Proterozoic Era

Proterozoic- The Eon of First Eukaryotes

All the articles of History of Life series

Oxygen Revolution

In the previous article, we reached the end of the Archean Eon. The beginning of Proterozoic was marked with a raising amount of oxygen in the atmosphere. It happened due to bacteria and revolutionized life on Earth. Firstly, Great Oxidation was a shock for anaerobic organisms, and a lot of them went extinct. Meanwhile, the geology of Earth started changing as well. The rise in oxygen levels is responsible for the appearance of more than half of minerals we can find on Earth today. Most of them were oxidized forms of previously existing rocks. Later on, organisms adapted and started using oxygen to oxidize nutrients in the process of cellular respiration- the very same that happens in almost all your cells. Some scientists also believe, that oxidative stress favored the line in which Eukaryotes developed. There were also many changes in continents, which caused further changes in organisms.

History of life

First Eukaryotic cells visualization
Image Courtesy: Fine Art America

Huronian- Proteorozoic Ice Age

Except for the life revolution, Oxygen Revolution also caused mass glaciation of the planet. When free oxygen appeared in the atmosphere, it reacted with methane creating carbon dioxide and water. Those two could not keep Earth as warm as methane did, which caused a drastic climate change. The Sun was way less luminous back then than it is today, so the new atmosphere started losing heat. The Earth froze, a lot of life vanished, it was also the longest ice age in history. However, due to the fact that water is less dense when solid, some life existed underwater. After it ended, another big change occurred, this time in life.

First Proterozoic Eukaryotes

Another important revolution in Proterozoic is the development of first Eukaryotic cells. The special feature of Eukaryotes is that their genetic material is closed within membrane, creating a nucleus. Other parts of the cell can also have membranes, which comes with many advantages. The theory of endosymbiosis explains the origin of internal membranes. At first, only organisms with membrane around the cell lived. Then somehow one of them ‘swallowed’ the other one, creating additional membrane inside the cell. If it happened twice, the double-membrane emerged. We can find double-membrane in chloroplasts of plants. Nucleus developed an ability to split into different gametes, which enabled passing mixed genetic code to younger generations. Some scientists mark beginning of sexual reproduction as the end of the eon. However, most connect it with the first multicellular organisms.

Evidence- first eukaryotic fossils from Proterozoic
Evidence- first eukaryotic fossils
Image courtesy: The Virtual Fossil Museum

Ediacara Fauna- The End of an Eon

In the latter part of Proterozoic life was flourishing in the water. First Eukaryotes were developing rapidly, and that led to one more major change in life. At some point cells started linking together, creating first organisms made of more than once cell. By about 600 million years ago the Ediacara fauna appeared (the name relates to species that existed in the Ediacaran Era). It was a complete sensation in those day biological world because they were built of more than one type of cell. They did not last long and vanished with increasing biodiversity. However, their appearance marked the very beginning of cell specialization and was also the first step towards organisms with skeletons. The appearance of those marked the end of Proterozoic and the beginning of our current eon- Phanerozoic.

Proterozoic Evidence

It is important to present the evidence at the end of every scientific discussion, so let’s look at that left from Proterozoic. The record shows a lot of sedimentary rocks from that period, which did not deform despite passing time, which proves the theory of moving continents. Evidence of Oxygen Revolution also includes sedimentary rocks, this time covered with hematite (oxidized mineral), which indicates the rise in oxygen levels. Older rocks never had that level of oxidation. There are also fossils of the first multicellular organisms and fossils of Ediacara. Scientists and artists also create a visualization of organisms, however, it is a concept based on evidence, not the evidence itself.

The Ediacara biota dominated the Proterozoic
Concept of Ediacara, based on fossils
Image Courtesy: Paleoexhibit

Previous in Series: The Archean and Hadeon Eons

Author’s Message

I hoped that the article helped you understand the last Precambrian eon. If you enjoyed it, please share with family and friends. In the next article, we will start discussing our current eon. If you want to learn more about the basics of evolution, make sure to check out my Theory of Evolution series. If you have any questions or propositions, feel free to contact me, you can find an email in the author’s section.

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