Human-Family Reunions
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Right Now The expanding Harvard universe IN THE DNA CAVE person, you have a personal map that shows where the little bits of Neanderthal ancestry are,” Reich explains. “Each person’s map is Human-Family Reunions quite di%erent…You might have Neander- thal ancestry at your beta-globin gene, and I might not, or vice versa.” ! "#$ trained interpreter, the hu- ern humans interbred with Neanderthals. In Europeans and East Asians, these man genome is a record of the hu- In fact, pieces of Neanderthal DNA live pieces summed to an average of about 2 man past. Ancient environments on today, in people of European and East percent of the genome. But the areas of Ne- T leave their imprint through natu- Asian heritage. anderthal ancestry were not randomly dis- ral selection; meanwhile, patterns of genetic In a paper published in Nature this Janu- tributed; in some regions of the genome, similarity among di%erent individuals hint ary, Reich’s lab identified the lingering in fact, nearly two-thirds of Eurasians at aspects of their shared history, ranging traces of this genetic mixture by comparing showed Neanderthal descent. Natural se- from family relationships to mass migra- the genome of a Neanderthal with those of lection seems to have favored the associ- tions that happened thousands of years ago. about 1,000 present-day humans. “In each ated genes, which disproportionately af- With the advent of pow- erful new sequencing tech- nology, genomes past and present are shedding new light on early human history. “Genetics tells you about the movements and relation- ships of people,” says David Reich, professor of genetics at Harvard Medical School. “These were dark spots, black spots, unknowns for a lot of archaeology.” The past that has come to light is more complex than researchers had suspected. In 2010, as part of the e%ort to sequence the Neander- thal genome, Reich and col- leagues came to a surprising conclusion: sometime in the distant past, not long after moving out of Africa, mod- The ancestors of modern humans interbred with other early hominids, including Neanderthals. VALERIA PETRONE Photograph by Tomislav Veic´ /Courtesy of the Krapina Neanderthal Museum, Croatia H&'(&') M&*&+,-$ 7 Reprinted from Harvard Magazine. For more information, contact Harvard Magazine, Inc. at 617-495-5746 RIGHT N OW (the sex chromosome named the Denisovans. The Denisovans, that is shared by males too, once interbred with the forebears of and females) in particu- modern humans—Polynesians and Ab- lar, Neanderthal DNA original Australians show evidence of was one-fifth as com- Denisovan ancestry—and Reich’s team mon as elsewhere—a now suspects that Neanderthals interbred well-established sign of with Denisovans as well. More genome the barriers to hybridiza- sequences, both ancient and modern, are tion that develop as two needed to understand all the hypothesized species diverge. Even so, admixtures, but the familiar human evo- the similarities necessary lutionary tree already seems more tangled for genetic mixture than previously thought. to have occurred Genetic evidence has its limits. It is dif- remained: “Ne- ficult or impossible, Reich says, to detect Early modern anderthals and migrations of closely related people us- humans likely encountered modern hu- ing genomic techniques. For instance, Neanderthals and mans are closer the powerful techniques that identified Denisovans after to each other pieces of Neanderthal DNA have not been migrating from Africa approximately than might have used successfully to detect a genomic 50,000 years ago. been thought be- signature from the barbarian invasion of fore,” Reich em- Rome, despite its occurring much nearer fect skin and phasizes. “They met, the present—though Reich thinks that hair, and Reich’s team hypothesize that they interbred…they pro- more careful study, combined with addi- they helped modern humans adapt to their duced o%spring, and those o%spring were tional samples of ancient DNA, may shed new, non-African environments. raised by one or both parents and contrib- light on this and other events in human Other details about the interbreeding uted to the modern human gene pool.” history. “Genomics tells you something have surfaced as well. Nothing in the ar- More broadly, Reich has found that that’s quite complementary to archaeol- chaeological record so far suggests when species boundaries among early hominid ogy,” says Reich. “It’s another very rich or where humans and Neanderthals met, groups were by no means fixed boundar- source of information about the past.” but using genetic evidence, Reich esti- ies. In 2010, the genome sequencing of an !.&"#$',-$ /0$ mates that their last encounters occurred ancient finger bone from a Siberian cave 47,000 to 65,000 years ago. Likewise, he identified an entirely new hominid group, )&(,) '$,1# 2$34,"$: and colleagues found that humans today distinct from both humans and Nean- http://genetics.med.harvard.edu/ are most closely related to a Neanderthal derthals, which Reich and collaborators reich/Reich_Lab/Welcome.html individual whose remains were found in the Caucasus Mountains, even though Ne- anderthals were known to have lived pri- GOLIATH FALLEN marily in Europe. Nevertheless, many aspects of the inter- breeding remain in the dark. “We don’t re- A Hemlock Farewell ally know how many matings there were,” says Reich. “We don’t know if it was Ne- anderthal males and modern human fe- #$ )&'. but delicate beauty of nearly 60 years, Foster says, and at last, the males, or the reverse, or equal numbers of hemlocks has inspired writers for hemlock population has reached a critical both.” Nor is it clear why the interbreed- centuries—Henry David Thoreau, point. There’s nothing science can do to ing occurred—whether it was consensual, T Emily Dickinson, Ernest Heming- stop it. The adelgid “basically sucks the or influenced by factors like warfare or a way, and Robert Frost among them. David sap out of the tree and deprives the tree need for mates at a time when modern hu- R. Foster, senior lecturer on biology and of nutrients and sugar for energy—that’s mans were likely outnumbered. director of the Harvard Forest, recently why it’s a slow process,” he explains. “It’s The evidence does show, he says, that joined that list with the publication of his not something that comes in and eats all “when modern humans and Neanderthals new book, Hemlock: A Forest Giant on the Edge the needles and just kills the tree out- met and mixed, they were already at the (Yale). His message is melancholy: within right—it takes a while because it’s basi- edge of biological incompatibility.” The the next 10 years, hemlocks in forests across cally competing with the tree for the tree’s recent study located genomic “deserts” the United States are projected to die o% own resources.” where Neanderthal DNA was rare, sug- completely. It seems ironic that these massive trees gesting that natural selection may have A tiny insect, the woolly adelgid, which could be laid low by something so small. purged Neanderthal gene variants harm- originated in Asia, is killing these giant Hemlocks can grow more than 150 feet ful for humans. On the X chromosome evergreens. It’s slowly been doing so for tall, with large canopies that shade forests MAP SOURCE: LEARNING ABOUT HUMAN POPULATION HISTORY FROM ANCIENT AND MODERN GENOMES, 8 J056 - A0*04" 2014 BY MARK STONEKING AND JOHANNES KRAUSE, AS PUBLISHED IN NATURE REVIEWS, 2011 Reprinted from Harvard Magazine. For more information, contact Harvard Magazine, Inc. at 617-495-5746.