July 18, 2002

Tell Me Another One


By Mark Hartwig

The big news last week was the unveiling of an ancient fossil skull. Nicknamed "Toumai," the skull was unearthed a year ago in Chad by French anthropologist Michel Brunet and his team of researchers. The skull is thought to be the oldest hominid fossil yet found. But what that means is anyone's guess.

At first blush, Toumai is a major shocker, being completely out of line with anthropologists' expectations. Judged to be about seven million years old, it is said to look more human than later "human ancestors," such as "Lucy."

Writing in the July 11 issue of Nature, anthropologist Bernard Wood, of George Washington University, observed, "It was the conventional assumption that the human-chimp common ancestor, and the earliest members of the chimp lineage … would have been adapted for life in the trees, with the trunk held either horizontal or upright and with the forelimbs adapted for knuckle-walking on large branches or on the ground. This would have been combined with projecting faces that accommodated elongated jaws bearing relatively small chewing teeth and, in males, large upper canine teeth ..."

In contrast, although Toumai has a chimp-sized cranium, it has a flat face, without the projecting snout. It also has relatively larger chewing teeth with small canines.

"If we accept these as sufficient evidence to classify [Toumai] as a hominid at the base, or stem, of the modern human [lineage] then it plays havoc with the tidy model of human origins," Wood said. "Quite simply, a hominid of this age should only just be beginning to show signs of being a hominid. It certainly should not have the face of a hominid less than one-third of its geological age."

According to Wood, this could mean that "all creatures with more primitive faces (and that is a very long list) would, perforce, have to be excluded from the ancestry of modern humans."

But don't hold your breath waiting for those primitive faces to disappear. If this goes like previous controversies in paleoanthropology, it will be years or longer before anything shakes out. The battle lines are already being drawn.

Some researchers, for example, question whether Toumai is really a hominid. Paleontologist Brigitte Senut, whose research team in Kenya discovered the six-million-year old "Millennium Ancestor," contends that the skull's features are characteristic of female gorillas. Martin Pickford, who co-led the Kenya team, adds that the Chad skull is distorted, having been subject to millions of years of pressure. Had the skull not been distorted, he claims, it would look like a "proto-gorilla."

There will also be interminable quarrelling over whether the new find is an actual human ancestor or merely a side branch.

Michele Brunet, of course, is pulling for the former view. He left no doubt about that when, at the unveiling ceremony in Chad, he announced, "From this moment the ancestor of humanity is Chadian. The cradle of humanity is in Chad. Toumai is your ancestor."

Berkeley paleonotologist Tim White is also certain that it's the common ancestor. In an interview on National Public Radio, he stated, "The fact of the matter is that it is the oldest and this skull that's been found is also the most primitive, and it's most like-because it is the oldest-the common ancestor that we know we had with the African apes."

On the other side of the fence, Bernard Wood said, "My guess is that it's neither a chimp nor a human ancestor - it's a creature that was living at the same time."

This is linked to his conviction, expressed by others as well, that human evolution was not a "tidy" affair, with hominid anatomy evolving only once, but a messy one, in which various hominid features were acquired independently by several lines of hominid and ape species.

This messy model, he writes in Nature, "would predict that at 6-7 million years ago we are likely to find evidence of creatures with hitherto unknown combinations of hominid, chimp and even novel features." As a result, some physical characteristics "are likely to be unreliable for reconstructing relationships because creatures can share features such as brow ridges without necessarily inheriting them from a common ancestor."

Although Toumai is a candidate for the stem hominid, "in my view it will be impossible to prove that it is."

So what are we supposed to make of Toumai?

A friend of mine has said that in his study of human evolution he has gone from skeptical to cynical. I'd have to say that I share his cynicism. The data is so scant and grip of assumptions so strong I have a hard time taking any of it seriously.

Toumai's story is surely imaginative. But reliable science? Uh-uh.

Recommended Resource
If you get a chance, be sure to check out Roger Lewin's book, Bones of Contention. It's a great antidote to the hype that often accompanies hominid-fossil finds. Here's a description from Amazon:

No area of science has a higher incidence of colorful personalities than paleoanthropology. The Leakey family and Donald Johanson are merely the best known of a vivid and contentious bunch that have not hesitated--indeed, have made every effort--to air their conflicts before a wider public. Roger Lewin's recently updated Bones of Contention is the only reliable field guide to these scientists, their characters, and controversies. Lewin never forgets that hominid fossil discoveries always involve both the self-image of humanity and that of individual scientists. Lewin is uniquely evenhanded (i.e. he thinks everyone is wrong from time to time), yet the all-star blurbs on the cover show that he retains the respect of the entire paleoanthropological community.

Copyright 2002 Mark Hartwig. All rights reserved. International copyright secured.
File Date: 7.18.02