Earth's Proterozoic Period

The Proterozoic Period marks the time when complex organisms began to make their first real foothold on Earth. It lasted from 2.5 billion to 543 million years ago, and is divided into three parts from earliest to latest: the paleoproterozoic, the mezoproterozoic, and the neoproterozoic.

In terms of geology, there is a much better geological record to study than the one in the Archean Period. By this Period, Earth's continents had largely coalesced, although not in the form we know them today. It would take hundreds of millions of years for the phenomenon of continental drift to move the continents into their present configurations. Earth's crust was considerable less viscous, and plate tectonics began to govern the majority of geologic processes, as they do today as well. Furthermore, the first glaciation appeared in the Proterozoic Period. Temperatures were low enough now to allow for frozen water deposits to form on the Earth's crust.

The most striking feature of the Proterozoic Period, and one that would have a significant effect on the course of life on Earth, was the buildup of oxygen in the atmosphere. Prior to the Proterozoic, Earth's atmosphere was composed largely of toxic elements such as hydrogen, ammonia, and methane. The buildup of oxygen can be attributed to the increase in photosynthetic organisms such as algae that converted the sun's light energy and carbon dioxide into sugars and oxygen. This process helped to reduce the greenhouse effect that was heating the planet (much like it does to Venus even today) and also pumped oxygen out into the atmosphere. Although photosynthetic organisms existed around the end of the Archean Period, it wasn't until around 2.3 billion years ago that oxygen levels in Earth's atmosphere broke one or two percent.

Multicellular life followed this increase in oxygen. Aerobic (oxygen-using) organisms are about 30-50 times more efficient at producing energy than anaerobic organisms who use fermentation and other chemical processes that do not involve oxygen. Thus, single-celled organisms were able to evolve into bigger forms. Many anaerobic bacteria that depended on large quantities of methane or hydrogen began to die out as this poisonous new oxygen began to invade the atmosphere they had depended on. Also during the Proterozoic Period, the first symbiotic relationships between mitochondria and chloroplasts with multicellular organisms began. This relationship would lay the groundwork for future plant and animal life on Earth. The evidence for single-celled and multicellular life comes from the presence of mounds of cellular formations in rocks known as stromatolites. Stromatolites are rare fossil finds, but became much rarer after about 750 million years ago due to the increasing abundance of organisms that fed on bacteria.

The first proper animals on earth might have shown up as early as one billion years ago. A rich tapestry of heterotrophs (organisms that consume other organisms) and autotrophs (organisms that manufacture their own food) was beginning to be woven beginning in the Proterozoic Period and would continue on its fantastic journey through evolution in later Eons.