bipedalism (see modern gibbons). In a fragmenting forest
>environment, those pre-hominids that were most adept at moving from tree
>to tree ON THE GROUND would have been selected for. An important thing
>to note here is that this model doesn't suggest that pre-hominids adapted
>bipedalism because living in open country was so wonderful. They did it
>because they needed to cross open country from one patch of trees to the
>next. As time went on, and aridification increased (e.g. terminal
>Miocene climatic event) the patches between trees became progressively
>larger and larger, selecting for more and more efficient bipeds.
>Eventually the adaptation to crossing open ground became effective enough
>that early hominids began other activities in open country (looking for
>food, etc.).
>
Hominids and modern chimpanzees share a common ancestor from about
7.5 mya. Gorillas branched off about 2 million years earlier. What you are
claiming for the ancestral hominds must also be true for chimpanzees and
gorillas; that their immediate ancestors were as arboreal as modern
gibbons. Even if this true, and I don't know if the fossil evidence
supports
the claim, the latter two never went through the major skeletal changes
required to support bipedalism when they moved to more open environments.
The problem with your scenario is that energy efficiency is far less
relevant
than other factors. The initial evolutionary imperative would have been to
minimise exposure on the ground rather than maximise energy efficiency.
Evolving a whole new mode of locomotion doesn't satisfy that imperative.
Walking fully upright rather than staying low doesn't satisfy that
imperative
either (those who claim that bipedalism makes it easier to spot predators
should realise that supposed advantage cuts both ways - it also makes it
much easier to be seen by predators).
Baboons and chimpanzees separately evolved similar strategies for living
in a savannah environment. They developed a social structure that
provided protection against predators and they retained good arboreal
skills. The males, especially in baboons, have powerful canines and
could certainly inflict significant damage on a leopoard.
It is hard to see how 100% bipedalism, reduced arboreal skills,
and increased visibility would prove a better strategy in the short-run.
>I can think of no oddities that wading and swimming fit in with. Please
>enlighten me. I'm not suggesting that early hominids didn't occasionally
>enter the water, but to postulate an aquatic existence as the precursor
>to all that is "hominid" flies in the face of all of the evidence I'm
>aware of.
It seems odd to me that humans have aquatic capabilities far greater
than those of any other apes or primates. Evolution doesn't fill your
kitbag with capabilities you never use nor ever used. In particular,
it doesn't equip you with conscious control over your breathing, if
you don't need it. It doesn't heighten your diving reflexes if you
don't need it. It doesn't give you a layer of subcutaneous fat if you
don't need it. It doesn't give you the ability to dive to 250 feet if you
never dive. It doesn't add salt to your tears and sweat if you never
needed to exude it. Yet, evolution bestowed all those useless gifts
on homo sapiens, and all in the last 7.5 million years. Despite having
98% of their DNA in common with us, chimpanzees share none of those
features with us. They also missed out on 100% bipedality,
eccrine sweating, and loss of most of their body hair.
Thus, when you propose a scenario for the evolution of bipedalism that
could equally well fit savannah chimpanzees yet fails to account for any
of
the other oddities, I have to wonder how plausible your scenario is.
Re Leopards and Taung. I should have checked my references
rather than relying on a memory showing premature signs of parity
errors. I got the name wrong and conflated another Australopithicene
fossil fragment with the Taung boy. The skull fragment showed two
puncture marks that match the space between a leopard's canines.
Pat Dooley
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