猛犸象灭绝源于气候变化

 

A DNA⁴ analysis shows that the number of creatures began to decrease much earlier than previously⁵ thought as the world's climate changed.

 

It also shows that there was a distinct population of mammoth in Europe that died out around 30,000 years ago.

 

The results have published in the Proceedings⁶ of the Royal Society B.

 

The view many researchers had about woolly mammoths is that they were a hardy, abundant species that thrived during their time on the planet.

 

But according to the scientist who led the research, Dr Love Dalen of the Swedish Museum of Natural History, the study shifts that view.

 

"The picture that seems to be emerging is that they were a fairly dynamic species that went through local extinctions, expansions and migrations⁸. It is quite exciting that so much was going on," he told BBC News.

 

Dr Dalen worked with researchers in London to analyse DNA samples from 300 specimens⁹ from woolly mammoths collected by themselves and other groups in earlier studies.

 

The scientists were able to work out how many mammoths existed at any given time from the samples as well as tracing their migration⁷ patterns. They looked at the genetic¹⁰ diversity in their samples - the less diverse the lower the population.

 

They found that the species nearly went extinct 120,000 years ago when the world warmed up for a while. Numbers are thought to have dropped from several million to tens of thousands but numbers recovered as the planet entered another ice age. 

 

The researchers also found that the decline that led to their eventual¹¹ extinction began 20,000 years ago when the Ice Age was at its height, rather than 14,000 years ago when the world began to warm again as previously thought.

 

They speculate that it was so cold that the grass on which they fed became scarce. The decline was spurred on as the Ice Age ended, possibly because the grassland¹² on which the creatures thrived was replaced by forests in the south and tundra¹³ in the north.

 

The reason they died out has been a matter of considerable scientific debate. Some have argued that humans hunted them to extinction while others have said that changes in the climate was the main factor.

 

A criticism of the climate extinction argument is that the world warmed well before the creatures became extinct and so that could not have been the cause.

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