The Cosmos is a gigantic flywheel, making 10,000 revolutions a minute. Man is a sick fly, taking a dizzy ride on it. Religion, is the idea, that the flywheel, was put there, just for us.
I regard the brain as a computer, which will stop working, when its components fail. There is no heaven, or afterlife, for broken down computers. That is a fairy tale for people afraid of the dark.
I believe we originated in a "Primordial Soup", and thus proceeded as would any other (initially) bacterial lifeform. Through the principles of Darwinian Natural Selection, our species "won out" against competitors, and has thus become the complex lifeform that we see here, today. In fact the human body is still made up of trillions of bacteria, itself. (Consider the Human Microbiome, for example). The other more complex lifeforms, also stand as "winners" in this all consuming and merciless race to be "the fittest" (In Darwinian terms "fittest" and "survival of the fittest' mean those organisms most able to reproduce (compared to competing species, and organisms, which then become extinct).....
RE: Door To The Other side
It's likely just oblivion, but that does not bother me, personally