Precambrian marine environment and the development of life.

The tropical thermocline must have existed since the ocean's depth exceeded 300 meters. The density gradient in this layer concentrated organic aggregates formed abiologically near the surface of the sea, and the low rates of diflusion across this layer permitted the accumulation of oxygen once the layer was populated by blue-green algae; thus the evolution of eukaryotes became possible within the layer. Because of rapid mixing over the shelves, the eukaryotes were restricted initially to the thermocline over deep water. The shelves could not be permanently inhabited by organisms requiring respiration until the oxygen level of the atmosphere was adequate. At this stage, the swimming Metazoa of the thermocline could adapt to a benthic environment on the shelves by developing exoskeletons.