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Bottlenecks made humans less diverse

Despite the sheer enormity of our planet, we humans are far more closely related than we should be.
/ Source: Discovery Channel

Despite the sheer enormity of our planet, we humans are far more closely related than we should be.

The evidence is all around us: Biologists, for example, have long known that measurements of skulls yield far more similarities among Asians, Europeans and Native Americans than among indigenous African populations. As humans migrated out of Africa, humanity lost much of its genetic diversity.

But exactly how this happened has become a matter of intense debate.

Now, a recent study from University of Cambridge researchers William Amos and J.I. Hoffman suggests that the limited genetic diversity we see among humans today came from two major evolutionary bottlenecks — or sudden declines in population — that lead to a loss of genetic diversity.

Amos and Hoffman found evidence of these events after they examined genetic samples from 53 distinct world populations. In the study, published in the October Proceedings of the Royal Society: B, the pair studied the frequency of two genetic indicators.

When Amos and Hoffman examined their samples, they found evidence of a bottleneck outside of Africa around 50,000 years ago. They also found a second major evolutionary bottleneck that occurred somewhere around the Bering land bridge, a narrow strip of land now submerged beneath the Bering Strait, which many anthropologists believe humans used to populate the Americas from Eurasia.

It's tempting to imagine sudden declines in human population at the hands of saber-toothed tigers or from a massive comet strike, but Amos says the likelier scenario is that humans diverged from one large population over and over again, leading to a series of what are known as founder effects. As humans leave big groups for smaller ones, they carry with them only a sample of the genes found in the larger group.

"It's almost like if you went into a town and took the first 15 people you met to go found another town," Amos explained. "The new town isn't going to fully represent all the surnames in the original town."

And what's true for names is also true for genes, say geneticists.

Amos and Hoffman's study is a controversial one in a controversial field of study. Geneticists vary widely in their interpretation of when and how humans migrated out of Africa -- and exactly what happened once they did. Amos and Hoffman suggest that humans began an exodus from Africa and ran into a bottleneck as they diverged and colonized Europe and Asia.

Robert Eckhardt, a geneticist from Pennsylvania State University, subscribes to a competing theory posed by researcher Alan Templeton of Washington University in St. Louis. Templeton suggests humans moved out of Africa in several major, distinct waves.

This theory contradicts the findings of Amos and Hoffman, who believe that colonists diverged from a single wave of emigrants. According to Templeton, these successive waves mated with the descendants from previous waves, accounting for the loss of diversity.

Eckhardt disagrees with the implications of Amos' and Hoffman's African bottleneck study, though he doesn't dispute their Bering land bridge findings. He is, however, wary of the computer program and raw data used in the study.

"It is pretty much the application of a canned program to a canned set of data," Eckhardt told Discovery News. "Subsequently, there are all sorts of separate bits of tinkering and adjustment to the process."

Amos is aware of the contentiousness of his field and the current limitations of his relatively young field of expertise. In the age of air travel and interracial marriage, tracking human ancestors through their modern descendants is difficult.

"There's a lot of information about history and past ancestors in the DNA of an individual," Amos said. "But we're in the early stages of unlocking that information."