Some time between 400,000 and 250,000 years ago, Homo erectus evolved into Homo sapiens, heralding the arrival of an archaic version of modern humans. Eventually, Homo sapiens replaced all other existing human species. How and why this happened isn’t fully understood.
There are two theories which attempt to explain how Homo sapiens replaced other species of human. The first, known as the “Out of Africa” model, says that sapiens evolved in Africa and migrated to other locations, replacing human species which already lived there. The second, called the “multiregional hypotheis” provides the opposition view that sapiens evolved more
or less simultaneously in multiple locations.
Currently the Out of Africa model, in which sapiens evolved in Africa and migrated to replace human populations in other locations is favored. However, there really isn’t enough fossil evidence to conclusively prove either of the two theories.
One of the earliest known specimens of Homo sapiens ancestors—Mitochondrial Eve—provides some interesting evidence for the Out of Africa model.
All human cells contain DNA in the form of 23 pairs of chromosomes. One chromosome in each pair comes from our mother, one from our father.
That’s not all the DNA we have, however. Within every cell are small loops of DNA which are contained within a cell’s mitochondria (tiny, self-contained “factories” in which the bulk of energy generation is carried out).
Unlike chromosomal DNA, children do not inherit mitochondrial DNA from both parents—it is inherited only from the mother. In addition, mitochondrial DNA does not undergo recombination as chromosomal DNA does. These two factors mean that mitochondrial DNA is a much more reliable source of statistically accurate information about humanity’s ancestral origins. The lack of recombination means that mitochondrial DNA is handed down through the matrilineal line virtually unchanged, except for those mutations which occur randomly.
Using this information, together with the base rate of mitochondrial DNA mutation, and information about mitochondrial DNA types present in current human populations, it has been calculated that Mitochondrial Eve—the most recent matrilineal ancestor of every human currently alive on the planet—lived around 140,000 years ago, most likely in Kenya, Ethiopia, or Tanzania.
All humans who are alive today have mitochondrial DNA which can be traced back to this one common ancestor. As if this in itself wasn’t of great enough interest, this evidence can actually provide support for the Out of Africa model of human evolution.
Family trees constructed using mitochondrial DNA show that certain subtypes of mitochondrial DNA—which are known to be the oldest—are confined to populations of people living in Africa. This in itself supports the idea that all of humanity is descended from populations which once lived on this continent. The idea of the Mitochondrial Eve bears this out, as she is believed to have lived in Africa at a time which predates the migration out of Africa.
Another piece
of evidence from mitochondrial DNA is that Africans have the greatest diversity in this form of genetic material. It is widely believed that this higher level of diversity has accumulated because humans have been living in Africa longer than anywhere else, thus providing further support for the Out of Africa model.