Don’t die and have lots of babies that also don’t die.
The whole evolution theory basically boils down to that being the purpose of life. If it weren’t, life would eventually die out. Can’t have life if there isn’t any new being born. This leaves us with a couple “simple” mechanics: Survive and procreate. Everything else you do on a daily basis is just some added spark to keep it interesting and smooth the process out a bit. Hell, the “survive” part is just there so you have enough time to procreate, so it isn’t even part of the core.
I was going to make some elaborate point about adding different mechanics to the mix, what with society having formed the way it has, but it would all be made moot by what I’ve written above. It’s all cover-up for the core task. Not always very directly, but the link’s definitely there.
Luckily, we have agency over our own actions and can choose not to do what nature would have us do. Humans are rebellious like that, and it’s neat.