Why did it take so long to leave Africa?
The extensive arid environments of northern Africa and the Middle East were a major barrier blocking movement out of Africa. Before they could spread out of Africa, our ancestors needed to develop physical and mental capabilities that would enable them to survive in these harsh environments where food and fresh water were highly seasonal resources.
Who left Africa first?
Homo ergaster may have been the first human species to leave Africa and fossil remains show this species had expanded its range into southern Eurasia by 1.75 million years ago. Their descendents, Homo erectus, then spread eastward and were established in South East Asia by at least 1.6 million years ago.
However, an alternate theory proposes that hominins migrated out of Africa before Homo ergaster evolved, possibly about 2 million years ago, prior to the earliest dates of Homo erectus in Asia. These hominins may have been either australopithicines or, more likely, an unknown species of Homo, similar in appearance to Homo habilis. In this theory, the population found at Dmanisi represent a missing link in the evolution of Homo erectus/Homo ergaster. Perhaps too, the evolution of African Homo ergaster occurred outside of Africa and considerable gene flow occurred between African and Eurasian populations.
This theory has gained more support in recent years due to DNA research. Evidence from a genetic study indicates an expansion out of Africa about 1.9 million years ago and gene flow occurring between Asian and African populations by 1.5 million years ago. More physical evidence is needed from key areas in Eurasia such as Iran, Afghanistan and Pakistan, but politics is currently making this difficult.
What made it possible to leave Africa?
Homo ergaster was the first of our ancestors to possess the physical and cultural attributes needed in order to disperse though the arid environments of northern Africa and the Middle East. These attributes included:
- a modern body shape with an efficient striding gait suited to travelling over long distances, although smaller statures are represented in the remains from Dmanisi
- a sufficiently developed intelligence to cope with unfamiliar environments, although did not require a brain size much bigger than Homo habilis, with an average brain size of 610cc
- improved technology to aid subsistence (Oldowan-style tools or Mode1 Technology have been found at sites in Dmanisi, Georgia, and northern China, both dating to 1.7 million years old)
- a diet that included more meat and which increased the food supply options in seasonally arid environments
Who left Africa next?
After the first early dispersals out of Africa, various other groups of early humans spread out of Africa as their populations grew. These dispersals were not regular or constant but instead occurred as waves of dispersal during periods with favourable climatic and environmental conditions.
These waves of dispersal out of Africa included movements eastward across southern Asia more than one million years ago and movements into western Europe within the last 900,000 years. Movements back into Africa also occurred.
More recently, modern humans began their dispersal out of Africa. Most experts conclude, from genetic and material evidence, that this occurred on a mass scale only within the last 60,000 years or so. The exception is in the Levant, where modern humans occupied the region about 120,000-100,000 years ago but did not seem to expand further than this. However, new evidence in the form of stone tools may cause a rethink of this scenario. Stone tools have been found in India dating to about 74,000 years old, in Yemen dating to between 70,000 and 80,000 years old, and in the United Arab Emirates dating to about 80,000 years old. Some of these tools resemble African Middle Stone Age technology, others are more like those used by Neanderthals in Europe and Homo sapiens and Neanderthals in the Levant. No human remains were found with the tools, but as Neanderthals have not been found in these regions, it is assumed the makers were modern humans.
By 100,000 years ago, humans had dispersed and diversified into at least three species. Our own species, Homo sapiens, lived in Africa and the Middle East, Homo neanderthalensis lived in Europe, and Homo floresiensis in southern Asia. DNA from human remains in Denisova cave, Russia, suggest a probable fourth species was also still extant when Homo sapiens was migrating through southern Asia about 60,000 years ago. Modern Melanesians have about 4% of this DNA. The species is unknown, but may be late surviving Homo heidelbergensis or a yet-to-be-discovered species. This diversity disappeared about 28,000 years ago, however, and only one human species now survives.