Re: AAH update (was: Bipe

Harry Erwin (
Tue, 04 Jul 1995 10:13:19 -0400

In article <3t7ar6$>, (Pat
Dooley) wrote:

> (J. Moore)
> << deletions>>
> ---trimmed---
> The recognition that humans probably didn't evolve bipedalism on the
> savannah is relatively recent. The various accounts of how bipedalism
> evolved that I have seen posted here and in the books that I have read
> (Leakey, Johanson etc.) don't give a clear description of how such a
> radical
> evolutionary change as human bipedalism might have arisen. The two major
> evolutionary problems are:
> (i) If bipedalism is such a great adaptation, why is it restricted to
> just one
> primate species. I know this comment will set off Nicholls in
> another
> long-winded exposition about how human bipedalism is just a slight
> exaggeration of an existing ape tendency towards bipedalism.

Actually, bipedalism has been an adaptation followed by perhaps a dozen
primate species, some simultaneously. However, the initial acquisition
does need a 'just so' story. That's why I'm suspicious that the earliest
hominids, perhaps 6 MYr ago, were not much bigger than gibbons and
significantly more arboreal than modern Pan/Gorilla, which appear to me to
have adaptations towards terrestrial quadrupedalism. That would give us an
ancestor with a propensity towards terrestrial bipedalism and with a low
enough weight that use of forelimbs in terrestrial locomotion would not
have significantly increased mobility.

> According to the DNA evidence, Chimps and Bonobos are
> our closest relatives, and we are more closely related to them
> than they are to Gorillas. You would never believe that was
> the case based on comparative anatomy. What is even more
> surprising is the short time frame in which those differences
> emerged. Most of the skeletal transformation occurred in the
> interval between the initial separation from the ape-line, say
> 7.5 mya, and the appearance of fully bipedal Australopithecus,
> say 4 mya. That is extremely rapid evolution, and could only
> have come about due to a major environmental change; a change
> that somehow seems to have bypassed just about every other
> mammal group in Africa over the same time scale.

There was a major environmental change at that time--deforestation as mean
temperatures decreased, grasslands spread, and conditions became more
continental. Investigate the spread of the 'Hipparion' fauna for some
insight. In any case, biological innovations can occur in the absence of
external causes. A number of groups experienced explosive radiations about
that time. We now know that the genetic clock varies in its speed very

> ---trimmed---

> Give it
> >up, Pat; it doesn't exist."
> You might try reading Richard Dawkins "The Blind Watchmaker" before
> making such a statement. It is a simple principle of evolution that
> its progress
> is not directed by the desirability of particular outcomes, but by
> the
> accumulation of small changes that are not, in themselves,
> disadvantageous.

It's _quite_ a bit more complex than you're indicating here. Almost all
features are at fixation at any given time.

> Eyes didn't evolve because 20:20 full colour vision is better than
> being blind;
> they evolved because every slight improvement from a patch of light
> sensitive skin through to modern eyes provided the possessor with a
> slight
> evolutionary advantage over its forbears.

That would imply continuous evolution down a selective gradient. That only
occurs for relatively short periods of time since it is exponentially

> So it is with bipedalism. You cannot argue that bipedalism evolved
> because
> human bipedalism is more efficient, by whatever measure, than
> knucklewalking.

It isn't. It's also no less efficient (Bishop-Cannings Theorem).
Interestingly, we lack evidence that knuckle-walking is as old as
bipedalism. Side question: what _did_ those Miocene ground apes _do_ on
the ground???

> For the purposes of the discussion, we can ignore small burrowing mammals,
> fully arboreal primates, spiny porcupines and the like. That said, we are
> left
> with animals that can:
> (i) run fast enough and long enough to stay ahead of predators,
> e.g. antelopes, zebras etc.
> (ii) are too large for predators to tackle
> e.g. rhinos
> (iii) have a social organisation that allows them to fend off predators
> e.g. chimpanzees and baboons.
> (iv) can run off to the nearest trees and climb rapidly
> e.g. baboons
> Human ancestors could be presumed to adopt some combination of (iii)
> and (iv). However, that supposition presents a number of problems.
> Firstly, none of the other primates that adopt such a strategy has evolved
> any of the human oddities such as bipedalism, hairlessness, sweating,
> and subcutaneous fat. Secondly, it fails to address the problem of
> disadvantageous intermediates. The ape in transition between a quadrupedal
> gait and a bipedal gait would be slower than the ape at either side of the
> transition. It's obvious that the last ape to reach the safety of a tree
> would be
> the first one eaten.

The large ground-dwelling primates (Gorilla, Theropithecus,
Gigantopithecus, and Archaeolemur) appear to have evolved a 'sitting'
strategy. Basically, they sat down to eat, and often moved from place to
place by 'scooting' quadrupedally on their arse. That's hardly as fast a
movement strategy as bipedalism. A bipedal ape would have been the first
one to _see_ the threat or opportunity, and so could have been the _first_
one to reach the tree or the food.

> >
> >Pa> Food gathering? Lots of problems with disadvantageous intermediate
> >Pa> forms.

Interestingly, the cercopithecines and colobines are believed to have been
originally terrestrial quadrupeds with arboreal skills. Their solution to
the food gathering problem was cheek pouches. Only in the largest forms do
we see full terrestrialism and loss of the cheek pouches. Holding food in
the hands or in the cheeks both work.


Harry Erwin
Home Page: (try again if necessary)
PhD student in comp neurosci: "Glitches happen" & "Meaning is emotional"