When and where did our species originate?
When and where modern humans arose is still debated.
Our species, Homo sapiens, has now spread to all parts of the world, but our origins are debated.There are several viewpoints on this but the two major and opposing opinions are the ‘Out of Africa’ model and the ‘Multiregional’ model.
Out of Africa model
The ‘Out of Africa’ model is currently the most widely favoured explanation accounting for the origins of modern humans. It suggests that modern humans originated in Africa within the last 200,000 years from a single group of ancestors. Modern humans continued to evolve in Africa and had spread to the Middle East by 100,000 years ago and possibly as early as 160,000 years ago. Modern humans only became well established elsewhere in the last 50,000 years. The ‘Out of Africa’ model has had a variety of names including:
- ‘The Garden of Eden’ hypothesis
- ‘Noah’s Ark’ hypothesis
- ‘Out of Africa 2’ hypothesis, which distinguishes the earlier and later dispersals of humans out of Africa. In this case, ‘Out of Africa 1’ refers to the initial dispersal out of Africa by Homo ergaster, whereas ‘Out of Africa 2’ refers to the later dispersal out of Africa by modern humans.
Did modern humans interbreed with other human species?
As modern humans spread, they replaced all other human species. Homo heidelbergensis was replaced by modern humans Africa and Europe, Homo erectus was replaced in Asia and Homo neanderthalensis was replaced in Europe.
The most extreme version of this model suggests that modern humans replaced the older humans without any interbreeding. Less extreme versions allow for some interbreeding between these populations but suggest that gene flow and mixing between these different species was extremely limited.
Groundbreaking analysis of the Neanderthal genome (nuclear DNA and genes) published in 2010 supports the less extreme 'Out of Africa' model. Results show that modern humans and Neanderthals did interbreed, although on a very limited scale. Researchers compared the genomes of five modern humans with the Neanderthal, discovering that Europeans and Asians share about 1-4% of their DNA with Neanderthals and Africans none. This suggests that modern humans bred with Neanderthals after moderns left Africa but before they spread to Asia and Europe. The most likely location is the Levant, where both species co-existed for thousands of years at various times between 50-90,000 years ago. Interestingly, the data doesn't support wide-scale interbreeding between the species in Europe, where it would have been most likely given their close proximity. Researchers are now questioning why interbreeding occurred on such a low scale, given that it was biologically possible. The answer may lie in cultural differences.
Another probable example of interbreeding between modern humans and other human species was also published in 2010. DNA from a tooth and finger bone excavated from Denisova cave in Russia, showed these remains belonged to a genetically distinct group of humans distantly related to Neanderthals and even more distantly related to Homo sapiens. The study also revealed that these 'Denisovans' interbred with the ancestors of modern Melanesians and Aboriginal Australians as the DNA of both groups today contains 4% to 6% Denisovan DNA (it has not been found in other Eurasian groups sampled). This may be due to rare encounters between modern humans and Denisovans as moderns migrated through South-East Asia and on to Melanesia. What species the Denisovans belonged to is unknown, but it is suggested they may be Homo heidelbergensis, whose remains have been found in the correct timespans and locations but whose DNA has yet to be extracted (and may never be due to the age of the remains).
Evolving different physical features
The different physical features now found in modern humans from different geographical areas around the world are believed to have evolved over only the last 60,000 years or so as a result of adaptations to different environments.
Evidence supporting the Out of Africa model
- the oldest known fossils of Homo sapiens are African
- fossil evidence indicates that modern humans quickly replaced earlier humans in Europe and western Asia.
- all living people show little genetic diversity. This is interpreted as being the result of a relatively recent replacement of earlier, more diverse populations.
- a variety of different DNA studies on modern humans all suggest a recent common ancestry from a small gene pool. Most of these point to Africa as the origin of this population
- DNA from contemporary humans can be used to produce maps of human movement throughout the world and show how long an indigenous population has lived in an area. These indicate modern human origins in Africa.
- analysis of the Neanderthal genome and comparisons with modern humans does support the view that the vast majority of genes of non-Africans came with the spread of modern humans that originated in Africa and then spread throughout the world.
The ‘Multiregional’ model suggests that when human ancestors first left Africa nearly two million years ago, they spread out and formed regional groups of early humans across Africa, Asia and Europe. Modern humans then evolved concurrently in all these regions rather than from a single group of humans in Africa. The ‘Multiregional’ model is also known as the ‘Regional Continuity’ model.
Did interbreeding occur?
Interbreeding between different regional populations did occur. Geographically separated populations remained genetically similar to one another through the genetic mixing that resulted from interbreeding and a single species was therefore maintained.
Although it is now known that interbreeding did occur between Neanderthals and some early modern humans, this is not on the scale that proponents of the Multiregional model claim. The amount of Neanderthal DNA found in some modern humans is too small to have made any significant difference.
The variety of modern humans
The different physical features that are found in modern humans from different geographical areas around the world are believed to have evolved over a very long period in Africa, Asia and Europe since the time when each region became settled.
Evidence supporting the Multiregional model
- there are similarities between some skull features found in modern humans and in ancient humans from the same regions.
- some modern Asians have features similar to those of some ancient humans from China (in the Out of Africa model these ancient humans are known as Chinese Homo erectus).
- some modern Australian Aboriginal people show similarities to some ancient humans from Indonesia (in the Out of Africa model these ancient humans are known as Indonesian Homo erectus)
- some modern Europeans show similarities to ancient humans from Europe (in the Out of Africa model these ancient humans are known as the Neanderthals, or Homo neanderthalensis).
- all living people show little genetic diversity. This is interpreted as being the result of continuous mixing of genes among regional populations.