The second half of Precambrian time is marked by a slow evolution of the Earth toward its modern appearance. But this period had its own precarious and bizarre events, just as did the Archean and Hadean eons before it.
The Proterozoic Eon (2500–550 Ma)
The Proterozoic is Earth's longest chapter. Early during this time, traces of modern plate tectonics appear in the rocks. In the late Proterozoic, the first proper animals arose. In between occurred several crises that involved the whole world.
Anaerobic life was not sustainable, gradually poisoning the environment with its waste product of oxygen. Dissolved iron ran out, and subduction brought oxidized material into the mantle, slowly changing its chemistry. Finally free oxygen persisted in the air and water, threatening all anaerobic life. This event is called the Oxygen Catastrophe.
New, more complex organisms that could live in the oxidizing environment arose. These later gave rise to the animals and plants, and the ancient anaerobes retreated. Today they thrive underground, still constituting the majority of Earth's biomass. The elements of a balanced biogeochemical cycle were in place.
Carbon-containing gases were pulled from the air, cooling the planet, and the world's first ice ages occurred. These appear to have covered the whole planet deep in ice, only to be melted as volcanic activity sent new carbon dioxide aloft. There appear to have been several of these "Snowball Earth" episodes in the later Proterozoic, during the Cryogenian Period.
The plate-tectonic motions of the continents are poorly known during the Proterozoic, but around 1000 Ma a supercontinent called Rodinia formed. It broke up starting around 750 Ma, and as continental fragments reached the north and south poles they probably contributed to the great ice ages. In the latest Proterozoic a new supercontinent called Pannotia came together.
Also around this time, multicelled animals evolved for the first time. Because none of them had hard parts, their fossils are very rare. A well-preserved fossil site in the Ediacara Hills of Australia has given its name to the Ediacaran Period, right at the end of the Proterozoic.
The Precambrian's End
What ended the Precambrian was a new wrinkle in the carbon cycle whose origin is still under discussion. Whether from evolutionary progress or from changes in geochemistry, animals gained the ability to build hard body parts of carbonate minerals.
At this point, at the start of the Cambrian Period, Earth had taken on its current form—in the life-filled oceans and oxygenated atmosphere. Coevolution of biosphere and lithosphere over billions of years led to that point. Anaerobes and oxygen-breathers had evolved complementary chemical cycles, and biogenic carbonates entered the plate-tectonic cycle of the crust and upper mantle with new efficiency.
Soon two more developments launched the Phanerozoic into the rapid pace of change it has had ever since the close of the Proterozoic: the rise of burrowing animals that cultivated the seafloor (giving rise to bioturbation) and the invasion of the land. With the new dynamism in Earth's evolutionary and biogeochemical processes, there was no looking back.
