Megabeasts' Sudden Death

TV Program Description
Original PBS Broadcast Date: March 31, 2009

Fifteen thousand years ago, North America was like the Serengeti on steroids, with mega-creatures roaming a continent teeming with incredible wildlife. But then, in a blip of geologic time, between 15 and 35 magnificent large types of animals went extinct. In this television exclusive, NOVA joins forces with prominent scientists to test a startling hypothesis that may finally explain these sudden and widespread extinctions—that a comet broke apart in the atmosphere and devastated North America 12,900 years ago. (See a video clip about the kind of damage a comet airburst could do.)

The program uses stunning computer animations to show what the continent may have been like thousands of years ago, with herds of woolly mammoths, hulking saber-toothed cats, giant ground sloths, and armadillo-like glyptodonts. Also on hand were the first well-documented humans in North America, known as the Clovis people. (Or were they the first? See our interactive map showing much earlier sites.)

The conjectured comet crash was practically yesterday compared to the dinosaur-killing asteroid of 65 million years ago, which humans were not around to see. But the Clovis people would have been there to witness this particular disaster unfold. While their remarkable stone-tool culture vanishes from the record at this point, we have no way of knowing how the comet strike affected these prehistoric Americans. (For examples of the striking Clovis culture, see Extraordinary Artifacts and Stone Age Toolkit.)

Whatever the culprit, something definitely happened to cause rapid extinctions across North America. One longstanding view is that the Clovis hunters arrived from Asia to find the North American big game easy prey to their sophisticated weaponry and hunting techniques. Within a few centuries they had wiped out the most vulnerable species. But geologist James Kennett of the University of California, Santa Barbara, argues that while comparable "overkills" occurred on many islands, the idea that bands of Stone Age hunters could annihilate a continent's-worth of big animals is absurd.

Another long-established theory proposes that a sudden, sharp flip back to ice-age climate conditions was responsible for the die-off. Evidence shows, however, that the big animals had survived similar episodes of severe climate stress before. A third idea, that a virulent disease or diseases wiped out the megafauna, has also been advanced. Now the comet-impact hypothesis introduces a fourth possible culprit for the extinctions. (In End of the Big Beasts, hear the strong opinions of a spokesman for each of the four camps, including Jim Kennett.)

Proposed by Kennett and others, the comet-impact hypothesis has been bolstered by unusual discoveries in a distinctive soil layer known as the "black mat," found at more than 50 sites across North America. The most intriguing clue is the presence of nanodiamonds, tiny gems believed to be forged only under the enormous heat and pressure of an extraterrestrial impact.

To test the hypothesis, NOVA brings leading climate scientist Paul Mayewski of the University of Maine and other experts to Greenland to hunt for such cosmic materials trapped in a narrow layer of ice associated with the time of the supposed impact. Mayewski likens the search to "worse than looking for a needle in a haystack." Amazingly, they find the layer and its inclusions.

In addition to Kennett and Mayewski, "Megabeasts' Sudden Death" features independent geologist Allen West, archeologist Vance Haynes of the University of Arizona, and physicist Mark Boslough of Sandia National Laboratories, an outspoken skeptic of the comet-impact scenario. (Follow the decades-long rumpus over the cause of the die-offs, in The Extinction Debate.)

One of the most moving moments in the program comes when Kennett sees evidence for his hypothesis materialize before his eyes on an electron microscope display. "Exciting isn't really the word," he says, choking up. "It's an experience you usually don't have much in your scientific career. Moments of intense discovery are very emotional for scientists."

Program Transcript
Program Credits

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Megabeasts' Sudden Death Megabeasts' Sudden Death

Did a comet wipe out North America's mammoths and other megafauna? Evidence in support of such a catastrophe, experts say, has turned up in such disparate places as Arizona (top, where geophysicist Allen West digs into the black layer he says marks the time it happened) and Greenland (bottom, where glacier expert Andrei Kurbatov digs for microscopic particles that may have originated during the supposed comet strike).

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