The Proterozoic Eon is the third of four eons and includes the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic eras. It makes up more than half of the history of life on Earth, with little known about the earlier eras. The atmosphere during the early Proterozoic had high carbon dioxide content, but about 2000 million years ago, oxygen-producing organisms evolved, causing an event known as the oxygen catastrophe. The end of the Proterozoic saw the appearance of single-celled ancestors of today’s animals, plants, and fungi.
The Proterozoic Eon is one of the four eons of time that make up the history of this planet. It is the third of four aeons, the first two with little life preserved, the last two with it. The aeons include the Hadean, Archean, Proterozoic and Phanerozoic. Proterozoic means “early life” and extends from 2500 million to about 542 million years ago. Traditionally, all eras prior to the Cambrian period (which began 542 million years ago) were informally referred to as the “Pre-Cambrian”, but more recently these time periods have been divided and named.
Originally, the Proterozoic Eon referred to the era before the emergence of numerous multicellular fossils. However, relatively recent discoveries of complex pre-Cambrian organisms called Ediacaran biota have thrown this convention into chaos. These appeared only a few tens of millions of years before the Cambrian, so the rough outline of the Proterozoic is still correct. The Proterozoic Eon is divided into three geological eras of approximately equal duration: the Paleoproterozoic, the Mesoproterozoic and the Neoproterozoic.
The Proterozoic makes up more than half of the history of life on earth. Although life originated before the Proterozoic, much of it is very poorly preserved and little is known about it. Even the fossil details of the Proterozoic are sketchy. The early part of the era was dominated, perhaps exclusively, by single-celled photosynthetic organisms that are the descendants of today’s cyanobacteria. These sometimes gathered in colognes that resembled beads on a string. Some of the more complex specimens were tightly integrated colonies of prokaryotic cells, reaching up to 1mm in size, considered to be among the earliest eukaryotes (complex and larger cells).
During the early Proterozoic, the atmosphere had 100 to 1000 times the carbon dioxide content of today, making it more like the atmosphere of Mars than the current Earth we are familiar with. Relatively abruptly, about 2000 million years ago, single-celled, photosynthesizing, oxygen-producing organisms evolved, releasing massive amounts of oxygen in an event known as the oxygen catastrophe. This rusted all the exposed iron on the surface, leaving behind geological evidence called banded iron formations. It also likely caused mass extinction among organisms unable to deal with such high concentrations of oxygen.
Towards the end of the Proterozoic, about 800 million years ago, the planet was hit by an ice age so severe that many paleontologists believe that most of the Earth was covered in glaciers. This period is called the Cryogenian due to its low temperatures. Surprisingly, life survived, perhaps in slightly warmer lakes deep below the freezing surface, such as present-day Lake Vostok in Antarctica.
By the end of the Proterozoic, the single-celled ancestors of today’s animals, plants, and fungi had appeared, as well as some primitive blob- and stem-like organisms. In general, data on Proterozoic organisms are very scarce compared to later eras. Lacking hard shells, these organisms did not keep very well. The most common remains are mysterious microfossils called acritarchs that come in a variety of shapes and sizes. They are thought to be the fossils of single-celled photosynthetic protists.
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