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We have an update this morning on a news story that's thousands of years old -it's the extinction of giant animals that once roamed the earth. Scientists do not know what killed off woolly mammoths and other animals after the last ice age - maybe the climate changed, maybe a comet struck the earth, maybe humans hunted them down.

Now, a study out today in Science magazine examines some evidence the animals left behind. Here's NPR's Richard Harris.

RICHARD HARRIS: After the end of the last ice age, North America was covered with large animals, even more diverse than we see in Africa today.

Ms. JACQUELINE GILL (Doctoral Student, University of Wisconsin in Madison): We had, you know, the iconic mammoths and mastodons but also beaver the size of black bear, you know, giant ground sloths. There were camels and horses in North America, which is often a surprise to many people. And then, of course, their predators, right, like the American lion, the short-faced bear, the dire wolves, etc.

HARRIS: Jacqueline Gill is getting her Ph.D. at the University of Wisconsin in Madison, and she's fascinated by how we came to have such diverse wildlife, and why those animals are all gone today.

Ms. GILL: By about 11,000 years ago we lose about half of the animals in North America larger than the size of a German shepherd, and that's a pretty big ecological event.

HARRIS: There aren't enough bones left to figure out in detail when exactly they all went extinct, but Gill and her colleagues hit upon a nifty method to track the abundance of grazing animals. The scientists study, of all things, a certain species of fungus. Spores of the fungus end up in lake sediments that date back some 15,000 years.

Ms. GILL: So, the spores from this fungus are preserved in the lake, and this fungus only grows on animal dung.

HARRIS: When there were lots of large animals, there was lots of dung. When the animals went away, so did the dung and the fungus that lived on those droppings. So, by studying the amount of this fungus in various layers of lake sediment, Gill and her group have pieced together a timetable that helps resolve how and when these animals died out.

One of the long-held ideas had been that a climate shift caused a change in vegetation, and the new plant matter was no good for these grazing animals, so they starved.

But no - Gill and company have disproved that by putting a date on the animals' decline. It was way back, more than 14,000 years ago.

Ms. GILL: And this happens before some of the widespread vegetation change that we see in the areas that we studied, which suggest to us that habitat loss is actually not a cause of the decline, but possibly a consequence.

HARRIS: That is, the vegetation might have changed after the browsing animals disappeared and stopped munching on it.

The new dates also help eliminate other ideas that have been argued over the years. John Alroy at the University of California, Santa Barbara, says it's clear that the extinctions couldn't have been caused by a cold snap called the Younger Dryas.

Mr. JOHN ALROY (University of California, Santa Barbara): These data show that the extinction happened well before that time, and there's also been a recent paper arguing that the extinction might have been caused by the impact of a comet, and that impact appears to have happened much, much later than the decline, at least this location.

HARRIS: So what does that leave? Well, human beings were moving into North America at that time, so Alroy says this is more evidence that hunters played a key role in driving those large animals to extinction.

Mr. ALROY: And now really the question is, exactly what did humans do to cause this large extinction?

HARRIS: Jacqueline Gill at Madison is interested in that question, too, because she notes human beings are now forcing another mass extinction as we reshape habitat around the world.

Richard Harris, NPR News.

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