The geographic origin of Homo sapien. A review

As promised this post will be another literary review of key papers which shape our understanding of human origins. Today we are going to examine opposing theories which attempt to explain how our species Homo sapien developed from the ancestral condition.

First let us paint a picture of the time period we are examining. It is 200,000 years ago; different species of the genus Homo are already dispersed all over the Old World (Africa, Asia and Europe): with H. erectus in Asia and Neanderthals in Europe. The climate is fluctuating between cold glacial and warmer interglacial periods and mega fauna dominate. This is the time where Homo sapiens make their debut appearance. Papers propose differing views of how this development into modern man came about; did H. sapiens evolve in Africa and replace all existing Homo species, or did they arise independently from pre existing local populations?

Anthropologist Allen Wilson was a proponent of the former view, the ‘Out of Africa’ hypothesis. In his 1987 paper he provides supporting evidence by analysing the mitochondrial DNA of different human populations from around the world. I realise many of you may not know what mitochondrial DNA is so I will give you a brief description: mitochondria exist in every cell in your body; they were once bacteria and act as oxygen factories. Mitochondria have a genome separate to the nuclear genome (chromosomes) we all know and love.  The brilliant thing about the mitochondrial genome is that they do not recombine like our chromosomes and instead passes intact directly from mother to offspring.  This allows the mitochondrial genome to be tracked back in time! Wilson chose 5 different populations from around the world: Africans, aboriginal Australians, Caucasians, Asians and aboriginal New Guineans and used their mitochondrial DNA to produce a restriction map. This map identifies the mutations which have occurred in each lineage, and upon comparison to one another can ultimately show where and when the original ancestor of all lineages existed. The conclusions made by Wilson are that this Eve of humanity existed about 200,000 years ago in Africa.

The consequence of this conclusion is that Homo sapiens must have evolved in Africa before spreading fully formed (with a brain size identical to our own) to colonise the rest of the world. Any species they may have met on their travels be it H. erectus or Neanderthals would have been outcompeted and driven to extinction.

Figure 1 A diagram of the two theories

The suggestions made by Wilson’s paper were controversial and were quickly attacked by proponents of a multi regional human evolution. A paper by Thorne and Wolpoff in 1992 argued that the methods used to calculate the 200,000 year old date of divergence was inaccurate. They suggested that the date of divergence of these 5 human populations should have actually been 1 million years ago. If this were true then the scenario would be very different: the original ancestor would have been much more primitive with a smaller brain size, each different regional population would have developed into a modern human separately and in situ. In Asia H. erectus would have developed into modern Asians and In Europe H. erectus would have developed into Neanderthals and then into modern Europeans; with interbreeding happening along the way.

Although this idea does seem quite outlandish Thorne and Wolpoff do provide some good evidence; the technology and culture of modern humans in Europe were identical to the Neanderthals they lived alongside including their burial systems and butchering of game. If Wilson’s idea were correct then it would be expected that the hand axe technologies of the African Eve population would spread alongside their population, however no hand axes are found outside of Africa. These two examples show that either the Out of Africa theory is incorrect or that the modern humans leaving Africa adapted to local practices and given up any former technology they had developed. 

Another important disparity discussed by Thorne ad Wolpoff’s paper is the lack of African cranial features found in Asia and Europe. Instead certain diagnostic characteristics of populations continue right up until the present day. Although the paper includes many examples, the best one comes from examination of Chinese fossils. The unique feature of a shovel like maxillary incisor is shared by both ancient specimens and the modern population of China; supporting an in situ transition from ancient to modern local population and not a replacement.

All of the evidence provided by the multi regional hypothesis is quite convincing, however the majority of the scientific community still support the Out of Africa theory. I believe this is mainly due to the lack of substantial fossil evidence to really support Thorne and Wolpoff’s claims. On top of this using mitochondrial DNA  to calibrate an ancestors age is a method approved by many to be accurate enough to use as evidence, and therefore the claim that Wilson’s method is inaccurate does not stand up to scrutiny. As is usually the case in a scientific debate more evidence is required to determine which paper is valid but I hope you have gained enough information from this review to form your own opinion on this topic.  

This review has been about the classic papers, for a more up to date analysis check out my updated version (30th November)..

References

Thorne, A.G., and Wolpoff, M.H., 1992. The Multiregional Evolution of Humans. Scientific American. 266(4) pp. 28-33.

Cann, R.L., Stoneking, M., and Wilson, A.C., 1987. Mitochondrial DNA and human evolution. Nature. 325(1) pp. 31-36.