Reich is a professor in the Department of Genetics at Harvard Medical School. This is a book intended to educate the layman on the recent technological advances in genetics. Specifically, it details how the mapping of the complete human genome, along with subsequent tests on samples from prehistoric and modern humans in the last decade, is changing the received wisdom about patterns of migration and inter-breeding of different populations from 70,000 to 1,000 years ago, previously based on techniques from archeology, linguistics, skeleton morphology, and anthropology.
The first five examples of complete ancient human genomes were only published in 2010- a few archaic Neanderthal genomes, an archaic Denisova genome, and a four-thousand-year-old individual from Greenland. Since then, hundreds more samples of genome-wide data have been analyzed, studied, and published. This was a vast improvement on previous techniques that only analyzed small stretches of the genome, such as mitochondrial DNA. “The genome is written out in twin chains of about three billion chemical building blocks…. What we call a gene consists of tiny fragments of these chains, typically around one thousand letters long.” By analyzing the complete genome, scientists have been able to divine much more detailed information about humanity’s collective past, historical migration patterns, and when human lineages broke off from one another and perhaps came back to co-mingle. “The most recent African ancestor of all the branches [of modern humans], “Mitochondrial Eve,” lived sometime after 200,000 years ago. The best current estimate is around 160,000 years ago.”
In this book, Reich first analyzes the relationship between modern humans and Neanderthals. “When we tested diverse present-day human populations, we found Neanderthals to be about equally close to Europeans, East Asians, and New Guineans, but closer to all non-Africans than to all sub-Saharan Africans…. We found that non-African genomes today are around 1.5 to 2.1 percent Neanderthal in origin.” This Neanderthal DNA probably mixed with modern human DNA somewhere between eighty-six thousand to thirty-seven thousand years ago, so we can be certain at least some Homo Sapiens and some Neanderthals were having children together at that point in history. But that was not all. Homo Sapiens were also inter-breeding with Denisovans. “We eventually estimated the separation between the Neanderthal and Denisovan ancestral populations to have occurred 470,000 to 380,000 years ago, and the separation between the common ancestral populations of both of these archaic groups and modern humans to have occurred 770,000 to 550,000 years ago.” From that point on, however, there was still inter-breeding between all these human sub-groups. “Interbreeding between Denisovan and New Guinean ancestors occurred fifty-nine to forty-five thousand years ago…. We estimated that about 3 to 6 percent of New Guinean ancestry derives from Denisovans…. The Denisovan-related ancestry in East Asians is about a twenty-fifth of that seen in New Guineans- it comprises about 0.2 percent of East Asians’ genomes, rising to up to 0.3-0.6 percent in parts of South Asia.”
Reich goes on to detail the recent discovery of a ghost population, no longer are alive in pure form today, but who have passed on parts of their genome to present-day humans. These are the Ancient North Eurasians. “There was a population living in northern Eurasia that was not the primary ancestral population of the present-day inhabitants of the region. Some people from this population migrated east across Siberia and contributed to the population that crossed the Bering land bridge and gave rise to Native Americans. Others migrated west and contributed to Europeans…. All told, more than half of the world’s population derives between 5 percent and 40 percent of their genomes from the Ancient North Eurasians.” Reich describes the process of migration (and the breeding that went along with it) as more akin to a trellis, than the more common tree branching metaphor. These populations separated and then often recombined as they moved about, traded with or invaded others, and relocated around the globe. In fact, the Mal’ta genome (a boy found in south-central Siberia, who lived about twenty-four thousand years ago) has strong genetic affinity to modern Europeans and Native Americans, but has little resemblance to modern-day Siberians. “The analysis of the Mal’ta genome made it clear that Native Americans derive about a third of their ancestry from the Ancient North Eurasians, and the remainder from East Asians. It is this major mixture that explains why Europeans are genetically closer to Native Americans than they are to East Asians.”
In Europe, further ghost populations were found in West Eurasia. “About ten thousand years ago there were at least four major populations in West Eurasia- the farmers of the Fertile Crescent, the farmers of Iran, the hunter-gatherers of central and western Europe, and the hunter-gatherers of eastern Europe. All these populations differed from one another as mush as Europeans differ from East Asians today.” The Bronze Age was the time when these distinct populations mixed and homogenized, perhaps as the result of technological and cultural integration as well. Furthermore, “five thousand years ago, the people who are now the primary ancestors of all extant northern Europeans had not yet arrived.” The Yamnaya were sheep and cattle herders who originally came from the Steppe. They promulgated, if not invented, the wheel and domesticated the horse. “In Germany, people buried with Corded Ware pots derive about three-quarters of their ancestry from groups related to the Yamnaya and the rest from people related to the farmers who had been the previous inhabitants of that region.” It is also likely that the Yamnaya were the ones who spread the origins of all Indo-European languages as they spread their genes, technology, and culture across Europe.
In India, Reich found the people today “are the outcome of mixtures between two highly differentiated populations, “Ancestral North Indians” (ANI) and “Ancestral South Indians” (ASI), who before their mixture were as different from each other as Europeans and East Asians today. The ANI are related to Europeans, central Asians, Near Easterners, and people of the Caucasus, but we made no claim about the location of their homeland or any migrations. The ASI descend from a population not related to any present-day populations outside India. We showed that the ANI and ASI had mixed dramatically in India. The result is that everyone in mainland India today is a mix, albeit in different proportions, of ancestry related to West Eurasians, and ancestry more closely related to diverse East Asian and South Asian populations. No group in India can claim genetic purity…. We found that West Eurasian-related mixture in India ranges from as low as 20 percent to as high as 80 percent.” This cut across all regions and castes, although the higher castes and males both tended to have a higher proportion of ANI ancestry. The Andamanese were the lone population with zero European genes at all, likely because of their secluded island location. However, within sub-castes in India, Reich did find long strings of identical genetic code, suggesting that “long-term endogamy as embodied in India today in the institution of caste has been overwhelmingly important for millennia.”
Reich’s analysis also provided revelations for the migration patterns of humans into the Americas. “There were at least two migrations that left a human legacy as far as South America and at least two whose impact was limited to northern North America.” These paths split more like tree branches than trellises, with little remixing after the initial separations. “The splits proceeded roughly in a north-to-south direction, consistent with the idea that as populations traveled south, groups peeled off and settled, remaining in approximately the same place ever since.” In the Americas, population displacement was not common. However, one unusual pattern was that “a sublineage of First Americans that originated well after the initial diversification of First American lineages in North America migrated back to Asia.” Migrations patterns are often not obvious, but the genetic codes do not lie.
Reich next moves on to East Asia, which “has been home to the human family for at least around 1.7 million years, the date of the oldest known Homo erectus skeleton found in China…. In Australia, archaeological evidence of human campsites make it clear that modern humans arrived there at least by about forty-seven thousand years ago, which is about as old as the earliest evidence for modern humans in Europe…. Chinese and Australians derive almost all their ancestry from a homogenous population whose ancestors separated earlier from the ancestors of Europeans.” As in West Eurasia, East Asian modern humans soon out-competed archaic humans and completely replaced them in every region that they migrated to. Today’s East Asians derive their ancestry from one of two ghost populations, one starting out around the Yangtze River Basin and the other from around the Yellow River. The Yellow River Ghost Population largely spread west, ending at the Tibetan plateau, while the Yangtze Ghost Population spread to southeast Asia and Taiwan. Often new genetic information in combination with traditional archaeology can yield revelations. For instance, in Japan “around twenty-three hundred years ago, mainland-derived agriculture began to be practiced and was associated with an archeological culture with clear similarities to contemporary cultures on the Korean peninsula…. Present day Japanese have about 80 percent farmer and 20 percent hunter-gatherer ancestry.” That genetic mix yields an approximate date of inter-breeding of sixteen hundred years ago. That means for hundreds of years, the two populations both lived on the islands of Japan, but with relatively little inter-breeding, until social segregation broke down, interbreeding began, and homogenization occurred.
As for Africa, “African genome sequences are typically about a third more diverse than non-African ones.” Most migration patterns moved in a north-to-south direction. “There is little if any sub-Saharan African related ancestry in ancient Near Easterners or Egyptians prior to medieval times.” Surprisingly, Ethiopian “caste” groupings have proven to have gone back generations further than even those in India. The modern Ari are the oldest example of strong endogamy that Reich has thus far come across, persisting at least forty-five hundred years. Another ghost population were the East African Foragers. “We also found that the East African Foragers were more closely related to non-Africans today than they were to any other groups in sub-Saharan Africa. The close relationship to non-Africans suggests that the ancestors of the East African Foragers may have been the population in which the Middle to Late Stone Age transition occurred, propelling expansions outside of Africa and possibly within Africa too after around fifty thousand years ago.”
Finally, Reich concludes his book with controversies associated with these genetic breakthroughs. Race may be a social construct, but it is still a touchy cultural subject. Racial variations are existent, if not determining. “Around 85 percent of variation in the protein types could be accounted for by variation within populations and “races,” and only 15 percent by variation across them.” Inter-breeding was also not propagated randomly. Cultural factors such as inequality played a role in whose genes were passed on. “The contribution of European American men to the genetic makeup of the present-day African American population is about four times that of European American women.” These numbers are even more skewed for the populations of South and Central America for obvious historical reasons. Somewhat less controversially, “8 percent of males in the lands that the Mongol Empire once occupied share a characteristic Y-chromosome sequence or one differing from it by just a few mutations.” The dating of such a “star-cluster” founder, estimated through the rate of the accumulation of mutations on the Y-chromosome, can be placed to between thirteen hundred and seven hundred years ago and then history can guess that Genghis Khan would be the best possible human match to fit such a profile. Finally, as of today the DNA revolution has been dominated by the testing of Europeans, both modern and archaic. “Of 551 published samples with genome-wide ancient DNA data as of late 2017, almost 90 percent are from West Eurasia.” That is because most state of the art techniques and labs were first developed in Europe, but also because DNA from warmer climates is more likely to degrade and harder to extract and many countries, such as India and China, limit the exportation of DNA material outside their borders. Scientists are still in the early days of this genetics revolution. Reich compares this technological breakthrough to the inventions of the microscope and of carbon dating in scientific significance. Genome-wide mapping is a burgeoning field and the future growth in new techniques is only going to expand the scope and scale of the DNA samples available for analysis.
No comments:
Post a Comment