Thursday, August 9, 2007

New Evidence on Human Evolution

The news media is very excited about new fossil discoveries from Kenya described in the current issue of Nature show that Homo habilis and Homo erectus lived contemporaneously. For example, the New York Times reports:

Scientists who dated and analyzed the specimens — a 1.44 million-year-old Homo habilis and a 1.55 million-year-old Homo erectus — said their findings challenged the conventional view that these species evolved one after the other. Instead, they apparently lived side by side in eastern Africa for almost half a million years.

If this interpretation is correct, the early evolution of the genus Homo is left even more shrouded in mystery than before. It means that both habilis and erectus must have originated from a common ancestor between two million and three million years ago, a time when fossil hunters had drawn a virtual blank.

Read it all here.

Mike Dunforth, a graduate student in the Department of Zoology at the University of Hawaii, Manoa offers some perspective about what this really means. His bottomline: the impact on traditional theories about human evolution is less than reported in the news media.

In the past, if you plotted the ages of the fossils of H. habilis, H. erectus, and H. sapiens on a graph, it would look more or less like this:

In this view, all of the H. habilis remains are older than all of the H. erectus, which are older than all of the H. sapiens fossils. If that's the case, it supports what's known as an anagenetic view of human evolution, where one ancestral species leads to the next, with no new lineages branching off along the way.

One of the two new finds, the H. habilis upper jaw, does not fit that nice, clean picture. If we go back to my first rough plot of fossils versus time, the new find would be represented with the orange circle:

If that's the case, if there were still H. habilis around after H. erectus first appeared, then the straight line depiction of human evolution doesn't work any more. Instead, we've got at least two possibilities. It's possible that both Homo habilis and Homo erectus both evolved from an as-yet unknown common ancestor.

Alternatively, it's possible that Homo erectus split off from Homo habilis, but that some H. habilis forms hung around for a while after the split.

If the authors' interpretation is correct, the new finds do change our view of the history of human evolution, but it's not really all that drastic a shift. The results are interesting, especially with the possibility that both H. erectus and H. habilis might have lived in the same area at the same time, but they're hardly revolutionary. That shouldn't be taken as a criticism of the scientists who did this work, by the way. Most science, including most of the interesting science, isn't revolutionary. It's simply small changes to our existing knowledge - the brick by brick construction of the library of human understanding.

Read it all here. Eric Michael Johnson also has good analysis here.

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