Archaic human admixture with modern humans
It has been asserted that, over the course of human history, there have been several instances of archaic human admixture with modern humans through interbreeding of modern humans with Neanderthals, Denisovans, and/or possibly other hominins. Neanderthal-derived DNA accounts for an estimated 1–4% of the Eurasian genome, but it is absent within the Sub-Saharan African genome. In Oceanian and Southeast Asian populations, there's a relative increase of Denisovan-derived DNA. An estimated 4–6% of the Melanesian genome is derived from Denisovans. Recent non-comparative DNA analyses—as there has been no specimens discovered—suggest that African populations have a genetic contribution from a now-extinct archaic African population.
Other studies carried out since the sequencing of the Neanderthal genome have cast doubt on the level of admixture between Neanderthals and modern humans, or even as to whether the species interbred at all. One study has asserted that the presence of Neanderthal or other archaic human genetic markers can be attributed to shared ancestral traits between the species originating from a 500,000-year-old common ancestor.
Through whole-genome sequencing, a 2010 draft sequence of the Neanderthal genome revealed that Neanderthals shared more alleles with Eurasian populations (e.g. French, Han Chinese, and Papua New Guinean) than with Sub-Saharan African populations (e.g. Yoruba and San). According to the study, the observed excess of genetic similarity is best explained by recent gene flow from Neanderthals to modern humans after the migration out of Africa. The proportion of Neanderthal-derived ancestry was estimated to be 1–4% of the Eurasian genome. In 2013, the same team of researchers revised the proportion to an estimated 1.5–2.1%. They also found that the Neanderthal component in non-African modern humans was more related to the Mezmaiskaya Neanderthal (Caucasus) than to the Altai Neanderthal (Siberia) or the Vindija Neanderthals (Croatia).
Although less parsimonious than recent gene flow, the observation may have been due to ancient population sub-structure in Africa, causing incomplete genetic homogenization within modern humans when Neanderthals diverged while early ancestors of Eurasians were still more closely related to Neanderthals than those of Africans to Neanderthals. On the basis of allele frequency spectrum, it was shown that the recent admixture model had the best fit to the results while the ancient population sub-structure model had no fit, thus confirming recent admixture as the most parsimonious and plausible explanation for the observed excess of genetic similarities between modern non-African humans and Neanderthals. This was likewise confirmed by data on the basis of linkage disequilibrium. More specifically, through the allele frequency spectrum, it was suggested that the best model was a recent admixture event that was preceded by a bottleneck event among modern humans.
No evidence of Neanderthal mitochondrial DNA has been found in modern humans. This would suggest that successful admixture with Neanderthals happened paternally rather than maternally on the side of Neanderthals. Possible hypotheses are that Neanderthal mtDNA had detrimental mutations that led to the extinction of carriers, that the hybrid offspring of Neanderthal mothers were raised in Neanderthal groups and became extinct with them, or that female Neanderthals and male modern humans did not produce fertile offspring.
Recent studies have shown a higher Neanderthal admixture in East Asians when compared to Europeans. It indicated that most-likely at least two independent events of gene flow must have taken place into early modern humans and that the early ancestors of East Asians experienced more admixture than those of Europeans after the divergence of the two groups. This is estimated to have been an additional 20.2% (95CI of 13.4–27.1%) of Neanderthal admixture in a second gene flow to East Asians. It is also possible, but less likely, that the difference was caused by dilution in Europeans by later migrations out of Africa. It may also be due to a lower negative selection in East Asians compared to Europeans. It has also been observed that there's a small but significant variation of Neanderthal admixture rates within European populations, but no significant variation within East Asian populations.
Through the extend of linkage disequilibrium, it was estimated that the last Neanderthal gene flow into early ancestors of Europeans occurred 47,000–65,000 years BP (conservatively 37,000–86,000 years BP). In conjunction with archaeological and fossil evidence, the gene flow is thought to likely have occurred somewhere in Western Eurasia, possibly the Middle East.
A 2012 study found that North Africans have a Neanderthal admixture rate lying between that of Eurasians (highest) and Sub-Saharan Africans (lowest). It has also shown a great variation within North Africans itself, depending primarily on the amount of Eurasian versus sub-Saharan African ancestry. However, there are indications that their Neanderthal admixture is not solely contributed by Eurasian introgression.
Studies found that there are large genomic regions with strongly-reduced Neanderthal contribution present in modern humans due to negative selection, partly caused by hybrid male infertility. These large regions of low Neanderthal contribution were most-pronounced on the X chromosome (with fivefold lower Neanderthal ancestry compared to autosomes) and contained relatively high numbers of genes specific to testes, meaning that modern humans have relatively few Neanderthal genes that are located on the X chromosome or expressed in the testes, consistent with the known fact that male infertility is affected by a disproportional large amount of genes on X chromosomes. It has also been shown that Neanderthal ancestry has been selected against in conserved biological pathways, such as RNA processing.
Genes affecting keratin were found to have been introgressed from Neanderthals into modern humans (shown in East Asians and Europeans), suggesting that these genes gave a morphological adaptation in skin and hair to modern humans to cope with non-African environments. This is likewise for several genes involved in medical-relevant phenotypes, such as those affecting systemic lupus erythematosus, primary biliary cirrhosis, Crohn's disease, optic disk size, smoking behavior, interleukin 18 levels, and diabetes mellitus type 2.
In a 2013 study, researchers found Neanderthal introgression of 18 genes (several of which are related to UV-light adaptation) within the chromosome 3p21.31 region (HYAL region) of East Asians. The introgressive haplotypes were positively selected in only East Asian populations, rising steadily from 45,000 years BP until a sudden increase of growth rate around 5,000 to 3,500 years BP. They occur at very high frequencies among East Asian populations in contrast to other Eurasian populations (e.g. European and South Asian populations). The findings also suggests that this Neanderthal introgression occurred within the ancestral population shared by East Asians and Native Americans.
Data from 2005 had previously shown that the D allele of microcephalin, a critical regulatory gene for brain volume, originated from an archaic human population. The results show that haplogroup D introgressed 37,000 years ago (based on the coalescence age of derived D alleles) into modern humans from an archaic human population that separated 1,1 million years ago (based on the separation time between D and non-D alleles), consistent with the period when Neanderthals and modern humans co-existed and diverged respectively. The high frequency of the D allele (70%) suggest that it was positively selected for in modern humans. The distribution of the D allele of microcephalin is high outside Africa but low in sub-Saharan Africa, which further suggest that the admixture event happened in archaic Eurasian populations. This distribution difference between Africa and Eurasia suggests that the D allele originated from Neanderthals. However, a 2010 study found that a Neanderthal individual from the Mezzena Rockshelter (Monti Lessini, Italy) was homozygous for an ancestral allele of microcephalin, thus providing no support to the theory that Neanderthals contributed the D allele to modern humans but does not exclude a possibility of a Neanderthal origin of the D allele. The 2010 Neanderthal genome study also could not confirm a Neanderthal origin of haplogroup D of the microcephalin gene.
There is evidence that some immune related genes are of Neanderthal origin. HLA-C*0702, found in Neanderthals, is common in modern Europeans and Asians but rarely seen in Africans. It is thought that this immune gene may have been picked up by humans after leaving Africa to help deal with European diseases that the Neanderthals had evolved defenses for.
According to a 1999 study, the early Upper Paleolithic burial remains of a modern human child from Abrigo do Lagar Velho (Portugal) featured traits indicating Neanderthal admixtures with modern humans dispersing into Iberia. Considering the dating of the burial remains (24,500 years BP) and the persistence of Neanderthal traits long after the transitional period from a Neanderthal to a modern human population in Iberia (28,000–30,000 years BP), it was suggested that the child may have been a descendant of an already heavily-admixed population.
In a 2006 study, researchers found that the early Upper Paleolithic modern human remains from Peştera Muierilor (Romania) of 35,000 years BP have the morphological pattern of European early modern humans, but possesses archaic and/or Neanderthal features, suggesting European early modern humans' admixture with rather than a full replacement of Neanderthals. These features include a large interorbital breadth, a relatively flat superciliary arches, a prominent occipital bun, an asymmetrical and shallow mandibular notch shape, a high mandibular coronoid processus, the relative perpendicular mandibular condyle to notch crest position, and a narrow scapular glenoid fossa.
The paleontological analysis of modern-human emergence in Europe has been shifting from considerations of the Neanderthals to assessments of the biology and chronology of the earliest modern humans in western Eurasia. This focus, involving early anatomically modern humans, suggest that they represent a variable mosaic of derived modern human, archaic human, and Neanderthal features.
In March 2013, new data from the late-Neanderthal jaw from the Mezzena rockshelter (Monti Lessini, Italy) indicated possible interbreeding in late Italian Neanderthals. The jaw (more specifically, a corpus mandibulae remnant) falls within the morphological range of modern humans, but also displayed strong similarities with some of the other Neanderthal specimens, indicating a change in late Neanderthal morphology due to possibly interbreeding with modern humans.
The hypothesis, variously under the names of interbreeding, hybridization, admixture or hybrid-origin theory, has been discussed ever since the discovery of Neanderthal remains in the 19th century, though earlier writers believed that Neanderthals were a direct ancestor of modern humans. Thomas Huxley suggested that many Europeans bore traces of Neanderthal ancestry, but associated Neanderthal characteristics with primitivism, writing that since they "belong to a stage in the development of the human species, antecedent to the differentiation of any of the existing races, we may expect to find them in the lowest of these races, all over the world, and in the early stages of all races".
Hans Peder Steensby in the 1907 article Racestudier i Danmark ("Race studies in Denmark") rejected that Neanderthals were ape-like or inferior, and, while emphasizing that all modern humans are of mixed origins, suggested interbreeding as the best available explanation of a significant number of observations which by then were available.
In the early twentieth century, Carleton Coon argued that the Caucasoid race is of dual origin consisting of Upper Paleolithic (mixture of H. sapiens and H. neanderthalensis) types and Mediterranean (purely H. sapiens) types. He repeated his theory in his 1962 book The Origin of Races.
Stan Gooch in Personality and Evolution (1973) and The Neanderthal Question (1977) develops a theory of Neanderthal/Cro-Magnon hybridization, based not on an examination of anatomy but of his understanding of modern human psychology and society, which he claimed owes a significant debt to Neanderthal culture. Gooch's theories were dismissed by academia. Gooch refined his theory in Cities of Dreams (1989) and The Neanderthal Legacy (2008).
A 2010 study has shown that Melanesians (e.g. Papua New Guinean and Bougainville Islander) share relative more alleles with Denisovans when compared to other of the studied Eurasians and Africans. It estimated that 4% to 6% of the genome in Melanesians derives from Denisovans, while no other Eurasians or Africans displayed contributions of the Denisovan genes. It has been observed that Denisovans contributed genes to Melanesians but not to East Asians, indicating that there was interaction between the early ancestors of Melanesians with Denisovans but that this interaction did not take place in the regions near southern Siberia, where as-of-yet the only Denisovan remains have been found. In addition, a 2011 study has also shown a relative increased allele sharing between Denisovans and Aboriginal Australians, compared to other Eurasians and African populations, consistent with relative high admixture between early ancestors of Melanesians and Denisovans.
In 2011, a study produced evidence that the highest presence of Denisovan admixture is in Oceanian populations, followed by many Southeast Asian populations, and none in East Asian populations. There is significant Denisovan genetic material in eastern Southeast Asian and Oceanian populations (e.g. Aboriginal Australians, Near Oceanians, Polynesians, Fijians, eastern Indonesians, Philippine Mamanwa and Manobo), but not in certain western and continental Southeast populations (e.g. western Indonesians, Malaysian Jehai, Andaman Onge, and mainland Asians), indicating that the Denisovan admixture event happened in Southeast Asia itself rather than mainland Eurasia. The observation of high Denisovan admixture in Oceania and the lack thereof in mainland Asia makes it likely that early modern humans and Denisovans interbred east of the Wallace Line that divides Southeast Asia.
However, in contrast to the previous results, more-recent research found indications that mainland Asian and Native American populations had 0.2% Denisovan contribution, albeit twenty-five fold lower than Oceanian populations. The manner of gene flow to these populations is currently unknown. After Oceanians, it has been observed that particularly Southeast Asians in general have affinity to Denisovans.
Findings indicate that the Denisovan gene flow event happened to the common ancestors of Aboriginal Philippines, Aboriginal Australians, and New Guineans. New Guineans and Australians have similar rates of Denisovan admixture, indicating that interbreeding with their common ancestors happened prior to their entry into Sahul (Pleistocene New Guinea and Australia), at least 44,000 years ago. It has also been observed that the fraction of Near Oceanian ancestry in Southeast Asians is proportional to the Denisovan admixture, except in the Philippines where there is a higher proportional Denisovan admixture to Near Oceanian ancestry. Reich et al. (2010) gave a possible explanatory model that suggest an early eastward migration wave of modern humans, some who were Philippine/New Guinean/Australian common ancestors that interbred with Denisovans, respectively followed by (1) divergence of the Philippine early ancestors, (2) interbreeding between the New Guinean and Australian early ancestors with a part of the same early-migration population that did not experience Denisovan gene flow, and (3) interbreeding between the Philippine early ancestors with a part of the population from a much-later eastward migration wave (the other part of which would become East Asians).
It has been shown that Eurasians have some but significant lesser archaic-derived genetic material that overlaps with Denisovans, stemming from the fact that Denisovans are related to Neanderthals—who contributed to the Eurasian gene pool—rather than from interbreeding of Denisovans with the early ancestors of those Eurasians.
The skeletal remains of an early modern human from the Tianyuan cave (near Zhoukoudian, China) of 40,000 years BP showed a Neanderthal contribution within the range of today's Eurasian modern humans, but it had no discernible Denisovan contribution. It is ancestral to many Asian and Native American populations, but post-dated the divergence between Asians and Europeans. The lack of a Denisovan component in the Tianyuan individual suggests that the genetic contribution had been always scarce in the mainland.
The immune system's HLA alleles have drawn particular attention in the attempt to identify genes that may derive from archaic human populations. Although not present in the sequenced Denisovan genome, the distribution pattern and divergence of HLA-B*73 from other HLA alleles has led to the suggestion that it introgressed from Denisovans into humans in west Asia. Indeed, half of the HLA alleles of modern Eurasians represent archaic HLA haplotypes, and have been inferred to be of Denisovan or Neanderthal origin. The apparent over-representation of these alleles suggests a positive selective pressure for their retention in the human population.
Archaic African hominins
Rapid decay of fossils in African environments have made it currently unfeasible to compare modern human admixture with reference samples of archaic African humans.
In 2011, after finding three candidate regions with introgression by searching for unusual patterns of variations—indicating a different origin—in 61 non-coding regions from two hunter-gatherers (Biaka Pygmies and San, shown significant for admixture in the data) and one West African agricultural group (Madinka, shown not significant for admixture in the data), researchers concluded that roughly 2% of the genetic material found in Sub-Saharan African populations was inserted into the human genome approximately 35,000 years ago from archaic hominins that broke away from the modern human lineage around 700,000 years ago. After a survey for the introgressive haplotypes across Sub-Saharan populations, it was suggested that the admixture event happened with archaic hominins that possibly once inhabited Central Africa.
In 2012, researchers studied high-coverage whole-genome sequences of fifteen Sub-Saharan hunter-gatherer males from three groups—five Pygmies (three Baka, a Bedzan, and a Bakola) from Cameroon, five Hadza from Tanzania, and five Sandawe from Tanzania—finding signs that the ancestors of the hunter-gatherers interbred with one or more archaic human populations, probably over 40,000 years ago. They also found that the median time of the most recent common ancestor of the fifteen test subjects with the putative introgressive haplotypes was 1.2–1.3 mya.
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