Where did humanity originate and how and when did it spread over the earth? It has now been clearly shown that primitive humanity originated in Africa, and our modern Homo sapiens branch arose there too. There were a few breakouts from Africa, and the branch that spread to Europe is now known as Neanderthals, and another branch that has left fossils in central Asia is known as Denisovans. There may be traces of those early people’s DNA still found in modern populations, but nearly all of the DNA that constitutes modern humanity’s DNA came from the last wave of people in Africa.
The first line of the blurb on the back cover of Deep Ancestry by Spencer Wells is clearly wrong. “The human race began 60,000 years ago with a single family in an African valley.” It appears to be true that modern humanity can trace their Y chromosome to a single male in Africa at that time, but that doesn’t mean there was only one male living at that time, or that he was in a family with a single woman. The other chromosomes in the modern human population almost certainly came from other people, as there is a mixing of chromosomes with every generation. Also, at every generation half of the chromosomes would come from a female and that would mean about forty-six chromosomes available at every new birth. It is generally accepted that there was a genetic bottleneck of about 10,000 breeding humans about 60,000 to 90,000 years ago. In future DNA historical research, it may be possible to trace these other chromosomes and observe common and disparate trails of migration.
That number seems frighteningly tiny compared to our modern population approaching eight billion. The population of many animals of our approximate weight have numbers in the low tens of thousands. The low numbers of humans during the bottleneck may have been a blessing because it meant that the unusual genetic qualities that permitted us to become human could spread throughout the entire population. It was at that time that many human qualities that could not have arisen from natural selection, or even sexual selection, came into being because of what I call Evish selection. That is, our ancestors were choosing mates that had the qualities we now call human, and we are still selecting our mates based on those same qualities.
The book is mostly about the spread of various DNA trails out of Africa, and it traces the various routes out of Africa using unique DNA markers taken from indigenous people now living in scattered places. It appears that the female mitochondrial DNA has taken some slightly different routes from the male Y chromosome DNA. The founding populations have a terrific advantage when coming into unpopulated areas, like the Americas, because it is easy for a small band of people to expand in a few generations of living in a world of absolute abundance. It takes a while to reach the carrying capacity of the land that is supporting them. Later genetic groups are entering a land already filled with people, and unless they have some superior qualities they are either driven back or slowly assimilated.
Many people are thrilled by factual history as I am by the processes that drive history.