At various times in Earth history, single-celled organisms threw their lot in with each other to become larger and multicellular, resulting, for instance, in the riotous diversity of animals. However, fossil evidence of these major evolutionary transitions is extremely rare.

Fossils reported this week in Science preserve stages in the life cycle of an amoeba-like organism dividing in asexual cycles, first to produce two cells, then four, eight, 16, 32 and so on, ultimately resulting in hundreds of thousands of spore-like cells that were then released to start the cycle over again. The organisms should not have been fossilized – they were just gooey clusters of cells – but they were buried in sediments rich in phosphate that impregnated the cell walls and turned them to stone.

Amoeba-like organism fossils shed light on evolutionary origin of animals - Science Codex