Cet article nous explique que les mammouths d'Amérique du Nord (aucun rapport avec les dirigeants du Parti socialiste!) auraient été exterminés par une comète (aucun rapport avec Ségolène Royal!) et non pas, comme on le supputait auparavant, par les chasses du peuple de "Clovis" (Clovis était le Roi des Francs) ;-) ...
ARCHAEOLOGISTS know little of what life was like for the first people in America. One thing they thought they knew, however, was that these people hunted mammoths and other large mammals until none was left. Once that had happened, they disappeared themselves.
But the Clovis people of 13,000 years ago, named after the place their characteristic arrowheads and spear-points were first found, may not be a bell-tolling example of the danger of technological progress after all. They may have gone, along with mammoths, short-headed bears, ground sloths and camels, because a comet exploded over their heads.
If that were true, North America would have suffered the first world-changing impact of an extraterrestrial object in the era of modern man. And because it happened so recently—“like yesterday” in the words of James Kennett of the University of California, Santa Barbara—any amateur-archaeologist could inspect the evidence for himself by digging deep enough. In many places in the United States and Canada, at a depth corresponding to 12,900 years ago, a few centimetres of charcoal will appear. This, according to Dr Kennett, is the product of wildfires that spanned the continent after an object roughly a kilometre across grazed the Earth, broke up into ever smaller pieces and deposited all of its oomph as heat into the atmosphere.
Mixed in with the charcoal, as Dr Kennett and a number of his colleagues reported to a meeting of the American Geophysical Union in Acapulco this week, are other bits that point to such an impact. There are glass-like beads made of carbon. To melt carbon requires a temperature of around 4,000°C. Such heat is hard to come by on Earth. And only this month it was discovered, by looking at the beads in an electron microscope, that they are chock full of diamonds a micron or less across. Finding the temperature and pressure to create such diamonds on the surface of the Earth would be impossible, unless a huge hammer-blow from space is meted out.
Something the researchers have not found in this layer is iridium, or at least not a lot of it. This metal is the fingerprint, the world over, of the impact some 65m years ago of an asteroid, in what is now Mexico. That collision caused the demise of the dinosaurs. A lack of iridium is the reason Dr Kennett and his colleagues think the collision they believe they have discovered was with a comet rather than an asteroid. Comets consist of “dirty ice”, rather than solid rock, and so contain little iridium.
That something big happened around 13,000 years ago was already known. Palaeontologists have a name for the sudden cooling of the climate of the northern hemisphere that started right then and lasted for over a thousand years. They call it the Younger Dryas.
Before the Younger Dryas, the world had been warming up as the last ice age came to an end. It was this warming that gave the Clovis people and their prey a chance to thrive. But as for the cause of the Younger Dryas, the eyes of science were, until now, directed not up at the sky, but down into the ocean.
Dr Kennett himself contributed to the body of work that suggests changing ocean currents caused the cooling. There is evidence that currents which bring warm water from the tropics to the Arctic, as the Gulf Stream does now, can be pushed around. Dr Kennett worked on the idea that at the start of the Younger Dryas a huge lake of fresh water suddenly breached a wall of ice that had confined it, and flowed into the northern Atlantic Ocean. That would have altered the heat- and salinity-driven dynamics of the area rapidly, stopping the warm current from travelling so far north.
According to Dr Kennett, the essence of this hypothesis is compatible with the cause of the change being an impact. The heat from the impact would have melted a lot of ice, releasing fresh water. So, by way of the ocean currents, the impact would initiate a long-term cooling. This would be in addition to a much less theoretical short-term cooling caused by smoke from the fires and dust thrown into the atmosphere by the impact. It would also explain why the lake required by the original hypothesis has not yet been located.
Once you look at the history of life around 13,000 years ago, things start coming together. Not all large mammals died out. Bison, for instance, kept roaming the plains of America. But Dr Kennett notes that present-day bison are not like the ones the Clovis people hunted. They are smaller—and they are remarkably alike in their genetic make-up. This suggests that all present-day bison are descended from a very small group of animals. According to the clock that ticks off the increase in diversity of the DNA of a group of animals due to the accumulation of random mutations, this “population bottleneck” occurred at the beginning of the Younger Dryas.
Of course, that might just mean that the Clovis people had not quite exterminated bison before they, themselves, disappeared. But if that were true, it would suggest that the Clovis culture vanished suddenly—which raises the question, why?
Dr Kennett suggests two approaches to answering this question. One is to look for evidence of population bottlenecks in other species, especially animals that were not hunted by the Clovis people. If found, such bottlenecks would indicate an external shock to the ecosystem. The other is to examine in detail the archaeological record just above the charcoal layer.
The successor of the Clovis culture, the Folsom culture, is also defined by its arrowheads. So far it has been impossible to decide if these arrowheads were the tools of new people from Asia who replaced their dead cousins, or of a remnant of Clovis people who changed their technology and survived. Now that it seems likely a comet has drawn a sharp line of charcoal between the two cultures, careful measurements should be able to show whether North America was, for a few centuries after the impact, as bereft of people as it has since been of mammoths.
May 24th 2007 | ACAPULCO
From The Economist print edition