[BigCanOfTuna]

The Selfish Gene - Richard Dawkings. (http://en.m.wikipedia.org/wiki/The_Selfish_Gene)

The idea that organisms evolved to be survival vehicles for our genes totally inverted and simplified my view of life. It gave me a plausible explanation for how we came to be, and put the final nail in religion's coffin (for me).

[hga]

Ditto for me, although not with quite the same effects. It ties into so many things in the real world; for us, e.g. "Worse is Better" (https://www.dreamsongs.com/WorseIsBetter.html) because it has survival value.