Some 12,900 years ago disaster struck the North American continent. The Earth's climate, which had been getting steadily warmer at the tail end of the retreating Ice Age, suddenly dropped catastrophically. The megafauna, such as camels and mammoth died out. And the first people to populate the American landscape saw their numbers drop by an estimated 75%. A number of explanations have been offered for why this happened. One of the latest is that cataclysmic event thought to have caused several other upheavals in the Earth's history, an impact from outer space.

13,000 years ago North America was a bountiful country. The glaciers, which until recently had covered much of the landscape, were in retreat. The climate is mild. Huge animals roam the plains. Herds of camel, horse, and the woolly mammoth, still drift across the prairie.

And there are people there, belonging to the so called Clovis (or Llano) culture, named for artifacts found near Clovis, New Mexico, who harvest of this bounty. They are expert stone-knappers, producing some of the finest blades, spear- and arrow-heads seen through the bulk of human history referred to as the Stone Age.

The origins of this Clovis culture, and the first human inhabitant in America, is something of a mystery in its own right. There are no corresponding finds of stone tools resembling the Clovis Point anywhere in Siberia or the Asian continent, where, until recently, it was thought all the pre-Columbian inhabitants of America come from, crossing the Bering land bridge during the late Ice Age. There are however similarities to the the older Solutrean culture in Western Europe.

This has led some to hypothesise that the very first people in America might have crossed over the Atlantic, during the Glacial Maximum, when a permanent ice rim connected the southwest coast of Ireland to the Grand Banks. It would be possible for groups of people, at the technological level of the times, to make the crossing, hugging the ice, in much the same way as present day Inuits. This theory has also found some support through recent study of mitochondrial DNA among some native North American tribes, who were found to have genetic markers in common with some present-day individuals in Europe identified by mtDNA Haplogroup X, which is unknown among the populations of eastern Asia or Siberia¹.

Whoever they were, and wherever they came from, they were doing very well in their new home. But then it all went pear shaped. The Big Chill returns with a vengeance. The large mammals die out. And the Clovis people disappear from the pages of history. What went wrong?

Some event took place at the start of what we now call the Younger Dryas period, that had a violent effect on the Earth's climate world wide. While the temperature dropped in the north, the hunter-gatherers of the Levant and Middle East saw their surroundings stricken by severe drought. Many hold that this drought was the impetus for the invention of agriculture in this region, rich in easily domesticated plants and animals, setting mankind on the road to urbanisation, trade, and modern civilisation. The Clovis people were not so lucky.

The generally accepted view in academic circles is that the 1,300-1,400 year cold period during the Younger Dryas paradoxically was caused by the rise in temperature. The melting glaciers that had covered Canada and large swaths of the United States had formed a giant fresh water sea, called Lake Agassiz, bigger than all of the present-day Great Lakes combined, in the center of North America. The remaining glaciation formed a dam, preventing a gradual run-off into the Atlantic. At some point, 12,900 years ago, that dam burst, and enormous quantities of cold fresh water drained into the North Atlantic in a deluge.

This had the effect of shutting down the North Atlantic thermohaline circulation, interrupting the normal flow of the Gulf Current. Some scientists worry we could see a repeat of this effect if the current melt-off of the Greenland ice sheet and the polar ice cap due to global warming were to proceed at too fast a pace. This theory explains most of the facts as we know them, and would do nicely. But some scientists are not so sure, believing there might have been an additional factor.

Impact events, meteors, meteorites, comets, and asteroids slamming into the Earth, have been blamed for quite a few sudden catastrophes, from the Tunguska event in historical times, to the impact in central America that is believed to have hastened the end of the age of the dinosaurs 65 million years ago. These hammers of the gods appeal to our imagination, and make for neat deus ex machina explanations for things that would otherwise seem inexplicable. That is of course not to say that they don't, on occasion, live up to their billing. A molten mountain smashing into the ground at screaming speeds will ruin anyone's day.

And now some scientists are theorising that an impact might have contributed to three events almost 13,000 years ago, the sudden cold period, the disappearance of 35 large species of mammals (the horse and camel actually evolved in the Americas, and survived by crossing the Bering land bridge, heading the opposite way as the migrating humans, before they became extinct in their native environment), and the collapse of the Clovis culture. The demise of the megafauna could be explained by the Clovis hunters driving them to extinction. And the traditional Lake Agassiz deluge and the subsequent colder climate would suffice to explain the rest. But great systemic breakdowns are ususally not due to a single isolated event, but to a confluence of blows. While the system might have survived one, the second, or even third, proves fatal.

At the joint assembly of the American Geophysical Union, which came together in Acapulco, Mexico on the 24th of May this year, a group of researchers presented their findings, which pointed to a major impact event on the North American continent at close to the start of the Younger Dryas period. A comet, some five kilometres across, entered the Earth's atmosphere, and exploded above the northern ice sheet, causing, among other damage, the catastrophic deluge of melt-water into the Atlantic.

While they have been unable to point to any central impact crater, this could be explained by the bolide disintegrating in the air, and the principal strike zone being covered by glaciation. They also point to four large trenches below the present Great Lakes as possible testaments to the impact. The so called Carolina bays, located in line with the lakes, as well as thousands of smaller features along the Atlantic coast, might also have been a result of the impact. But some geologists protest that there are other, and far more plausible explanations for these features.

The researchers think they have found chemical traces of the impact in geological strata on 25 different locations, 9 of which are Clovis sites. They also claim to have found traces as far away as Belgium. In a thin layer of soil, they have found minute spheres of glass and carbon (nano-diamonds), which are related to meteor impacts. They've also found high levels of Irridium, and other materials more common in space debris than here on Earth.

In addition they've found a layer of ash, across the whole continent, as if a gigantic fires had raged across the landscape. The ash is laid down just below the Younger Dryas, according to their studies. Did the Clovis people, and the giant mammals perish in horrific fires, followed by darkness and a long cold period? All we know for sure is that the number of arrowheads, and probably the number of people, fall by 75% following this period. The America that emerged from the catastrophe, would be totally different from the one that had once been.



¹ For more on this, see "Lost World: Rewriting Prehistory: How New Science Is Tracing America's Ice Age Mariners" by Tom Koppel.