Question

Geologists have divided Earth's history into time units, which are largely based on?

Answer 1

Fossils, at least for the Phanerozoic - the last 600 million years ago. This is because marine species in particular can easily spread worldwide, so if you find a certain fossil in two different types of rock in two places in the world you know the rocks were deposited at around the same time. How precise it is depends on how long the fossil species lived for. This dating often gives finer divisions than radiometric dating can.