Why did multicellular animal life take so long to appear?
The proterozoic lasted for close to two billion years and only at the very end, in the Ediacaran period do we get some tantalizing hints at a start for multicellular life. But what took so long? The answer may lie in the conditions on earth just prior to the Ediacaran when the continental land masses were located on the equator. Remember they are barren rock and one thought is that they reflected sunlight back without warming the planet; the result the earth cooled. It’s also possible the erosion of the continental land masses removed large quantities of carbon dioxide from the atmosphere as carbonate salts. Yet another suggestion is that primary productivity in the ancient oceans was so large that sufficient carbon dioxide was removed. In either case removing the greenhouse gas carbon dioxide would have cooled the planet and in all three scenarios the earth started to cool and large glaciers formed at both poles and continued to grow. Glaciers also reflect most of the sun’s energy bac
