About 15,000 years ago, North America was home to an astonishing number of large plant-eating mammals -- giant sloths, mastodons, mammoths. A thousand years later, they were all gone, wiped from the face of the Earth with sudden finality.

Scientists, including University of Wyoming researchers, have floated possible explanations for this mass die-off, including climate change and a cataclysmic asteroid impact. But now, a team of American researchers may be closing in on the answer, hidden in the thousands-year-old muck of an Indiana lake.

To track the population of large herbivores, scientists analyzed the fossil pollen, charcoal and fungus in ancient sediments beneath Appleman Lake, a 35-foot-deep body of water left behind when the last ice age ended 20,000 years ago. The research focused on the amounts of the fungus Spromiella present in the sediments, according to Jacquelyn Gill, a graduate student at the University of Wisconsin, and a co-author of the paper that appeared in Friday's issue of the journal Science.

Because the fungus is commonly found in the dung of large plant-eaters, its prevalence in the fossil record should be a direct measure of population density, Gill said.

By tracking the rise and fall of the amounts of fungus in soil samples, the research team found that the decline of the large mammals started about 14,800 years ago and was virtually complete a thousand years later.

"About 13.8 thousand years ago, the number of (fungus) spores drops dramatically," Gill said.

In the end, 34 types of large animals disappeared.

According to Gill and the other researchers, from UW and Fordham in New York, these dates eliminate several possible reasons for the mass extinction.

The first is habitat loss from changing climate. Around this time, tree species such as black ash, elm and ironwood began spreading across the North American landscape. According to Gill, the big mammals' die-off predated this change. In fact, the loss of the big herbivores may have helped precipitate it. Without the large plant-eaters to keep them in check, the tree species were free to colonize the countryside.

Another theory suggested that a comet or meteor impact that occurred about 12,900 years ago could have wiped out the big mammals in the same way that a similar but larger impact is believed to have killed off the dinosaurs. The new timeline shows the extinction event was over when that impact took place, Gill said.

A third theory held that the animals were wiped out by a so-called "blitzkrieg" of hunting by Clovis culture human beings. The Clovis culture is distinguished by the fluted spear points used by hunters to bring down large animals. The fluting caused an injured animal to bleed freely, making a kill more certain once the animal was injured, Gill said.

But according to Gill, the die-off was under way before the Clovis hunters arrived.

"This was already happening before humans adopted the Clovis toolkit," Gill said.

The new evidence doesn't mean humans didn't play a central role in the decline of megafauna, said Christopher Johnson, a professor of biology at James Cook University in Australia who wrote an accompanying perspective article in the same journal.

Even if the Clovis hunters, with their advanced technology, did not start the decline, "They were certainly part of the story, and probably account for the final demise," Johnson said. "My view of all the evidence ... is that human hunting was the sole cause of the megafaunal extinction, and that other factors such as climate change had nothing to do with it."

Johnson believes pre-Clovis humans and the Clovis era hunters were the same population. Analysis of the DNA in the feces of pre-Clovis native Americans show a direct connection between the two populations, he said.

Further evidence that Clovis people finished off the campaign of extinction began by their ancestors is that, according to Johnson, the distinctive Clovis weaponry disappeared after only 400 years.

"The cause of this could have been the final extinction of megafauna, which forced a change of lifestyle focused on hunting of much smaller game," Johnson said.

Gill isn't ready to point to human predation as the cause of the extinction.

"What we've been able to do is start to eliminate some hypotheses" for the die-off, she said. "We're not ruling out humans so far."

She said her research is continuing at the lake and more discoveries may be coming.