Ardi: Looking at the Latest Missing Link

Virtually everyone with any access to news last week probably heard about Ardi, a 4.4 million year old skeleton of a human ancestor found in Ethiopia. However, given the tendency of the mainstream media to cover every ancient primate discovery as "Scientists discover 'missing link' which 'changes everything'" those who don't track these things can easily become confused, or even rather suspicious of the whole thing.

So, what is Ardi, and why is this discovery a big deal?

Ardi is a 45% complete skeleton of a female individual from the hominin species Ardipithecus ramidus. This is not a new species: we've known about Ardipithecus ramidus since a small number of bones from a member of the species was found in 1992 and formally described and named in 1994. Living about 4.4 million years ago, Ardipithecus ramidus is also not the oldest human ancestor known or a common ancestor between humans and our apparent closest genetic living relatives, the chimps. However, the excitement about Ardi (found along with less complete remains of a number of other Ardipithecus ramidus individuals and also fossil evidence about the plants and animals present in their environment) is not just hype. It is a very important find. Here's why:

Very Complete, Very Old
Invariably, Ardi has been compared to the other famous hominid find, Lucy who made headlines back in the 70s. However, Ardi is both more complete than Lucy and also over a million years older. Lucy was a 40% complete skeleton, about 3.2 million years old, belonging to the species Australopithecus afarensis.

We have a few fossil finds from hominid species which are older than Ardi, but we don't know nearly as much about these species because the finds are much more fragmentary. Sahelanthropus tschadensis lived 6-7 million years ago, but the only fossils found so far of it are a partial skull. Orrorin tugenensis lived 6 million years ago, but all we have is a leg bone and a few fragments. So while basically all we know about these earlier species is that we have a few scraps of bone from a creature that looks to be a hominid and doesn't belong to any other known species, we now have a very clear idea of what Ardipithecus ramidus looked like, and thus what hominids living 4.4 million years ago were like.

A Missing Link
Is Ardi a "missing link"? Well, she (and the other remains found in the same place -- much more partial remains of 35 other individuals) is certainly a missing link in the sense that these fossils provide us with a lot of fascinating information about a certain stage in hominid evolution. But there is no single "missing link" in the hominid ancestry chain. Fossils of primates in general are so rare that piecing together the more distant periods of human ancestry is very, very hard. While the charts we see in books and articles suggest seamless lines of descent, the actual evidence we have is often quite fragmentary, and even the links of the chain that we do have are often only partial. One stage or even a whole species may be represented by only a partial skull or most of a leg -- enough to tell it's different from known species, but not enough to have a very complete picture of the species. The below chart (excuse my poor freehand drawing skills) shows the problem, and why there's often dispute among biologists as to where the actual branches are, and whether we're descendants or cousins of some hominid species.

What is often referred to as "the missing link" is the hope of finding a species which appears to be a direct ancestor of both modern chimps and modern humans. Ardipithecus ramidus is not such a link, and indeed, some researchers are suggesting that Ardi points to that common ancestor being more ancient that previously believed.

What Ardi Tells Us
One of the most interesting things about Ardi is what she seems to indicate about human/chimp divergence. It had been widely assumed at one point that the common ancestor between humans and primates probably looked a lot like a chimp. Our DNA shows that we're closely related to chimps, and because we often have difficulty not thinking about evolution in terms of "progress" (especially when we're talking about ourselves) it's natural to think of chimps as the "ancient" form and to talk about "humans evolving from chimps".

Lucy knocked a bit of a hole in this thinking back in the 70s by showing that upright posture went back to Australopithecus afarensis 3+ million years ago, putting to rest the already crumbling idea that hominids prior to Homo erectus had been "knuckle draggers".

Now we have Ardi, who despite having a big toe that would have allowed her to grip things thing her feet, has a pelvis and legs which are clearly adapted to walking upright 4.4 million years ago. Even the leg bones we have from Orrorin tugenensis 6 million years ago appear to suggest a bi-pedal posture (though it's harder to know from such incomplete remains). So with Ardi's well preserved skeleton for confirmation, it's starting to look very much like human ancestors have been bipedal for a very long time. Large brains and other adaptations are later, but it would appear that it may have been the chimps and gorillas who developed adaptations for arboreal life, and in the process shifted to walking on all fours and putting weight on the knuckles of their hands -- rather than these being features that our ancestors shed.

Ardi did have proportionally much longer arms than more modern human ancestors, and her fingers were long for gripping branches. Her feet could still grip better than ours can (though not as well as modern great apes). Her brain was about the same size as that of a chimp, and she stood about four feet tall (the height of my seven-year-old.) But while she probably did not possess any of the traits that we see as uniquely human (language, higher consciousness, reason, complex tool-making, etc.) she looked less "like an ape" than expectations would have been in the past.

For more detailed information, the following are interesting links:

At long last, meet Ardipithecus ramidus

Ardipithecus: We Meet At Last

And if you really want the mother lode, the journal Science (which put out a special issue with all the original research papers on Ardi) has taken the unprecedented step of making all of the papers available on their site if you fill out a free registration. The Science Magazine Ardipithecus site is here.