The Herto fossils will have a tremendous impact on a long-standing debate in palaeoanthropology: 鈥淲hen, where and how did modern humans arise?鈥�
Two competing theories have been slugging it out for more than twenty years. The first is the 鈥渕ultiregional鈥� hypothesis, which argues that Homo sapiens arose from Homo erectus ancestors in many parts of the Old World simultaneously over a time span of hundreds of thousands of years. Among the most vocal champions of this theory are Milford Wolpoff of the University of Michigan in Ann Arbor and Alan Thorn of the Australian National University in Canberra.
The rival hypothesis, called 鈥淥ut of Africa鈥�, sees things very differently. It says that early modern humans arose just once quite recently, a little more than 100,000 years ago, in a single place, probably Africa. Descendants of that population spread throughout the Old World, replacing more archaic forms. Chris Stringer of London鈥檚 Natural History Museum is the chief protagonist of this theory.
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If the Out of Africa model is correct, then you should expect to find the earliest forms of modern humans in Africa. Candidate fossils up to 300,000 years old have been found in eastern and southern Africa, but they were usually fragmentary and hard to put an date on. These factors gave critics of the hypothesis enough wiggle room to argue that the case was not proven, and that multiregionalism was the preferred interpretation. However, the Herto fossils make it much more difficult to dispute the Out of Africa view. The fossils are beautiful specimens, and they have been accurately dated to 160,000 years old.
H. erectus first appeared in Africa almost two million years ago, and rapidly expanded its range into Asia and, later, Europe. Various forms of so-called archaic sapiens then popped up in the Old World, most famously the Neanderthals in Europe. To the multiregionalists, the Neanderthals were an intermediate form, on the way to truly modern humans in Europe.
But the fact that the Herto people were so strikingly modern long before the 鈥渃lassic鈥� Neanderthals lived in Europe makes that tough to argue. And DNA evidence culled from Neanderthal bones in recent years indicates that they were evolutionary dead ends that contributed little, if anything, to modern humans (New Scientist, 17 May, p 14).
Moreover, genetic evidence from mitochondrial DNA and other sources has been building inexorably over the past several decades, giving ever stronger support to the Out of Africa model. Most recently, this data has been interpreted to indicate an African origin of about 160,000 years ago. The Herto fossils support that.
Proponents of the multiregional view insist that the new fossils are not the nail in the coffin for their idea. Wolpoff, for instance, argues that although they represent one of the ancestors of modern Europeans, there could well have been others. He told New Scientist that early humans spreading from Africa could have interbred with Neanderthals in Europe and H. erectus in Asia to create modern populations.
But proponents of the Out of Africa theory say that the Herto discovery settles the question of 鈥渨here, when and how?鈥� at the heart of the debate. Although it is always risky in science to say 鈥渃ase closed鈥�, that鈥檚 how it looks.