Why did the Earth Before the Dinosaurs Look So Terrible?

Why did the Earth Before the Dinosaurs Look So Terrible?
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Did you know that you are most likely an alien on this planet.
Some scientists believe that the seeds of life may have been planted here by an asteroid or a comet, carrying the building blocks of life across the vast expanse of space. This tantalizing idea raises the possibility that life on Earth might not be entirely ‘Earthly’ at all. Perhaps, in a way, we are all aliens.
But whether life began here or was delivered from the stars, every living thing on our planet shares a remarkable connection. Hidden deep within our cells is the evidence of a single ancestor… an organism that lived billions of years ago and gave rise to all forms of life on Earth.
Could we really be aliens? And what bizarre creatures came after the first life on Earth appeared?