It is generally accepted that our ancestors were African and the world was populated by migration out of Africa . There may have been a number of major migrations out of Africa but we would, given our understanding of human behaviour assume that migration did not go on all the time. Recent studies have shown that even the great divide that is the Sahara desert was once a lush forest through which could sustain humanoid life and from that there is evidence of Southern Europe being populated via this route at some point.
The migration that eventually populated the world really was the migration of a small group of Africans, a member of which is the person described as Mitochondrial Eve as in the biblical 'Adam and Eve', the maternal ancestor of all non-African peoples. The paternal Adam appears much later in the picture. Whether this small group of Africans were taking part in a much bigger migration will probably never be known. When we talk about humans, we really are talking about a sub-species of the human family Homo sapiens sapiens. But other members of the great ape families already existed in other parts of the world as we spread out; themselves most probably the result of much earlier migrations or even the result of evolution in those locations, the Neanderthals are an example of this.
Our ancestors eventually replaced these great ape populations and the debate continues as to whether they played a part in the eventual decline of those populations. Another debate rages about whether we inter bred with these other humanoids. Recent evidence suggesting that all non-African people have Neanderthal DNA probably proves that we did. Some Chinese anthropologists (people who study people) are convinced that the Chinese are descendants of a different branch of the human tree but that has yet to be proved. It may be possible that there may be have been further mixing in the Chinese case maybe with Homo erectus, the most famous fossil known as Peking man.
The Africans have proved to be the most genetically diverse people proving the archaeological evidence that it is the cradle of homo sapiens which has led to the out of Africa theory. However, Africa was home to a number of human like creatures and with the severe lack of DNA and fossil evidence, it is difficult to determine if homo sapiens was indeed the product of several, possible inter-breeding events.
What is obvious is that human beings were not averse to mixing it up then. This behaviour continues throughout our history to present day as we shall we see over the course of this project. Long term survival of a species is a product of genetic variation that allows for adaption to changing environments and mixing with genetically different individual seems to be nature's way of doing it. Nature also provides the barrier that stops the mixing of two different species - all humans belong to the same species.