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 Could have happened? Sure. Did happen? No. 

Why? We have never found a breakpoint in the evolutionary chain of animals that would suggest an "insertion" of a brand new species that has never had a parent in the evolutionary chain. 

Yes. Humans, plants, animals, bacteria, and fungii share a common ancestor. We can trace all living organisms back to that ancestor. 

If we were seeded, that common single-cell bacterium-like ancestor was the seed. That alien seed is so far away from primates that it would be virtually impossible for two separate evolutionary chains to end up with the same structure as primates, 4 billion years later.