A genetic study of African hunter-gatherers suggests modern humans evolved in southern Africa rather than in the east as previously thought, scientists report this month.
The team, led by Brenna Henn of Stanford University California, carried out the largest genetic study of modern hunter-gatherers living in Africa so far. They also compared the DNA with samples from other African populations and a European population.
They analysed single nucleotide polymorphisms (SNP), which are small variations in DNA sequence between groups of people. SNPs can help highlight genetic relationships, evolutionary divergence and even disease.
The results revealed that Namibian and Khomani Bushmen of southern Africa, Biaka Pygmies of Central Africa and the Sandawe of East Africa, have the highest levels of genetic diversity in the world, the team says.
Genetic diversity is an indicator of how ancient a people are. In principle, the more ancient the population, the more time it has had to build up diversity.
Using the genetic data and the geographical locations of the hunter-gatherer groups, the team deduced that the origin of modern humans, Homo sapiens, was in southern Africa.
So far, most evidence has suggested that modern humans evolved in East Africa around 200,000 years ago.
For example, fossils of modern humans more than 150,000 years old are known from Ethiopia. And genetic evidence shows all people outside Africa share groups of genes found most commonly in east African populations.
Until now, there had been very few large-scale genetic studies on African hunter-gatherers and so this research adds important data for the study of human evolution.
Prof Chris Stringer, human origins expert at the Natural History Museum, comments on this research. ‘This is a landmark study, with far more extensive data on single nucleotide polymorphisms in hunter-gatherer groups than we have ever had before.’
However, Stringer is cautious about localising origins from the genetic study. ‘The ranges of these people are currently quite limited, but rock paintings by ancient populations linked to the Bushmen hint that they were once far more widespread.
‘It seems more likely that the surviving hunter-gatherer groups are now localised remnants of populations that formerly ranged across much of sub-Saharan Africa 60,000 years ago.’
Prof Chris Stringer is one of the architects of the Out of Africa theory, which explains how all humans living today share a recent African origin and that those outside Africa migrated out in small groups during the last 60,000 years.
He discusses these topics in his upcoming book The Origin of our Species published this summer and adds, ‘I no longer think there was a single ‘‘Garden of Eden’’ where we evolved, and distinct populations in ancient Africa probably contributed to the genes and behaviours that make up modern humans.’
The evidence for the first ancient humans, the earliest members of the genus Homo, comes from fossils discovered in East Africa between 1.9 and 2.4 million years ago.
The modern form of our species, Homo sapiens, is thought to have evolved in Africa around 200,000 years ago and descended from those first humans.
Areas in Africa other than the east have also been proposed as being the centre of evolution of modern humans and their behaviour, as Stringer explains. ‘Caves on the South African coast such as Blombos and Pinnacle Point have suggested these were early centres for aspects of behavioural modernity, such as the production of jewellery, the use of pigments, and marine exploitation.
‘In the last few years discoveries from North Africa and Israel have suggested that the north of the continent was important in our evolution too.’
Genetic evidence suggests that modern humans left Africa between 55,000 and 60,000 years ago. But that was not the end of the human family tree, as shown by two genetic studies that have revealed intimate relations with other human species.
In May 2010, scientists obtained the first genetic code of Neanderthals, our closest relatives who died out about 30,000 years ago.
They showed that modern humans outside of Africa share about 2.5% of their genetic material with Neanderthals, meaning modern humans probably interbred with Neanderthals soon after they left Africa around 60,000 years ago.
Another ancient human population recognised from DNA in a 40,000 year old finger bone and tooth has also left its mark. In December last year, scientists revealed the group, called Denisovans, must have interbred with our species, leaving behind a genetic trace in people living in Australasia today.