Monday, November 2, 2009

A chimp in our ancestry?

The recent announcement of a new 4.4 million year old human ancestor, named Ardipithecus, has led to a flood of papers, debates, and publicity. Found many years ago initially by Yohannes Haile-Selassie working in a team led by Berkeley anthropologist Tim White, this interesting set of specimens, and its unusually well-representative ecological environment (including diverse floral and faunal fossils), deserved the attention it was given. Holly described these finds, and the ensuing excitement, here.

We're triggered to write this post by having listened to the BBC radio program "Material World", in which White and Haile-Selassie were interviewed when attending a Royal Society gala meeting to announce and celebrate the finds.

Part of the discussion was about 'missing links', as is so often the case when fossil finds are the subject. The discussion eventually turned to a recent and rather sensationalized story about a truly remarkable find in Germany, a 47 Million year old primate named Ida (image above). This was touted in the media (though not in the Science article last May itself) as a fabulous missing link, our earliest known ancestor that separates apes and humans.

More recently, a find in Egypt of the fossilized jaw of a similar creature, called Afradapis (figure at left), put Ida into a broader and less sensationalized framework, of lemurs and other ancestors of monkeys, apes, and humans.

Ida was remarkable on its own, without the hype. In particular, the idea that one find revolutionizes our understanding becomes less and less tenable the more pieces of the history we have. To truly overturn a century's accumulated knowledge in paleontology (not to mention comparative embryology, genetics, and morphology) would take something that even our over-adrenalized media-run society would recognize as really, truly, genuinely new! Not this one.

It should be interesting enough that the fossil was complete, with fur and all. It didn't need to be our Lost Grandmomma. Well, actually, Ida was a yearling female, probably never reproducing and hence not the ancestor of anyone at all; that's another lesson: what we mean, if we were to speak carefully, is that Ida is a member of a group among which were distant members of lineages that survive today, such as ourselves. The lesson, one that is perhaps not in the interest of the media, or even that of some scientists to learn, is not to claim too much.

As Holly said in her earlier post, the Ardi finds present a kind of pastiche of changes in different traits, rather than a simple linear trend from our common ancestor with the chimp and gorilla. The interpretations are already controversial and debated, but that doesn't diminish the relevance of the finds.

In the interview, Tim quite properly and enthusiastically praised Charles Darwin's prescient predictions about human origins, and that earlier fossils would be more primitive than recent fossils relative to modern humans. But one needs to be careful about such statements.

When you read Darwin, you regularly see very cogent arguments, and diverse facts impressively brought to bear. Darwin's books presented a consistent, multi-faceted view and his predictions like that related to human origins could be strikingly on target as we understand things today. But we should be careful not to use a post hoc sieve to make Darwin into something other than what he was. He could be incredibly prescient, but he could also be dramatically wrong, and some of his arguments seem now to be thoroughly naive. His reputation rightly survives his gaffes (some of which were simply
extending his theory beyond the data available at the time). But, while this is not meant to denigrate Darwin in any way, he was human and selective after-the-fact praise could suggest that he was more than the already remarkable person he actually was.

There are other trip-wires in evolutionary discussions, too, that we should be wary of. The Material World moderator, Gareth Mitchell, referred to the surprising lack of resemblance of Ardi to chimps. Even Tim, easily one of the world's leading paleoanthropologists, agreed that Ardi was surprisingly unchimplike. But that was then, and chimps are now. We should be more careful: the question should more properly have been the degree to which Ardi resembled our common ancestor with the chimp, not today's chimp. Did he show the way in which those lines diverged? Were its traits more like what we would expect in a line from ancestor to chimp, as if the more distinctive human traits came along later?

These slips of the evolutionary tongue are easy to make. If one wishes to argue that humans have changed much more than chimps relative to our common ancestor, then that's what should be said. But if we're sloppy, we can rightfully be accused, by creationists for example, of saying we descended from chimps--as suggested by the famous lampoon of Darwin drawn roughly when his Origin was published.

While we note here the other issues that arose, overall this story about these specimens was excellent, including its correctly noting that the lemur fossil was improperly and prematurely hyped, yet is still an excellent addition to our knowledge. Tim and Yohannes described the arduous nature of the finds and the value of the other, less glamorous plant and animal finds--and the patience it takes to assemble a synthesis of such delicate, piece-meal, fortuitous data, something that Holly also described in her post.

The program is worth listening to as a quick, well-done thumbnail sketch of new finds. We only note here the other issues that arose.


Jennifer said...

When is someone going to get some DNA from one of these ancient fossils and trace actual humans to it to find the familial link? And thus PROVE evolution

Ken Weiss said...

Well, Jennifer, in a sense they actually have (and the first such find, produced by Svante Paabo's group in Germany, was given a confirmatory test by Anne and our lab here at Penn State).

Neandertals are either our ancestors or our very close relatives who did not contribute to our own genomes. We have extensive DNA sequence from several individuals. Some Nanderthal variants are only found today in Europeans (the fossils were from Europe). A small percentage of the sequence contains nucleotides that are found in us today, but not in chimps, and hence reflect human ancestry.

But you can't tie it to individuals for several reasons. Many individuals today, and back then, will have shared any sequence elements that were not entirely unique to themselves. And chromosomes recombine--the two copies in an individual exchange pieces--each generation. That means that you can't really trance a whole genome sequence to a single individual, any more than you yourself can be traced back in time to just one individual (you have, for example, two parents, 4 grandparents, etc.)

But by far the clearest evidence is that the Neanderthal sequences (and those from some morphologically modern human-looking ones), all about 30,000 to 40,000 years old, are clearly separated from chimpanzee and gorilla (and every other present-day species) sequences. They, collectively at least, are our ancestors by direct DNA descent from them, as a group, to us as a group.

That essentially is proof. Any other explanations would have to be concoted and contorted, and are far less convincing.

Of course, we have the actual bones, too, which shoe the same thing.

Jennifer said...

Ken, hmmmm.... you have a psychic on staff so you have tried to "trance a whole genome sequence to a single individual,". I think Houdini used to try that. And I wasn't aware that neadrathals wore shoes?
(Picking on your spell checking) Actually, I understood what you had to say. Thank you

Ken Weiss said...

Ah, well, a hasty response. I also spelled neandert(h)al variably. Neandertals did not wear shoes that we can _show_. They may have undergone trances. We could not _trace_ that back.