The last woolly mammoths on Earth died from bad luck, not inbreeding

Around 10,000 years ago, a handful of woolly mammoths found themselves stranded on an island in the Russian Arctic, off the Siberian coastline. In the following millennia, this tiny herd, perhaps as few as eight individuals, grew to a stable population of between 200 and 300 mammoths before becoming extinct around 4000 years ago. They were the last known population of woolly mammals on Earth – and if it weren’t for bad luck, it is possible they could have survived into the modern era.

We know the story of these mammoths thanks to a genetic study conducted by Love Dalén at Stockholm University in Sweden and his colleagues. The team examined the DNA of 14 mammoths from Wrangel Island, plus seven from the mainland population prior to the small group being isolated by rising sea levels due to melting ice sheets – altogether covering 50,000 years of genetic history.

The researchers’ analysis shows that, despite the small number of individuals, inbreeding wasn’t the reason for the mammoths’ demise. Dalén says the population was successfully purging major harmful genetic mutations, even though minor ones were accumulating.

“We can show that, in all likelihood, inbreeding and genetic diseases did not cause the population to gradually decline towards extinction,” he says. “The population was doing OK despite the inbreeding.”

However, the team found that individual mammoths were affected by genetic diseases, and this negative impact at the individual level kept going for thousands of years. “This means that endangered species today, who in most cases were bottlenecked very recently, are likely to continue suffering from genetic diseases for hundreds of generations into the future,” says Dalén.

He points to the Tasmanian devil as another example of a species that has become isolated on a large island after the mainland population went extinct and that is now suffering from low genetic diversity. This, in turn, affects the immune system, says Dalén. When this is reduced, a population is more susceptible to decline when facing a new pathogen, such as the facial tumor disease that affects the devils.

“It seems that natural selection was effective in removing potentially lethal mutations, but other, less severe ones gradually mounted,” says Adrian Lister at the Natural History Museum in London.

“Whether this contributed to eventual extinction is uncertain, but it could have done, perhaps in combination with environmental change,” says Lister.  “There are lessons here for monitoring the genetic health of endangered species today.”

Exactly what drove the mammoths to extinction is unclear, but intriguingly, the availability of freshwater lakes and rivers on Wrangel suggests they potentially could have survived for longer than they did, unlike a similarly isolated group that went extinct 5600 years ago due to drought.

“All these things like diseases, short-term climate events, tundra fires, are things we consider random events,” says Dalén. “And since they are random, there was nothing inevitable about them happening and therefore had they not happened, maybe mammoths would have survived on Wrangel until today, assuming humans didn’t kill them off when they finally arrived.”