Miracle was not happy in just existing so it decided it needed to replicate itself to leave a posterity after it died.   

One of Miracle's progeny determined that two-cells are probably better than one so it divided with the result that were now 2 two-celled organisms.   

Time passed and eventually one of Miracle's two-celled progeny determined that if two is good, millions is better, so it organized all the one and two cell organisms into a tissue.   

Again time passes and eventually one tissue realized it did not have any real purpose in being, so it decided it should become an organ with a function.   

It is not clear which organ first developed from the tissues but in course of time, hearts, livers, kidneys and brains were seen just hanging around earth.   

After generations of organs lived and died unto themselves, the brain organs got together and decided there should be some coordination amongst all the organs.   It conceived the idea of a system whereby the various organs could function at a higher level.   

And so, there were single cell organisms, tissue organisms, organ organisms and system organisms everywhere on earth.   

It eventually occurred to the nervous systems that there must be more to life than just pumping blood, eliminating waste, breathing, reproducing, etc.   So, the systems organized under the direction of the nervous system to become a body.   I believe that first body was a worm.   

Since it is getting late I will speed things up:
   → some of the worms decided to be fish
   → some of the fish decided to be turtles
   → some of the turtles decided to be monkeys
   → some of the monkeys decided to be humans

And that, my child, is how evolution occurred.