LETTERS TO THE EDITORS
Out of Africa?
To the Editors:
"We Are All Africans" by Pat Shipman (Marginalia, November–December) is very interesting and thought-provoking. Its support for the African Eve theory gives a partial picture of the origin of modern man.
The African Eve theory is based on the hypothesis that Homo sapiens sapiens appeared in Africa more than 100,000 years ago and migrated out of Africa into Europe and Asia sometime afterward. The key problem with the African Eve theory is the replacement of the indigenous population in the other parts of the world by Homo sapiens sapiens recently migrated out of Africa. Replacement of the Neandertals in Eurasia is not difficult to accept, though some may still have reservations. Replacement of the descendants from Homo erectus in Asia, especially East Asia, is questionable. This part of Asia was not totally covered by glaciers as in Europe during the Pleistocene. In view of the large landmass, abundant food supply and tool technology, including the use of bamboo, the population in East Asia must have far exceeded Neandertals. Their replacement by a small group of hominids from Africa is very difficult to envision. Furthermore, no evidence shows that the stone-tool culture in East Asia was altered as a result of the recently arrived Homo sapiens sapiens from Africa.
The Multiregional theory is based on the migrations of Homo erectus from Africa over a million years ago. They settled in different parts of Europe and Asia, with periodic gene exchanges between the groups. Through time, they evolved into the modern population. Neandertals, as discussed in the article, form only one branch of the multiregional population. Also, the early man described in the African Eve theory is really a part of the multiregional population unless "replacement" can be proved.
Dating of the genetic events, such as the time hominids migrated out of Africa, is very uncertain. Mutation is not a constant event, and assuming a constant rate for mutation to be used in this calculation of dating genetic events could lead to serious errors. Since both the African Eve and Multiregional theories are based on African origin, any genetic data without proper dating would support either theory.
A serious effect of ignoring hominid evolutionary processes beyond 100,000 years ago is the loss of input data to research. For example, the effect of bipedalism would not be included in the study of evolutionary psychology and other evolution-related science, since bipedal development is believed to have occurred much earlier than 100,000 years ago.
Flemington, New Jersey
Dr. Shipman replies:
Dr. Chou raises some important questions that have puzzled several readers.
He asks first about evidence for the replacement of archaic humans around the world by modern humans who arose in Africa, a key tenet of the Out of Africa theory. He finds the replacement of the Asian population of Homo erectus by "a small group of hominids from Africa . . . difficult to envision." Although we humans often judge ideas on their plausibility, plausibility is not a rigorous test of the validity of scientific ideas.
Whether we can envision it readily or not, the replacement of one species by another invasive species has happened repeatedly on large landmasses with abundant food supplies. For example, a few pairs of English starlings were released in Central Park in New York City in the 1890s; starlings are now a widespread and extremely numerous species across the United States, which is contributing to the decline of native species of swallow, bluebird and woodpecker. These indigenous species nest, as do starlings, in tree cavities, but apparently the invading species is better at monopolizing an essential and limited resource. Replacement often occurs by indirect competition.
What evidence might we find of such a replacement? Three possibilities come to mind.
First, as Dr. Chou suggests, the style of tools in a geographic area might change if tool-making invaders arrived. Frustratingly, it is very difficult to know who made any particular stone tool or type of tool. Hominids are very good imitators, and the knowledge of how to make a particular type of tool is not biologically encoded, so far as we know. The archaeological record does not give us clear evidence either of replacement or of its lack.
Second, a change in anatomy or morphology of the fossils should reveal whether an indigenous species has evolved or whether another species has moved in from elsewhere. Unfortunately, Out of Africanists and Multiregionalists cannot agree on whether there is good evidence of morphological continuity in the fossil record of the genus Homo within any single geographic region.
Third, we can turn to genetics. Evolution in situ will leave evidence of genetic continuity; replacement by another lineage will not show such continuity. This is where the comparison of mitochondrial DNA (mtDNA) extracted from various hominids has provided strong evidence of replacement. As I discussed in my article, the only test yet completed compared mtDNA from three European hominids: archaic Neandertals; anatomically modern but ancient Cro-Magnons; and living, modern Europeans. If Multiregionalists are correct, these three populations ought to be more similar to each other in mtDNA than they are to hominids from other geographic regions—but that is emphatically not the case. For confirmation, similar studies should be conducted on Asian and African fossil and modern humans. Since extracting mtDNA from fossils is technically difficult, time-consuming and expensive, we shall have to wait for those further tests.
Dr. Chou is right when he observes that dates based on molecular clocks are uncertain. Fortunately, dates based on fossils are very accurate, when the specimens have been buried in sediments with the appropriate geochemical attributes. The occurrence of fossils with modern anatomy thus provides a good check on the molecular dates. This is why the most compelling answers come from the consilience of genetic and fossil evidence.
Dr. Chou fears that evolutionary developments in our lineage that are older than the origin of modern humans will be ignored, to our peril. I agree wholeheartedly. The human species as a biological entity is deeply rooted in primate, mammalian and vertebrate evolution. Anyone wanting to develop a sense of who we are must take that ancient ancestry into full account.