Mammals, which for 150 million years had been small, rodent-sized creatures, rapidly evolved to massive proportions in the wake of the Cretaceous-Tertiary extinction 65 million years ago.
Geological timekeeping continues to be a lively science, with new methods emerging all the time.
Radioactivity also gave the history of life an absolute calendar.
By measuring the atoms produced by these breakdowns inside rocks, physicists were able to estimate their ages (right).
And by comparing the ratios of those atoms to atoms from meteorites, they could estimate how long ago it was that the Earth formed along with the rest of the solar system.