13.8 billion years ago, matter condensed into an infinitely tiny space suddenly violently expanded for reasons unknown and possibly incomprehensible. This matter (behaving either by its constitution or in accords with mysterious forces) was spread out, and formed giant congregations based upon chemical properties or else spread out sparsely, reacted, solidified however transiently, or underwent a series of admixtures and repulsions, such as the ones from solar nebulae that lead to the formation of our little planet some 4.5 billion years ago. At first our planet was a molten inferno. It all went something like this:
http://nbmpub.com/blog/wp-content/uploads/2012/11/history-of-earth_spread.jpg
http://whitchurchmeadow.org.uk/wp-content/uploads/2009/11/Geological_time_spiral.jpg
Eventually things stabilized enough for phospholipids to form with nucleic acids inside of them, and then enough of those lipids collided and their contents interacted, until eventually an intricate nucleotide chain capable of spitting out copies of itself formed, and it began spitting out copies of itself which spit out copies of themselves (however imperfect), until so many were spit out and they all had changed so much that they filled the atmosphere with oxygen as a byproduct of their metabolic processes, changing our atmosphere and the way the biosphere functions at a fundamental level.
Then these life forms by random mutation developed ways to utilize oxygen as an energy source by putting it through a series of chemical reactions which yielded ATP for mitochondrion, and in a flash a myriad of possibilities opened up as more energy was made available for chemical reactions within these membrane-bounded self-replicating things.
Eventually these bound membranes started forming into gigantic collectives, some of them sustaining themselves by tapping into the naturally available energy resources, others by taking advantage of the ones that did just that by consuming them, and then those that consumed other things began consuming each other. The carnivorous being developed brains over time which allowed them to better propagate their genes. These brains became very complex as the competitive relationship between species and the mating competition within species became ever more fierce.
I gotta go to work so I have to end how babies are born in this paragraph. 16-year-old Johnny has been fucking his girlfriend for months because he the brain components of a reptile telling him to procreate, but an underdeveloped frontal lobe not advising him that he should be wearing a condom because he isn't ready to be a dad. And then bam, a child is born. People with fully-developed frontal lobes have babies too, of course, and by choice. Has a lot to do with where the reptilian complex is situated in the brain...OK, gotta go to work.