What is the History of Evolution Before Animals?
Animals (multicellular life) evolved about 600 million years ago, during the Ediacaran period. Life in general existed much earlier than that, at least 3.8 billion years ago, and possibly as far as 4.2 billion years ago, just 300 million years after the formation of the Earth itself. Thus, about seven-eighths of the history of life occurred before animals came onto the scene. The period of evolution before animals spans at least 3 billion years. Evolution before animals is thought to have been relatively slow. Some organism similar to modern-day cyanobacteria appeared at the beginning, taking up the Earth’s carbon and forming it into photosynthesizing biomass. This is evidenced by isotopic studies of various rocks — since life preferentially intakes the lighter isotope of carbon, high abundance of this light isotope in rock is considered indicative of life. Diamonds dated to 4.2 billion years ago found in Australian zircons have had this isotope ratio, leading some scientists to argue
