Neanderthal extinction

From Wikipedia, the free encyclopedia

Neanderthal extinction began around 40,000 years ago in Europe, after anatomically modern humans had reached the continent. This date, which is based on research published in Nature in 2014, is much earlier than previous estimates, and it was established through improved radio carbon dating methods analyzing 40 sites from Spain to Russia. The survey did not include sites in Asia, where Neanderthals may have survived longer.^[1]

Hypotheses on the fate of the Neanderthals include violence from encroaching anatomically modern humans,^[2] parasites and pathogens, competitive replacement, competitive exclusion, extinction by interbreeding with early modern human populations,^[3] and failure or inability to adapt to climate change. It is unlikely that any one of these hypotheses is sufficient on its own; rather, multiple factors probably contributed to the demise of an already widely-dispersed population.^[4][5]

Coexistence prior to extinction

Neanderthal tools

modern human tools

In research published in Nature in 2014, an analysis of radiocarbon dates from forty Neanderthal sites from Spain to Russia found that the Neanderthals disappeared in Europe between 41,000 and 39,000 years ago with 95% probability. The study also found with the same probability that modern humans and Neanderthals overlapped in Europe for between 2,600 and 5,400 years.^[1] Modern humans reached Europe between 45,000 and 43,000 years ago.^[6] Most recent numbers based on improved radiocarbon dating indicate that Neanderthals disappeared around 40,000 years ago, which overturns older carbon dating which indicated that Neanderthals may have lived as recently as 24,000 years ago,^[7] including in refugia on the south coast of the Iberian peninsula such as Gorham's Cave.^[8][9] Inter-stratification of Neanderthal and modern human remains has been suggested,^[10] but is disputed.^[11]

Possible cause of extinction

Violence

Some authors have discussed the possibility that Neanderthal extinction was either precipitated or hastened by violent conflict with Homo sapiens. Conflict and warfare are virtually ubiquitous features of hunter-gatherer societies, including conflicts over limited resources, such as prey and water.^[12] It is therefore plausible to suggest that violence, including primitive warfare, would have transpired between the two human species.^[13] The hypothesis that early humans violently replaced Neanderthals was first proposed by French palaeontologist Marcellin Boule (the first person to publish an analysis of a Neanderthal) in 1912.^[14] Several finds in both Homo-sapiens and Neanderthal bones indicate inter-species aggression from injuries (grooves in the bones themselves) that could only have come from spear or other projectile tips crafted with prevalent tool-making methods contemporary to the time.^{[15][unreliable source?]}

Parasites and pathogens

Another possibility is the spread among the Neanderthal population of pathogens or parasites carried by Homo sapiens.^[16] Neanderthals would have limited immunity to diseases they had not been exposed to, so diseases carried into Europe by Homo sapiens could have been particularly lethal to them while Homo sapiens were relatively resistant. If it were relatively easy for pathogens to leap between these two similar species, perhaps because they lived in close proximity, then Homo sapiens would have provided a pool of individuals capable of infecting Neanderthals and potentially preventing the epidemic from burning itself out as Neanderthal population fell. An examination of human and Neanderthal genomes and adaptations regarding pathogens or parasites may shed light on this issue.

Competitive replacement

Sapiens and Neanderthal skulls

Species specific disadvantages

Slight competitive advantage on the part of modern humans has accounted for Neanderthals' decline on a timescale of thousands of years.^[17]

Generally small and widely-dispersed fossil sites suggest that Neanderthals lived in less numerous and socially more isolated groups than contemporary Homo sapiens. Tools such as Mousterian flint stone flakes and Levallois points are remarkably sophisticated from the outset, yet they have a slow rate of variability and general technological inertia is noticeable during the entire fossil period. Artifacts are of utilitarian nature, symbolic behavioral traits are undocumented before the arrival of modern humans in Europe around 40,000 to 35,000 years ago.^[18][19]

Jared Diamond, supporter of competitive replacement points out in his book The Third Chimpanzee that the genocidal replacement of Neanderthals by modern humans is comparable to patterns of behavior that occur whenever people with advanced technology clash with less advanced people.^[20]

Division of labor

In 2006, two anthropologists of the University of Arizona proposed an efficiency explanation for the demise of the Neanderthals.^[21] In an article titled "What's a Mother to Do? The Division of Labor among Neanderthals and Modern Humans in Eurasia",^[22] it was posited that Neanderthal division of labor between the sexes was less developed than Middle paleolithic Homo sapiens. Both male and female Neanderthals participated in the single occupation of hunting big game, such as bison, deer, gazelles and wild horses. This hypothesis proposes that the Neanderthal's relative lack of labor division resulted in less efficient extraction of resources from the environment as compared to Homo sapiens.

Anatomical differences and running ability

Researchers such as Karen L. Steudel of the University of Wisconsin have highlighted the relationship of Neanderthal anatomy (shorter and stockier than that of modern humans^{[citation needed]}) and the ability to run and the requirement of energy (30% more).^{[23][not in citation given]}

Nevertheless, in the recent study, researchers Martin Hora and Vladimir Sladek of Charles University in Prague show that Neanderthal lower limb configuration, particularly the combination of robust knees, long heels and short lower limbs, increased the effective mechanical advantage of Neanderthal knee and ankle extensors, thus reducing the force needed and the energy spent for locomotion significantly. The walking cost of the Neanderthal male is now estimated to be 8–12% higher than that of anatomically modern males, whereas the walking cost of the Neanderthal female is considered to be virtually equal to that of anatomically modern females.^[24]

Other researchers, like Yoel Rak, from Tel-Aviv University in Israel, have noted that the fossil records show that Neanderthal pelvises in comparison to modern human pelvises would have made it much harder for Neanderthals to absorb shock and to bounce off from one step to the next, giving modern humans another advantage over Neanderthals in running and walking ability.^[25]

Modern humans' advantage in hunting warm climate animals

Pat Shipman, from Pennsylvania State University in the United States, argues that the domestication of the dog gave modern humans an advantage when hunting.^[26] The oldest remains of domesticated dogs were found in Belgium (31,700 BP) and in Siberia (33,000 BP).^[27][28] A survey of early sites of modern humans and Neanderthals with faunal remains across Spain, Portugal and France provided an overview of what modern humans and Neanderthals ate.^[29] Rabbit became more frequent, while large mammals – mainly eaten by the Neanderthals – became increasingly rare. In 2013, DNA testing on the "Altai dog", a paleolithic dog's remains from the Razboinichya Cave (Altai Mountains), has linked this 33,000-year-old dog with the present lineage of Canis lupus familiaris.^[30]

Interbreeding

Human-Neandertal mtDNA

Neanderthal DNA extraction

Interbreeding can only account for a certain degree of Neanderthal population decrease. A homogeneous absorption of an entire species is a rather unrealistic idea. This would also be counter to strict versions of the Recent African Origin, since it would imply that at least part of the genome of Europeans would descend from Neanderthals, who left Africa at least 350,000 years ago.

The most vocal proponent of the hybridization hypothesis is Erik Trinkaus of Washington University.^[31][32] Trinkaus claims various fossils as hybrid individuals, including the "child of Lagar Velho", a skeleton found at Lagar Velho in Portugal.^[33] In a 2006 publication co-authored by Trinkaus, the fossils found in 1952 in the cave of Peștera Muierilor, Romania, are likewise claimed as hybrids.^[34]

Genetic studies indicate some form of hybridization between archaic humans and modern humans had taken place after modern humans emerged from Africa. An estimated 1–4% of the DNA in Europeans and Asians (e.g. French, Chinese and Papua probands) is non-modern, and shared with ancient Neanderthal DNA rather than with sub-Saharan Africans (e.g. Yoruba and San probands).^[35]

Modern-human findings in Abrigo do Lagar Velho, Portugal allegedly featuring Neanderthal admixtures have been published.^[36] However, the interpretation of the Portuguese specimen is disputed.^[37]

Jordan, in his work Neanderthal, points out that without some interbreeding, certain features on some "modern" skulls of Eastern European Cro-Magnon heritage are hard to explain. In another study, researchers have recently found in Peştera Muierilor, Romania, remains of European humans from ~37,000–42,000 years ago^[38]who possessed mostly diagnostic "modern" anatomical features, but also had distinct Neanderthal features not present in ancestral modern humans in Africa, including a large bulge at the back of the skull, a more prominent projection around the elbow joint, and a narrow socket at the shoulder joint. Analysis of one skeleton's shoulder showed these humans, like Neanderthals, did not have the full capability for throwing spears.^[39]

The Neanderthal genome project published papers in 2010 and 2014 stating that Neanderthals contributed to the DNA of modern humans, including most humans outside sub-Saharan Africa, as well as a few populations in sub-Saharan Africa, through interbreeding, likely between 50,000 and 60,000 years ago.^[40][41][42] Recent studies also show that a few Neanderthals began mating with ancestors of modern humans long before the large "out of Africa migration" of the present day non-Africans, as early as 100,000 years ago.^[43] In 2016, research indicated that there were three distinct episodes of interbreeding between modern humans and Neanderthals: the first encounter involved the ancestors of non-African modern humans, probably soon after leaving Africa; the second, after the ancestral Melanesian group had branched off (and subsequently had a unique episode of interbreeding with Denisovans); and the third, involving the ancestors of East Asians only.^[44]

While interbreeding is viewed as the most parsimonious interpretation of the genetic discoveries, the authors point out they cannot conclusively rule out an alternative scenario, in which the source population of non-African modern humans was already more closely related to Neanderthals than other Africans were, due to ancient genetic divisions within Africa.

Among the genes shown to differ between present-day humans and Neanderthals were RPTN, SPAG17, CAN15, TTF1 and PCD16.

Neanderthal DNA Comparison (SharedDNA)

Climate change

Snowbound, painting of Neanderthals in a blizzard, Charles R. Knight, 1911

Neanderthals went through a demographic crisis in Western Europe that seems to coincide with climate change that resulted in a period of extreme cold in Western Europe. "The fact that Neanderthals in Western Europe were nearly extinct, but then recovered long before they came into contact with modern humans came as a complete surprise to us," said Love Dalén, associate professor at the Swedish Museum of Natural History in Stockholm. If so, this would indicate that Neanderthals may have been very sensitive to climate change.^[45]

Natural catastrophe

A number of researchers have argued that the Campanian Ignimbrite Eruption, a volcanic eruption near Naples, Italy, about 39,280 ± 110 years ago (older estimate ~37,000 years), erupting about 200 km³ (48 cu mi) of magma (500 km³ (120 cu mi) bulk volume) has contributed to the extinction of Neanderthal man.^[46] The argument has been developed by Golovanova et al.^[47][48]

Although Neanderthals had encountered several Interglacials during 250,000 years in Europe^[49] inability to adapt their hunting methods caused their extinction facing H. sapiens competition when Europe changed into a sparsely vegetated steppe and semi-desert during the last Ice Age.^[50] Studies of sediment layers at Mezmaiskaya Cave suggest a severe reduction of plant pollen.^[48] The damage to plant life would have led to a corresponding decline in plant-eating mammals hunted by the Neanderthals.^[48][51][52]

Behavioral modernity

From Wikipedia, the free encyclopedia

 

Upper Paleolithic (16,000-year-old) cave painting from Lascaux cave in France

Behavioral modernity is a suite of behavioral and cognitive traits that distinguishes current Homo sapiens from other anatomically modern humans, hominins, and primates. Although often debated, most scholars agree that modern human behavior can be characterized by abstract thinking, planning depth, symbolic behavior (e.g., art, ornamentation, music), exploitation of large game, and blade technology, among others.^[1][2] Underlying these behaviors and technological innovations are cognitive and cultural foundations that have been documented experimentally and ethnographically. Some of these human universal patterns are cumulative cultural adaptation, social norms, language, and extensive help and cooperation beyond close kin.^[3][4] It has been argued that the development of these modern behavioral traits, in combination with the climatic conditions of the Last Glacial Maximum, was largely responsible for the human replacement of Neanderthals and the peopling of the rest of the world.^[2][5]

Arising from differences in the archaeological record, a debate continues as to whether anatomically modern humans were behaviorally modern as well. There are many theories on the evolution of behavioral modernity. These generally fall into two camps: gradualist and cognitive approaches. The Later Upper Paleolithic Model refers to the idea that modern human behavior arose through cognitive, genetic changes abruptly around 40,000–50,000 years ago.^[6] Other models focus on how modern human behavior may have arisen through gradual steps; the archaeological signatures of such behavior only appearing through demographic or subsistence-based changes.^{[1][2][7][8][9]}

Definition

To classify what traits should be included in modern human behavior, it is necessary to define behaviors that are universal among living human groups. Some examples of these human universals are abstract thought, planning, trade, cooperative labor, body decoration, control and use of fire. Along with these traits, humans possess a heavy reliance on social learning.^[10][11] This cumulative cultural change or cultural "ratchet" separates human culture from social learning in animals. As well, a reliance on social learning may be responsible in part for humans' rapid adaptation to many environments outside of Africa. Since cultural universals are found in all cultures including some of the most isolated indigenous groups, these traits must have evolved or have been invented in Africa prior to the exodus.^{[12][13][14][15]}

Archaeologically, a number of empirical traits have been used as indicators of modern human behavior. While these are often debated^[16] a few are generally agreed upon. Archaeological evidence of behavioral modernity includes:^[2][6]

• Burial
• Fishing
• Figurative art (cave paintings, petroglyphs, dendroglyphs, figurines)
• Systematic use of pigment (such as ochre) and jewelry for decoration or self-ornamentation
• Using bone material for tools
• Transport of resources over long distances
• Blade technology
• Diversity, standardization, and regionally distinct artifacts
• Hearths
• Composite tools

Critiques

Several critiques have been placed against the traditional concept of behavioral modernity, both methodologically and philosophically.^[2][16] Shea (2011) outlines a variety of problems with this concept, arguing instead for "behavioral variability", which, according to the author, better describes the archaeological record. The use of trait lists, according to Shea (2011), runs the risk of taphonomic bias, where some sites may yield more artifacts than others despite similar populations; as well, trait lists can be ambiguous in how behaviors may be empirically recognized in the archaeological record.^[16] Shea (2011) in particular cautions that population pressure, cultural change, or optimality models, like those in human behavioral ecology, might better predict changes in tool types or subsistence strategies than a change from "archaic" to "modern" behavior.^[16] Some researchers argue that a greater emphasis should be placed on identifying only those artifacts which are unquestionably, or purely, symbolic as a metric for modern human behavior.^[2]

Theories and models

Late Upper Paleolithic Model or "Revolution"

The Late Upper Paleolithic Model, or Upper Paleolithic Revolution, refers to the idea that, though anatomically modern humans first appear around 150,000 years ago, they were not cognitively or behaviorally "modern" until around 50,000 years ago, leading to their expansion into Europe and Asia.^[6][17][18] These authors note that traits used as a metric for behavioral modernity do not appear as a package until around 40–50,000 years ago. Klein (1995) specifically describes evidence of fishing, bone shaped as a tool, hearths, significant artifact diversity, and elaborate graves are all absent before this point.^[6] Although assemblages before 50,000 years ago show some diversity the only distinctly modern tool assemblages appear in Europe at 48,000.^[17] According to these authors, art only becomes common beyond this switching point, signifying a change from archaic to modern humans.^[6] Most researchers argue that a neurological or genetic change, perhaps one enabling complex language, such as FOXP2, caused this revolutionary change in our species.^[6][18]

Alternative models

Contrasted with this view of a spontaneous leap in cognition among ancient humans, some authors like Alison S. Brooks, primarily working in African archaeology, point to the gradual accumulation of "modern" behaviors, starting well before the 50,000 year benchmark of the Upper Paleolithic Revolution models.^[1][2][19] Howiesons Poort, Blombos, and other South African archaeological sites, for example, show evidence of marine resource acquisition, trade, and abstract ornamentation at least by 80,000 years ago.^[1][7] Given evidence from Africa and the Middle East, a variety of hypotheses have been put forth to describe an earlier, gradual transition from simple to more complex human behavior. Some authors have pushed back the appearance of fully modern behavior to around 80,000 years ago in order to incorporate the South African data.^[19]

Others focus on the slow accumulation of different technologies and behaviors across time. These researchers^[1][2] describe how anatomically modern humans could have been cognitively the same and what we define as behavioral modernity is just the result of thousands of years of cultural adaptation and learning. D'Errico and others have looked at Neanderthal culture rather than early human behavior for clues into behavioral modernity.^[5] Noting that Neanderthal assemblages often portray traits similar to those listed for modern human behavior, researchers stress that the foundations for behavioral modernity may in fact lie deeper in our hominin ancestors.^[20] If both modern humans and Neanderthals express abstract art and complex tools then "modern human behavior" cannot be a derived trait for our species. They argue that the original "human revolution" theory reflects a profound Eurocentric bias. Recent archaeological evidence, they argue, proves that humans evolving in Africa some 300,000 or even 400,000 years ago were already becoming cognitively and behaviourally "modern". These features include blade and microlithic technology, bone tools, increased geographic range, specialized hunting, the use of aquatic resources, long distance trade, systematic processing and use of pigment, and art and decoration. These items do not occur suddenly together as predicted by the "human revolution" model, but at sites that are widely separated in space and time. This suggests a gradual assembling of the package of modern human behaviours in Africa, and its later export to other regions of the Old World.

Between these extremes is the view – currently supported by archaeologists Chris Henshilwood,^[21] Curtis Marean,^[22] Ian Watts^[23] and others – that there was indeed some kind of 'human revolution' but that it occurred in Africa and spanned tens of thousands of years. The term "revolution" in this context would mean not a sudden mutation but a historical development along the lines of "the industrial revolution" or "the Neolithic revolution".^[24] In other words, it was a relatively accelerated process, too rapid for ordinary Darwinian "descent with modification" yet too gradual to be attributed to a single genetic or other sudden event. These archaeologists point in particular to the relatively explosive emergence of ochre crayons and shell necklaces apparently used for cosmetic purposes. These archaeologists see symbolic organisation of human social life as the key transition in modern human evolution. Recently discovered at sites such as Blombos Cave and Pinnacle Point, South Africa, pierced shells, pigments and other striking signs of personal ornamentation have been dated within a time-window of 70,000 – 160,000 years ago in the African Middle Stone Age, suggesting that the emergence of Homo sapiens coincided, after all, with the transition to modern cognition and behaviour.^[25] While viewing the emergence of language as a 'revolutionary' development, this school of thought generally attributes it to cumulative social, cognitive and cultural evolutionary processes as opposed to a single genetic mutation.^[26]

A further view, taken by archaeologists such as Francesco D'Errico^[27] and João Zilhão,^[28] is a multi-species perspective arguing that evidence for symbolic culture in the form of utilised pigments and pierced shells are also found in Neanderthal sites, independently of any "modern" human influence.
Cultural evolutionary models may also shed light on why although evidence of behavioral modernity exists before 50,000 years ago it is not expressed consistently until that point. With small population sizes, human groups would have been affected by demographic and cultural evolutionary forces that may not have allowed for complex cultural traits.^{[8][9][10][11]} According to some authors^[8] until population density became significantly high, complex traits could not have been maintained effectively. It is worth noting that some genetic evidence supports a dramatic increase in population size before human migration out of Africa.^[18] High local extinction rates within a population also can significantly decrease the amount of diversity in neutral cultural traits, regardless of cognitive ability.^[9]

Highly speculatively, bicameral mind theory argues for an additional, and cultural rather than genetic, shift from selfless to self-perceiving forms of human cognition and behavior very late in human history, in the Bronze Age. This is based on a literary analysis of Bronze Age texts which claims to show the first appearances of the concept of self around this time, replacing the voices of gods as the primary form of recorded human cognition. This non-mainstream theory is not widely accepted but does receive serious academic interest from time to time.

Archaeological evidence

Africa

Before the Out of Africa theory was generally accepted, there was no consensus on where the human species evolved and, consequently, where modern human behavior arose. Now, however, African archaeology has become extremely important in discovering the origins of humanity. Since human expansion into Europe around 48,000 years ago is generally accepted as already "modern",^[17] the question becomes whether behavioral modernity appeared in Africa well before 50,000 years ago, as a late Upper Paleolithic "revolution" which prompted migration out of Africa, or arose outside Africa and diffused back.

A variety of evidence of abstract imagery, widened subsistence strategies, and other "modern" behaviors have been discovered in Africa, especially South and North Africa. The Blombos Cave site in South Africa, for example, is famous for rectangular slabs of ochre engraved with geometric designs. Using multiple dating techniques, the site was confirmed to be around 77,000 years old.^[29] Beads and other personal ornamentation have been found from Morocco which might be as old as 130,000 years old; as well, the Cave of Hearths in South Africa has yielded a number of beads significantly before 50,000 years ago.^[1]

Expanding subsistence strategies beyond big-game hunting and the consequential diversity in tool types has been noted as signs of behavioral modernity. A number of South African sites have shown an early reliance on aquatic resources from fish to shellfish. Pinnacle Point, in particular, shows exploitation of marine resources as early as 120,000 years ago, perhaps in response to more arid conditions inland.^[7] Establishing a reliance on predictable shellfish deposits, for example, could reduce mobility and facilitate complex social systems and symbolic behavior. Blombos Cave and Site 440 in Sudan both show evidence of fishing as well. Taphonomic change in fish skeletons from Blombos Cave have been interpreted as capture of live fish, clearly an intentional human behavior.^[1]

Humans in North Africa (Nazlet Sabaha, Egypt) are known to have dabbled in chert mining, as early as ~100,000 years ago, likely for use as tools.^[30][31]

Europe

While traditionally described as evidence for the later Upper Paleolithic Model,^[6] European archaeology has shown that the issue is more complex. A variety of stone tool technologies are present at the time of human expansion into Europe and show evidence of modern behavior. Despite the problems of conflating specific tools with cultural groups, the Aurignacian tool complex, for example, is generally taken as a purely modern human signature.^[32][33] The discovery of "transitional" complexes, like "proto-Aurignacian", have been taken as evidence of human groups progressing through "steps of innovation".^[32] If, as this might suggest, human groups were already migrating into eastern Europe around 40,000 years and only afterward show evidence of behavioral modernity, then either the cognitive change must have diffused back into Africa or was already present before migration.

In light of a growing body of evidence of Neanderthal culture and tool complexes some researchers have put forth a "multiple species model" for behavioral modernity.^[5][20][34] Neanderthals were often cited as being an evolutionary dead-end, apish cousins who were less advanced than their human contemporaries. Personal ornaments were relegated as trinkets or poor imitations compared to the cave art produced by H. sapiens. Despite this, European evidence has shown a variety of personal ornaments and artistic artifacts produced by Neanderthals; for example, the Neanderthal site of Grotte du Renne has produced grooved bear, wolf, and fox incisors, ochre and other symbolic artifacts.^[34] Though burials are few and controversial, there has been circumstantial evidence of Neanderthal ritual burials.^[20] There are two options to describe this symbolic behavior among Neanderthals: they copied cultural traits from arriving modern humans or they had their own cultural traditions comparative with behavioral modernity. If they just copied cultural traditions, which is debated by several authors,^[5][20] they still possessed the capacity for complex culture described by behavioral modernity. As discussed above, if Neanderthals also were "behaviorally modern" then it cannot be a species-specific derived trait.

Asia

Most debates surrounding behavioral modernity have been focused on Africa or Europe but an increasing amount of focus has been placed on East Asia. This region offers a unique opportunity to test hypotheses of multi-regionalism, replacement, and demographic effects.^[35] Unlike Europe, where initial migration occurred around 50,000 years ago, human remains have been dated in China to around 100,000 years ago.^[36] This early evidence of human expansion calls into question behavioral modernity as an impetus for migration.

Stone tool technology is particularly of interest in East Asia. Following Homo erectus migrations out of Africa, Acheulean technology never seems to appear beyond present-day India and into China. Analogously, Mode 3, or Levallois technology, is not apparent in China following later hominin dispersals.^[37] This lack of more advanced technology has been explained by serial founder effects and low population densities out of Africa.^[38] Though tool complexes comparative to Europe are missing or fragmentary, other archaeological evidence shows behavioral modernity. For example, the peopling of the Japanese archipelago offers an opportunity to investigate the early use of watercraft. Though one site, Kanedori in Honshu, does suggest the use of watercraft as early as 84,000 years ago, there is no other evidence of hominins in Japan until 50,000 years ago.^[35]

The Zhoukoudian cave system near Beijing has been excavated since the 1930s and has yielded precious data on early human behavior in East Asia. Though disputed, there is evidence of possible human burials and interred remains in the cave dated to around 34-20,000 years ago.^[35] These remains have associated personal ornaments in the form of beads and worked shell, suggesting symbolic behavior. Along with possible burials, numerous other symbolic objects like punctured animal teeth and beads, some dyed in red ochre, have all been found at Zhoukoudian.^[35] Though fragmentary, the archaeological record of eastern Asia shows evidence of behavioral modernity before 50,000 years ago but, like the African record, it is not fully apparent until that time.

Recent African origin of modern humans

From Wikipedia, the free encyclopedia

Map of the migration of modern humans out of Africa, based on mitochondrial DNA. Colored rings indicate thousand years before present.

In paleoanthropology, the recent African origin of modern humans, also called the "Out of Africa" theory (OOA), recent single-origin hypothesis (RSOH), replacement hypothesis, or recent African origin model (RAO), is the dominant^[1][2] model of the geographic origin and early migration of anatomically modern humans (Homo sapiens).

The model proposes a "single origin" of Homo sapiens in the taxonomic sense, precluding parallel evolution of traits considered anatomically modern in other regions,^[3] but not precluding limited admixture between H. sapiens and archaic humans in Europe and Asia.^{[note 1]} H. sapiens most likely developed in the Horn of Africa between 300,000 and 200,000 years ago. The "recent African origin" model proposes that all modern non-African populations are substantially descended from populations of H. sapiens that left Africa after that time.

There were at least several "out-of-Africa" dispersals of modern humans, possibly beginning as early as 270,000 years ago, and certainly during 130,000 to 115,000 ago via northern Africa.^{[4][5][6][7][8][9]} These early waves appear to have mostly died out or retreated by 80,000 years ago.^[10]

The most significant "recent" wave took place about 70,000 years ago, via the so-called "Southern Route", spreading rapidly along the coast of Asia and reaching Australia by around 65,000–50,000 years ago.^{[11][12][note 2]} while Europe was populated by an early offshoot which settled the Near East and Europe less than 55,000 years ago.^[13][14][15]

In the 2010s, studies in population genetics have uncovered evidence of interbreeding of H. sapiens with archaic humans both in Africa and in Eurasia,^[16] which means that all modern population groups, both African and non-African, while mostly derived from early H. sapiens, to a lesser extent are also descended from regional variants of archaic humans.

Proposed waves

"Recent African origin," or Out of Africa II, refers to the migration of anatomically modern humans (Homo sapiens) out of Africa after their emergence at c. 300,000 to 200,000 years ago, in contrast to "Out of Africa I", the migration of archaic humans from Africa to Eurasia between roughly 1.8 to 0.5 million years ago.

Since the early 21st century, the picture of "recent single-origin" migrations has become significantly more complex, not just due to the discovery of modern-archaic admixture but also due to the increasing evidence that the "recent out-of-Africa" migration took place in a number of waves spread over a long time period. As of 2010, there were two main accepted dispersal route for the out-of-Africa migration of early anatomically modern humans: via the "Northern Route" (via Nile Valley and Sinai) and the "Southern Route" via the Bab al Mandab strait.^[17]

• Posth et al. (2017) suggest that early Homo sapiens, or "another species in Africa closely related to us," might have first migrated out of Africa around 270,000 years ago.^[18]
• Finds at Misliya cave, which include a partial jawbone with eight teeth have been dated to around 185,000 years ago. Layers dating from between 250,000 and 140,000 years ago in the same cave contained tools of the Levallois type which could put the date of the first migration even earlier if the tools can be associated with the modern human jawbone finds.^[19][20]
• An Eastward Dispersal from Northeast Africa to Arabia during 150–130 kya based on the finds at Jebel Faya dated to 127 kya (discovered in 2011).^[4][5] Possibly related to this wave are the finds from Zhirendong cave, Southern China, dated to more than 100 kya.^[17] Other evidence of modern human presence in China has been dated to 80,000 years ago.^[10]
• The most significant dispersal took place around 70,000 years ago via the so-called Southern Route, either before^[21] or after^[14][15] the Toba event, which happened between 69,000 and 77,000 years ago.^[21] This dispersal followed the southern coastline of Asia, and reached Australia around 65,000-50,000 years ago. Western Asia was "re-occupied" by a different derivation from this wave around 50,000 years ago, and Europe was populated from Western Asia beginning around 43,000 years ago.^[17]
• Wells (2003) describes an additional wave of migration after the southern coastal route, namely a northern migration into Europe at circa 45,000 years ago.^{[note 3]} This possibility is ruled out by Macaulay et al. (2005) and Posth et al. (2016), arguing for a single coastal dispersal, with an early offshoot into Europe.

Northern Route dispersal

Beginning 135,000 years ago, tropical Africa experienced megadroughts which drove the humans from the land and towards the sea shores, and forced them to cross over to other continents.^{[22][note 4]}
Modern humans crossed the Straits of Bab el Mandab in the southern Red Sea, and moved along the green coastlines around Arabia, and thence to the rest of Eurasia. Fossils of early Homo sapiens were found in Qafzeh cave in Israel and have been dated 80,000 to 100,000 years ago. These humans seem to have either become extinct or retreated back to Africa 70,000 to 80,000 years ago, possibly replaced by southbound Neanderthals escaping the colder regions of ice-age Europe.^[23] Hua Liu et al. analyzed autosomal microsatellite markers dating to about 56,000 years ago. They interpret the paleontological fossil as an isolated early offshoot that retracted back to Africa.^[24]

The discovery of stone tools in the United Arab Emirates in 2011 indicated the presence of modern humans at least 100,000 and 125,000 years ago,^[4] leading to a resurgence of the "long-neglected" North African route.^{[5][25][6][7]}

In Oman, a site was discovered by Bien Joven in 2011 containing more than 100 surface scatters of stone tools belonging to the late Nubian Complex, known previously only from archaeological excavations in the Sudan. Two optically stimulated luminescence age estimates place the Arabian Nubian Complex at approximately 106,000 years old. This provides evidence for a distinct stone age technocomplex in southern Arabia, around the earlier part of the Marine Isotope Stage 5.^[26]

According to Kuhlwilm and his co-authors, Neanderthals contributed to modern humans genetically around 100,000 years ago, from humans which split off from other modern humans around 200,000 years ago.^[27] They found that "the ancestors of Neanderthals from the Altai Mountains and early modern humans met and interbred, possibly in the Near East, many thousands of years earlier than previously thought".^[27] According to co-author Ilan Gronau, "This actually complements archaeological evidence of the presence of early modern humans out of Africa around and before 100 ka by providing the first genetic evidence of such populations."^[27] Similar genetic admixture events have been noted in other regions as well.^[28]

In China, the Liujiang man (Chinese: 柳江人) is among the earliest modern humans found in East Asia.^[29] The date most commonly attributed to the remains is 67,000 years ago.^[30] High rates of variability yielded by various dating techniques carried out by different researchers place the most widely accepted range of dates with 67,000 BP as a minimum, but does not rule out dates as old as 159,000 BP.^[30] Liu, Martinón-Torres et al. (2015) claim that modern human teeth have been found in China dating to at least 80,000 years ago.^[31]

Southern Route dispersal

Coastal route

Red Sea crossing

By some 70,000 years ago, a part of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East. It has been estimated that from a population of 2,000 to 5,000 individuals in Africa, only a small group, possibly as few as 150 to 1,000 people, crossed the Red Sea.^[32][33] The group that crossed the Red Sea travelled along the coastal route around Arabia and Persia to India, which appears to be the first major settling point.^[34] Wells (2003) argued for the route along the southern coastline of Asia, across about 250 kilometres (155 mi)^{[dubious – discuss]}, reaching Australia by around 50,000 years ago.

Today at the Bab-el-Mandeb straits, the Red Sea is about 20 kilometres (12 mi) wide but 50,000 years ago sea levels were 70 m (230 ft) lower (owing to glaciation) and the water was much narrower. Though the straits were never completely closed, they were narrow enough and there may have been islands in between to have enabled crossing using simple rafts.^[35][17] Shell middens 125,000 years old have been found in Eritrea,^[36] indicating the diet of early humans included seafood obtained by beachcombing.

The dating of the Southern Dispersal is a matter of dispute.^[21] It may have happened either pre- or post-Toba, a catastrophic volcanic eruption that took place between 69,000 and 77,000 years ago at the site of present-day Lake Toba. Stone tools discovered below the layers of ash disposed in India may point to a pre-Toba dispersal but the source of the tools is disputed.^[21] An indication for post-Toba is haplo-group L3, that originated before the dispersal of humans out of Africa and can be dated to 60,000–70,000 years ago, "suggesting that humanity left Africa a few thousand years after Toba".^[21] New research showing slower than expected genetic mutations in human DNA was published in 2012, indicating a revised dating for the migration to between 90,000 and 130,000 years ago.^[37]

Western Asia

A fossil of a modern human dated to 54,700 years ago was found in Manot Cave in Israel, named Manot 1,^[38] though the dating was questioned by Groucutt et al. (2015).

South-Asia and Australia

It is thought that Australia was inhabited around 65,000-50,000 years ago. As of 2017, the earliest evidence of humans in Australia is at least 65,000 years old,^[11][12] while McChesney stated that

...genetic evidence suggests that a small band with the marker M168 migrated out of Africa along the coasts of the Arabian Peninsula and India, through Indonesia, and reached Australia very early, between 60,000 and 50,000 years ago. This very early migration into Australia is also supported by Rasmussen et al. (2011).^[13]

Fossils from Lake Mungo, Australia, have been dated to about 42,000 years ago.^[39][40] Other fossils from a site called Madjedbebe have been dated to at least 65,000 years ago.^[12]

East Asia

Tianyuan man from China has a probable date range between 38,000 and 42,000 years ago, while Liujiang man from the same region has a probable date range between 67,000 and 159,000 years ago. According to 2013 DNA tests, Tianyuan man is related "to many present-day Asians and Native Americans".^{[41][42][43][44][45]} Tianyuan is similar in morphology to Minatogawa Man, modern humans dated between 17,000 and 19,000 years ago and found on Okinawa Island, Japan.^[46][47]

Europe

According to Macaulay et al. (2005), an early offshoot from the southern dispersal with haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinai. This group then branched, some moving into Europe and others heading east into Asia.^[14] This hypothesis is supported by the relatively late date of the arrival of modern humans in Europe as well as by archaeological and DNA evidence.^[14] Based on an analysis of 55 human mitochondrial genomes (mtDNAs) of hunter-gatherers, Posth et al. (2016) argue for a "rapid single dispersal of all non-Africans less than 55,000 years ago."

Genetic reconstruction

Mitochondrial haplogroups

Within Africa

Map of early diversification of modern humans according to mitochondrial population genetics (see: Haplogroup L).

The first lineage to branch off from Mitochondrial Eve is L0. This haplogroup is found in high proportions among the San of Southern Africa and the Sandawe of East Africa. It is also found among the Mbuti people.^[48][49] These groups branched off early in human history and have remained relatively genetically isolated since then. Haplogroups L1, L2 and L3 are descendants of L1-6 and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descend from L3. L3 is about 84,000 years old and haplogroup M and N are about 63,000 years old.^[14] The relationship between such gene trees and demographic history is still debated when applied to dispersals.^[50]

Of all the lineages present in Africa, only the female descendants of one lineage, mtDNA haplogroup L3, are found outside Africa. If there had been several migrations, one would expect descendants of more than one lineage to be found. L3's female descendants, the M and N haplogroup lineages, are found in very low frequencies in Africa (although haplogroup M1 populations are very ancient and diversified in North and North-east Africa) and appear to be more recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and became the dominant haplogroups after the departure through the founder effect. Alternatively, the mutations may have arisen shortly afterwards.

Southern Route and haplogroups M and N

Results from mtDNA collected from aboriginal Malaysians called Orang Asli and the creation of a phylogentic tree indicate that the hapologroups M and N share characteristics with original African groups from approximately 85,000 years ago and share characteristics with sub-haplogroups among coastal south-east Asian regions, such as Australasia, the Indian subcontinent and throughout continental Asia, which had dispersed and separated from its African origins approximately 65,000 years ago. This southern coastal dispersion would have occurred before the dispersion through the Levant approximately 45,000 years ago.^[14] This hypothesis attempts to explain why haplogroup N is predominant in Europe and why haplogroup M is absent in Europe. Evidence of the coastal migration is thought to have been destroyed by the rise in sea levels during the Holocene epoch.^[51] Alternatively, a small European founder population that had expressed haplogroup M and N at first, could have lost haplogroup M through random genetic drift resulting from a bottleneck (i.e. a founder effect).

The group that crossed the Red Sea travelled along the coastal route around Arabia and Persia until reaching India.^[34] Haplogroup M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred.^[34] Sixty percent of the Indian population belong to Haplogroup M. The indigenous people of the Andaman Islands also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from the mainland. They are evidence of the coastal route of early settlers that extends from India to Thailand and Indonesia all the way to Papua New Guinea. Since M is found in high frequencies in highlanders from New Guinea and the Andamanese and New Guineans have dark skin and Afro-textured hair, some scientists think they are all part of the same wave of migrants who departed across the Red Sea ~60,000 years ago in the Great Coastal Migration. The proportion of haplogroup M increases eastwards from Arabia to India; in eastern India, M outnumbers N by a ratio of 3:1. Crossing into Southeast Asia, haplogroup N (mostly in the form of derivatives of its R subclade) reappears as the predominant lineage.^{[citation needed]} M is predominant in East Asia, but amongst Indigenous Australians, N is the more common lineage.^{[citation needed]} This haphazard distribution of Haplogroup N from Europe to Australia can be explained by founder effects and population bottlenecks.^[52]

Autosomal DNA

A 2002 study of African, European and Asian populations, found greater genetic diversity among Africans than among Eurasians, and that genetic diversity among Eurasians is largely a subset of that among Africans, supporting the out of Africa model.^[53] A large study by Coop et al. (2009) found evidence for natural selection in autosomal DNA outside of Africa. The study distinguishes non-African sweeps (notably KITLG variants associated with skin color), West-Eurasian sweeps (SLC24A5) and East-Asian sweeps (MC1R, relevant to skin color). Based on this evidence, the study concluded that human populations encountered novel selective pressures as they expanded out of Africa.^[54] MC1R and its relation to skin color had already been discussed by Liu, Harding et al. (2000), p. 135. According to this study, Papua New Guineans continued to be exposed to selection for dark skin color so that, although these groups are distinct from Africans in other places, the allele for dark skin color shared by contemporary Africans, Andamanese and New Guineans is an archaism. Endicott et al. (2003) suggest convergent evolution. A 2014 study by Gurdasani et al. indicate that higher genetic diversity in Africa was caused by relatively recent Eurasian migrations into Africa.^[55]

Pathogen DNA

Another promising route towards reconstructing human genetic genealogy is via the JC virus (JCV), a type of human polyomavirus which is carried by 70–90 percent of humans and which is usually transmitted vertically, from parents to offspring, suggesting codivergence with human populations. For this reason, JCV has been used as a genetic marker for human evolution and migration.^[56] This method does not appear to be reliable for the migration out of Africa, in contrast to human genetics, JCV strains associated with African populations are not basal. From this Shackelton et al. (2006) conclude that either a basal African strain of JCV has become extinct or that the original infection with JCV post-dates the migration from Africa.

Admixture of archaic and modern humans

Evidence for archaic human species (descended from Homo heidelbergensis) having interbred with modern humans outside of Africa, was discovered in the 2010s. This concerns primarily Neanderthal admixture in all modern populations except for Sub-Saharan Africans but evidence has also been presented for Denisova hominin admixture in Australasia (i.e. in Melanesians, Aboriginal Australians and some Negritos).^[57]

The rate of admixture of Neanderthal admixture to European and Asian populations as of 2017 has been estimated at between about 2%–3%.^[58]

Archaic admixture in some Sub-Saharan African populations hunter-gatherer groups (Biaka Pygmies and San), derived from archaic hominins that broke away from the modern human lineage around 700,000 years, was discovered in 2011. The rate of admixture was estimated at around 2%.^[59] Admixture from archaic hominins of still earlier divergence times, estimated at 1.2 to 1.3 million years ago, was found in Pygmies, Hadza and five Sandawe in 2012.^[60][61] Archaic admixture in West African agricultural populations (Mende and Yoruba) was found in 2017.^[62][63]

Stone tools

In addition to genetic analysis, Petraglia et al. also examines the small stone tools (microlithic materials) from Indian subcontinent and explains the expansion of population based on the reconstruction of paleoenvironment. He proposed that the stone tools could be dated to 35 ka in South Asia, and the new technology might be influenced by environmental change and population pressure.^[64]

History of the theory

Classical paleoanthropology

The frontispiece to Huxley's Evidence as to Man's Place in Nature (1863): the image compares the skeleton of a human to other apes.

The cladistic relationship of humans with the African apes was suggested by Charles Darwin after studying the behaviour of African apes, one of which was displayed at the London Zoo.^[65] The anatomist Thomas Huxley had also supported the hypothesis and suggested that African apes have a close evolutionary relationship with humans.^[66] These views were opposed by the German biologist Ernst Haeckel, who was a proponent of the Out of Asia theory. Haeckel argued that humans were more closely related to the primates of South-east Asia and rejected Darwin's African hypothesis.^[67][68]

In the Descent of Man, Darwin speculated that humans had descended from apes, which still had small brains but walked upright, freeing their hands for uses which favoured intelligence; he thought such apes were African:

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates, existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.

— Charles Darwin, Descent of Man^[69]

In 1871 there were hardly any human fossils of ancient hominins available. Almost fifty years later, Darwin's speculation was supported when anthropologists began finding fossils of ancient small-brained hominins in several areas of Africa (list of hominina fossils). The hypothesis of recent (as opposed to archaic) African origin developed in the 20th century. The "Recent African origin" of modern humans means "single origin" (monogenism) and has been used in various contexts as an antonym to polygenism. The debate in anthropology had swung in favour of monogenism by the mid-20th century. Isolated proponents of polygenism held forth in the mid-20th century, such as Carleton Coon, who thought as late as 1962 that H. sapiens arose five times from H. erectus in five places.^[70]

Multiregional origin hypothesis

The historical alternative to the recent origin model is the multiregional origin of modern humans, initially proposed by Milford Wolpoff in the 1980s. This view proposes that the derivation of anatomically modern human populations from H. erectus at the beginning of the Pleistocene 1.8 million years BP, has taken place within a continuous world population. The hypothesis necessarily rejects the assumption of an infertility barrier between ancient Eurasian and African populations of Homo. The hypothesis was controversially debated during the late 1980s and the 1990s.^[71] The now-current terminology of "recent-origin" and "Out of Africa" became current in the context of this debate in the 1990s.^[72] Originally seen as an antithetical alternative to the recent origin model, the multiregional hypothesis in its original "strong" form is obsolete, while its various modified weaker variants have become variants of a view of "recent origin" combined with archaic admixture.^[73] Stringer (2014) distinguishes the original or "classic" Multiregional model as having existed from 1984 (its formulation) until 2003, to a "weak" post-2003 variant that has "shifted close to that of the Assimilation Model".^[74][75]

Genetics

In the 1980s, Allan Wilson together with Rebecca L. Cann and Mark Stoneking worked on genetic dating of the matrilineal most recent common ancestor of modern human populations (dubbed "Mitochondrial Eve"). To identify informative genetic markers for tracking human evolutionary history, Wilson concentrated on mitochondrial DNA (mtDNA), passed from mother to child. This DNA material mutates quickly, making it easy to plot changes over relatively short times. With his discovery that human mtDNA is genetically much less diverse than chimpanzee mtDNA, Wilson concluded that modern human populations had diverged recently from a single population while older human species such as Neanderthals and Homo erectus had become extinct.^[76] With the advent of archaeogenetics in the 1990s, the dating of mitochondrial and Y-chromosomal haplogroups became possible with some confidence. By 1999, estimates ranged around 150,000 years for the mt-MRCA and 60,000 to 70,000 years for the migration out of Africa.^[77]

From 2000–2003, there was controversy about the mitochondrial DNA of "Mungo Man 3" (LM3) and its possible bearing on the multiregional hypothesis. LM3 was found to have more than the expected number of sequence differences when compared to modern human DNA (CRS).^[78] Comparison of the mitochondrial DNA with that of ancient and modern aborigines, led to the conclusion that Mungo Man fell outside the range of genetic variation seen in Aboriginal Australians and was used to support the multiregional origin hypothesis. A reanalysis on LM3 and other ancient specimens from the area published in 2016, showed it to be akin to modern Aboriginal Australian sequences, inconsistent with the results of the earlier study.^[79]