Re: disadvantageous intermediates?

alex duncan (
5 Aug 1995 17:21:54 GMT

In article <3vstbl$> pete, VINCENT@REG.TRIUMF.CA

AD >`The point here is that the earliest hominids didn€t have to be very
AD >`bipeds to take advantage of the open spaces between trees that
AD >`at the end of the Miocene. All they had to be able to do was move
AD >`one patch of trees to the next before being gobbled by a predator.
AD >`you can€t imagine them doing that, then the patches of trees in your
AD >`are too far apart. Move them closer together and try it again.
>I don't want to make Pat's arguments for him, but his argument is
>not the strawman you have built, because in your argument, you use
>as an example a mutation which is immediately advantageous, rather
>than disadvantageous. Thus you and he are talking at crosspurposes,
>as he clearly has stated that he has no problem with the development
>of characteristics which have marginal advantage. His argument,
>which you skillfully continue to ignore, is that a partially
>bipedal ape is worse off than one fully bipedal or one fully
>quadrupedal. This is a simple sentiment to understand, it is the
>same one my son voiced to me a month or so ago, and the reason for
>it is also simple.

I think Pat's arguement was exactly what I've outlined. Pat can't
imagine a "partially evolved biped" living in a mosaic environment (the
"argument from personal incredulity"). Perhaps you might explain how you
know a partially bipedal ape would be at a disadvantage compared to
either a committed quadruped or a committed biped. Chimps are "partially
bipedal", and other than gibbons, are the most successful extant apes.
Can you demonstrate that a creature who was a little more "partially
bipedal" than a chimp would necessarily be at a selective disadvantage?

>It contains an assumption that the bipedal
>hominids evolved from a quadrupedal ancestor, by slowly straightening
>the hip, lifting the upper body from the ground, implying that
>on the way to full bipedalism they went through a stage where
>the upper body was partially bent forward from the hip, in a
>crouch, which would have been hell on the lower back, and awkward
>for rapid movement.

Actually, you are reading something into my argument that I did not put
there. I've always been disturbed by the idea that bipedalism might
evolve from quadrupedalism. (Despite what I say above about chimps, I
suspect that if you put chimps in an environment w/out trees, their
adaptive response would be to elaborate their quadrupedal capabilities.
But the point is that we can't really demonstrate the results, and we
don't know for sure that a slightly more bipedal chimp would necessarily
be at a selective disadvantage.)

>It seems to me that this sentiment is also perfectly valid, and
>that there is no way in hell that we achieved bipedalism by that
>route. If I was a quadruped, it would take one hell of an impetus
>to get me to go around suspending the front half of my body with
>my lower back muscles all the time. No, I believe the source
>of the problem is that we were never quadrupedal. If we were
>brachiating suspensory feeders, who habitually ran along tree
>branches on our rear legs in upright posture, then we would
>already be essentially bipedal by the time we hit the ground.

Yes, but we would still be "partial bipeds" in the sense that this type
of bipedalism would be a lot less efficient than what we see in modern
humans. (Note: I'm not saying we were not fully committed bipeds -- just
not as good at it as modern humans.)

>We had straightened our backs by hanging from our hands,
>millions of years before we left the trees. The pongids would
>then be the ones who chose to head back toward quadrupedalism,
>by making use of their long arms for locomotion, while we
>found other uses for them.

For the most part I agree with you here. I suspect that when we find the
most ancient hominid, we will not see a lot of similarities to chimps in
the postcrania. My post was in response to the impression I get from Pat
that there was just no way we could evolve into bipeds without going into
the water. Pat seems to think that any movement into open country for a
"partially evolved biped" would be disastrous. In fact, my post really
didn't have much to do with HOW early hominids became "partially evolved
bipeds." It was aimed at Pat's assumption that such a creature couldn't
exist in a terrestrial/arboreal environment.

Alex Duncan
Dept. of Anthropology
University of Texas at Austin
Austin, TX 78712-1086