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11.12 Genetics

Molecular biologists starting with Allan Wilsonon have produced observations relevant to human evolution.

11.12.1 Age of the common ancestor

By estimating the rate at which mutations occur in mtDNA, the age of the common ancestral mtDNA type can be estimated. Assuming a rate of 2%-4% per million years, this implies that the common ancestor of all surviving mtDNA types existed 140,000-290,000 years ago. This observation is robust, and this common direct female line ancestor of all extant humans has become known as mitochondrial eve. The observation that the mtMRCA is the direct matrilineal ancestor of all living humans does not mean either that she was the first anatomically modern human, nor that no other female humans lived concurrently with her. This is because other women might have lived at the same time and passed their genes down to living humans, but that their mitochondrial lineages was lost over time. This could be due to random events such as producing only male children.

11.12.2 African origin for modern humans

There is evidence that modern human mtDNA has an African origin. The broad study of African genetic diversity headed by Dr.Sarah Tishkoff found the San people to express the greatest genetic diversity among the 113 distinct populations sampled, making them one of 14 “ancestral population clusters”. The research also located the origin of modern human migration in south-western Africa, near the coastal border of Namibia and Angola.

11.12.3 Lineage descent

MtDNA types (haplogroups) do not cluster by geography, ethnicity or race, implying multiple female lineages were involved in founding modern human populations. Many closely related lineages spread geographically and many populations contain distantly related lineages. Take, as an example, mtDNA type 49, a lineage whose nearest relative is not in New Guinea, but in Asia (type 50). Asian lineage 50 is closer genealogically to this New Guinea lineage than to other Asian mtDNA lineages. Six other lineages lead exclusively to New Guinean mtDNAs, each originating at a different place in the tree (types 12, 13, 26-29, 65, 95 and 127-134). This small region of New Guinea (mainly the Eastern Highlands Province) thus seems to have been colonised by at least seven maternal lineages. In the same way, we calculate the minimum numbers of female lineages that colonised Australia, Asia and Europe. Each estimate is based on the number of region-specific clusters in the tree. These numbers, ranging from 15 to 36 will probably rise as more types of human mtDNA are discovered.”

11.12.4 Y chromosome findings

The Y chromosome is much larger than mtDNA, and is relatively homogeneous; therefore it has taken much longer to find distinct lineages and to analyse them. Conversely, because the Y chromosome is so large by comparison, it holds more genetic information. Y chromosome studies show similar findings made with mtDNA. The estimate for the age of the ancestral Y chromosome for all extant Y chromosomes is given at about 70,000 years ago and is also placed in Africa; this individual is sometimes referred to as Y chromosome Adam. The difference in dates between Y chromosome Adam and mitochondrial Eve is usually attributed to a higher extinction rate for Y chromosomes due to greater differential reproductive success between individual men, which means that a small number of very successful men may produce a great many children, while a larger number of less successful men will produce far less children.

11.12.5 Human accelerated regions

Human accelerated regions are areas of the genome that differ between Humans and Chimpanzees to a greater extent than can be explained by genetic drift over the time since the two species shared a common ancestor. Thus these regions show signs of being subject to natural selection leading to the evolution of distinctly human traits.