Giant cave bears froze to death during the last Ice Age in Europe about 28,000 years ago, according to a study on Wednesday that cleared human hunters of driving them to extinction thousands of years later.

The largely vegetarian bears, weighing up to a ton and bigger than modern polar bears or Kodiak bears, apparently died off as a sharp cooling of the climate led to a freeze that killed off the fruits, nuts and plants they ate.

The bears vanished 27,800 years ago, or about 13,000 years earlier than previously believed, the scientists in Austria and Britain said in a study of bear remains using radiocarbon dating including at hibernation sites in the Alps.

"There is little convincing evidence so far of human involvement in extinction of the cave bear," they wrote in the journal Boreas. Some past reports have suggested that the cave bears' demise was linked to over-hunting.

Cave bears ranged from what is now Spain to the Ural Mountains, and were one of several large creatures -- such as the woolly mammoth, woolly rhinoceros, giant deer and cave lion -- to vanish during the Ice Age that ended 10,000 years ago.

"Our work shows that the cave bear ... was one of the earliest to disappear," Martina Pacher, one of the co-authors at the University of Vienna, said in a statement.

"Other, later extinctions happened at different times within the last 15,000 years," she said. Previous studies had errors in dating samples and sometimes confused remains of cave bears with those of brown bears, which still survive.

"A fundamental question to be answered by future research is: why did the brown bear survive to the present day, while the cave bear did not?" said Anthony Stuart, the other author at the Natural History Museum in London.

Answers might involve differing diets, hibernation habits, geographical ranges, habitat and perhaps hunting by people, he said.