The big bang theory's back. But this time the theory doesn't involve the cosmos, just a comet.

Some scientists hypothesize that relatively recently in our geological history a comet collided with Earth. And they're not talking about the collision 65 million years ago that did in the dinosaurs. They're talking about a collision 12,900 years ago, which did away with woolly mammoths, saber-tooth tigers and giant sloths, among some three dozen large mammals.

Tuesday, at 8 p.m., PBS, Channel 2 airs Last Extinction. The hour-long NOVA-produced program features commentary from several scientists, including Peter Schultz, professor of geological sciences at Brown.

"This is like a CSI mystery," he says. "We're still picking up pieces and analyzing them. We think something happened and it's still a debate what exactly happened."

Scientists postulate that a comet struck Earth, probably in North America. The impact would have been equivalent to several nuclear bombs. There would have been widespread fire, the destruction of many plants and the death of animals that lived off them. Then, scientists say, there was a small ice age.

There's "a major climate change and some animals don't make the change," the show reports. Humans, obviously, made the change, but needed to bundle up. The temperature on Earth, according to geologic records, dropped 18 degrees in two years.

The basis for the show and its theory is scientific research, coupled with scientific interpretation.

"It has generated quite a bit of controversy in the field, some of it not necessarily polite," says David Fastovsky, a professor of sciences at URI, who specializes in the study of dinosaur extinction. "But that's OK."

Fastovsky is not a participant in the show. However, he is receptive to what it says.

"I don't reject the theory out of hand. As a scientist, I am interested. But I can't say the theory is without criticism."

As a basis for their theory, scientists used the Greenland ice sheet, which the show calls "a frozen library of information of the Earth's history." Snow captured in a glacier, scientists say, reflects the atmospheric conditions of a given year, including temperature and particulate matter.

And what's discovered in the ice in Greenland is also discovered in the ground of North America, a geological layer known as the "Black Mat." In it, scientists found iridium, an element rarely found on earth, yet found in elevated levels in the Black Mat at some 50 sites across North America. Iridium, which is often an indicator of meteors and comets, is the same element that was found in the geological layer correlating with the extinction of the dinosaurs 65 million years ago.

And, more tellingly, the Black Mat also contains hexagonally shaped microscopic diamonds, which don't occur naturally on earth, scientists say. These diamonds are formed only by a high-pressure blast caused by an extraterrestrial force, such as a comet collision.

"This raises the bar a bit," Schultz says. "The discovery of hexagonal nanodiamonds softens the criticism and makes the story much more interesting."

The comet-collision theory was first formally presented two years ago. Before that, some scientists theorized that humans, namely the Clovis people who used stone-tipped spears, may have hunted woolly mammoths and other large mammals to extinction. But the Last Extinction show dismisses that idea. The number of humans was too few and the size of North America was too great. And the hunting theory doesn't explain the temperature change in the earth and the sudden appearance of iridium and nanodiamonds.

The comet-collision theory offers an explanation, but it has a shortcoming: Where's the crater?

Answering this question is Schultz's primary role in the program. He offers two theories why there's no12,900-year-old crater on Earth.

Well, the comet could have broken up upon entering the earth's atmosphere, Schultz says. A big comet could become millions of small fragments, which would have relatively little impact on the Earth.

"The Earth would recover from this very easily, just a little bit of rain and a little bit of weather and you'd lose the evidence," Schultz says in the program.

The second explanation Schultz offers is that the comet collided with a glacier. The comet broke up on impact, and so did the glacier.

"You lose the evidence," Schultz says in the program. "The ice acted as this flak jacket."

All scientists aren't completely embracing either explanation, according to Fastovsky. The comet break-up theory has a weakness.

"If you destroy the comet too much, then its catastrophic effect becomes undone," Fastovsky says. "You can't break it up too small."

And the glacier explanation defies science, according to Fastovsky.

"Science has to be testable. That is a fundamental criterion in science. If this impact occurred on land, but all traces of it are gone, then it can't be supported and it's not science."

This is not to say that Fastovsky doesn't like the comet-collision theory, just that it needs more research.

"The theory that an asteroid killed the dinosaurs was first seen as implausible or absurd. But as time went on, the data showed the theory was correct. So, right now we're at 'Wow, that's interesting. Let's get more data.' "

So that's what scientists are doing, Schultz says, gathering more data. A scientific theory, he says, must be based on indisputable proof, not popularity.

"This isn't American Idol. This is a case of investigation."

And further investigation will take time. In the meantime, Schultz says the scientific suspicion that a comet collided with the Earth relatively recently should give us pause.

"If this was a comet and it was big, it would suggest we need to worry about comets as well. Comets are like a long-distance relative coming to your house unannounced. They are hiding out there in the solar system."

Maybe the next time a comet makes an unwelcome visit to earth, Schultz says, scientists may have developed some sort of defensive response to keep it from entering our atmosphere. But right now, we don't know when the next comet could come.

"It won't be tomorrow. But maybe that's what the dinosaurs said the day before. This means a sudden self-awareness of the human race. We can't control the weather on Earth or in the solar system. In this case, it would be hard rain. Objects from space hitting the earth I'd call hard rain."