Comment by dieselgate

This is actually a fairly reasonable theory given the information we currently have access to - not "spores" per se but rather by amino acids entering our atmosphere. Check out the "panspermia theory" as well as the "Murchison meteorite", I believe there may have been amino acids found on other meteorite samples since and perhaps one within the past 5 years or so. It's possible though our planet is very "ripe" for harboring life and there are multiple avenues through which it has evolved over time.