Ophiolites emplaced in collisional orogens prior to 1 Ga generally have a thicker magmatic crust than younger complexes. This fact suggests a model in which Mesoproterozoic oceanic crust was two or three times as thick as at present, possibly owing to partial melting of a more fertile mantle. Earth had less continental freeboard and 90%-95% marine inundation, producing a much more uniform worldwide temperature distribution. Neoproterozoic oceanic crust thinning increased ocean basin volume and continental freeboard, and caused a drop of average sea level relative to the level of the continents. The resultant increased erosion, sediment deposition, upwelling, and consequent biological productivity and burial led to increased atmospheric oxygen, which permitted the evolutionary development of animals.