When and where did our species arise? Over the past two decades molecular evolutionists have pursued this question. DNA evidence from the mitochondrion, an independent element inside every cell that acts as the “powerhouse” of the cell, has figured prominently in these studies, providing the raw data for producing evolutionary trees and molecular clocks to illustrate a theory of the development of diverse human populations. The mitochondrial family tree of humans has suggested that our roots lie in Africa; but, so far, this theory have had only weak statistical support. Another theory is that modern humans arose simultaneously in different regions of the world.
Recently, Dr. Gyllensten of the University of Uppsala in Sweden and his colleagues have conducted the most thorough analysis yet of diversity in human mitochondrial DNA*. The results support the view that modern humans originated in Africa. The analysis of mitochondrial DNA (mtDNA) has become an important tool in this debate on human evolution. This is due to the fact that mtDNA is inherited solely from the mother and does not change as much as cellular DNA from generation to generation, and therefore can offer evidence of human mitochondrial lineages*.
Dr. Gyllensten and his colleagues have described the global mtDNA diversity in humans based on analyses of the complete mitochondrial genomes of 53 people of diverse geographical, racial and linguistic backgrounds. Each sequence in these genomes is much longer than those previously studied. The result is a robust family tree* rooted in Africa, which indicates the exodus* from Africa within the past 100,000 years (recent in evolutionary terms). With this result, the scientific opinion further shifted towards the claim that modern humans, Homo sapiens, originated in Africa.
Our closest living relatives are African apes, so why is an African origin for modern humans controversial? The reason is that our immediate ancestors, now extinct, are known to have wandered out of Africa as early as two million years ago. The main alternative to a model of an African origin is a multiregional model that holds that modern humans arose simultaneously in Africa, Europe and Asia from these ancestors. Proponents of this view argue that the fossil record indicates transitions between, for example, Neanderthals (H. neanderthalensis*) and modern humans in Europe, and between H. erectus* and modern humans in Asia. However, the existence of non-African traditional fossils is debatable, and there is genetic evidence that Neanderthals did not widely interbreed with modern humans even though the two coexisted for at least 10,000 years. Such coexistence is good evidence for recognizing the two as Separate species.
The key point of the mitochondrial evidence for an African origin has been several African mitochondrial lineages that go far back in the evolutionary trees of human populations, even though they have only had weak statistical support. Gyllensten’s team also found this pattern, but obtained a stronger family tree by collecting a larger data set than in previous studies. Interpreted literally, the tree indicates that some Africans are genetically closer to Europeans and Asians than to other Africans. However, the history of a single gene or molecule may not always mirror that of the population, and other molecular studies place Africans in a single group. Together, these studies suggest that the founding population leaving Africa carried with it a group of mitochondrion alleles — alternative forms of the same gene — and that African populations continued to interbreed after the exodus.
Gyllensten and colleagues estimate that the divergence of Africans and nonAfricans occurred 52,000 plus or minus 28,000 years ago, shortly followed by a population expansion in non-Africans. This date may even be a bit too recent. Other genetic markers indicate an exodus from Africa around 100,000 years ago, which would be more consistent with fossil and archaeological evidence of modern humans outside Africa. But, no single genetic marker can indicate that event precisely, and the mitochondrion date is approximately range. Some nuclear DNA markers have suggested earlier dates for the exodus from Africa, so more data is needed to provide a fuller picture. Nonetheless, most of the genetic evidence indicates that there were only about 10,000 breeding individuals for a long time before the recent expansion of modern humans outside Africa. Such a small population size is incompatible with the multiregional model, which would require many more individuals to maintain genetic movement among continents.
Another question is when H. sapiens arose in the first place. Molecular clocks would be well suited to address that question if our closest relative were living. But the closest relative to modern humans, whether H. erectus or some other species, is unfortunately extinct. The earliest fossils of modern H. sapiens are 130,000 years old, so that is the most recent time boundary for the origin of our species. Studies of ancient DNA provide hints to the older time boundary. The split between H. neanderthalensis (a species which is not necessarily our closest relative) and H. sapiens has been indicated by a DNA clock at 465,000 years ago. So our species probably arose somewhere between 130,000 and 465,000 years ago. An estimate of 200,000 years ago is not unreasonable given the transition seen in the African fossil record between ancient and modern humans around that time.
The research results by Gyllensten and colleagues have indicated that the field of mitochondrial population genomics will provide a rich resource of genetic information for evolutionary studies. Nevertheless, mtDNA is only one aspect of the analysis of human evolution and only reflects the genetic history of females. For a detailed view, a combination of genetic systems is required. With the human genome project reaching completion, the ease by which such data may be generated will increase, providing us with an ever more detailed understanding of our genetic history.
Gyllensten and colleagues have used sequences from a large number of complete mitochondrial genomes to address these evolutionary questions, an approach that could be called population genomics. Genes responsible for physical and behavioral traits will probably be found and their allelic histories provide additional information. Molecular evolutionary trees and time estimates will have greater precision, all of which will help to clarify our evolutionary history.
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