Evolution in General and Human Evolution in Particular

There's a major new genetic study out that says modern humans don't have a single point of origin:

Scientists have revealed a surprisingly complex origin of our species, rejecting the long-held argument that modern humans arose from one place in Africa during one period in time.

By analyzing the genomes of 290 living people, researchers concluded that modern humans descended from at least two populations that coexisted in Africa for a million years before merging in several independent events across the continent. (NY Times, Nature)

If you think that sounds really weird, that's because of the way the origin of new species has been thought about and taught for most of the past 150 years. The first model came from Darwin himself, after he studied the finches of the Galapagos. His idea what that groups of finches became isolated from each other, either geographically (on different islands) or behaviorally (relying on different foods) and so gradually evolved in distinct directions. The basic idea was isolation ►speciation. Part of the reason this became dogma was that in the absence of Mendelian genetics Darwin and his allies could not explain how a new, favorable trait could spread through a large population, rather than being swamped by existing variants.

That isn't gone from the science; geographical and behavioral separation probably play a role in the formation of many species. But lately a lot more attention has been payed to whole large populations evolving in common ways. Consider Homo erectus, which existed for more than a million years across vast areas of Africa and Eurasia but gradually evolved toward larger brains and more dexterous mouths. Modern genetic studies have also taught us how common inter-species breeding is, and how readily species can incorporate genes they bring in from outside.

So working under the old model, people thought that fully modern humans evolved from some small population that had gotten separated from the mass of African hominids. The new model suggests that modern humans emerged from at least two distinct populations, Stem 1 and Stem 2, that were not entirely separated from each other, and that the transition took place over a few hundred thousand years.

This study is based on modern genomes so it is not definitive; it will need to be confirmed with ancient DNA and fossils. But this is definitely the way evolutionary science is trending.