The Cambrian explosion: evolution of complex life
Snowball Earth events coincided with the rise of oxygen in the atmosphere and this was crucial to the emergence of life.
Life evolved from the
- microscopic (not visible to the eye) microbial organisms that did not respire oxygen
- microscopic oxygen-respirers that appeared after the rise of atmospheric oxygen.
Then there was the emergence of
- macroscopic (visible to the eye) oxygen-respirers that branched into plants and animals on land and eventually to humans.
An explosion of life
The oceans mainly contained tiny microscopic life until about 575 million years ago when the first simple but complex animals appeared.
At about 541 million years ago, the full range of marine animal lineages we know today emerged. This set the stage for the global appearance of animals and plants on land between 495-417 million years ago.
The appearance of so many new species in the fossil record is remarkable and sudden. This is referred to as the Cambrian explosion and changed life on Earth permanently.
This burst of new life can definitely be seen as an extreme event.
What causes life to evolve new forms?
Massive changes in the environment are closely linked to the evolution of life. The rise of oxygen in the atmosphere would have wiped out a large number of oxygen-sensitive microorganisms that had evolved when oxygen wasn’t present.
The extinction of these non-oxygen respirers led to oxygen-respirers to evolve and colonise the oxygenated niches.
What happened next?
Even after the extreme events of Snowball Earth and oxygen rises, large changes in our planet’s environment continued to occur.
These include large-scale environmental catastrophes such as large-scale volcanism that led to mass extinctions. This promoted the emergence of new dominant lineages. One of these new lineages ultimately produced us modern humans.
For the purposes of this course, the key thing to remember here is how the extreme Snowball Earth events coincided with great rises in the atmosphere and this created conditions for complex life to emerge.
Our planet developed an oxygen rich atmosphere and this made our planet habitable to complex forms of life.
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