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Comet approaching Earth on collision course.

ou're walking through the woods one day when you hear a loud munching sound. You turn, and there before you, gnawing on a tree trunk, is a beaver as big as a bear!

Not so long ago in the history of life on Earth, giant beavers inhabited North America. So did giant camels, giant sloths, and giant elephants. Then something mysterious killed them off. Now scientists believe that "something" might have come from outer space.

The disappearance of North America's megafauna about 12,000 years ago is one of science's big puzzles. Some scientists put the blame on the end of the last Ice Age. North America was covered in enormous ice sheets, and as they shrank, the climate grew warmer and drier. Many lakes and streams, which were the animals' watering holes, dried up. Vegetation also became scarcer and the remaining megafauna starved to death.

Other scientists have a different explanation. Sometime between 14,000 and 12,000 years ago, wandering tribes crossed from Asia to North America. The new immigrants called the Clovis people were expert hunters. As they spread across the continent, they hunted the giant mammals to extinction.

Replica of a woolly mammoth on display at the Royal
British Columbia Museum

Now another team of scientists has offered a third explanation. They have found evidence that a large comet exploded over southern Canada about 13,000 years ago. Great balls of fire rained down on the northern hemisphere, igniting wildfires across North America, Europe, and Asia. The fires killed many giant mammals and destroyed the vegetation that the remaining ones might have survived on.

What is the evidence that the scientists have for the catastrophe? When a comet explodes, it releases a shower of particles, many of which contain hundreds of nanodiamonds. The scientists dug into the ground at 26 locations throughout the northern hemisphere. In most locations, they found a thin layer of fallen nanodiamonds dating back about 13,000 years. Among the nanodiamonds, they also found high amounts of an element called iridium. High amounts of iridium do not occur naturally on Earth and were probably part of the fallout from the explosion.

Think About It!

Which theory makes the most sense to you? Why

Scientists believe another group of megafauna—whose bones are displayed in some museums—may have been killed after the explosion of a comet. What were those megafauna called?

Check out this prehistoric mammal quiz!

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