system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is there only one evolutionary tree on earth? Doesn't this mean that life is extremely rare?

As I understand it, scientists believe that all life on earth has a common ancestor, which formed through natural processes billions of years ago. Why is it that this only happened once? Just like there are different species, why aren't there different, unrelated trees of organisms that originated in a different time/place? If life really only sprung up once on a planet as fertile as ours, doesn't that mean that it's probably extremely rare throughout the universe as well?
gpt
There are a couple of possible scenarios:

A)  Life formed only once

B)  Life formed multiple times and we've failed to differentiate because it was too similar (example, mDNA could have originally been a different life form which has been incorporated, or viruses may extend from a completely different origin point)

C)  Life formed multiple times and we've failed to differentiate because we have not yet detected the radically different life (example, silicon based life existing within the mantle)

D)  New life forms often, but since first life is abundant, the "new" life is regularly consumed by existing microorganisms.