Alaska's native horse was killed off by food shortages caused by climate change - not human hunters, scientists say.
humans may not have been responsible for the horses' demise
Researchers found the horses shrank in size before their extinction 12,500 years ago, which fits with the theory that they did not have enough to eat.
Humans were unlikely to have had a hand in the horses' demise, they claim, because fossil records show man had hardly arrived on the scene by then.
The research is published in the scientific journal Nature.
Until about 20,000 to 10,000 years ago, the Mammoth Steppe in Alaska contained a rich panoply of large mammals.
While London and New York lay submerged under glacial ice, the Mammoth Steppe provided a cold grassy haven to woolly mammoths, lions, bison, sabre tooth tigers - and the Alaskan horse.
Then a catastrophic event happened. In a 10,000-year period, 70% of North America's large mammals died. Something killed those warm blooded giants, although quite what - or who - remains a matter for discussion.
There are two main suspects: our own species, Homo sapiens, or climate - or perhaps a mixture of both.
Around 12,000 years ago, human migrants - known as the Clovis people - entered the New World across a land bridge from Asia.
These people are often implicated in the late Pleistocene extinction, and indeed fossil records prove they did hunt some of the animals that died out - like mammoths.
But many experts believe climate change was the biggest culprit.
At the end of the ice age, the environment in North America changed dramatically. It got warmer - and wetter. The cold dry grassland morphed into tundra, which is largely characterised by unpalatable plants.
Large mammals like the horse - which needs a great bulk of food and has a slow breeding cycle - suffer the most from such changes.
"Horses are almost obligatory grazers," wrote Professor Dale Guthrie, from the University of Alaska, Fairbanks. "And these northern horses appear to have been the most specialized grassland-dependent component of the large mammal fauna."
In an attempt to find out what happened to the Alaskan horse, Professor Guthrie looked at how the size of their foreleg bones changed over time.
He radiocarbon-dated a series of fossilised bones, from as far back as 27,000 years.
He noticed that as time progressed the bones became smaller - in other words, the horses shrank before they went extinct.
The timing of this reduction in size coincides with the decline of grassland, which suggests the horses were suffering from a shortage of food.
Professor Guthrie believes this shrinkage, along with the date of the horses' disappearance, exonerates man.
After all, evidence for the first human settlement can be dated to 12,000 years ago - a long time after the horses' went extinct.
"There is a hiatus of over 500 radiocarbon years between the last dated Alaskan horses and the earliest undisputed human artefacts," wrote Professor Guthrie.
"Even if we allow that human hunters may have been present in Alaska at levels below archaeological visibility when horses became extinct, the idea that such a low density of hunters could have caused horse extinction requires an unlikely scale of overkill performance."