The strange lack of variation in the modern human genome

In 2007 there were an estimated 6,600,000,000 members of the species Homo sapiens living on planet Earth. No other creature larger than a rat can boast anywhere near this number. Within that huge and apparently ever-growing number of people there are a few genetic differences (“races”) in skin colour, shape and colour of eyes, form and colour of hair, height of stature etc that are based on minute variations in an otherwise almost identical genome. By contrast: it has been estimated that a small troupe of chimpanzees (the nearest relative of to Homo sapiens ) contains more genetic variation than that of all humans alive today. This is a strange state of affairs that can only have one reasonable explanation: there had to be the mother of all genetic bottlenecks in our relatively recent past. A genetic bottleneck means that the number of reproducing individuals in a species is reduced for whatever reason. A complete bottleneck is, of course, extinction. The early humans of very roughly 100,000 years ago (which on an evolutionary timescale is very recently) appeara to have missed extinction by the narrowest of margins.

In our article on the the Toba volcanic eruption of 73,000 years ago (see also map of “southern route migration” below) we asuggest that this eruption could have killed off most of the waves of early human migrants making their way out of Africa at the time and reducing their already low genetic variety still further in comparison to the populations they left behind. Toba must have played a part in this early human drama but the volcano is unlikely to be the whole answer. The left-behind Africans have more genetic variety than those who migrated out of Africa, but compared with most higher animals, even the African genetic variety is very small. The mystery of the lack of variety in the human genome clearly has not yet been fully explained.

While human overall genetic diversity is very low compared to other higher animals, what human genetic diversity there is, is highest in Africa south of the Sahara. This fact is his is one of the major arguments in favour of the “Out of Africa” theory.

The chart below shows the female lineage (mtDNA) for the descendants of the northern route of the Great Migration (see below).

The male line of descent through the Y-DNA chromosome is less often used to establish human descent because male haplogroups are limited to the Y chromosome and are much less numerous and much more difficult to extract and interpret than the female mtDNA.

Gen-HaplogroupsF

The mtDNA gene flow shows the routes various modern people took to get to where they are now and where groups split off from a source population.

Areas that the migrants vacated completely or where they had no contact with other groups have not, of course, left genetic traces of their former presence.

GenFlow-World

Geneticist Pääbo wrote in 2001:

‘…the gene pool in Africa contains more variation than elsewhere, and the genetic variation found outside of Africa represents only a subset of that found within the African continent. From a genetic perspective, all humans are therefore Africans, either residing in Africa or in recent exile.’

In this context, “recent” can indeed mean more than 80,000 years.

Local adaptations known as “human races” also show very low levels of genetic variation within-and-between populations. Only 10% of the limited human genetic variation is accounted for by differences between populations and this even in comparison to the nearest human relatives, the apes. These very odd genetic facts support an extremely recent origin and a rapid population expansion for Homo sapiens – once they had spread all over the world.

So, why haven’t we started our story on this Web-site with Africa where it all began?

Sub-Saharan Africa is the continent of the people who stayed behind, who did not leave 80,000 years ago when the ancestors of most of the rest of mankind left Africa. Staying behind, the Africans developed such an enormous variety of cultures, tribes, races and additional genetic variations (from the pygmies of the Congo to the Maasai of Kenya, from the Ethiopians to the Khoisan of South Africa) that we must admit to terminal trepidation at the very thought of tackling this overwhelming mass of evidence and variety, not made easier by a dearth of reliable literature and research. Really, we thought to ourselves (honestly we did! right boys and girls?) we need to leave something for our successors and their children and children’s children to do. And so it came to pass that we have started with the Andamanese.

For similar reasons we have decided to concentrate not on the African home our ancestors left behind first, but on their way into the bigger world. The most archaic and longest-isolated group of modern humans known (also one of the most neglected until recently!) on the long human Migration Out of Africa is to be found on an isolated group of islands belonging to India: the Andamanese Negrito.We have named this web-site after them and have given them pride of place. Which does not mean that other remnant or major population groups are less important. We hope to get round to all of them eventually. Promise!

Next: The Naked Ape: from animal to human in Africa