Meet 'Ardi,' the hottest fossil on the international scene

Scientists in the U.S. and Ethiopia have unveiled fossils from a 4.4 million year old human ancestor. Her name: Ardi. The revelation: The early ancestors to humans were more modern than today's apes and chimps. Ardi represents a middle ground in human evolution and bumps "Lucy," the fabled 3.2 million year old fossil, out of science's limelight.

According to the American Association for the Advancement of Science, the research surrounding Ardi, will appear in the journal Science on Friday. A package of 11 detailed papers "will be the first comprehensive, peer-reviewed description of the Ardipithecus fossils, which include a partial skeleton of a female, nicknamed 'Ardi.'"

The AAAS describes Ardi:

The last common ancestor shared by humans and chimpanzees is thought to have lived six million or more years ago. Though Ardipithecus is not itself this last common ancestor, it likely shared many of this ancestor's characteristics. For comparison, Ardipithecus is more than a million years older than “Lucy,” the partial female skeleton of Australopithecus afarensis. Until the discovery of the new Ardipithecus remains, the fossil record contained scant evidence of other hominids older than Australopithecus.

Through an analysis of the skull, teeth, pelvis, hands, feet and other bones, the researchers have determined that Ardipithecus had a mix of “primitive” traits, shared with its predecessors, the primates of the Miocene epoch, and “derived” traits, which it shares exclusively with later hominids.

Because of its antiquity, Ardipithecus takes us closer to the still-elusive last common ancestor.

The Washington Post paints Ardi in words:

Ardi lived 4.4 million years ago in the woodlands of East Africa. She spent most of her time in the trees. She stood about four feet tall, weighed 110 pounds, and had long arms, short legs, and a grasping big toe that was perfect for clambering branch to branch. She ate in the trees, raised her offspring in the trees, slept in the trees.

But sometimes she came down to the ground, and stood upright. She could walk on two legs. She was, in a sense, taking baby steps on a journey that would change the world.

In an interview, Tim White, a prof at the University of California, Berkeley (hear White's comments on Ardi), who led the research team that found Ardi explained how her remains were painstakingly removed from a treasure trove of fossils. Ardi falls into a prehuman species called Ardipithecus ramidus, which isn't chimp, but isn't human either.

Simply put, Ardi is a bit of an evolutionary tweener. White explains (transcript):

Well, ever since the days of [Charles] Darwin and [Thomas] Huxley, it's been appreciated, first based on anatomy and later of course on genetics, independently on genetics, that our closest living relatives are chimpanzees - African apes that live today in west Africa.

The issue for Darwin was the lack of a fossil record, and in fact, he devoted an entire chapter in his book, On the Origin of Species, to imperfections in the geological record in general. And the human fossil record in those days was extremely imperfect. There were a few Neanderthals known. Well, over the hundred-some [150] years since the Origin of Species was published, people have been collecting human fossils from all over the world, Europe and Asia and Africa, and we had a fairly good fossil record that went back to the time of Lucy and beyond, to between 3 and 4 million years ago. Now, that doesn't get us far enough back to reach that last common ancestor that we shared with chimpanzees.

So, it's been a kind of an elusive organism, as it was for Darwin. We just had a lack of fossils. What we found in Ethiopia at 4.4 million years ago is the closest we've ever come to that ancestor along our own line - unfortunately, the lines leading to modern chimps and gorillas are not even represented in the fossil record. These animals live in places that don't produce a good fossil record. So we have virtually no fossil record for these modern apes. But we have a really good one for fossil humans. Well, really good except it doesn't get us back far enough.

That's really one of the things that lead us in the Middle Awash to want to explore these older deposits, to see what we would find because ever since Darwin, people have sort of assumed that modern chimpanzees haven't evolved very much, that the last common ancestor was more or less like a chimpanzee and that it's been the hominid branch of the family tree, the human lineage, if you want, that's done all of the evolving. But without a fossil record, it's very hard to test that.

So this new evidence, coming as close as we've ever come to that last common ancestor really allows us to infer what that creature was like.

The find raises a bevy of questions. Ardi didn't necessarily evolve into Lucy. It's possible that Ardi's species wasn't a direct human ancestor. However, the Ardi find does indicate that species splintered making human evolution more complicated.

White says:

The other interesting thing as far as Ardipithecus is concerned is to try to understand that transition, from that grade of hominid into the Australopithecus grade - where did it happen, when did it happen, how did it happen, why did it happen?

And then, the other great transition, getting into the genus Homo - stone tools appear at 2.6 [million years ago], hominids expand beyond Africa by 2 million years ago. What's going on there? Very interesting time there because you have multiple lineages of hominids. Very specialized Australopithecus that's still there, still evolving, still just being Australopithecus and then these other forms that have larger brains, that have stone-tool technology and, interestingly that are starting to get involved with scavenging from large-mammal carcasses.

With any luck Ardi will have more tales to reveal for scientists and make us all a bit smarter.

This post was originally published on Smartplanet.com