The Ordivician 487.7 million - 443.8 million years ago
The age of compromise
The Ordovician only exists as a geological era to stop an argument between two Victorian geologists (thankfully, both were dead by the time the compromise was suggested, as neither would have approved of it).
During this period, the Earth's continents were concentrated in the southern hemisphere and largely covered with shallow seas. Colonial coral reefs got going, and the first jawed fish make their appearance. Despite the earth being an ice capped water world, life might have made it onto land for the first time.