Beyond The Savanna Mentality

Discover, October 31, 2005

Tim White of the University of California at Berkeley has been at the frontier of paleoanthropology since the 1970s, when he helped discover fossils of Australopithecus afarensis, a species that includes the famous “Lucy” skeleton. Since then he has focused his efforts on a small region of central Ethiopia, where he discovered a variety of fossils, from 5.5-million-year-old hominids to some of the earliest members of our own species, dating back 160,000 years.

How do you manage to find so much history in such a small region?

W: The reason that we’re able to do it is that it’s just a geological nightmare. You have a patchwork quilt of different aged sediments on the surface. You can step across a fault and step back 2 million years. It allows you to look through many different windows in one small area.

How is our view of early hominid evolution changing?

W: We had become so used to finding hominids in open country that there was an expectation that all hominid origins could be related to a shift from forest to savanna. You could characterize it as the savanna mentality. Now we’re finding an association of early hominid fossils with faunas more characteristic of woodlands, of more closed habitats. We’ve also learned that the idea that the last common ancestor of hominids was like a chimpanzee is just wrong. The more closely we approach that last common ancestor with real fossils, we’re learning that its browridge is shaped differently from any chimpanzee’s, and its canine teeth are much smaller. Chimpanzee incisor teeth are very broad, and they use them for eating fruits. We don’t see that in any of the oldest hominids. This is saying that chimpanzees evolved that specialization after the split with our ancestors. We’re able to put together a picture of the earliest hominids quite different from a projection one might have made from Lucy–a picture that allows us to see ourselves as a specialized primate.

What technology advances are changing the way you study evolution?

W: The global positioning satellite system. With GPS, we no longer have to worry about the position of a fossil. Some of the biggest blunders in the history of paleoanthropology were made by people who lost the place where a fossil came from. There’s no excuse for that anymore. The other big advance is in geochemical dating. Consider an analogous situation. Let’s say you’re interested in Columbus’s exploration of the Caribbean, and you go into the library and someone has removed all the dates from the documents. We face a similar challenge. It is the order of the succession that is really important–particularly when you’re trying to understand evolution.

What big developments do you anticipate?

W: The major progress will come from knowledge about how anatomy is formed. It’s the integration of data from fossils and developmental biology. For example, when you look at the length of your thighbone, you might ask: Why is the human thighbone so much longer than the chimpanzee thighbone? We can answer that question in a very simplistic way right now. The growth plates on the end of the chimpanzee femur fuse earlier. But we’re at a stage where we can ask new questions: What signaling between cells is leading to that?. How easy is it for the femur to be extended? And then we can apply those insights to the fossil record. When did the femur lengthen? Another great example is the hand. In a chimpanzee or gorilla hand the metacarpals [palm bones] are really long. In humans they’re quite shod. Again, how that happens genetically is a question that can be addressed in the laboratory. Then we can take that knowledge and play it against the fossil record. Lucy’s hand had a relatively short thumb. When was it in human evolution that the thumb elongated? How do the genetic controls over thumb length differ from the controls over the other digits? It’s that synthesis that I see coming in 20 years. But right now it’s way early in the game. Attributing a particular gene to the expansion of the brain is just kind of silly. Downstream, though, it’s going to be fascinating. Of course, no amount of work on the human and chimpanzee genome could have ever resulted in the knowledge about a 3.2-million-year-old hominid ancestor in terms of its anatomy, its ecology, or its behavior. You don’t discover a Lucy in the molecular laboratory.