Reich's genetics research focuses primarily on finding complex genetic patterns that cause susceptibility to common diseases among large populations, rather than looking for specific genetic markers associated with relatively rare illnesses.[citation needed]
Reich's research team at Harvard University has produced evidence that, over a span of at least four million years, various parts of the human genome diverged gradually from those of chimpanzees.[10] The split between the human and chimpanzee lineages may have occurred millions of years later than fossilized bones suggest, and the break may not have been as clean as previously thought. The genetic evidence developed by Reich's team suggests that after the two species initially separated, they may have continued interbreeding for several million years.
A final genetic split transpired between 6.3 million and 5.4 million years ago.[11]
Reich's 2009 paper Reconstructing Indian population history[12] was a landmark study in the research on India's genepool and the origins of its population. Reich et al. (2009), in a collaborative effort between the Harvard Medical School and the Indian Centre for Cellular and Molecular Biology (CCMB), examined the entire genomes worth 560,000 single-nucleotide polymorphisms (SNPs), as compared to 420 SNPs in prior work. They also cross-compared them with the genomes of other regions available in the global genome database.[13] Through this study, they were able to discern two genetic groups in the majority of populations in India, which they called "Ancestral North Indians" (ANI) and "Ancestral South Indians" (ASI).[note 1] They found that the ANI genes are close to those of Middle Easterners, Central Asians and Europeans whereas the ASI genes are dissimilar to all other known populations outside India.[note 2][note 3] These two distinct groups, which had split ca. 50,000 years ago, formed the basis for the present population of India.[14]
A follow-up study by Moorjani et al. (2013) revealed that the two groups mixed between 1,900 and 4,200 years ago (2200 BCE–100 CE), after which a shift to endogamy took place and admixture became rare.[note 4] Speaking to Fountain Ink, David Reich stated, "Prior to 4,200 years ago, there were unmixed groups in India. Sometime between 1,900 to 4,200 years ago, profound, pervasive convulsive mixture occurred, affecting every Indo-European and Dravidian group in India without exception." Reich pointed out that their work does not show that a substantial migration occurred during this time.[15]
Metspalu et al. (2011), representing a collaboration between the Estonian Biocenter and CCMB, confirmed that the Indian populations are characterized by two major ancestry components. One of them is spread at comparable frequency and haplotype diversity in populations of South and West Asia and the Caucasus. The second component is more restricted to South Asia and accounts for more than 50% of the ancestry in Indian populations. Haplotype diversity associated with these South Asian ancestry components is significantly higher than that of the components dominating the West Eurasian ancestry palette.[16]
Human genetic map (2011)
Reich was a co-leader, along with statistician Simon Myers, of a team of genetics researchers from Harvard University and the University of Oxford that made the most complete human genetic map then known in July 2011.[17]
Interbreeding of Neanderthals and humans (2010–2012)
Reich's research team significantly contributed to the discovery that Neanderthals and Denisovans interbred with modern human populations as they dispersed from Africa into Eurasia 70,000–30,000 years ago.[18]
Genetic markers for prostate cancer
Reich's lab received media attention following its discovery of a genetic marker which is linked to an increased likelihood of developing prostate cancer.[19] Reich has also argued that the higher incidence of prostate cancer among African Americans, compared to European Americans, appears to be largely genetic in origin.[20]
Indo-European origins
Reich has suggested that the Indo-European languages may have originated south of the Caucasus, in present day Iran or Armenia:
"Ancient DNA available from this time in Anatolia shows no evidence of steppe ancestry similar to that in the Yamnaya (although the evidence here is circumstantial as no ancient DNA from the Hittites themselves has yet been published). This suggests to me that the most likely location of the population that first spoke an Indo-European language was south of the Caucasus Mountains, perhaps in present-day Iran or Armenia, because ancient DNA from people who lived there matches what we would expect for a source population both for the Yamnaya and for ancient Anatolians. If this scenario is right the population sent one branch up into the steppe – mixing with steppe hunter-gatherers in a one-to-one ratio to become the Yamnaya as described earlier – and another to Anatolia to found the ancestors of people there who spoke languages such as Hittite."[20]
Eurasian back-migrations
Reich in 2018 demonstrated based on genetic evidence, West Asian geneflow into modern Afro-Asiatic speaking African populations could support a diffusion of these languages from the Middle East:
"New insights are already emerging from ancient DNA, which makes it possible to document ancient migrations between the Near East and North Africa that could have spread languages, culture, and crops. In 2016 and 2017, my laboratory published two papers showing that a shared feature of many East African groups, including ones that do not speak Afroasiatic languages, is that they harbor substantial ancestry from people related to farmers who lived in the Near East around ten thousand years ago. Our work also found strong evidence for a second wave of West Eurasian-related admixture this time with a contribution from Iranian-related farmers as might be expected from a spread from the Near East in the Bronze Age-and showed that this ancestry is widespread in present-day people from Somalia and Ethiopia who speak Afroasiatic languages in the Cushitic sub-family. So the genetic data provide evidence for at least two major waves of north-to-south population movement in the period when Afroasiatic languages were spreading and diversifying, and no evidence of south-to-north migration."[20]
Software tools
Reich has developed ADMIXTOOLS 2, an R software package primarily used for analyzing admixture, in collaboration with Nick Patterson.[21]
^Reich (2009): "We analyze 25 diverse groups to provide strong evidence for two ancient populations, genetically divergent, that are ancestral to most Indians today. One, the "Ancestral North Indians" (ANI), is genetically close to Middle Easterners, Central Asians, and Europeans, while the other, the "Ancestral South Indians" (ASI), is as distinct from ANI and East Asians as they are from each other."
^Moorjani et al. (2013): "Most Indian groups descend from a mixture of two genetically divergent populations: Ancestral North Indians (ANI) related to Central Asians, Middle Easterners, Caucasians, and Europeans; and Ancestral South Indians (ASI) not closely related to groups outside the subcontinent."
^Moorjani et al. (2013): "We report genome-wide data from 73 groups from the Indian subcontinent and analyze linkage disequilibrium to estimate ANI-ASI mixture dates ranging from about 1,900 to 4,200 years ago. In a subset of groups, 100% of the mixture is consistent with having occurred during this period. These results show that India experienced a demographic transformation several thousand years ago, from a region in which major population mixture was common to one in which mixture even between closely related groups became rare because of a shift to endogamy."
^ScienceNews.org – 'Hybrid-Driven Evolution: Genomes show complexity of human-chimp split: Not only did the evolutionary parting of human from chimpanzee ancestors occur more recently than had been indicated by previous data, but it also played out over an extended period during which forerunners of people and chimps interbred', Bruce Bower, Science News (May 20, 2006)
Metspalu, Mait; Romero, Irene Gallego; Yunusbayev, Bayazit; Chaubey, Gyaneshwer; Mallick, Chandana Basu; Hudjashov, Georgi; Nelis, Mari; Mägi, Reedik; Metspalu, Ene; Remm, Maido; Pitchappan, Ramasamy; Singh, Lalji; Thangaraj, Kumarasamy; Villems, Richard; Kivisild, Toomas (2011), "Shared and Unique Components of Human Population Structure and Genome-Wide Signals of Positive Selection in South Asia", The American Journal of Human Genetics, 89 (6): 731–744, doi:10.1016/j.ajhg.2011.11.010, ISSN0002-9297, PMC3234374, PMID22152676
Moorjani, P.; Thangaraj, K.; Patterson, N.; Lipson, M.; Loh, P. R.; Govindaraj, P.; Singh, L. (2013), "Genetic evidence for recent population mixture in India", The American Journal of Human Genetics, 93 (3): 422–438, doi:10.1016/j.ajhg.2013.07.006, PMC3769933, PMID23932107
