460-430 million years ago
The Ordovician period was a time when life flourished. But towards its end, the world cooled dramatically and ice sheets spread from the poles. The ensuing ice age is called the Andean-Saharan, because the evidence of it comes from the Andes mountains and the Sahara desert. The deep freeze led to the second-worst mass extinction on record, the Ordovician-Silurian. Most life was still confined to the sea, and 85% of marine species were wiped out. In the aftermath, fish became much more common.
See other: History of Life