About 10,000 years ago, a handful of woolly mammoths washed up on an island off the Siberian coast in the Russian Arctic. Over the next few thousand years, this small group of perhaps eight animals grew into a stable population of 200-300 animals before going extinct about 4,000 years ago. These mammoths are the last known population of woolly mammals on Earth, and may have survived into modern times but for some bad luck.

The history of these mammoths has been clarified through genetic research. Love Darren A team from Stockholm University in Sweden and their colleagues looked at DNA from 14 mammoths from Wrangel Island and seven from the mainland, dating back to before melting ice sheets caused sea levels to rise and isolate them, covering a combined genetic history of 50,000 years.

The researchers’ analysis found that despite the small population, inbreeding wasn’t the cause of the mammoth’s extinction: While small genetic mutations accumulated, Dallen said, the population was good at eliminating larger, harmful ones.

“We can demonstrate that it’s unlikely that inbreeding or genetic disease caused the population to slowly decline and go extinct,” he said. “Despite the inbreeding, the population did well.”

But the team found that individual mammoths were affected by genetic diseases, and that this negative impact at the individual level had been ongoing for thousands of years. “This means that today’s endangered species, which in most cases were at risk of extinction until very recently, are likely to continue to suffer from genetic diseases for hundreds of generations to come,” says Dallen.

Dallen points to the Tasmanian devil as an example of a species that became isolated on a large island after mainland populations became extinct, leading to reduced genetic diversity. This in turn affects the immune system, Dallen says, which puts the species at greater risk of population decline when faced with new pathogens, such as the facial tumour disease that attacks Tasmanian devils.

“Natural selection appears to have been effective in eliminating potentially lethal mutations, but other, less severe mutations gradually increased,” they said. Adrian Lister At the Natural History Museum, London.

“We’re not sure whether this led to eventual extinction, but it’s possible that, combined with environmental changes, it did,” Lister said. “There are lessons here for monitoring the genetic health of endangered species today.”

The exact cause of the mammoths’ extinction is unknown, but interestingly, Wrangel Island contained freshwater lakes and rivers, which suggests that the mammoths may have been able to survive longer than similarly isolated groups that became extinct 5,600 years ago due to drought.

“Diseases, short-term weather events, tundra fires — all of these are thought to be random events,” Dallen says. “Because they’re random, they’re not inevitable. So if they hadn’t happened, the mammoths might have survived to this day, assuming humans hadn’t killed them when they arrived on Wrangel Island.”