Genome study deepens mystery of what doomed Earth's last mammoths
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[June 28, 2024]
By Will Dunham
(Reuters) - About 4,000 years ago, the last of Earth's woolly mammoths
died out on a lonely Arctic Ocean island off the coast of Siberia, a
melancholy end to one of the world's charismatic Ice Age animals. But
what doomed this last mammoth population on Wrangel Island? A new
genomic analysis deepens the mystery.
The study offers the fullest account to date of the inbreeding,
deleterious mutations and low genetic diversity experienced by this
population during 6,000 years of isolation on the island but concluded
that, despite previous suggestions, these factors are unlikely to have
doomed the Wrangel mammoths.
"This suggests that something else, and very sudden, caused the
population to collapse," said evolutionary geneticist Marianne Dehasque
of Uppsala University in Sweden, lead author of the study published on
Thursday in the journal Cell.
The researchers examined genome data obtained from the remains of 14
Wrangel mammoths and seven mammoths from a Siberian mainland population
ancestral to the island dwellers, dating to up to 50,000 years ago.
As the Ice Age eased, the dry steppe tundra where mammoths long had
thrived transformed, gradually from south to north, into wetter
temperate forests amid rising global temperatures, confining these
animals to Eurasia's northernmost reaches.
"This is probably also how mammoths eventually ended up and became
isolated on Wrangel Island, which lost its connection to the mainland
around 10,000 years ago due to rising sea levels. It may have even been
a single herd that populated the island," Dehasque said.
The genome data indicated that the population isolated on mountainous
Wrangel originated with at most eight individuals, then grew to 200 to
300 mammoths within about 20 generations - around 600 years - and
remained stable.
The study detected reduced diversity in a group of genes crucial to the
immune system. But while the mammoths slowly accumulated moderately
harmful mutations, the most deleterious defects were disappearing from
the population, apparently because individuals carrying these were less
likely to survive and reproduce.
The study included no genomes from the population's final 300 years, but
such remains have now been unearthed and genomic analysis is planned.
Previous studies had attributed the extinction to accumulated genetic
defects.
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A view of the landscape of Wrangel Island in the Arctic Ocean off
the coast of Siberia, Russia, where the world’s last woolly mammoths
lived until about 4,000 years ago, with the remnants of a woolly
mammoth’s tusk sticking out of the ground, is seen in this
photograph from July 2017. Love Dalen/Handout via REUTERS
"The reason we don't think inbreeding, low genetic diversity or
harmful mutations caused the population to be doomed is that if that
had been the case, the population should have gone through a gradual
decline in size, where it dwindled toward extinction with an
accompanying increase in inbreeding and loss in diversity," said
evolutionary geneticist Love Dalén of the Centre for Palaeogenetics,
a collaboration between Stockholm University and the Swedish Museum
of Natural History.
"But this is not what we see. There is virtually no change in
inbreeding levels or genetic diversity throughout the 6,000 years
the mammoths were isolated on the island. This means that the
population size was stable through time," Dalén added.
Human hunting also does not appear to have been the culprit.
"I agree that the mystery of the mammoth's demise continues. From
archeological evidence, we know that humans only arrived 400 years
after mammoths went extinct," Dehasque said.
"Fire hearths and habitation structures would be easy to find, as
well as flint shards, reworked bones and tusks, et cetera. But there
is simply no trace of humans having interacted with the mammoths on
Wrangel," Dalén added.
An infectious disease, possibly brought to the island by birds, is
one possibility.
"Perhaps the mammoths would have been vulnerable to that given the
reduced diversity we identified in the immune system genes.
Alternatively, something like a tundra fire, a volcanic ash layer or
a really bad weather season could have caused a really bad growth
year for the plants on Wrangel. Given how small the population was,
it would have been vulnerable to such random events," Dalén said.
"In other words, it seems to me that maybe the mammoths just got
unlucky. Had it not been for such bad luck, we would perhaps still
have had mammoths around today," Dalén added.
(Reporting by Will Dunham in Washington, Editing by Rosalba O'Brien)
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