Well my theory is that the knowledge to build computers with Artificial Intelligence was sent to us by aliens from outside our solar system in order to propagate their species and enslave us. No spaceships required, just let the existing inhabitants build up the structure for artificial life and let it go. Humans will become second class citizens in their own world while the different artificial intelligent codes battle it out for supremacy.
Then we not need brains and live in harmony. Yeah right.