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Post by Admin on Apr 13, 2018 18:47:14 GMT
Recent genetic studies have demonstrated differences across populations not just in the genetic determinants of simple traits such as skin color, but also in more complex traits like bodily dimensions and susceptibility to diseases. For example, we now know that genetic factors help explain why northern Europeans are taller on average than southern Europeans, why multiple sclerosis is more common in European-Americans than in African-Americans, and why the reverse is true for end-stage kidney disease. I am worried that well-meaning people who deny the possibility of substantial biological differences among human populations are digging themselves into an indefensible position, one that will not survive the onslaught of science. I am also worried that whatever discoveries are made — and we truly have no idea yet what they will be — will be cited as “scientific proof” that racist prejudices and agendas have been correct all along, and that those well-meaning people will not understand the science well enough to push back against these claims. This is why it is important, even urgent, that we develop a candid and scientifically up-to-date way of discussing any such differences, instead of sticking our heads in the sand and being caught unprepared when they are found. To get a sense of what modern genetic research into average biological differences across populations looks like, consider an example from my own work. Beginning around 2003, I began exploring whether the population mixture that has occurred in the last few hundred years in the Americas could be leveraged to find risk factors for prostate cancer, a disease that occurs 1.7 times more often in self-identified African-Americans than in self-identified European-Americans. This disparity had not been possible to explain based on dietary and environmental differences, suggesting that genetic factors might play a role. Self-identified African-Americans turn out to derive, on average, about 80 percent of their genetic ancestry from enslaved Africans brought to America between the 16th and 19th centuries. My colleagues and I searched, in 1,597 African-American men with prostate cancer, for locations in the genome where the fraction of genes contributed by West African ancestors was larger than it was elsewhere in the genome. In 2006, we found exactly what we were looking for: a location in the genome with about 2.8 percent more African ancestry than the average. When we looked in more detail, we found that this region contained at least seven independent risk factors for prostate cancer, all more common in West Africans. Our findings could fully account for the higher rate of prostate cancer in African-Americans than in European-Americans. We could conclude this because African-Americans who happen to have entirely European ancestry in this small section of their genomes had about the same risk for prostate cancer as random Europeans. Did this research rely on terms like “African-American” and “European-American” that are socially constructed, and did it label segments of the genome as being probably “West African” or “European” in origin? Yes. Did this research identify real risk factors for disease that differ in frequency across those populations, leading to discoveries with the potential to improve health and save lives? Yes. While most people will agree that finding a genetic explanation for an elevated rate of disease is important, they often draw the line there. Finding genetic influences on a propensity for disease is one thing, they argue, but looking for such influences on behavior and cognition is another. But whether we like it or not, that line has already been crossed. A recent study led by the economist Daniel Benjamin compiled information on the number of years of education from more than 400,000 people, almost all of whom were of European ancestry. After controlling for differences in socioeconomic background, he and his colleagues identified 74 genetic variations that are over-represented in genes known to be important in neurological development, each of which is incontrovertibly more common in Europeans with more years of education than in Europeans with fewer years of education.
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Post by Admin on Jul 14, 2018 18:36:24 GMT
Reich’s work as a leader of prehistoric population studies includes the discovery that all people of non-African descent carry small amounts of Neanderthal DNA, showing that Homo sapiens – at one stage – must have interbred with this long-dead species of ancient humans. Reich was also involved in uncovering the existence of Denisovans, a previously unknown species of ancient humans, using DNA found in fossil scraps in a Siberian cave. In addition, he has discovered that 5,000 years ago northern Europe was overrun by invaders from central Asia, a migration of profound importance – for those newcomers became the first people of the British Isles. These remarkable recreations of our past are outlined in Reich’s book Who We Are and How We Got Here, in which he chronicles the spectacular rise of ancient DNA studies in the last few years. Thanks to this remarkable new science, we now know that about 70,000 years ago, our planet was remarkably rich in terms of its human variety. It was populated by modern humans, Neanderthals – and the Denisovans who, Reich has recently discovered, must have existed as at least two separate varieties: Siberian Denisovans and the more recently discovered Australo-Denisovans from south-east Asia. In addition, we also know that the Hobbit folk – Homo floresiensis, a race of tiny humans whose remains were discovered in 2003 – were then thriving in Indonesia. In those not too distant days, there were many ways to be a human, it transpires. The ingrained notion – that there has only ever been one species of human being, Homo sapiens – is a latterday fiction born of our own self-important view of ourselves. Think instead of the bar scene from Star Wars with all those various people playing and drinking, says the Israeli palaeontologist Yoel Rak. That gives a far better flavour of our evolutionary past. In making constant new discoveries about humanity, Reich and his Harvard team are now plunging into uncharted academic waters. “We are going out on a limb on so many different studies,” he says. “It is very lonely and somewhat terrifying. We don’t have the comfort of standing on the shoulders of others. We are the first. That’s why I worry.”
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Post by Admin on Jul 15, 2018 18:39:57 GMT
Reich’s influence in this field has been immense and the output of his department monumental. This year alone he has been involved in producing an analysis that reveals the existence of a previously unknown group of ancient Native Americans from fossil remains uncovered in Alaska; a study that shows the ancient British people who built Stonehenge and other great neolithic monuments were almost completely replaced by invaders from central Asia 5,000 years ago; and a paper that indicates there were at least two waves of settlers, from Taiwan and then Papua New Guinea, which were responsible – 3,000 years ago – for the settling of one of the last pockets of the planet to be reached by humans, Vanuatu. Ancient DNA studies are overturning our oversimplified vision of our past and are the outcome of a late 20th-century revolution in molecular biology that gave scientists the power to study DNA, the material from which our genes are made, with startling precision. For the first time, the exact structure and makeup of a gene could be determined and the detailed origins of many inherited illnesses and cancers outlined, setting in motion the slow, ongoing task of developing new treatments. By contrast, the study of ancient DNA, which uses the same basic technology, began late but has since flowered far more dramatically. “It is in the area of shedding light on human migrations – rather than in explaining human biology – that the genome revolution has been a runaway success,” says Reich. The field’s hesitant start is understandable. In samples from living animals, DNA exists in long, healthy, easily analysed strands. However, DNA starts to decay the moment an organism dies and those strands quickly fragment. And the longer the passage of time, the shorter the fragments become. This disintegration poses problems. If, for example, you want to study Neanderthals, who dominated Europe for around 400,000 years and who were close in evolutionary terms to Homo sapiens, DNA from their fossils is going to be in minuscule pieces. The last member of this doomed species died more than 40,000 years ago, after all. Genetic material taken from Neanderthal fossils is also likely to be contaminated with large amounts of DNA from bacteria and vegetation – and sometimes from researchers. Nevertheless, scientists have persevered and in 2007, geneticist Svante Pääbo, of the Max Planck Institute for Evolutionary Anthropology, decided to assemble a team of experts to sequence a Neanderthal genome that would be billions of DNA units in length. Reich, an innovator in the field of studying population mixtures, was asked to join and has since played a key role in the fledgling field’s remarkable development.
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Post by Admin on Jul 16, 2018 18:30:24 GMT
Clean rooms were built, advanced gene sequencers purchased and DNA extracted from Neanderthal bones that had been found in Vindija cave in Croatia. A Neanderthal genome was slowly spliced together from pieces of DNA only a few dozen units in length. It was a brilliant achievement though Reich makes clear progress was halting. “The Neanderthal sequences we were working with had a mistake approximately every 200 DNA letters,” he reveals in his book. These errors were not due to differences between humans and Neanderthals, it should be pointed out, but to errors made in analysing DNA. It was Reich’s task to get round these problems and help create a meaningful genome of a Neanderthal. From that, scientists could assess just how closely we were related to these ancient people. His tests succeeded and subsequently showed, to everyone’s surprise, that many modern humans carry small amounts of Neanderthal DNA in their genomes. “Non-African genomes today are around 1.5 to 2.1% Neanderthal in origin,” he says. So yes, Homo sapiens and Neanderthals had a common ancestor, about 500,000 years ago, before the former evolved as a separate species – in Africa – and the latter as a different species in Europe. Then around 70,000 years ago, when modern humans emerged from Africa, we encountered the Neanderthals, most probably in the Middle East. We briefly mixed and interbred with them before we continued our slow diaspora across the planet. In doing so, those early planetary settlers carried Neanderthal DNA with them as they spread out over the world’s four quarters. Hence its presence in all those of non-African origin. By contrast, Neanderthal DNA is absent in people of African origins because they remained in our species’s homeland. Reich has since established that such interbreeding may have occurred on more than one occasion. More importantly, his studies show that “Neanderthals must have been more like us than we had imagined, perhaps capable of many behaviours that we typically associate with modern humans”. They would, most likely, have had language, culture and sophisticated behaviours. Hence the mutual attraction. That itself is intriguing. However, there is another key implication of Reich’s work. Previously, it had been commonplace to view human populations arising from ancestral groupings like the trunk of a great tree. “Present populations budded from past ones, which branched from a common root in Africa,” he states. “And it implies that if a population separates then it does not remix, as fusions of branches cannot occur.” But the initial separation of the two lines of ancient humans who gave rise to Neanderthals and to Homo sapiens – and then their subsequent intermingling – shows that remixing does occur. Indeed, Reich believes it was commonplace and that the standard tree model of populations is basically wrong. Throughout our prehistory, populations have split, reformed, moved on, remixed and interbred and then moved on again. Alliances have shifted and empires have fallen in a perpetual, sliding global Game of Thrones.
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Post by Admin on Jul 17, 2018 18:27:07 GMT
An illustration is provided by the puzzling fact that Europeans and Native Americans share surprising genetic similarities. The explanation was provided by Reich who has discovered that a now nonexistent group of people, the Ancient North Eurasians, thrived around 15,000 years ago and then split into two groups. One migrated across Siberia and gave rise to the people who crossed the Bering land bridge between Asia and America and later gave rise to Native Americans. The other group headed west and contributed to Europeans. Hence the link between Europeans and Native Americans. No physical specimen of the Ancient North Eurasian people had ever been discovered when Reich announced their existence. Instead, he based his analysis on the ghostly impact of their DNA on present-day people. However, the fossil remains of a boy, recently found near the Siberian village of Mal’ta, have since been found to have DNA that matches the genomes of Ancient North Eurasians, giving firmer physical proof of their existence. “Prior to the genome revolution, I – like most others – had assumed that the big genetic clusters of populations we see today reflect deep splits of the past. But in fact the big clusters today are themselves the result of mixtures of very different populations that existed earlier. There was never a single trunk population in the human past. It has been mixtures all the way down.”
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