You should look into Dalores Cannon's work. She discusses the very exact things you are curious about. Most of the information in her books is salvaged from the subconscious minds of individuals in deep states of hypnosis, accessing buried memories of the hereafter. You might find it interesting.
The universe, or should i say multi-verse, contains an infinitely diverse amount of intelligent life. And there are more than just "physical incarnations". Chances are you have incarnated in non-human bodies before.
In a sense, the big bang event did happen. But its not over with, it is/has/will be happening forever. Time is a big bump in the road to understanding the nature of reality. You are still ONE, and you are still simulating the illusion of separation as well.
And as far as transmigration is concerned... Consciousness passes through all forms of reality. But once it has evolved out of a certain from associated with a certain "threshold" of awareness, it does not again return to the "lower" or less complex form of consciousness, except in rare instances.