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.

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.

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."

Details of the New Idea

The following details of the new idea were provided by Benny Peiser and Michael Paine and appear here with only minimal editing for style and clarity:

The findings are calculated on the basis of the generally accepted "impact rate" (i.e. the rate of cosmic impacts calculated from terrestrial and lunar impact craters together with the currently observable flux of asteroids and comets in the solar system). A computer simulation of cosmic impacts over a 5 million-year period was chosen to give an indication of the environmental disruptions that have occurred during the evolution of our species.

These consequences can be categorized into:
A. Local -- devastation over a radius of tens of (miles) kilometers. No serious regional or global consequences

B. Moderate regional -- devastation over a radius of hundreds of miles (kilometers) -- the size of a small country. Short-term regional climatic problems.

C. Severe regional -- devastation over 600 miles (1,000 kilometers) (the size of a large country). Severe regional climatic disruption. Mild, short-term global climatic disruption -- year without summer.

D. Moderate global -- devastation over thousands of miles (kilometers) -- continental. Severe global climate disruption lasting several years. Global food chain failures

E. Severe global -- global firestorms from ballistic entry of impact debris. Extreme worldwide climate disruption for decades to centuries. Extinctions.
For everything except the last category, the effects on early human populations depend on proximity to the impact -- a matter of luck. In addition to climate disruption (mainly darkness and cooling), the larger impacts could lead to global warming due to the greenhouse effect (water and carbon dioxide), loss of the ozone layer (particularly with ocean impacts that propel chlorine into the upper atmosphere), acid rain and toxins.

The simulation looked at the worst event in each of 5,000 millennia. It therefore gives an underestimate of the total number of impacts.

The program recognizes five outcomes of an asteroid or comet colliding with Earth:

* The object skims the atmosphere and flies harmlessly back into space. This happened in 2 percent of the millennia.
* The object explodes above land in an airburst similar to an atomic explosion. This happened in 17 percent of the millennia.
* The object impacts the land and forms a crater. This happened in 11 percent of the millennia.
* The object explodes in an airburst above an ocean. This was the most frequent outcome, accounting for 41 percent of millennia. Fortunately, until recently, most of these impacts would have been harmless to land dwelling creatures.
* The object impacts the ocean, forming tsunami and, possibly, ejecting vast quantities of water and salt into the atmosphere. This happened in 28 percent of the millennia. (Larger impacts may also reach the ocean floor and cause similar effects to a land impact)
Over the period of the simulation some 57 percent of millennia suffered an impact that would potentially have consequences for land-dwelling creatures. In most cases they would only be affected when they were close to the impact site. The situation is different now with significant human populations living in low-lying coastal areas.

Size impactor (The letters refer to the typical environmental consequences.)

1,650 to 2,950 feet (500 to 900 meters) (C): 108 events

0.6 to 0.9 miles (1 to 1.5 kilometers) (C/D): 24 events

1 mile (1.6 kilometers +) (D/E): 13 events

Craters (The letters refer to the typical environmental consequences.)

Over the 5,000 millennia a total of 552 craters were formed on land. Of these:

* 477 were less than 3 miles (5 kilometers) in diameter (A);
* 64 were between 3 and 6 miles (5 and 10 kilometers) in diameter (B);
* nine were between 6 and 12 miles (10 and 20 kilometers) in diameter (C);
* two were more than 12 miles (20 kilometers) (D).
There were also six ocean impacts that could be expected to produce moderate-to-severe global climate disruption (D/E), particularly destruction of the ozone layer. Three of these involved transient craters more than 31 miles (50 kilometers) in diameter and would probably have penetrated to the ocean floor.

Therefore, during this simulation severe climate disruption occurred, on average, every million years (i.e., two land impacts and three ocean impacts).

The findings by Peiser and Paine are underpinned by a significant number of large impact craters. So far, 32 impact craters have been discovered that are younger than 5 million years. One is 32 miles (52 kilometers) in diameter, three are between 6 and 12 miles (10 and 20 kilometers), one is between 3 and 6 miles (5 and 10 kilometers) and 25 are less than 3 miles (5 kilometers) in diameter. However, it should be noted that it takes very unusual conditions to preserve craters of this size for more than a few hundred thousand years.

Additional background material is provided by the authors here.