A new study into the North American landscape 15,000 years ago has debunked several theories about why giant sloths, mastodons and mammoths died out.
Researchers from the University of Wisconsin-Madison studied fossil pollen, charcoal and dung fungus spores in an Indiana lake to give an idea of population density.
They 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," said co-author Jacquelyn Gill.
These dates eliminate several possible reasons for the mass extinction.
The first is the spread of trees caused by a changing climate. But according to Gill, the die-off of the big mammals predated this change - and may even have helped bring it about. With the large plant-eaters out of the picture, the trees 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 - but, again, the timeline shows the extinction event was already over when that impact took place, Gill said.
A third theory held that the animals were wiped out by overhunting by Clovis culture humans.
But according to Gill, the die-off was already underway before the Clovis hunters arrived. "Our data are not consistent with a rapid, 'blitzkrieg' overkill of large animals by humans," she said.
The research appears in Science.