The woolly mammoths of Wrangel island were survivors. Trapped on a hunk of rock in the Arctic Ocean after rising sea levels cut them off from present-day Siberia, they were the last of their species to go extinct. Palaeontology textbooks have explained their eventual demise around 4,000 years ago as a classic case of extinction through inbreeding, in which severely damaging genetic mutations spread through an isolated population and kill it off. New work published in Cell , a journal, on June 27th reveals that the textbooks are wrong.

Genetic diversity can be thought of as an insurance policy for a species. If there are enough unrelated individuals in a population, there is a good chance that one will possess a heritable trait capable of protecting it from a novel threat. So long as that individual is capable of passing that trait on to its offspring, the species can avoid extinction. When small isolated populations are forced to breed with one another over multiple generations, however, harmful mutations become inescapably concentrated. Known as mutational meltdown, this phenomenon has the power to wipe out entire populations.