OK, I appreciate that posting serious answers in this thread is a cardinal sin but I cannot resist. This is how it goes.

Given a choice, every organism would choose to live in a place with perfect conditions. The problem is, what is perfect for me is also perfect for John, Fred and Mary. Which means the good places will soon become overcrowded and each individual is left with three choices. Die, push someone out of be pushed out and learn to live on the edge, where the conditions are a bit less perfect.

Those who learn to accept the edge will over time learn to like it there and beget a new generation for whom that environment is "optimal" - and the whole cycle repeats. Given a long enough time, every possible environment will become populated and those living in diametrically opposing kinds of environment will call each other "extremophiles".

That applies to any population, microbial or human.