It is doubtful that the derived character of knuckle walking in chimps (both species) evolved after the separation of chimps and humans. You see, gorillas are also knuckle walkers. Thus, such a proposal suggests that this unique method of walking evolved in two separate species. Humans and chimps probably separated some 6 or 7 million years ago. They separated from the gorillas around 10 million years ago. It's not likely that both gorillas and chimps developed identical methods of locomotion, including the structure of the wrist that locks when these knuckle walkers put their knuckles on the ground, after being separate species for 5 million years or so.
Let me explain what I'm talking about. Lean down and put your knuckles on the ground similar to what a football player does in a down stance. Keep your thumb along the side of your hand. Now lower the heel of your hand to the ground. Easy, right? Neither a chimp nor a gorilla can do that. There is a knob-like protrusion on the distal end of the radius of all three of these species that prevents the hyper-extension of the wrist. We humans lack this protrusion and can thus flex our wrists forwards and backwards.
Several years ago an anatomist who was familiar with both human and chimp anatomy had an opportunity to examine "Lucy" (AL 288-1), whether the actual fossil or a cast I do not know, and observed a protrusion on the end of the radius similar to that of chimps and gorillas. At the time, I was in communication with two graduate paleoanthropologists who were as surprised as everyone else by the paper. Both of them confirmed that it was true. It had never been noticed before.
More recently, when "Lucy" began her tour of the US at the Houston Museum of Natural Science, I had an opportunity to observe the actual fossil from mere inches away. On my second visit I went armed with life-sized pictures of human wrists. I can confirm that "Lucy" did have this wrist locking extension on the end of the radius.
I can only conclude that knuckle walking is a derived trait that has been lost in humans and retained in related species. Otherwise we are postulating that gorillas first separated, then chimpanzees (and then chimpanzees separated into two species) from the lineage that lead to mankind. Additionally we are postulating that after all of these separations, all of them developed identical structures that lock the wrist. After that, the lineage that led to humans lost this mechanism. We know that "Lucy" was a biped. Why on earth would that species have developed this wrist walking mechanism along with the three other species, only to lose it again. Is it not more logical to understand that the wrist-locking mechanism was vestigial in "Lucy"?
So, is this mechanism missing in "Ardi"? Then "Ardi" cannot be ancestral to "Lucy". Both can be, and obviously are, transitional between our common ancestor with apes and us. But we can't state to a logical certainty that either is our ancestor.
That's how I understand it. If I am wrong, then I would appreciate knowledgeable correction.
Edited by pandion, : No reason given.
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