Ardipithecus ramidus

What a day to be stuck in airplanes for hours on end; I had to slurp in a bunch of files on my iPhone and then look at them on that itty-bitty screen, just to catch up on the story of Ardipithecus. Fortunately, you can just read Carl Zimmer's excellent summary to find out what's cool about it.

For a summary of a summary: it's another transitional fossil in our lineage. Ardipithecus ramidus is old, 4.4 million years or so — so it's well before Lucy and the australopithecines. The latest result is a thorough analysis of a large number of collected specimens that shows it is an interesting mosaic of traits: it was bipedal, but not quite so well adapted to terrestrial locomotion as we are, and it had feet with an opposable big toe. And of course it had a small brain, only a little larger than a chimpanzee's.

Digital representations of the Ar. ramidus cranium and mandible. (A to D) The ARA-VP-6/500 and downscaled ARA-VP-1/500 composite reconstruction in inferior, superior, lateral, and anterior views (in Frankfurt horizontal orientation). (E) Individual pieces of the digital reconstruction in different colors. Note the steep clivus plane intersecting the cranial vault on the frontal squama (as in Sts 5 and not apes). (F and G) Lateral and superior views of the ARA-VP-1/401 mandible (cast). (H and I) Lateral and superior views of the ARA-VP-6/500 left mandibular corpus with dentition.

Ardipithecus is clearly different from (but related!) to us, and it's also very different from a chimpanzee. One thing I'm finding baffling in all the commentary is the argument that this somehow shows that the last common ancestor of humans and chimpanzees would have been very unchimpanzee-like, and perhaps closer in morphology to us than to modern chimps. I'm not buying it. Has anybody actually ever suggested that chimpanzees have been in a state of relative stasis for 6 million years? Chimps have evolved in parallel with us for all of that time, so that argument is addressing a non-controversy, or at least, an argument that should have been recognized as silly all along.

We're also going to have to push the fossil record back another couple of million years to get to that last common ancestor, and there's no reason to presume that Ardi's ancestors weren't also rather different from Ardi. We also need to know more about the breadth of the primate family tree at that time; was Ardi a weirdly specialized sub-branch, or actually representative of a wider trend in the ape species that would lead to us? I think this image is a nice way to illustrate Ardipithecus's place in the family tree.

Evolution of hominids and African apes since the gorilla/chimp+human (GLCA) and chimp/human (CLCA) last common ancestors. Pedestals on the left show separate lineages leading to the extant apes (gorilla, and chimp and bonobo); text indicates key differences among adaptive plateaus occupied by the three hominid genera.

Don't get me wrong: Ardipithecus is a magnificent addition to our family album, and the author's of the multiple papers that have come out have done a very impressive job of analysis and documentation. We can all jump up and down with joy at these new data, and we can rightly point to this species and say, "Transitional form! Boo-ya, creationists!"

Unfortunately, I'm also seeing the press mangling the story already. National Geographic says, Oldest "Human" Skeleton Found—Disproves "Missing Link", which is annoying. The article itself isn't bad, but can we just kill the "missing link" nonsense altogether? It's as if the only way some science journalists can grasp a new discovery is by relating it to a misbegotten misconception.

The prize for the very worst coverage has to go to Metro News and the Torstar News Service (is that from the Toronto Star?). They put up an article titled New theory may answer missing link question, which opens with the bizarre assertion, Man didn't descend from apes. There is no new theory here. There is new evidence and further data documenting the details of one lineage's descent. And if you put the phrase "missing link" in your headline any more, we're going to have to put a silly hat on your editors and make them sit in a corner.

But the very worst part is this misinterpretation of the suggestion that the LCA of humans and chimps would have had characters we consider human-like. I guarantee you that this will be the core of the creationist response to Ardi in the near future.

The four-foot, 110-pound female's skeleton and physiological characteristics bear a closer resemblance to modern-day humans than to contemporary apes, meaning they evolved from humanlike creatures -- not the other way around.

Brace yourself, gang. The creationists are going to be claiming that this shows humans were created first, and all of these other hairy beasts the paleontologists are digging up are just degenerate spawn of the Fall.


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