DNA and ancient remains suggest that most mammoths had died out around 10,000 years ago - but why did they go extinct?
Credit: Wikipedia
ADELAIDE: A new analysis of ancient mammoth DNA backs climate as the main culprit in their extinction, partially exonerating palaeolithic human hunters.
DNA extracted from the remains of woolly mammoths (Mammuthus primigenius) suggests that a trend of decreasing genetic diversity had already taken hold prior to the onslaught of human hunting, according to research revealed today in the journal Current Biology.
"A picture is emerging of extinction not as a sudden event at the end of the last ice age, but as a piecemeal process over tens of thousands of years, involving progressive loss of genetic diversity," said lead author, palaeobiologist Ian Barnes from the University of London, England.
"For the mammoth, this seems much more likely to have been driven by environmental rather than human causes, even if humans might have been responsible for killing off the small, terminal populations that were left," he added.
Extinction question
The mammoth was mostly extinct by the end of last ice age 10,000 years ago – although a small population did cling to survival on Wrangel Island, north of Siberia, until around 3,700 years ago.
The mammoth is thought to have appeared around 150,000 years ago, adapting to the frigid conditions of an earlier ice age.
Adaptations to the extreme cold included a thick, shaggy hair, small ears to reduce heat loss, and an insulating layer of fat. These relatives of Asian elephants had distinctive, curved tusks, up to five metres long, and were around three metres tall at the shoulder.
To help answer the question of why the mammoth died out, Barnes and his team studied the DNA of mammoth populations in present-day Siberia and Alaska. They sequenced samples of mammoth mitochondrial DNA – which is carried down the maternal line – from the bones, teeth and ivory of 41 individual mammoths.
The researchers were able to identify for the first time, distinct lineages of mammoths from Siberia and Alaska. Only one mammoth from Europe was analysed in this study limiting the conclusion that can be drawn from it, but the researchers suggest that there could be a third European lineage too.
Mammoths were able to reach the Americas from Asia via a Bering Strait landbridge that opened up with low sea levels during the ice age, said Barnes.
These mammoth populations in Alaska and Siberia became isolated from each other during a period of warmer climate, when higher sea levels submerged the land bridge. Then around 100,000 years ago, the land bridge reopened as the climate cooled, allowing the two groups intermingle once more.
DNA further suggests that at around 43,000 years ago, the original Siberian lineage died out, leaving only the lineage of the Alaskan immigrants alive at the time of the mammoth's last gasp. This builds on previous work using the fossil record, also by Barnes, and co-author Adrian Lister of University College London.
Set up for extinction
"Around 20,000 years ago, before humans start to become a real predation threat to most of the [large] species in Siberia, we find a significant cooling across the Northern Hemisphere, with an increase in the size of glaciers, and considerable environmental change, and this seems to have been important in setting up the conditions for extinction too," said Barnes.
As the climate warmed 12,000 years ago, forests once again took over the steppe-tundra of Europe and the mammoths vanished. The last population of mammoths survived in Siberia until around 10,000 years ago.
"This study is a great first step in understanding the complexities of the recently extirpated mammoth populations," commented Hendrik Poinar, an anthropologist at McMaster University in Ontario, Canada. Poinar has worked on sequencing parts of the mammoth genome.
Past insights
However, "it's too early, with too few individuals to really tell what happened to the mammoths, whether or not the population was structured at all, and when the migrations to North America took place," cautioned Poinar. "But with more data, from more mammoths, this should become clearer."
"At a time when we should be very concerned about the potential extinction of many existing large mammals, studying those that occurred in the geologically recent past can provide many insights," said Lister. "Our work, together with that of others, shows that the conditions for extinction can be set up long before the actual extinction event."
