All modern humans belong to a single species, Homo sapiens. There are phenotypic differences, such as skin color, associated with people living in different regions. However, all H. sapiens are genetically similar enough to produce offspring together. How did these different phenotypes arise?
Multiregional Evolution
Some anthropologists propose that modern humans evolved in parallel all over Earth from different populations of Homo erectus. For this process to result in a single species of modern humans, as actually exists, constant gene flow between the different populations would be necessary. Without exchanges of genes during the transition from H. erectus to H. sapiens, the different populations would tend to speciate into separate groups, in response to local environmental pressures.
If the multiregional hypothesis is correct, it would suggest that regional differences in phenotype have been developing for well over a million years. Some investigators claim that Asian fossils of Homo erectus show the high cheek bones seen in modern Homo sapiens living in Asia.
Out of Africa
The more widely supported hypothesis, the recent-African-Origin hypothesis, states that modern Homo sapiens originated in Africa only about 100,000 to 200,000 years ago and then, like Homo erectus before them, left Africa. They colonized the world, displacing and causing the extinction of Homo erectus and early Homo sapiens, such as the Neanderthals.
The recent-African-origin hypothesis gets much of its support from studies of the genes found in the mitochondria. Because mitochondria reproduce asexually, their genes are not subject to the mixing caused by gene flow and meiosis. In humans were one large population dating back to over a million years ago, we should find human mitochondria that show a million years of accumulated mutational differences. Instead, most human mitochondria have very similar genes. The period of time needed for mitochondria to accumulate the differences actually seen is only 100,000 to 200,000 years-far short of a million years. Because all humans mitochondria are so similar, supporters of this hypothesis infer that all modern humans came from one small group in Africa a fairly short time ago- 100,000 or 200,000 years ago.
According to the existing fossil evidence, Neanderthals dies out about 30,000 years ago. Some anthropologists hypothesize that the Neanderthals were killed off by Cro-Magnons. Others hypothesize that the two groups interbred. What evidence would you look for to evaluate these two hypotheses?
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