Introduction

Around 100,000 years, the world was inhabited by a heterogeneous group of hominins: the Neanderthals in Europe and Western Asia, Denisovans and possibly other primitive hominins in East Asia, and more modern-looking humans in Africa and the Middle East. By around 30,000 years ago, most of this diversity had disappeared and modern humans occupied much of the Eurasian continent. How did this transition occur?

Definition

The Out-of-Africa Origins model (also Recent African Origins) was developed by several workers during the 1980s and was discussed in some detail by Chris Stringer and Peter Andrews (Stringer & Andrews 1988). The hypothesis is that the modern form of Homo sapiens, or anatomically modern humans (and probably also fundamental aspects of modern human behavior), had evolved in Africa by at least 150,000 years ago. Around 60,000 years ago, modern humans left Africa, replacing archaic hominins outside of the continent with restricted amounts of interbreeding.

Key Issues/Current Debates/Future Directions/Examples

The origins and spread of modern humans have been hotly debated throughout the history of paleoanthropology. There are several hypotheses that explain modern human origins. The first is the Out-of-Africa or Recent African Origins hypothesis (Fig. 1). This hypothesis states that modern humans and modern human behavior evolved in Africa and then spread from Africa around 60,000 years ago, replacing archaic hominins with restricted amounts of inbreeding (Fig. 1). The second hypothesis is the regional continuity or multiregional model and proposes that modern humans evolved in parallel across the globe from the hominins that lived there at the time, with gene flow between the different lineages. A third hypothesis is the assimilation model. This model proposes a middle ground and states that modern humans evolved in Africa but they merged with the local hominins, resulting in hybrid populations. With advances in scientific methods and additions to the fossil record, these hypotheses are ever changing, but their essence remains the same.

Out-of-Africa Origins, Fig. 1
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Three models of modern human origins. Left, Out-of-Africa model; middle, multiregional model; right, assimilation model

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Below are the arguments supporting the Recent African Origins model. Evidence from the fossil record indicates that the African continent gave rise to modern humans. Both morphological and behavioral modernity evolved in Africa between 200,000 and 100,000 years ago. These people would then have spread throughout Africa and across the rest of the world, replacing the other hominins with little gene flow between them. Genetic evidence from mitochondrial and Y-chromosome DNA supports this hypothesis, while nuclear DNA is more ambivalent.

Fossil Evidence

The oldest modern human fossils come from East Africa and date to about 160–195,000 years. Fossils from Omo and Herto show some primitive features but, at the same time, some derived Homo sapiens characteristics such as a rounded skull with high forehead, a chin, and a short face that is tucked under the large brain case. There is fossil evidence from other parts of Africa that modern humans also lived there at least 100,000 years ago. At Klasies River Mouth in South Africa, for example, the remains of several individuals have been found that suggest some modern affinity. Although the fossils are fragmentary, archaeological evidence from the same site has been described as presenting aspects of behavioral modernity. More complete modern human fossils from Qafzeh and Skhul Caves in Israel are dated to around 110,000 years. Environmental evidence has shown that modern humans were present in this region during a warm and wet period, possibly between phases of Neanderthals occupation. Other Afro-Arabian mammal species are also present in the archaeological deposits from that time and suggest that this was a small-scale migration into the Levant due to favorable climatic conditions that suited the African fauna.

The earliest fossil evidence for morphologically modern humans outside of Africa comes from sites such as Lake Mungo in Australia (∼42,000 BP), Niah Cave in Sarawak (∼40,000 BP), Tianyuan Cave in China (∼40,000 BP), and European sites such as Cavallo Cave and Kents Cavern (42–45,000 BP (Benazzi et al. 2011; Higham et al. 2011). Accompanying some of these fossils is evidence of modern human tools and artifacts. The exodus out of Africa is believed to have been firstly via a coastal route into eastern Asia and down into Australasia. Australia appears to have been a stepping stone for the peopling of the Pacific Islands by 5,000 years ago. Northeast Asia served as the gateway to North and South America by 20,000–15,000 years ago. To date, no morphologically modern humans have been well dated outside of Africa to more than 45,000 years ago, apart from those at Skhul and Qafzeh in the Near East.

Genetic Evidence

Until recently, it was impossible to extract intrinsic DNA from fossils. Therefore, until the end of the twentieth century, genetic research in the field of paleoanthropology was limited to the study of modern human and living primate DNA. Modern human diversity in different types of DNA (mitochondrial, Y-chromosome, and nuclear [autosomal] DNA) enables us to look at the demography of humans’ ancient ancestors, the size of the ancestral population, and the number of humans that may have left Africa to populate the rest of the world. DNA can also be used to estimate dates for some key events.

Mitochondrial DNA (mtDNA) is found in the mitochondria, which are small organelles that are involved in energy processing inside the cell. mtDNA is inherited by both males and females from their mother’s ovum. Mitochondrial DNA does not undergo recombination but can carry a novel mutation in the child. Many of these mutational changes are introduced at a slow, but steady rate. Despite its clear signal of ancestry and descent, mtDNA studies have received criticism because they cover a limited amount of genomic coding compared to the whole DNA sequence. At the same time, the small size of the genome and its limited coding means that it is easier to sample and sequence, and mutations can be easily tracked. Results of mtDNA analysis show that the African lineages contain the greatest diversity, indicating that they had the longest time to diverge and are therefore the oldest lineages. This is consistent with Africa being the place of origins of all modern humans. Because the amount of mutation is proportional to time passed, mtDNA can be used to estimate the date when the “mother of all living mtDNA” or “mitochondrial Eve” lived (Fig. 2) and how large the pool of ancestral females was. Although mtDNA mutation rates are not perfectly understood, and therefore dating is still approximate, results from mtDNA studies show that the ancestral population of humans must have been in Africa and perhaps consisted of around 10,000 individuals, with the latest estimate for the mitochondrial origin dating to around 150,000 years (Endicott et al. 2010).

Out-of-Africa Origins, Fig. 2
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Map showing the spread of model human groups as traced using mitochondrial DNA (numbers refer to 1,000 years ago) Routes shown are notional, not precise (From Lone Survivors etc. Credit: Silvia Bello)

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Y-chromosomal DNA is inherited by males from their fathers and carries very few genes. It can only be used to infer the evolutionary history of males. It has received less attention than mtDNA, but in recent years, the Y-chromosomal DNA for a large worldwide sample has been fully sequenced and has yielded some important data on human history. The theoretical “Adam” had been estimated to have lived approximately 70,000 years ago, though recent research has doubled that age estimate, and the deepest branches again appear in Africa, supporting an African origin for all modern humans.

The study of autosomal or nuclear DNA has also been used to reconstruct human population history, and analyses of different sequences have largely supported the results from mitochondrial and Y-chromosomal DNA in confirming the same pattern: the greatest amount of variation is present in Africa and DNA diversity steadily declines with overland distances from Africa. This decline in diversity is also picked up in the morphometry of skulls in populations from different parts of the world.

In 1997 the first ancient mtDNA was extracted from Neanderthal bone. Since then, some 30 Neanderthal fossils (Green et al. 2008) have been sequenced, showing that mtDNA diversity in Neanderthals was much lower than that of modern humans. In addition, the results strongly indicate that Neanderthals made no lasting contribution to the human mtDNA gene pool. This does not mean, however, they did not contribute to the nuclear DNA of humans.

In 2010, a nearly complete Neanderthal nuclear genome was published for the first time (Green et al. 2010). That same year, the nuclear DNA of the Denisovans, a previously unknown population that has distinct DNA from modern humans and Neanderthals, was published. This population is known only from a phalanx and two teeth, but DNA analysis has indicated that this population may have been relatively widespread in Southeast Asia before modern humans migrated into the area, somewhat like Neanderthals were widespread throughout Europe and Western Asia when modern humans arrived there (Reich et al. 2010).

Contrary to the results of the mtDNA, nuclear genome analysis indicates that humans interbred with both Neanderthals and Denisovans. Between about 1 % and 2 % of Neanderthal DNA is still present in non-African DNA today, and Denisovan DNA is additionally present in the genome of modern-day Australasians at a rate of around 4 %. These levels of interbreeding are low, but significant and ongoing analysis is focused on the ways in which these genetic differences between Neanderthals, Denisovans, and modern humans would have manifested phenotypically and how some similarities might have been advantageous to modern humans (e.g., protection from endemic diseases).

The genomic evidence confirms that humans have a predominantly recent African Origin. Yet, at the same time, it is evident that considered globally, Homo sapiens is not entirely Out-of-Africa and that a limited amount of interbreeding with populations outside of Africa had occurred.

Cross-References

Archaic Homo sapiens

Central and West African Middle Stone Age: Geography and Culture

East Asia: Early Homo Fossil Records

Europe: Early Homo Fossil Records

Fossil Records of Early African Homo

Fossil Records of Early Modern Humans

Homo heidelbergensis

Homo neanderthalensis

Human Evolution: Molecular Timescale

Human Evolution: Theory and Progress

Journal of Human Evolution

Pinnacle Point: Excavation and Survey Methods

Southern and East African Middle Stone Age: Geography and Culture

West and Central Asia: Early Homo Fossil Records