As recently as 1970, some evolutionary scientists believed there was no single point of origin for modern humans. Instead, they believed, we evolved globally all over the world. That view, known as multi-regionalism, suggests that human evolution took place seamlessly from Homo erectus to modern humans.
In the last four decades, however, many new evolutionary theories have been created and revised. The "Out of Africa" theory, for example, maintains that humans emerged rapidly in one small part of Africa and then spread to replace all other humans within and outside the continent.
Chris Stringer has a theory of his own. He is a researcher at the Natural History Museum in London and author of "Lone Survivors: How We Came to be The Only Humans on Earth." The book suggests that distinct humans coexisted and competed across the African continent, exchanging genes, tools and behavioral strategies.