The Creation myth of course claims that we are all descended from the genes of just two individuals (really one, since Adam’s rib couldn’t possibly have had different genes to the rest of him). And since creation was on 23rd of October, 4004 BC according to Bishop James Ussher, the human race must have come to be as a consequence of a great deal of incest among the descendants of Adam and Eve.
Myths aside, I was wondering – with all the new DNA evidence that is now being produced – how far back in time we need to go to find the earliest common ancestors of all humans alive today?
The Out of Africa, single migration theory postulated that all humans alive (outside of Africa) were descended from just one group – perhaps as small as 200 individuals – who left Africa around 70,000 years ago and then expanded to populate the world. According to this theory our earliest common ancestors were then mitochondrial Eve and Y-chromosomal Adam who lived in Africa between 100,000 and 200,000 years ago (but not concurrently). But this theory of a single “out of Africa event is now being shown to be unsatisfactory on a number of counts.
- It is silent about how many were left in Africa and why their evolution continued in such a similar manner to the little, genetically cramped group which left.
- it is becoming increasingly evident that though AMH originated in Africa, there were probably many “Out of Africa” events. There were also many earlier “Out of Africa” events of homo erectus which then led to the evolution of Neanderthals in central Asia and points west into Europe, and the Denisovans in central Asia and points south-eastwards. Furthermore, once AMH had appeared in Africa there were groups which left – more than once – before and after the Toba eruption (74,000 years ago).
- Populations outside Africa today all show a few percentage of either Neanderthals or Denisovans or of anther unknown ancient human species. This may have been another human species concurrent with the Neanderthals and the Denisovans. African AMH then had a number of admixture events with these other species outside of Africa (probably in central asia around 30,000 – 50,000 years ago) to give modern humans.
- There were probably many Back to Africa events of AMH groups from outside of Africa, back into Africa which also influenced the genetic pool within Africa. (But these back-mixture events were not sufficient to introduce detectable traces of Neanderthal and Denisovan genes into African AMH).
African AMH is thought to have first come into being around 200,000 years ago in Africa. But it is thought that the ancestors of the Neanderthals and the Denisovans left Africa about 500,000 years ago. This immediately means that mitochondrial Eve and Y-chromosomal Adam could not have been our earliest common ancestors.
Our earliest common ancestry must go back to a time before African AMH and to a time when the ancestors of Neanderthals and Denisovans were still in Africa. And that takes us back to the time of homo erectus and to not less than some 500,000 years ago.