However, not every definition of life considers all of these properties to be essential. Human-made analogs of life may also be considered to be life. The fossil record retains evidence of many of these older species.
Continents formed, then broke up and reformed as the surface of Earth reshaped over hundreds of millions of years, occasionally combining to make a supercontinent. Roughly 750 million years ago, the earliest known supercontinent Rodinia, began to break apart. The moon formed roughly 20 million years later. Initially molten, the outer layer of the Earth cooled, resulting in the solid crust.
Since the Cambrian explosion there have been five distinctly identifiable mass extinctions. The last mass extinction occurred some 66 million years ago, when a meteorite collision probably triggered the extinction of the non-avian dinosaurs and other large reptiles, but spared small animals such as mammals. Over the past 66 million years, mammalian life diversified.