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Got into a discussion with a friend who is a biologist about the "human nature" argument against communism. My best shot was saying that we used to live in a sort of proto communism so the evidence of it working are there. He didn't accept that argument and basicly said that due to natural selection, competition etc. and that all social structures eventually disolve.

I didn't have good ideas on how to respond after that.

EDIT: Forgot the question, how could I have defended this argument more?

EDIT2: I read each and every comment anyone posts, I just can't respond to all of you, thanks so much for the explanations it really goes to show how awesome this community is.