Does Darwin need reinventing again?

If asteroid experts are sometimes a mile apart on their view of history -- and they are -- then evolutionary theorists live on different continents.

Followers of Charles Darwin have long believed that failed branches of our ancestry reflect a common mode of evolution, whereby species are gradually replaced by more advanced species that adapted because of their superior genetic fitness.

But in recent decades, a different view called punctuated equilibria has taken hold. This theory, first put forth in the 1970s by Stephen J. Gould and Niles Eldredge, expects sharp changes in evolution.

In either scenario, luck plays a role. And both fit within the most famous of Darwinian themes, survival of the fittest. But the rapid shifts assumed in punctuated equilibria, be they caused by sudden disasters or other means, are thought to be the mechanism by which one species replaces another.

"There has been debate for over 100 years on whether evolution is gradual or punctuated," said Balding.

And the debate continues. Recent fossil findings have some researchers leaning back toward the gradual approach to human evolution.

Peiser said his study supports punctuated equilibria, and helps explain why "almost all hominids, i.e. the 14 known species of human ancestors, have become extinct during the last 5 million years."

But Wesleyan's Ellen Thomas said it is not even known that there were 14 species.

"The human fossil record is incomplete, and it is not easy to agree on which fossils belong to different species," Thomas said in an e-mail interview. "The experts disagree wildly."

Thomas echoed other scientists in pointing out that there is no fossil evidence -- neither of human remains in Africa nor marine organisms, which leave a much more complete record -- that reveal any mass die-offs during the 5 million-year period covered in Peiser's study.

"And if the extinctions affected humans, they should show up in the extinction record of other organisms as well," Thomas said. "The paper just shows that many impacts, many of which could have been damaging, possibly occurred."

But Peiser argues that no expert on near-Earth asteroids, the space rocks known to exist in our region of the solar system, questions that "many such global disasters must have occurred." Yet he said "all textbooks on human evolution completely ignore the occurrence of catastrophic impacts."

Other forces of evolution

While Peiser and Paine suggest that comets or asteroids are a driving force behind evolutionary change, it is the climatic consequences of impacts that are the would-be crushing mechanisms for fledgling species. Other researchers have long debated possible links between climate change and human evolution.

For example, cold periods are suspected of forcing migrations that created small, isolated groups that could have evolved significantly but then died out. One such period may have occurred as recently as 71,000 years ago. But firm links between climate and serious evolutionary changes elude researchers.

One recent international study, released earlier this year and led by Jeremy Marlow of Newcastle University, showed evidence of a significant cooling of the climate 2 million years ago that the authors said "adds weight to the theory that climate change played a significant part in the evolution of early humans."

Further clouding the possibilities, recent findings have hinted at the possibility that the worst extinctions might require multiple killing mechanisms, such as when an impact, or perhaps several, happens to occur during a time of heavy volcanic activity.

Irony in our existence

In an ironic preface to the whole argument, it's possible that asteroids and comets were responsible for life in the first place. A growing movement among astrobiologists suggests that rocks from space brought critical building blocks that stimulated the initial biological activity in the earliest primordial soup billions of years ago.

But regardless of whether cosmic messengers helped make us who we are, there is one thing researchers seem to agree on: Given the evidence that our ancient ancestors were clustered in a relatively small area (in Africa) you are somewhat lucky to be reading about all this.

"Asteroids certainly had the opportunity to wipe out man at his roots," said Jack G. Hills, an asteroid specialist at Los Alamos National Laboratory. "Only good luck prevented it."

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