Molecular Anthropology

Mitochondria are energy-harvesting organelles in the cell. They are inherited only from the mother, and so track maternal inheritance in the same way that the Y chromosome tracks paternal inheritance. Like microsatellite DNA, mitochondrial DNA accumulates mutations faster than chromosomal coding DNA.

One of the earliest and most famous mitochondrial studies was used to address a central question in anthropology: Where and how did modern humans originate?

The Homo genus itself is universally believed to have originated in Africa. Groups of Homo erectus are known to have migrated out of Africa, populating Europe and Asia between one and two million years ago. H. erectus gradually changed in character, so that by about half a million years ago, it had taken on some more modern characteristics. Anthropologists call these groups "archaic" modern humans. They include the Neandertals, who lived in Europe and the Middle East from 150,000 to 28,000 years ago. Did modern humans evolve from these older populations in several different regions simultaneously? Or did they arise from a small group in Africa, and spread out from there? If so, did they mix with less advanced local populations (such as Neandertals), or replace them entirely?

The scientists who performed the mitochondrial DNA study (Rebecca Cann, Mark Stoneking, and Allan Wilson) reasoned that populations that had been in one place for only a short period of time would show very little variation in their mitochondrial DNA, since they all shared a relatively recent common ancestor. This would be the case in a modern human population if it had only recently migrated into the area in which it is found. (Such relative genetic homogeneity in newly formed populations is known as the founder effect.) In contrast, populations that have remained in place for long periods have much more ancient common ancestors, and therefore have more mitochondrial DNA variations.

To perform their analysis, the scientists collected samples from different ethnic groups from all over the world. They found that the populations with the greatest amount of sequence variation were in sub-Saharan Africa, indicating these were the groups with the most ancient ancestry. All other groups had much less variation, indicating more recent arrivals of those groups in those regions. Cann, Stoneking, and Wilson went on to estimate the date at which all these groups had their most recent common ancestor. Using a figure of 2 to 4 percent sequence divergence per million years, they estimated that the most recent common ancestor lived approximately 200,000 years ago.

The simplest explanation, they argued, was that ancestors of the non-African Homo sapiens migrated out of Africa about 200,000 years ago to populate other regions, over time replacing the nonmodern humans (H. erectus, Neandertals, and possibly others) already living in these regions. They argued that the relatively short time since the divergence of all modern humans was too brief to support the alternative hypothesis, that each local group of archaic humans had independently evolved modern traits, a model called multiregional evolution.

The conclusions drawn in this study are still controversial. Numerous other studies have been done since, and the data have been subjected to multiple different analyses. Some studies suggest differing dates for the most recent common ancestor (ranging from 100,000 to 400,000 years ago), and others suggest that an exclusive African origin is not the only possible interpretation of the data.

It is important to keep in mind that the vast number of comparisons that must be made in such studies require computer programs, not only to make the comparisons, but to draw from them the simplest "family tree" that fits. Much of the controversy surrounds the assumptions that must be built into these programs in order to generate results. The mutation rates by which events are timed (the "molecular clock") are also not known with precision, leading to further uncertainties about the exact timing of migrations.