re: Chance
Purpose is the opposite of chance ("hasard" in french).
Humans (and animals) do things on purpose, but not nature.
It is a matter of understanding what causes and consequences are, then there's less room for purpose (actually, there should be none).
There are plenty of things we don't understand (in a way, luckily, otherwise we'd stop searching) but that doesn't mean we should be relying on a non man-made purpose.