LIFE × EVOLUTION

Algae Take Over the Ocean

~659 to 645 million years ago

Proterozoic · Neoproterozoic · Cryogenian

The open ocean changed hands. Until the Cryogenian, bacteria were the only primary producers that counted in it, and the molecular fossils of algae are barely present. In the narrow warm interval between the two snowball glaciations, 659 to 645 million years ago, algal steroids become both abundant and varied. The trigger appears to have been the meltwater: a snowball ends by flushing the continents, and the nutrients that arrived broke the cyanobacteria's hold.

Why it matters

An alga is a far larger parcel of food than a bacterium, and it sinks. More of what is fixed at the surface reaches whatever is living below: on modern seafloor with no oxygen in the water, adding algal debris alone is enough to wake the single-celled hunters. The ocean did not simply grow more crowded. It began to deliver.

Dating & uncertainty

The evidence is molecular: steroid molecules left by algae become abundant in the narrow warm window between the Sturtian and Marinoan glaciations. Molecular fossils can migrate through rock, so the dating of the transition rests on careful sampling.

Sources

1. Brocks et al. (2017), Nature · The rise of algae in Cryogenian oceans and the emergence of animals
2. Knoll (2017), Nature · Biogeochemistry: food for early animal evolution
3. Mills et al. (2025), Science Advances · The rise of algae promoted eukaryote predation in the Neoproterozoic benthos

Earth history as one day

20:30:583 h 29 min before midnight

if Earth’s 4.54 billion years were compressed into a single day