Around 34,000 years ago, woolly mammoths went extinct from parts of Europe, only to be replaced by… woolly mammoths. The two groups—the disappearing individuals and their substitutes—belonged to the same species. If you looked at their fossils, you probably couldn’t tell them apart. Their genes, however, reveal them to be part of two genetically distinct lineages, one of which suddenly displaced the other.

These giant beasts—the woolly mammoth, the cave and short-faced bears, the steppe bison, the Eurasian cave lion, the woolly rhino, and the Irish elk—are now extinct. There have been some two centuries of debate over what killed them. Was it a blitzkrieg of incoming humans, with our insatiable appetites and sharp spears, or was it