Out of Africa: The Origin of Our Species (Part 9)

Early Human Migration

Note: I started reading Africa: A Biography of the Continent by John Reader. The following posts will basically summarize what I find interesting in the book as I’m reading it. None of the ideas or thoughts are of my own.

The African fossils found so far are up to 100,000 years old, while the non-African counterparts have all been significantly younger. This tells us that anatomically modern humans from Africa were ancestral to all non-African populations and their modern descendants. Gunter Brauer, a German anthropologist, published his "Afro-European sapiens hypothesis" in 1984, in which he concluded that anatomically modern humans had evolved in East Africa from the pre-existing hominid stock not less than 150,000 years ago. From there the early human species, homo sapien sapien, spread rapidly throughout the African continent to the Nile valley reaching the Delta, the Mediterranean, the Middle East, and from there to the continents of Europe, Asia, Australia, and the Far East.

Map showing early human migration (Wiki). Numbers accompanied by arrows indicate how many million years ago the migration of homo sapiens happened.
Red: Homo Sapiens Yellow: Neanderthals Green: Earlier hominid

The fossil record shows that modern humans were in the Middle East by 100,000 years ago. Populations that migrated north from that point on were established in Europe 40,000 years ago. And those that turned east reached Australia by 35,000 - 50,000 years ago at the latest, and those who reached China did so before 30,000 years ago. From Asia, when the sea levels were low, groups of modern humans crossed the Bering Straits into North America between 30,000 and 15,000 years ago and from there to the tip of South America by 12,000 years ago.

In addition to fossil records, the past history of human populations can be constructed from genes. A group of geneticists at the University of California at Berkley analyzed the mitochondrial DNA of different groups of people around the world, and found that more mutations had occurred among Africans than among or between any other groups. The mitochondrial DNA molecules are identical in every cell of an individual. Mitochondria reproduces by cloning; asexually, by division. But the mitochondria are only inherited from the female parent because in the sperm cells they disintegrate at fertilization. Clonal reproduction and female inheritance leave mtDNA unaffected by the combination of genes that occurs in the reproduction of nuclear DNA.

Mitochondria in a cell

The Berkley geneticists found the greatest degree of variation in mtDNA among indigenous people in Africa, and significantly less variations among non-Africans. In fact, the mtDNA of an individual born in England and another born in New Guinea was more alike than the mtDNA of two individuals from Nigeria. This shows that a greater time-depth of mutation was preserved among people in Africa, while everyone else shared a predominance of mutations which had accumulated in the relatively recent past. From this, the geneticists concluded that the entire population of the modern world was descended from a relatively small group of people that left Africa about 100,000 years ago.

In addition to that, the geneticists found that every human being alive today carries the mtDNA of just one African woman who lived more than 10,000 generations ago. She wasn't the only woman alive at the time (i.e. she wasn't the Biblical Eve). It just happened that her particular mtDNA steadily became dominant as some maternal lineages disappeared with each succeeding generation. Not every woman produces daughters who would go on carrying the mtDNA. The geneticists referred to this ancestor as "our common mother," but later she became known as "The African Eve."