You seem to be appealing to instincts or to the facts of how we evolved. But that's the naturalistic fallacy. The question isn't whether we'll be forced to behave in some way, but whether there's a good reason for choosing some life plan. Explaining why we behave as we do isn't the same as justifying the behaviour for being good rather than just probable. Naturalistic explanation isn't the same as moral justification. So no, I don't think science handles the existential question of how we should live.