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
