The rapid extinction of many large mammals 10,000 years ago may be attributed to a radical climate change at that time, but not to human overkill, a U.S. scientist said on Wednesday.

The Pleistocene to Holocene extinction between 13,000 and 10,000 years ago has been recognized by scientists as "the sixth great extinction." Mammals, which had been blossoming for 60 million years, retreated for the first time because of this catastrophic extinction.

Since the beginning of the Holocene corresponds with the early Neolithic age of humans, the degree to which humans were involved in the big extinction has always been a heated debate among paleontologists.

Many scientists believe that humans, the most mighty and agile game hunters at that time, may have killed off some big mammals almost simultaneously. This hypothesis is called a "Blitzkrieg model."

Others indicated that early human colonization may have destructed some keystones of the ecological system, leading to mammals' extinction. Or, humans may have induced some diseases deadly to other mammals.

But according to Dale Guthrie, a professor at the University of Alaska, the climate, not humans, should be responsible. His findings appeared in the May 10 issue of the journal Nature.

Guthrie addressed the question with a detailed look at the fauna of that period of Alaska and the Yukon Territory. He classified the radiocarbon dates of more than 600 bones of various animals species, including mammoth, horse, wapiti, bison, moose, and human.

Among these species, noted Guthrie, mammoths and horses became extinct, but wapiti, bison and moose survived and thrived. This fact suggests the faunal change was a result of ecological and vegetational change, rather than human-induced "overkill," he said.

"Species that survived the Pleistocene, for example, bison (Bison priscus, which evolved into Bison bison), wapiti (Cervus canadensis) and, to a smaller degree, moose (Alces alces), began to increase in numbers and continued to do so before and during human colonization and before the regional extinction of horse (Equus ferus) and mammoth (Mammuthus primigenius)," Guthrie wrote in the Nature paper.

"These patterns allow us to reject, at least in AK-YT (Alaska-Yukon), some hypotheses of late Pleistocene extinction: 'Blitzkrieg' version of simultaneous human overkill, 'keystone' removal, and 'palaeo-disease,'" he added.

"Hypotheses of a subtler human impact and/or ecological replacement or displacement are more consistent with the data. The new patterns of dates indicate a radical ecological sorting during a uniquely forage-rich transitional period, affecting all large mammals, including humans," Guthrie concluded.

Source:Xinhua