In the 5 million years or so that it took for apes to become human, many human-like branches of the evolutionary tree were lopped off. Scientists have long wondered why these other hominid species, estimated to number a dozen or more, didn't make it.

Were those who came to travel to the Moon and ponder their very origin the logical and inevitable victors in the most important of all Darwinian struggles?

Or did we just get lucky?

A newly presented mathematical argument suggests that the birth of Homo sapiens was guided by catastrophic asteroid or comet impacts, which created climate conditions that competing species, frankly, couldn't handle.

It also holds that our human ancestors avoided early elimination by the statistical skin of their rotting teeth.

"The reason that Homo sapiens have survived in spite of these global disasters has little to do with the traditional explanations given by neo-Darwinists," said Benny Peiser, a social anthropologist at Liverpool John Moores University. "It is sobering to realize that we are alive due to cosmic luck rather than our genetic makeup."

Peiser bases his argument on the fact that populations of hominids and early modern humans were extremely small. "Had any of these impacts occurred in the proximity of these population groups, we might also have gone the way of the dodo," he said.

The study's assumptions and calculations have met with strong caution and even sharp criticism among scientists who specialize in evolution, as well as asteroid experts.

Adaptive advantage

David Balding, a professor of applied statistics at University of Reading in the U.K., said the idea that human survival is due to "cosmic luck" does not compute:

"Perhaps we were lucky in avoiding a massive impact, but perhaps it was our adaptive advantage that helped us survive modest regional impacts whereas our hominid cousins did not," said Balding, whose own research focuses on human evolution.

But some called the new scenario plausible. It has not been published in a peer-reviewed journal but it is based on impact estimates that are generally accepted by the asteroid research community, though there are disagreements over the precise number of times a large asteroid or comet has struck our planet.

Peiser laid the idea out earlier this month at a conference, "Celebrating Britain's Achievements in Space." He worked with Michael Paine, a volunteer for the Planetary Society in Australia who ran impact scenarios through a computer program. (Paine has written freelance stories for SPACE.com in the past.)

The researchers concluded that there would have been 20 "globally devastating" impacts during the past 5 million years, with effects strong enough to have had "a catastrophic and detrimental effect" on human evolution. Five million years ago is roughly the time when hominids diverged from other apes, though some recent controversial evidence puts the split as far back as 6 million years ago.

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