The first animals
Sponges — colonies of cells that divide the labour
See it on the timeline →In depth
Fossils of early sponges are contested, so the key evidence is chemical: 24-isopropylcholestane, a molecule made today only by demosponges, occurs in oils and rocks from Oman dated to around 650 million years and older. Molecular clocks point further back still, towards 800 million years. The coincidence with rising oxygen is unlikely to be accidental: an animal that pumps water through its body needs far more oxygen than a single-celled organism. Sponges differ from everything before them in having *differentiated* cell types — the same DNA doing different jobs.
For children
Somewhere in the dark ocean, some cells began cooperating in a new way: some pumped water, some caught food, some built the skeleton. That is a sponge — the simplest animal there is. Every animal on Earth, including you, descends from something like it.
Connects to
Part of Terra Saga — the story of Earth on a timeline. terrasaga.org