During the Ediacaran period, about 600 million years ago, animals (multicellular life) evolved. Life existed much earlier, at least 3.8 billion years ago, and possibly as far back as 4.2 billion years ago, only 300 million years after the Earth’s formation. As a result, about seven-eighths of the history of life took place before animals appeared. Before animals, there was at least a 3 billion-year period of evolution.
Prior to the appearance of animals, evolution is thought to have been relatively slow. At first, some organisms resembling modern-day cyanobacteria appeared, absorbing carbon from the atmosphere and converting it to photosynthesizing biomass. This is demonstrated by isotopic studies of various rocks; because life prefers the lighter carbon isotope, a high abundance of this light isotope in rock is thought to indicate life. This isotope ratio has been found in diamonds dated to 4.2 billion years ago found in Australian zircons, leading some scientists to speculate that simple life may have existed at that time.
Prior to the appearance of animals, evolution took place almost entirely in the oceans. Life developed oxyphotosynthesis around 2.7 billion years ago, pumping large amounts of oxygen into the atmosphere, which was mostly carbon dioxide and water vapor at the time. The “Oxygen Catastrophe” lasted over 500 million years, and when it was over, the Earth’s oxygen percentage was roughly equivalent to today’s. This is referred to as a “catastrophe” because it would have killed many organisms that were poisoned by oxygen. The transformation of the Earth’s atmosphere from anoxic to oxic state is one of the most significant events in the history of evolution before animals.
The evolution of eukaryotic, or complex cells, was another significant event in evolution prior to the appearance of animals. Any cell that is more complex than a bacterial cell is referred to as a eukaryote. Eukaryotic cells are about 10 times larger than bacterial cells and contain organelles, or specialized parts. Some organelles, such as the cell’s “power plants,” are likely bacterial cells that were captured, retained, and replicated in later eukaryotic cells. Between 1.6 and 2.1 billion years ago, eukaryotic cells appeared. Red algae were most likely the first eukaryotic cells.
The Cryogenian glaciations, which occurred between 790 and 630 million years ago, were another significant event prior to the evolution of animals. Glaciation was so severe during this time period that evidence of thick ice at the equator exists. Some scientists even believe the oceans have frozen over, a scenario dubbed “Snowball Earth.” Whether or not the oceans froze, such a cold environment would have been hostile to multicellular life, and it’s interesting that it seems to have emerged relatively quickly after the glacial period ended.