Earth was a very different place in the Proterozoic Eon (2500 to 541 million years ago).
Temperatures cooled down significantly compared to the Archean Eon just before. And not a lot of life could survive in these harsh conditions.
Although some microscopic bacteria known as cyanobacteria existed, they did so without oxygen.
These bacteria were known for releasing oxygen. This oxygenation of Earth resonated into the Proterozoic Eon and becomes a central theme.
The oxygen crisis for anaerobic cyanobacteria
Without oxygen, only anaerobic life existed in the Proterozoic Eon. Microscopic bacteria known as cyanobacteria flourished.
The key to their survival was that they didn’t need oxygen to exist. Cyanobacteria photosynthesize sunlight and convert it into energy.
But as a waste product, they released oxygen. So for millions of years, cyanobacteria generated oxygen. Eventually, they filled the oceans with oxygen.
The irony of cyanobacteria is that the oxygen they released was toxic to them. As a whole, the number of anaerobic organisms dropped in the Proterozoic Eon.
The reason why this event is called an “oxygen crisis” is because they threatened their own existence through their own waste oxygen.
Cyanobacteria didn’t completely vanish in the Proterozoic Eon. By hiding in low oxygen environments, cyanobacteria avoided their mass extinction.
Snowball Earth from an oxygen-filled atmosphere
Another key event in the Proterozoic Eon was that Earth completely froze over. Again, oxygen played a key role in forming a “Snowball Earth” or “Slushball Earth”.
Remember that the atmosphere before the great oxygenation event was filled with methane.
One thing that methane did very well was trap heat in the atmosphere. Actually, it’s one of the most efficient greenhouse gases that exists.
So when oxygen combined with methane, it produced carbon dioxide. Because there was less methane in the atmosphere, the greenhouse effect wasn’t as strong.
Without heat trapped in the atmosphere, Earth froze for about 300,000,000 years.
The presence of an ozone layer
What makes Earth unique is its ozone layer. It’s essential to life on Earth because it protects us from harmful radiation from the sun.
After the Great Oxygenation Event, oxygen atoms could bond and form ozone. Over time, Earth’s ozone layer thickened in the upper atmosphere.
Before the full presence of an ozone layer, life was restricted to shallow water. This is because water shielded harmful radiation.
When ozone started protecting Earth from deadly UV rays, this enabled life to diversify on land. But it wasn’t until the Paleozoic Era when life truly took flight on land.
The emergence of eukaryotes and multicellular organisms
We know that Earth had the right conditions to support life. In the Proterozoic Eon, there is evidence that eukaryotes and multicellular organisms appeared on Earth.
What makes eukaryotes different from prokaryotes is they are complex cells. Their genes are stored in the form of DNA in a nucleus.
Scientists believe eukaryotic cells evolved when one simple cell engulfed another through endosymbiosis. Then, multicellular life developed because multiple cells benefited by cooperating like an ant colony.
Because these organisms did not fossilize, it still remains unclear when eukaryotes and multicellular organisms evolved on Earth. Long before, the Last Universal Common Ancestor (LUCA) existed which share the same origin as us.
Proterozoic Eon highlights
In the Proterozoic Eon, Earth completely froze into a Snowball Earth or Slushball Earth. It was one of Earth’s many ice ages.
Because of the oxygen-enriched atmosphere, the ozone layer thickened and cyanobacteria were threatened.
There’s evidence that eukaryotes and multicellular organisms evolved. Because they did not fossilize, we don’t truly understand.
It was in the Phanerozoic Eon when hard-shelled species arrived after the Cambrian Explosion.
Proterozoic Eon Chronostratigraphic Chart