Anthropologists have said human beings evolved in a straight line from Homo habilis to Homo erectus to us, Homo sapiens, over two million years. But fossil bones found in 2000 near Lake Turkana in Kenya are changing that view, says University College London's Fred Spoor, a lead author of the study. (Read more about the finding.)
How can a single bone, such as a Homo habilis upper jaw, help reconstruct the path to Homo sapiens?
In the past, we didn't know how long Homo habilis had survived. After we found this upper jaw, we were surprised when the geologists told us that it was only 1.44 million years old. Both Homo erectus and Homo habilis are seen in the fossil record beginning at about 1.9 million years ago. Instead of one evolving into the other, they overlap for about 450,000 years, an enormously long period. They must have survived side by side in the same geographical area for a long time and somehow found different ecological niches so they would not directly compete—maybe by focusing on a different diet or habitat, like chimpanzees and gorillas today.
What does that overlap mean for the linear view of human evolution?
It means Homo erectus and Homo habilis are really sister species, with a common ancestor older than 2 million years.
Now that you are back out in the field in Kenya, what are you hoping to find?
We are working in areas between about 1.6 and 2 million years in age. Ideally we would like to work between 2 and 3 million years, when that hypothetical common ancestor of Homo habilis and Homo erectus lived. But the parts around Lake Turkana that have sediments from that period really don't show much bone. That is a general problem in eastern Africa. We know very little about this particular earlier time period.
Your group also found a Homo erectus skull. What's special about that?
It is a young adult, a very small one. This shows that there is size variation in Homo erectus. If you look at modern primates with sexual dimorphism [the males are much larger than the females], groups are often organized around a single dominant male, a silverback in the case of gorillas. Possibly Homo erectus is less modern human-like in its behavior than we thought.
Not the classic picture of human evolution, is it?
No. This research shows we evolved in the same way other mammals did—in an experimental, unpredictable, quirky way, with side branches that die out after a long time, and then something else comes along. That differs from the human-centric way of thinking in the classic cartoon of a chimpanzee evolving into a person, as if there was a directing hand steering chimpanzee-looking creatures to modern humans.