A new analysis of an exquisitely preserved fossil that lived half a billion years ago suggests that arachnids—spiders and their close kin—evolved in the ocean, challenging the widely held belief that their diversification happened only after their common ancestor had conquered the land.

Spiders and scorpions have existed for some 400 million years, with little change. Along with closely related arthropods grouped together as arachnids, they have dominated Earth as the most successful group of arthropodan predators. Based on their fossil record, arachnids appeared to have lived and diversified exclusively on land.