Woolly mammoths and other large, lumbering beasts faced extinction in north America long before early humans perfected their skills as spearmakers, scientists say.

The prehistoric giants began their precipitous decline nearly 2,000 years before the Clovis people turned stone fragments into sophisticated spearpoints at the end of the last ice age.

The animals, which included mammoths, elephant-sized mastodons and beavers the size of black bears, were probably picked off by more inept hunters who only much later developed specialised weapons when their prize catches became scarce.

"Some people thought humans arrived and decimated the populations of these animals in a few hundred years, but what we've found is not consistent with that rapid 'blitzkrieg' overkill of large animals," said Jacquelyn Gill, a PhD student at the University of Wisconsin, Madison, who led the research team.

Archaeological evidence shows that humans developed advanced spearheads around 13,000 years ago. The Clovis people of North America crafted speartips with deep grooves that made wounds bleed freely. With these, hunters did not have to kill their prey on the spot, but could wait for the beasts to bleed to death.

The rise of the Clovis culture was thought to coincide with the demise of the woolly mammoth and other slow-moving giants on the continent, leading many researchers to suspect the animals died at the ends of the hunters' spears.

Gill's team rules this out by putting a more accurate date on the decline and fall of woolly mammoths and more than 30 other large mammals that dominated the landscape as the ice sheets retreated from North America. Among them were giant sloths the size of SUVs.

To date the animals' slide to extinction, the scientists examined sediment cores from a lake in Indiana. The deepest sediments were laid down in the distant past, while more recent sediments were nearer the surface.

Specifically, the scientists measured levels of a fungus that is known to thrive in the excrement of giant herbivorous mammals and nowhere else. They reasoned that more fungal spores meant more dung, which in turn reflected a larger population of roaming mammals. The sediments also held ancient pollen and charcoal dust, which gave the team clues about the predominant plant life and frequency of wildfires.

Writing the US journal Science, the researchers describe how the amount of mammal dung started to fall around 14,800 years ago, long before advanced spearheads became commonplace. The animals had been almost completely wiped out a thousand years later.

"We know there were people who pre-dated the Clovis culture who were butchering mammoths in the area. What we're suggesting is the declines happened before the Clovis toolkit was adopted. These earlier people had tools, but they probably weren't as sophisticated," said Gill.

Chris Johnson, a population ecologist at James Cook University in Queensland, Australia, said the shortage of mammoths and other easy targets might have forced early humans to improve their weapons. "People were still hunting them but this was more challenging, so they developed somewhat better tools for the job," he said.

Another theory, that the larger beasts were wiped out by an asteroid strike around 13,000 years ago, also looks unlikely in view of the latest study. By improving their hunting techniques, early humans seem to have played a major role in finishing off the woolly mammoths and nine other mammal species that weighed over a tonne.

The study is among the first to reveal the environmental consequences of such a catastrophic decline in species. Pollen and charcoal recovered from the sediment cores show that wildfires became far more common and that the variety of plant life changed dramatically, as the nutritious and easily digestible trees and shrubs that were eaten by the mammals grew back.

"For the first time we've got a linkage between this major ecological event, the disappearance of these large animals, and evidence of the environmental consequences," said Jack Williams, a co-author on the study.

• This article was amended on 4 December 2009. The original omitted to specify that the research pertained only to North America. This has been

corrected.