A bit long-winded; I’ll try to be brief. And will the right kind of scientist happen by and correct the inevitable mistakes, ‘cuz I read a lot, but don’t have the degree.
MYA = Million Years Ago
5MYA a common simian ancestor speciated.
What’s that? Multiple populations lost touch with each other and drifted genetically to the point they could no longer interbreed.
One descendant led to the chimpanzee. Don’t ask me about bonobos, apes, great apes, monkeys, etc. Suffice it to say that homo sapiens and pan troglodytes have a common ancestor from about then.
Well established as a forest species, homo sapiens’ progenitor found a mix of trees and savannah such that going to the ground worked pretty well. Due to circumstances, the forest faded away but the population stayed put, and adapted to ground life.
During this period changes to feet and pelvis made walking upright much easier.
Around 2MYA the climate went through several rough patches, where the modus operandi wired into the genome stopped being advantageous, and the ability to suss out new ways to find food, water, and shelter turned out to be a real advantage.
From then until relatively recently, about every .2MY (200,000 years) the weather would do another jump-shift. That’s on the order of ten different times when having a more reactive brain paid off. That’s where a relatively small number of DNA changes led to brains that develop in a much more organized 3D matrix, and a bigger-deeper matrix, resulting eventually in today’s brain case size.
In fact the neanderthal variety was capable of interbreeding but apparently incapable of generating sophisticated speech. A risky change that lets a human choke on his own vomit also facilitated much more sophisticated vocalizations. This in turn allowed the development of complex speech, hence complex culture.
Homo neanderthal had somewhat bigger brain cases than we did then or do today. But without the gift of gab, they “got voted off the island” for being too poorly organized to resist homo sapiens’ much greater ability to coordinate and scheme.
.250 to .225 MYA a single female line of mitochondrial DNA survived a really really harsh weather period. We have the right to call her Eve. Most large mammal species in Africa have DNA that traces back to farthest-possible single ancestors that are also about that recent. We can tell by the relatively small degree of dissimilarity in non-critical DNA stretches.
About .075 MYA a single male line of Y chromosomes wiped out all the others. All men today carry a Y chromosome that differs from any other Y on the planet by only about 75K years of accumulated genetic drift. So he’s Adam. He’s also African. I am going out on a limb and guessing that some other DNA advantage is what made that progenitor the ultimate winner in the race to reproduce. All of that individual’s progeny outcompeted all other progeny.
For what it’s worth, this is a layman’s account.
Oh, and Another Thing 