michaelm@vax.3Com.Com (Michael McNeil) (03/26/88)
In article <4736@aw.sei.cmu.edu> firth@bd.sei.cmu.edu.UUCP (Robert Firth) writes: >In article <1125@3comvax.3Com.Com> michaelm@3comvax.UUCP (Michael McNeil) >writes: > >>I might also add that since our closest living relatives are the >>gorillas and chimpanzees, which are much more markedly dimorphic, >>humans most probably lost much dimorphism during our evolution. > >Well, since both pongos and chimpanzees are more highly differentiated >than humans (and read that as "more evolved" if you wish), it is not >impossible that the opposite is the case - that they have gained >some dimorphism rather than that we have lost some. I agree it's *not impossible* that chimpanzees and gorillas gained dimorphism rather than humans having lost it, but it's less probable. The reason is due to the fact that humans, chimpanzees, and gorillas apparently are roughly equally divergent from a common ancestor. In such a three-way branching situation, if two species share a trait -- such as the knuckle-walking behavior of chimpanzees and gorillas -- it is quite likely that our common ancestor possessed that trait as well. The alternative -- that convergent evolution independently invented a trait within different species -- is of course possible, but generally considered somewhat less likely, depending on the trait's complexity. Follow-ups to sci.bio and talk.origins. Michael McNeil 3Com Corporation Santa Clara, California {hplabs|fortune|idi|ihnp4|tolerant|allegra|glacier|olhqma} !oliveb!3comvax!michaelm ... It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present. But if (and oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a proteine compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed. Charles Darwin, 1871
straney@msudoc.ee.mich-state.edu (Ronald W. DeBry) (04/01/88)
I *think* that I correctly sorted out all the >>>>>>s - I hope so. Michale McNeil wrote: >>>I might also add that since our closest living relatives are the >>>gorillas and chimpanzees, which are much more markedly dimorphic, >>>humans most probably lost much dimorphism during our evolution. Robert Firth replied: >>Well, since both pongos and chimpanzees are more highly differentiated >>than humans (and read that as "more evolved" if you wish), it is not >>impossible that the opposite is the case - that they have gained >>some dimorphism rather than that we have lost some. > I don't know what you mean by several of your terms here. Did you intend "pongo" to mean gorilla? _Pongo_ is the genus name for orangutans. Currently, as a family name (Pongidae), it refers to orangs, gorillas and chimps (its only our keen sense of anthropocentrism that keeps a separate family for humans, there ought to be no such thing as the Hominidae). Anyway, next, what do you mean by "more highly differentiated" than humans? More different from what? A common ancestor? More different from each other than either is to humans? If I had to pick one as "more different from a common ancestor" I would pick humans, on the basis of their radically enlongated development compared to any other primate. Even so, that has no necessary bearing on any other character. It could, of course, be true that the developmental differences have pleiotropic effects on sexual dimorphism, but we don't have anything like the data needed to show that. Of course, just because your resoning wasn't perfect, that doesn't guarantee that your conclusion must be wrong. In fact, it certainly is possible that chimps and gorillas have gained dimorphism. (by the way - where did the measures of dimorphism come from in the first place? I mean, is it really true that humans are less dimorphic than the rest of the Pongidae?) Michael McNeil replied (to Robert): >I agree it's *not impossible* that chimpanzees and gorillas gained >dimorphism rather than humans having lost it, but it's less probable. >The reason is due to the fact that humans, chimpanzees, and gorillas >apparently are roughly equally divergent from a common ancestor. In >such a three-way branching situation, if two species share a trait -- >such as the knuckle-walking behavior of chimpanzees and gorillas -- it >is quite likely that our common ancestor possessed that trait as well. >The alternative -- that convergent evolution independently invented a >trait within different species -- is of course possible, but generally >considered somewhat less likely, depending on the trait's complexity. No iron-clad guarantee that the answer you get will be right, but the best approach in this situation is to look at the next "outgroup", in this case the orang. If they are strongly dimorphic, then it strengthens the case that humans became less so. Another by the way: The causes (meaning selective pressures) that lead to any given level of sexual dimorphism are one of the big, hotly contested areas in evolutionary population dynamics. Theories are about a dime a dozen, and equations purporting to support of theory or another go for about 50 cents :-). Ron DeBry Dept. of Zoology Michigan State University
malc@tahoe.unr.edu (Malcolm L. Carlock) (04/05/88)
Concerning the debate over whether humans have become less sexually
dimorphic, or the other pongidae (gorillas, chimps, orangs) more dimorphic:
An important characteristic that distinguishes adult humans from adult
members of the other pongidae is the fact that the opening at the base of
the skull through which the spinal cord passes (sorry, I can't remember
what it's called) remains nearly centered at the bottom of the skull
throughout the life of the individual, helping to allow an upright posture.
In the case of the other pongidae, infants are born with this opening
in the central position (as in humans), but as an infant matures, the opening
migrates toward the rear of the skull, resulting in a more horizontal attachment
of the head to the body, in harmony with a more hunched ("apelike") adult
posture.
The result of all this is that adult humans tend to resemble INFANT chimps/
gorillas/orangs MUCH more than they resemble the adults of those groups
(large head relative to body, small face, spinal cord opening at bottom of
skull, etc.)
What is all this leading to? Well . . .
A common characteristic of mammals, including apes, is that sexual dimorphism
prior to puberty is almost nonexistent (other than obvious differences in the
construction of sexual organs).
Since adult humans tend to resemble infant apes, and infant apes display far
less sexual dimorphism than adult apes, then it would seem that humans have
indeed "lost" some of their sexual dimorphism relative to the other pongidae.
. . .
The phenomenon of one species actually being an underdeveloped form of another
species (in this case, humans being (physically) "underdeveloped" apes) is
known as neoteny, and much evidence has been accumulated which supports the
neotenic view of human development. For an interesting discussion of neoteny,
see Stephen Jay Gould's book, "The Mismeasure of Man" (well worth reading in
any case).
Cordially,
Malcolm L. Carlock
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