Did space rocks set the human stage?

No one argues that asteroids can be devastating when they tangle with Earth. An impact 65 million years ago is widely believed to have spurred the demise of the dinosaurs and many other animals and plants.

But efforts to tie other, more ancient mass extinctions to impacts remain inconclusive. While extinctions are clearly identifiable in fossil records, impact evidence seems not to survive the millennia as well. So impact estimates are based largely on the Moon -- a nearby archive of countless craters that have not eroded much over time.

Still, because scientists have not witnessed a severe impact, the presumed effects are speculative.

If an asteroid larger than a kilometer (0.6 miles) hit the planet, it would cause instant death across a wide area near the site of impact, and researchers generally agree that drastic climate changes could last a year or more. Even our protective ozone layer could be damaged, studies have shown.

But the precise consequences of these effects are not known. It is thought that long-term climate change could make life impossible for many species, which in turn would cause mass death that might move up entire food chains.

Peiser suggests another possible effect: "The abrupt loss of the ozone layer and the sudden release of toxins may even affect the DNA in some unknown manner, thus triggering macro-mutations, including the sudden reorganization of entire genomes."

Ellen Thomas, a Wesleyan University research professor who examines how climate change affects evolution, said few evolutionists would by this argument of quick, significant changes in the genetic blueprints. Instead, macro-mutations are seen by many as a genetic dead end.

"Macro-mutations can hardly ever lead to evolution," Thomas said. "They lead to non-viable organisms."

Basic numbers questioned

Of course to affect human evolution in any fashion, a space rock first has to hit Earth. But "no one knows how many impacts took place, or when, or with what severity, over the past 5 million years," said David Morrison, an asteroid expert at NASA's Ames Research Center in California.

Morrison told SPACE.com that instead of the 20 potentially devastating impacts assumed by the study, he expects there were probably only five or 10 with enough energy to create global environmental effects.

"But we know very little about specific impacts in this time frame, and virtually nothing at all about their actual environmental effects," Morrison said, adding that there is "no evidence of an impact associated with a hominid extinction."

Morrison did not discount the whole idea, however.

"I would be surprised if impacts had not had some influence on early hominid populations and perhaps evolution," he said. "On the other hand, I am not convinced that impacts led to numerous extinctions in the past 5 million years. This is all interesting speculation, but specific data are lacking on either impacts or extinction events and there is no known correlation between the two."

Peiser counters that the estimates used in the study are "very conservative." He acknowledges that shortcomings in the human fossil record (fossils on land erode more easily than those in the oceans) "are far too big to allow any direct correlation between impact catastrophes and hominid extinction." But he said that the study shows that "impact catastrophes that occurred during the crucial period of human evolution should no longer be ignored."

Still, it is clear that more research will be needed before any consensus emerges.

"What [Peiser and Paine] may have added," said Balding, the statistics professor, "is some quantitative simulations to make more precise some well established speculations."

Speculation about evolution is nothing new. And the more one delves into the nitty-gritty of our own past, the stronger the criticism gets over Peiser's attempt to reinvent Darwin.

Next Page: Does Darwin Need reinventing again?