Re: Bipedalism and other factors

Harry Erwin (
Tue, 04 Jul 1995 11:11:39 -0400

In article <3t7fjl$>, (Pat
Dooley) wrote:

> 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,

It does. Not quite as arboreal as gibbons but definitely as arboreal as
orangs. Proconsul africanus for example.

> the latter two never went through the major skeletal changes
> required to support bipedalism when they moved to more open environments.

Gorillas followed a different strategy, also adopted by Theropithecus,
Archaeolemur, and Gigantopithecus, of seated feeding, and quadrupedal
'scooting' on one's arse. There are suspicions (presented in a Nature
editorial in the same issue where A. ramidus was announced) that the
bipedal ramidine apes were the ancestors of Pan and possibly Gorilla.

> 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.

To minimize _risk_, not exposure. They are very different.

> 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).

The advantage is very much with the prey in this exchange. There are
species that deliberately show themselves to predators that they've
detected. Apparently it is selectively advantageous in dealing with ambush
predators to let them know that they've been seen so they don't waste your
time and energy. I did a lot of work in ESS theory in this area about 10
years ago (presented at the Sheffield ESS workshop in 1987), and it was
clear that efficient information exchange between predator and prey was to
the clear advantage of _both_ if the ambush was not going to work.

> 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.

Chimps have reduced canines relative to their arboreal ancestors! The
behavioral _differences_ between chimps and baboons are marked.

> It is hard to see how 100% bipedalism, reduced arboreal skills,
> and increased visibility would prove a better strategy in the short-run.

See above. Joel Brown's thesis addressed this, as did my ESS research.

> >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.

Actually it can, but you have to understand how innovations reach
fixation. The term you should check out is 'hitch-hiking.'

> ---trimmed---

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