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with all the tiny meteorites and asteroids that would hit earth w/o an atmosphere, it could be very easy.
LONG ago, earth didnt have one, so things did land on the surface. All it would take is one out of billions of hits to have one take hold and start to survive.
It wouldve had to have been bacteria that were photosynthetic as well, but even if it wasnt, over a long enough period of time (not long in the terms of duplicating bacteria) it could change and adapt.
The fact that long enough ago there wasnt an atmosphere, leaves anything open to chance.
On a basic level, all cells duplicate the same way...that isnt a coincidence.
And essentially, all animals are the same (vertabrates i mean)
Look at organ systems and even the layout of the skeleton...its all very similar.
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