You are way off base, all humans split off in one event in the past ~6 million years ago, the differences that you see since then are minor due to recent geographical isolation of populations in the last 200,000 years. We are the result of one speciation event only.
In terms of evolution, there is no difference between those differences that historically have been called races. In fact, your history is evidence of the lack of evolutionary distance and that while isolation of populations has occurred over the last 200k years, our interaction with other ancestral populations will only reduce the differences in populations.
Although this tells us when the changes occurred, the unanswered question is, what caused the humans to continue to change, while the other branches more less remained in steady state? All the Apes still hang in trees, while humans fly planes.
One possible way to answer this is with a variation of natural selection. One possible change in selective pressure, would be if the early pre-humans had formed a working relationship with another species; canine. Dogs were domesticated about 15,000 years ago, so these initial dogs, would not be domesticated but wild dogs that worked with the pre-humans in a cooperative way, that was mutually beneficial.
Based on the dating of the diagram above, there were major climate events and ice ages from the time these branches appear to the present. Having a relationship with dogs, would give these pre-humans a selective advantage during ice ages.
The value of this is pre-humans, could learn skills that were not innate, but which came from another apex species. Dogs are good pack hunters and the extra meat would increase the protein in the pre-human diet, so their brain could grow. With the dogs not yet domesticated, the pack interactions with the pre-humans would offer a more personal natural selective pressure.
For example, packs of dogs will fight among themselves for pecking order, like a football team at practice. If you include the pre-humans in this pack dynamics, they will need to learn to fight with the dogs and other humans, gaining skills to defending themselves and hunt, while migrating into new territories as a pack; chase and ambush. Pre-humans standing upright, would make him more intimidating in pack fights.
I had a dog who liked to chew sticks. When he chewed larger sticks; branch length, he often made a pointy stick. The caveman pounding a stick to a point with a rock is like copying a dog chewing a stick with his molars. The pre-human would not learn that from watching apes.
To start this journey of change, all that would need to happen is a group of transitional apes or prehuman, finding a litter of puppies, with no mother. They built a bond since puppies are friendly and needy. But since Apes and Dogs were enemies, both species rejected this odd couple. so they had to leave and find a new home, together. As the puppies grew up some will be too wild and some more cooperative; mutual selection and deselection. As time goes on and new generations appear, their bonds get stronger. The symbiosis add a whole new range of experience, that was not written on Ape or Dog DNA, but would become the two future composites; modern human and domestic dog.
Humans have two centers of consciousness; inner self and ego. This is unique to humans, with the ego appearing a little bit after the time of dog domestication. However, dogs and some other higher domestic animals, like cats and horses, etc., appear to have evolved a virtual secondary due to their human interaction and their selection for this purpose; work dogs. This virtual secondary can become programmed to act like a virtual ego; seeing eye dog. When someone treats their dog like as baby, the dog's virtual ego is populated, and the dogs begins to act from this program to where it appears to be leading the human, so she/he can enjoy coddle him.
If a domesticated dog was to become feral, the virtual secondary become less conscious, and the dog become more from inner self, which is better for survival.
