You can see Clovis spear points, long polished leaves of agate and chalcedony, in Tacoma’s Washington State History Museum. Clovis points, named after the town in New Mexico where they were first found more than 70 years ago, have pushed back the date of human settlement in North America to at least 13,200 years ago.

The Tacoma spear points formed part of a cache unearthed in East Wenatchee 22 years ago by a couple of farm workers digging a trench through an orchard. Others have turned up virtually all over the continent. Some archaeologists think the Clovis people scattered quickly across North America; others think it was their technology that scattered quickly — among people who were already living here.

The Clovis culture that created the distinctive points disappeared 12,900 years ago, and the giant sloths and mammoths and a host of other Ice Age fauna they hunted disappeared along with them, at exactly the same time. They all vanished right at the start of the cold spell known as the Younger Dryas, which interrupted 5,000 years of warming at the end of the last ice age, plunging the northern hemisphere back into cold for another 1,100 years.

So, where did they all go? An intriguing theory has been getting lots of attention since a one-page article by Douglas J. Kennett, a University of Oregon archaeologist, and his colleagues, appeared in Science in January.

”People have been puzzling about the extinctions for a very long time,” Kennett says. Some scientists think that climate change did in the mammoths and all those other critters, and that Clovis hunters vanished along with their prey. But why, Kennett asks, would whole genera of animals “highly adapted to Ice Age conditions” die of cold? And why would they all die at once?

Other scientists suggest that the first human hunters killed the last of the ice age megafauna, basically hunting themselves out of business. There are two problems with that. One is political. If those early hunters were forebears of current Native Americans and Native Americans were the "first conservationists," then they just couldn't have killed off whole species. The other is basically logistical. Mammoths and mastodons weren't the only critters that disappeared. Sixteen whole genera vanished. Kennett doubts the small populations suggested for North America 13,000 years ago were large enough to pull this off.

Regardless of why the Clovis hunters and their prey disappeared, why should a world growing warmer suddenly head back to ice? There have been 10 glaciations and warmings in the past 700,000 years, explains University of California Santa Barbara paleoclimatoligist James Kennett (who happens to be Douglas Kennett’s father). This abrupt cooling seems to have been unique. All else being equal, he says, “the Younger Dryas should not have occurred,” No long-term pattern explains it. The earth was getting warmer. The amount of solar radiation reaching the northern hemisphere was near its peak. “So why have a cooling?”

You may remember Al Gore warning against the perils of global climate change. He suggested that an accelerated melting of the Greenland ice cap could send enough fresh water into the north Atlantic to disturb the so-called North Atlantic conveyor, the northbound current of relatively warm ocean water that moderates the temperature of North America and northern Europe.

In fact, fresh water has screwed up the North Atlantic conveyor before. As the last ice age waned, the huge Laurentide Ice Sheet started melting, releasing huge volumes of fresh water that formed the giant Lake Agassiz in what is now central Canada and the northern Midwest, and flowed down the Mississippi Valley to the Gulf of Mexico.

Then, suddenly, 12,900 years ago, the flow of meltwater shifted. Lake Agassiz largely drained. The freshwater torrent started flowing east, perhaps along the St. Lawrence valley, to the north Atlantic, and north to the Arctic. The conveyor was indeed disturbed, and cold weather returned to much of the northern hemisphere for another thousand years. That was the Younger Dryas. James Kennett suggests the change in ocean currents that caused the Younger Dryas was the prototype of the process Gore describes.

The big question isn’t why ice melted, why currents changed course, why cultures vanished or species went extinct. These things happen. The question is why they all happened so abruptly — and all at once.

Diamonds may hold the clue. Something strewed tiny diamonds across the continent and perhaps across much of the hemisphere 12,900 years ago. First, scientists found tiny spherules of carbon, little dark balls light enough to float — which is how you separate them from the soil in which they lie — but large enough for the largest of them to be seen with the naked eye as they roll around in the bottom of a glass vial. They split open readily. When scientists examined the insides with a powerful transmission electron microscope, they saw the diamonds. They have now identified several kinds; none is large enough to be seen with the naked eye.

The diamonds must have been scattered across North America just when those people and all those critters disappeared. Everywhere, evidence of Clovis culture “stops with the nanodiamond layer,” Douglas Kennett explains. “Why,” he asks, “isn’t there continuity across the nanodiamond layer?”

Douglas Kennett envisions a “swarm” of particles, perhaps a comet that had already broken up before it reached the earth’s atmosphere and fractured further when it hit the air, striking in many places, generating the intense heat and high pressure needed to create diamonds. It started fires, producing a huge shock wave, accelerating the demise of the ice sheet, blowing diamonds, soot and other particles high into the stratosphere.

The impact would have done two things to change the climate: by sending those particles into the stratosphere, it would have blocked sunlight for years; and by somehow hammering the Laurentide Ice Sheet, it would have changed the North Atlantic conveyor. The first would have dropped northern hemisphere temperatures in a hurry, killing or stunting a lot of plants that occupied critical lower rungs of the extinct fauna’s food chains, but its effects would have been short-lived. The second wouldn’t have happened as quickly — changing the course or velocity of a major ocean current would be like changing the course or velocity of a major ocean liner, only slower — but its effects would have lasted a long, long time.

The impact theory may not exclude other processes that scientists have pondered. There’s no doubt those early hunters killed mammoths and some of the other critters with which they shared the continent. If the comet fragments hitting the earth wiped out most of the remaining ice age fauna, some of the surviving hunters may have wiped out the last of the surviving mammoths. Or they might already have depleted animal populations to the point at which the effects of the impact easily finished them off.

Who knows what really happened 12,900 years ago? Even if the impact theory turns out to be the right answer, it doesn’t have to be the exclusive answer. “I don’t think that this [cosmic impact theory] replaces all other explanations,” Douglas Kennett says. “We would never rule out very stressed out humans . . . hunting . . . remnant populations of these animals.”