New evidence casts doubt on the theory that sabre-toothed cats,
mammoths and other big North American mammals were driven to
extinction by human hunting.
Genetic analysis of bison remains shows their populations
started to crash around 37,000 years ago - long before humans
arrived in the New World [even given the disputed maximum 30,000
years limit for their first arrival].
Researchers claim that climate change and other factors are more
likely culprits in the extinction.
An international team has published its findings in Science
Until as recently as 20,000 years ago, North America had a range
of large mammals to rival the wildlife of present-day Africa.
The continent was home to woolly mammoth and mastodon, horses,
camels, giant ground sloths and bear-sized beavers, as well as
By about 10,000 years ago, most of these animals were gone. Some
70 North American species disappeared - three-quarters of them large
This so-called "megafaunal extinction" has been blamed by some
on human hunters who appear in North America around 12,000 years
But the latest research seriously questions this hypothesis.
Scientists extracted mitochondrial DNA from 442 bison remains found
in the US, including Alaska; and in Canada, Siberia and China.
Mitochondrial DNA comes from the cell's "power houses" and is
inherited through the maternal line only.
Some of the best-preserved material used in the study was
unearthed from beneath the Alaskan permafrost by gold miners, some
of whom even kept the remains refrigerated until the scientists came
to claim them.
Scientists stand a much better chance of extracting useful DNA
sequences from such frozen remains.
From this ancient genetic material, Alan Cooper at the
University of Oxford, UK, and colleagues were able to reconstruct a
genetic history of bison over a period of around 150,000 years.