From: Kevin Bluck (kevin.bluck@mail.com)
Date: Wed Jul 17 2002 - 11:18:43 MDT
> >The best hallmark of direct human ancestry in a fossil in my mind is the
> >presence of an upright two-legged gait. The other apes are all quadrupeds,
> >although they can move awkwardly on two legs for a limited distance.
>
>Yes, *but* - chimps and gorillas knuckle-walk, while the ancestral state
>is palm-walking. This strongly suggests chimp and gorilla ancestors were
>bipedal at some point. Since all the African ape fossils from about 2 to
>6 million years ago were bipeds, I think that was the primitive state for
>all African apes. Human went all-the-way upright, and other apes
>almost-all-the-way knucklewalking.
Perhaps. It still leaves open the question of *why* humans would abandon
quadripedality altogether. This didn't just happen for no reason, given
bipedality's considerable disadvantages.
However, if one thing is clear, it is that hominid fossils are very
difficult to find, some geologies are very adverse to forming fossils at
all, and there are huge gaps in the record. I think it is more likely that
the direct ancestors of gorillas and whatnot simply haven't been found,
possibly because they lived in regions where fossils did not form well or
have been destroyed by geologic forces.
>Gibbons, incidentally, are bipeds too. It's not a human-only
>characteristic. We're just the best at it.
I think it is more accurate to say that gibbons are capable of
approximating bipedality. They can also walk quadripedally with the same
facility, which isn't saying much; they're not very good at either method.
Their primary method of locomotion is brachiation. They rarely touch the
ground, branch-swinging making up more than 90% of their total locomotion.
Their "bipedality" can easily be dismissed as a fluke - they have no need
to walk any long distance, they are very flexible, they have good balance,
and so they are democratic about their preferred gait when they decide to
shamble over a few feet along a branch. Who knows, they may even do it just
for fun, like a kid will balance on a curb.
Humans, on the other hand, are true bipeds. We have abandoned
quadripedality altogether. It is more difficult for us to walk on all fours
than it is for a chimp to walk upright. Such a radical departure didn't
"just happen". There would have been a transition during which we weren't
particularly good at either. That had to take place in an environment
forgiving of our clumsiness. I don't believe tree-dwelling is the answer.
If you spend all your time in trees, as the gibbon, it seems to be you
would simply stay bad at walking by any method. If you spend significant
time on dry land, even if you are a regular tree-climber, it still favors
the more stable quadripedality. The farther you move from the forest, the
greater the pressure to go on all fours. Baboons are a good example of what
happens when a primate is kicked out of the forest.
That said, certainly a brachiating past is a *preadaption* to a bipedal
gait. The ability to stretch upright makes bipedality in the human mode a
realistic possibility. Certainly, if our ancestors had been non-brachiators
like, say, horses, they would not have stood on their hind legs in the
water. They would have become wallowers, like a hippo.
> >For such pressures to produce a bipedal gait and
> >then opposite pressures to revert later descendants back to knuckle-walking
> >quadripedalism seems to me vanishingly unlikely.
>
>No; knuckle-walking has lots of speed advantages, it's just hard to carry
>things.
Not really. Apes carry things all the time with no obvious difficulty. They
just adopt a modified one-knuckle gait that is still way more efficient
than two legs. Or they use their mouth, also quite common. It's not like
they have bedroom furniture to rearrange. ;-)
>The spectacular expansion of Erectus could have driven all the
>australopithicines out of their niches. Indeed, it's about when Erectus
>shows up that the australopithicines-that-look-like-gorillas go "poof" and
>the gorillas appear.
Possible. However, besides the regression of the upright gait, there is the
issue of diet. Australopithecus seems to have enjoyed a human-style
omnivorous diet, while gorillas are leaf-eaters, living on stuff that would
be indigestible to humans. I just don't see two major attributes swinging
one way, then back the other. Especially with the geneticists' evidence
backing it up, I believe the hominid/ape split predates Australopithecus
and the onslaught of the Homo species simply drove the Australopithecines
into extinction.
>Sapiens leaves huge shell middens which no prior primate did, so Sapiens
>was doing some swimming prior humans didn't. So, we have an adaptation to
>help with dunking.
The bones of "Lucy" were discovered at what then would have been the Sea of
Afar, among the eggs of turtles and crocodiles and the remains of crabs at
the periphery of a flood plain close to what would have been Africa's coast
at the time. This is commonly dismissed as coincidence, but why should we
just assume that the australopithecines were not amphibious? There is
certainly nothing to indicate that they weren't, just our humanocentric
prejudices. The line of reasoning seems to be "We're not water animals
(really?) so Australopithecus must not have been either." I think the
question is important enough that it deserves more than a hand-waving
dismissal, which is all most anthropology professors will give it.
This could be another answer to the "missing fossil" question you have. If
these hominids were in fact waders, they would tend to die near the water.
Inundation in mud is one of the best methods of forming fossils, and so
perhaps we are finding a relatively high percentage of those fossils. The
gorilla ancestors presumably would have been living in similar environments
to modern gorillas, which might have been less conducive to fossil
formation and/or preservation.
> >In my opinion, the answer to the question of human origin all begins with
> >our wacky habit of walking exclusively on two legs. It's ridiculous, when
> >you think about it. Damn poor engineering. It exacts an enormous physical
> >price.
>
>Based on gibbons, it seems a natural ground transportation method for a
>suspensory climber. If the australopithicines needed to climb, they had
>good reason to walk that way.
I don't think gibbons *have* a "natural ground transportation method". It's
not natural for them to be on the ground at all. You might as well say that
flopping about is the natural method for a fish to move across the land.
Ability to move on the land, whether gibbons or fish, is not important to
their survival and reproduction, and so there is no natural selection
pressure for them to optimize that ability.
There are a great number of primate and other mammal species that do move
about on the ground as well as in the trees. None of them have adopted a
bipedal gait which they use for moving any great distance. Evolution's
unanimous vote on this matter should tell us that walking on two legs is
*not* a natural method for any class of tree-dwelling mammal that needs to
walk any distance greater than a few feet on dry land.
Something unique happened to our ancestors, something that drove them into
adopting a thoroughly *unnatural* gait. That is why I believe solving the
question of bipedality solves everything about our origins. To my mind, the
most plausible explanation presented thus far is wading. Wading in deep
water both facilitates and encourages an upright posture for an animal
capable of assuming it. All the other explanations seem to be based on
sentiments of "because we could, we just did. It's evolution!" As humans,
we have a superiority complex, and we tend to reason rather circularly that
bipedality is naturally superior because we are naturally superior, and
after all we are bipedal, so it must be superior. Remember those "ascent of
man" drawings in the textbook, how the lowly ape slowly stood up to become
the confident, kingly man striding into the future? (Always a man, too,
never a woman.)
Let's discard our species-centrism for a moment and consider whether
walking upright might actually be a sort of evolutionary kludge, a clumsy
solution to a traumatic change, not a mark of superiority at all.
--- Kevin
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