The stories of human evolution and migration have always fascinated me. I wrote a Blog post about the book “Early Indians” that described where my ancestors in India came from. Now I just finished reading a book that provided a lot of material for “Early Indians.” This one, titled “Who we are and how we got here: Ancient DNA and the new science of the human past,” written by David Reich, is a more scholarly work, difficult to digest sometimes, but it is full of surprises and mysteries.
The two breakthroughs applied to human history are the ability to sequence human genome effectively and efficiently, and doing it from bone fragments found in different parts of the world. Combining it with the traditional techniques; such as ability to date a specific archeological site (using Carbon dating), a vast database of anatomical measurements of parts of humans (such as skull), and advances in linguistics; allows us now to look back in an unprecedented way.
Here are some of the highlights of what the study of ancient DNAs have revealed thus far.
Encounters with ancient humans: An amazing fact is that we, modern humans, interbred with Neanderthals. We have plenty of jokes about Neanderthals, those cousins of modern humans, who had large heads but not very smart. They lived in Europe (Neanderthal is the location in Germany---Neander Lake—where they first bone fragments were found), and went extinct. However to discover that almost all of us (all except the Africans), have some Neanderthal DNA was a new discovery. There is not much of their DNA in us---the maximum is only 2%---but it is there.
Another branch of ancient humans, whose DNA we carry are Denisovans, named after Denisova Caves in Siberia, where a pinky bone belonging to one of them was found. They are cousins of Neanderthals, but some of us carry as much as 5% of their DNA. The funny part is that those with the highest concentration live in New Guinea and Australia, nowhere close to Siberia. This startling discovery led to figuring out that there were actually two types of Denisovans, Siberian type and Australian type. So, seventy thousand years back, world had four types of populations: modern humans, Neanderthals, Siberian-Denisovans, and Australo-Denisovans. (There actually is a fifth, small humans, nicknamed “hobbit,” which lived in Indonesia as long ago as seven hundred thousand years.)
What is interesting about such interbreeding is that the evolution of humans did not follow a tree like structure, starting from a trunk and branching out. It is more like a trellis, branching and remixing.
Humanity’s Ghosts: A prevailing theory until this new technology came about was that the ancestors of Native Americans were East Asians. They are the ones that crossed Bearing Sea when the sea levels were down due to an ice age. The new findings reveal that one third of the ancestry of Native Americans is North Eurasians, and the remaining East Asians. Interestingly, the same North Eurasian group mixed with ancestral Europeans to create North Europeans. So, the Native Americans and the Europeans share a common ancestry. How bizarre! Even more bizarre is that there was no such group identified before. So, it was a ghost group.
The mystery was solved when bones of a boy were found in south-central Siberia at a place called Mal’ta. This twenty four thousand year old genome was what one would expect that of the ghost population to be like. Mystery solved. However, there are additional ghost groups identified using the new technologies and they are yet to find their own Mal’ta boys.
Europeans: Europe of ancient times lived in many population centers during the hunter-gatherer phase and moved around. That much has been discerned by analyzing ancient DNAs. “We should not expect the people who lived in any one place in the past to be direct ancestors of those who live there today.” Driving their movements were natural phenomena such as a super volcano near the present day Naples in Italy (much before Vesuvius) and the glaciers created by an ice age that divided west and east of Europe. Relative homogeneity in the European population was brought about by domestication of plants and animals, which began between twelve and eleven thousand years ago in southern Turkey and northern Syria.
A major change in European people took place some five thousand years ago when people from steppes (a grass land that stretches eight thousand kilometers from central Europe to China) migrated west. These people belonged to Yamnaya culture that herded sheep and cattle. Such mass migration possible was made possible by a major innovation---that of wheel. Another innovation---horse---made cattle herding more efficient. Combining a wheeled wagon with an animal allowed the Yamnaya folks to travel.
This expansion also spread a major group of languages---the Indo-European Languages--- throughout Europe. One clue for the common root shared by most (not all) European languages is the words used for describing a wagon and its parts. Well, if these folks used wheels and wagons to move about, they had to have common words describing them.
Even Indian languages are a part of the family, as was discovered by judge serving in British India. A scholar of Greek and Latin, he learned Sanskrit, and as they say, the rest is history. So, the steppe pastoralists had broken through to India too! This is a great example of how linguistics works hand-in-hand with archeology and genetics to create accurate maps of human migration.
Even though a single genetically coherent group was responsible for spreading many of the Indo-European languages, this fact does not support the concept of “Aryan purity” promoted by Hitler. The Europeans, including the blue-eyed blond Germans are the result of mixture of many groups.
Indians: India is another place where the DNA based research confronted local views on why people in North India speak languages that are members of the Indo-European family. “It is simple,” they say. “India was the cradle of civilization and people speaking those languages migrated out of it to go to Europe.” This view, espoused primarily by folks in India who are promoting Hindu supremacy, is not consistent with the findings of research done by David Reich and his fellow researchers, as mentioned above. As it stands, the findings prove that one part of Indian population resulted from mixing of people from Central Asia, Europe, and Near East, while the other part, from East Asians (Chinese). The only place with pure ancient Indian blood is in the tiny island of Andaman.
Another set of interesting discoveries relate to the cast system in India. Organized in two levels, the cast system is composed of four major Varnas and at least forty six hundred Jatis. People tend to marry within their subgroups, creating the degree of genetic differentiations among Indians many times larger than in Europe. India is composed of a large number of small populations, according to Reich.
Marriages within the same Jati provide a fertile ground for rare diseases. This is tragic but also a goldmine. According to Reich, “The history of India presents an important opportunity for biological discovery. As finding genes for rare recessive diseases is cheap with modern genetic technology. All it takes is access to a small number of people in a jai group with the disease whose genome can then be sequenced.”
When all is said and done, the genetic research show that Europe and South Asia are similar in many ways. “They both were affected by two successive migrations. The first migration was from the Near East around nine thousand years ago, which brought farmers who mixed with local hunter gathers. The second was from the steppe after about five thousand years ago, which brought pastoralists, who probably spoke Indo-European languages, who mixed with the local farmers they encountered along the way.”
Native American ancestors: Moving on to America, the genetic research has shown that vast majority of Native Americans descended from groups of people that crossed over from Siberia during the ice age when the ocean levels were low. They came across Bering Strait via an ice-free corridor some thirteen thousand years ago but also along a second route, a coastal one, some sixteen thousand years ago. No archeological remains have been found of the coastal route because it is all covered by one hundred meters of water at this time.
A big surprise was the discovery of Native American population in the Amazon area that share ancestry with Australians, New Guineans, and people of Andaman Islands, mentioned above as the place where original Indians can be found. This indicates that there was another “ghost” population, called “Population Y” that most likely came from the Bering Strait route, however the timing of that event is unknown. This group perhaps was more widely distributed in the Americas but was then marginalized by the expansion of other groups.
Reich postulates that there were two more waves of people who crossed Bering Strait. One group, five thousand years ago formed Paleo-Eskimo lineage, the other, just a thousand years go, created a Neo-Eskimo group that displaced Paleo-Eskimos. As the names suggest, these are the ancestors of the current folks who live in the northern part of the continent. What is interesting is that there is evidence of reverse migration. Some folks went back to Asia along Bering Strait.
Reich mentions opposition he and other researchers encountered while studying genetics of Native Americans in the Continental United States. There is general mistrust of Western science among the Native population stemming from the way some early samples were collected. Also, in 1990 US Congress passed the Native American Graves Protection and Repatriation Act (NAGPRA) that requires institutions to return artifacts including bones, to Native American tribes. This reduces the opportunity to study ancient DNA analysis.
East Asians: Moving on the East Asia and Australia, the ancient DNA reveals that interbreeding of modern humans with Neanderthals created two groups, the European hunter-gatherers and the East Asians. This happened some 49,000-54,000 years ago. The East Asian group than interbred with the Denisovans to produce Papuans and Australians. All this happened quickly within a 5,000-year interval.
From 50,000 to 10,000 years ago, the hunter-gatherer East Asian group that moved east, diversified and spread south toward India, north east toward the Americas, and south east toward Australia. Two very divergent populations from this initial dispersion developed agriculture, about 9,000 years ago, independent of those in Turkey and north Syria mentioned earlier.
One of these two groups farmed along the Yangtze River, the other along the Yellow River. Analyzing DNA of population from Southeast Asia and Taiwan, it has been discovered that their ancestry can be traced back to a homogenous ancestral population that lived along Yangtze River. Since no ancient DNA from Yangtze River has been found, it is a “Yangtze River Ghost Population.” Similarly, the ancestry of Han Chinese---the world’s largest group of more than 1.2 billion people---can be traced back to a “Yellow River Ghost Population.” The Tibetan speakers also emerged from this second ghost population it is believed.
The East Asian group moving toward Australia, mentioned above, populated Indonesia, Australia and New Guinea. This first wave, 47,000 years ago, was followed by two other waves, one from Taiwan that populated the Southwest Pacific Islands (5,000-3,000 years ago), and the last one, originating from Papua New Guinea about 2,400 years ago that went to the same islands and beyond.
Africa: The story of human evolution in Africa is very complex, as might be expected from the continent’s huge size, its varied landscape, and the antiquity of our species there. As a lot of focus of research thus far has been for people who came out of Africa 50,000 years go to populate the rest of the world, what happened to them before or after that fateful event has not received as much attention. A lot of work henceforth has been based on linguistics, identifying similarities and differences in languages spoken in the continent to develop hypotheses on human development and migrations. The study of ancient (and current) DNA is still in its infancy and that is proving some of the hypotheses while bringing up new possibilities.
Broadly speaking, at some unknown time hundreds of years ago, a West African lineage split from the rest, and then some 200,000 to 300,000 years ago two other groups emerged, the East African foragers and the South African foragers. The former group is of specific interest to us, the non-Africans, as they had a pivotal role in our history. Subsequently, agriculturalists from the West Central Africa spread out and went south as well as east. This movement of people is called Bantu Expansion and it was discovered based on linguistics. This, and three other expansions occurred in the past few thousand years ago. As Reich says, the story is till being written as more research is done.
Disruptive Genome: Reich concludes the book talking about several areas where genome research has caused disruptions in current thinking of equality and race. For example, the genetic data confirms that powerful men have the potential to have a far greater impact than powerful women. One study points that a small number of powerful males living during the Mongol period succeeded in having an impact on billions of people living in East Eurasia today. One single person left many millions of direct male descendants across the territory that Mongol occupied. That male could be Genghis Khan based on the estimated rate of accumulation of mutations on the Y chromosome. How about that for the dominance of male?
One area where the genomic revolution is playing an important role is identifying genetic differences among different groups of people. Reich and his colleagues applied this method to find that African American men had genetic profile that led to a higher incidence of prostate cancer than the rest of the population. When these findings were presented, there was a strong opposition from the audience, including accusation of racism against him. Thus the well -meaning effort that can potentially improve health ran into a buzz saw.
Reich now admits that, “There are nontrivial average genetic differences across populations in multiple traits and the race vocabulary is too ill defined and too loaded with historical baggage to be helpful.” He says, “We need to come up with a new way of thinking that can accommodate differences caused by genetic traits, rather than deny that differences can exist and so find ourselves caught without a strategy once they are found in the future.” He categorically denies the clams made by people like Hitler that were based on racial superiority, but cautions against the pendulum swinging too far in the other direction, denying that there can be differences among genes of different groups of people.
As the revolution in understanding our past using ancient DNA continues, Reich expects that during the next decade much will be revealed about the past of people in Central Asia, South Asia, East Asia, and Africa. This will supplement what has already been learned about West Eurasia, Americas and Pacific Islands. This technology will also help estimate human population sizes at different times in the past. Yet another application is in the area of human biology and pace of human adaptation. The impact of natural selection during different times of human evolution can then be studied. medical advances that allow people with genetic conditions that in the past would have prevented them from surviving or having families would affect the natural selection.
Reich concludes by saying that, “In this field, the pursuit of truth for its own sake has had the effect of exploding stereotypes, undercutting prejudices, and highlighting the connections among people not previously known to be related.”
Indeed.
