Out Of Africa

Modern humans emerged as a completely new species in Africa just 100,000 to 200,000 years ago. About 100,000 years ago, began to migrate into the rest of the world. Instead of interbreeding with the locals, modern human replaced them, presumably driving all other human tenants on Earth to extinction.

In 1987, Mark Stoneking, Allan Wilson and Rebecca Cann, at the University of California, Berkeley, focused on the genetic material, or mitochondria DNA that comes from the mother alone and it records a pristine family history from one mother to the next.

Mitochondria DNA offers a quick-ticking molecular clock and by comparing the number of mutations that have collected in separate populations, geneticists can infer when the populations split from each other.
Wilson and his colleagues compared selected sequences of mtDNA from a group of people representing African, Asian, Australian, Caucasian and New Guinean ethnic groups. Among these people they found 133 variants of mtDNA. Next, they arranged these different mitochondrial types into an evolutionary tree. That tree showed a trunk splitting into two major branches. One branch consisted only of Africans, the other included some modern Africans and some people from everywhere else. According to the researchers, a population of Africans, the first modern humans, forms the trunk and longest branch of the tree. The second branch represents a subgroup that left Africa and later spread out to the rest of the world.

                                       Homo Sepian

                                     Homo Heidelbergensis

                                  Homo Erectus

                                        Neanderthal Man

The researchers also found that all of the mtDNA, even from far regions of the world, was similar. This suggested that the molecular clock has not been ticking long enough to accumulate appreciable differences in our DNA. In other words, our species is young.But the African samples had the most mutations. This too implied that the African lineage is the oldest, that all modern humans trace their roots back to Africa.

To find the date, the researchers searched for a sequence of mtDNA that all the subjects shared. This part must have come from a common ancestor of all modern humans and must have given rise through mutation to the 133 variants in people today. Using the known rate of mutation among other primates, the researchers calculated how much time that ancestral mtDNA would have taken to mutate into the 133 variants.

From this, they traced the mtDNA back to a single, common female ancestor who lived in Africa between 140,000 and 290,000 years ago. A new species called Homo sapiens split off from Homo erectus in Africa somewhere between 140,000 and 290,000 years ago. Eve was born among this pioneering group of modern humans.

At some point, one of her mitochondria genes mutated. She then passed it on through her lineage to other modern humans. Thousands of years later, people carried the gene out of Africa. Eventually, their descendants replaced all other humans on the planet, including the Neanderthals.This model was based on a certain genetic evidence, Mitochondrial Eve represent the hypothesis that modern humans can be traced back to an ancestral population that lived in Africa closed to 150,000 years ago.