Specifically, the mammoths (Mammuthus primigenius) were afflicted by genetic diseases, likely brought about by a lack of genetic diversity. As their numbers declined and the pool of available mates grew ever smaller, detrimental genetic mutations increased, resulting in an increasingly unhealthy herd.
The evidence is compelling, as it's not just genome sequencing. Scientists actually raised the mammoths' genes from the dead, and placed them in elephant embryo cells in the lab to see how well they functioned.
The answer is: Not well at all. The genes were sad, stumbling, broken things that could have seriously impaired important functions, such as male fertility, and the mammoths' sense of smell.
"The key innovation of our paper is that we actually resurrect Wrangel Island mammoth genes to test whether their mutations actually were damaging (most mutations don't actually do anything)," said evolutionary biologist Vincent Lynch of the University at Buffalo.
"Beyond suggesting that the last mammoths were probably an unhealthy population, it's a cautionary tale for living species threatened with extinction: If their populations stay small, they too may accumulate deleterious mutations that can contribute to their extinction."
The death of the mammoth on Wrangel Island has been the subject of a number of studies. Last year, isotope analysis of the bones and teeth of the animals - which can reveal what the deceased ate over the course of their life - pieced together dramatic changes in the mammoths' diet that point to dramatic environmental changes.
Prior to that research, scientists had conducted complete genome sequencing on Wrangel Island woolly mammoths along with earlier, more healthy mammoth populations. The results were published in 2017; in those genomes, the scientists found "accumulation of detrimental mutations ... consistent with genomic meltdown."
The new research builds on that 2017 paper. "The results are very complementary," Lynch said.
"The 2017 study predicts that Wrangel Island mammoths were accumulating damaging mutations. We found something similar and tested those predictions by resurrecting mutated genes in the lab."
Lynch and his colleagues identified detrimental mutations by comparing the genome of the Wrangel Island mammoths to their living relatives - three Asian elephants (Elephas maximus).
They also compared it to the genomes of two other mammoths - one from 44,800 years ago, and the other from 20,000 years ago, when the populations were large and hale.
From these comparisons, they were able to identify mutations related to defects in sperm morphology; neurological development; insulin signaling; and olfactory receptors.