When and where did our species originate?
Our species, Homo sapiens, has now spread to all parts of the world but it's generally believed that we originated in Africa by about 200,000 years ago. We interacted with local archaic human populations as we colonised the globe.
During the 1980s and 1990s the fossil record, while growing, was not able to clearly demonstrate whether Homo sapiens evolved from local ancestors across the globe or originated in a single region and then dispersed. DNA studies generally supported the single origin theory, but were still in their infancy. The two major and opposing opinions were known as the ‘Out of Africa’ model and the ‘Multiregional’ model.
Out of Africa or single origin model
The ‘Out of Africa’ model was the most widely favoured explanation accounting for the origins of modern humans. It suggested 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 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.
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 suggested that modern humans replaced the older humans without any interbreeding. Less extreme versions allowed for some interbreeding between these populations but suggest that gene flow and mixing between these different species was extremely limited.
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.
The ‘Multiregional’ model suggested 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. 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. 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.
Other names for the Multiregional model:
The ‘Multiregional’ model is also known as the ‘Regional Continuity’ model.
Latest findings – including new fossils and improved DNA research and dating techniques – confirm the complexity of modern human (Homo sapiens) origins. Evidence still suggests that all modern humans are descended from an African population of Homo sapiens that spread out of Africa about 60,000 years ago but also shows that they interbred quite extensively with local archaic populations as they did so (Neanderthal and Denisovan genes are found in all living non-Africa populations) and these local populations contributed to our species’ success. So, while the general basis of the original Out of Africa model prevails, it requires extensive revision.
Recent African Origin With Hybridisation
Evidence indicates that Neanderthal and Denisovan traits emerge in Eurasia, while Homo sapiens traits emerge in Africa. Africa and Eurasia are isolated until H. sapiens disperses and interbreeds with the other two (and possibly some other unknown archaic species). Modern humans essentially absorb and replace the fragmentary local populations. The low percentage of Neanderthal and Denisovan genes found in living humans indicates a replacement process was most likely.
What remains unclear is how ancestral modern human populations were interacting in Africa. Did Homo sapiens originate from a small local population that then spread, as some believe, or via interbreeding between multiple groups across a wide area?
New fossil discoveries suggest that modern human physical traits did not emerge as one suite but were gradual. A skull from Jebel Irhoud in Morocco dated to about 300,000 years old, controversially assigned to Homo sapiens, had a modern-looking face but an elongated, archaic-looking braincase. This suggests our globular braincase evolved later and not as part of a fully modern suite of features.
Based on this and other early Homo sapiens remains, including those from Herto and Omo Kibish in Ethiopia, the African Multiregionalism Model of H. sapiens evolution was proposed in 2018 by a group including Eleanor Scerri and Chris Stringer (who proposed the original Out of Africa model). They suggest that early Homo sapiens showed great diversity and that rather than a single origin, our species emerged from admixture among numerous populations within Africa.
While most researchers agree with the RAOWH model, there are some that propose a different theory of interactions among human species and the role these interactions had in modern human origins. This theory differs in the way it explains how the DNA of Homo sapiens mixed with local populations outside Africa. Essentially, while some H. sapiens traits originated in Africa, it was when populations spread into Eurasia and extensively interbred with Neanderthals and Denisovans, that the evolution of new modern traits occurred. Thus, this model proposes, modern human origins involve a high degree of assimilation with archaic Eurasian populations within the last 100,000 years.
How do we relate to other archaic humans given that we clearly interbred and the offspring also produced viable ancestors?
Homo sapiens share physical features and genetic characteristics that distinguish them from other humans and merit their classification as a separate species. However, some believe the fact we interbred with Neanderthals, for instance, means they should be classified as the same species under the biological species concept. However, this definition of species has limitations, particularly when it comes to defining human species.