A Proper Study of Mankind

A Proper Study of Mankind: A Survey of Human Evolution December 24th 2005, in The Economist
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  Republication, copying or redistribution by any means is expressly prohibited without the prior written permission of The Economist  The properstudy ofmankind A survey of human evolution December 24th 2005  1 S EVEN hundred and forty centuries ago, give or take a few,the skies darkened and the Earth caught a cold. Toba, a vol-cano in Sumatra, had exploded with the sort of eruptiveforce that convulses the planet only once every few mil-lion years. The skies stayed dark for six years, so much dust did theeruption throw into the atmosphere. It was a dismal time to bealive and, if Stanley Ambrose of the University of Illinois is right,the chances were you would be dead soon. In particular, thepopulation of one species, known to modern science as Homo sa-piens , plummeted to perhaps 2,000 individuals.The proverbial Martian, looking at that darkened Earth, wouldprobably have given long odds against these peculiar apes mak-ing much impact on the future. True, they had mastered the art oftool-making, but so had several of their contemporaries. True, too,their curious grunts allowed them to collaborate in surprisinglysophisticated ways. But those advantages came at a huge price, fortheir brains were voracious consumers of energya mere 2% ofthe body’s tissue absorbing 20% of its food intake. An interestingevolutionary experiment, then, but surely a blind alley.This survey will attempt to explain why that mythical Martianwould have been wrong. It will ask how these apes not only sur-vived but prospered, until the time came when one of them couldweave together strands of evidence from elds as disparate as ge-ology and genetics, and conclude that his ancestors had gonethrough a genetic bottleneck caused by a geological catastrophe.Not all of his contemporaries agree with Dr Ambrose aboutToba’s eect on humanity. The eruption certainly happened, butthere is less consensus about his suggestion that it helped form thebasis for what are now known as humanity’s racial divisions, bybreaking Homo sapiens into small groups whose random physicalquirks were preserved in dierent places. The idea is not, how-ever, absurd. It is based on a piece of evolutionary theory calledthe founder eect, which shows how the isolation of small popu-lations from larger ones can accel-erate evolutionary change, be-cause a small population’saverage characteristics are likelyto dier from those of the largergroup from which it is drawn. Likemuch evolutionary theory, this is just applied common sense. Butonly recently has such commonsense been applied systematicallyto areas of anthropology that havetraditionally ignored it and some-times resisted it. The result, whencombined with new techniquesof genetic analysis, has been arevolution in the understandingof humanity’s past. And anthropology is not theonly human science to have beeninfused with evolutionary theory.Psychology, too, is undergoing amakeover and the result is a sec-ond revolution, this time in theunderstanding of humanity’spresent. Such understanding has been of two types, which oftenget confused. One is the realisation that many human activities,not all of them savoury, happen for exactly the same reasons as inother species. For example, altruistic behaviour towards relatives,indelity, rape and murder are all widespread in the animal king-dom. All have their own evolutionary logic. No one argues thatthey are anything other than evolutionarily driven in speciesother than man. Yet it would be extraordinary if they were not sodriven in man, because it would mean that natural selection had The proper study of mankind New theories and techniques have revolutionised our understanding of humanity’s past and present, says Georey Carr Human evolution Christmas special The Economist December24th2005 1 Also in this section   Christmas survey   The long march of everyman It all started in Africa. Page 4 Meet the relatives Alarge and diverse family. Page 6 If this is a man Why it pays to be brainy. Page 7  The concretesavannah Evolution and the modernworld. Page 9 Starchild Evolution is still continuing. Page 11  12 Humanevolution 2Christmas specialThe Economist December24th2005 somehow contrived to wipe out their genetic underpinnings,only for them to re-emergeas culturally determined phenomena.Understanding this shared evolutionary history with otherspecies is important; much foolishness has owed from its denial.But what is far more intriguing is the progress made in under-standing what makes humanity dierent from other species:friendship with non-relatives; the ability to conceive of what oth-ers are thinking, and act accordingly; the creation of an almost un-imaginably diverse range of artefacts, some useful, some merelydecorative; and perhaps most importantly, the use of language,which allows collaboration on a scale denied to other creatures.There are, of course, gaps in both sets of explanations. And thiseld of research being a self-examination, there are also manycontroversies, not all driven by strictly scientic motives. But theoutlines of a science of human evolution that can explain human-ity’s success, and also its continuing failings, are now in place. It is just a question of lling in the canvasor the cave wall. 7 The long march of everyman It all started in Africa O UT of Africa, always something new, wrote Caius Pli-nius Secundus, a Roman polymath who helped to inventthe eld of natural history. Pliny wrote more truly thanhe could possibly have realised. For one ne day, some-where between 85,000 and 60,000 years before he penned thosewords, something did put its foot over the line that modern geog-raphers draw to separate Africa fromAsia. And that somethingor, rather,somebodydid indeed start some-thing new, namely the peopling of theworld.Writing the story of the spread ofhumanity is one of the triumphs ofmodern science, not least because theink used to do it was so unexpected.Like students of other past life forms,researchers into humanity’s prehis-toric past started by looking in therocks. The rst fossilised human to berecognised as such was unearthed in1856 in the Neander Valley near Dus-seldorf in Germany. Neanderthalman, as this skeleton and its kin be-came known, is now seen as a cousinof modern humans rather than an an-cestor, and subsequent digging has re-vealed a branching tree of humanitywhose root can be traced back morethan 4m years (see next article). Searching for human fossils,though, is a frustrating exercise. Formost of their existence, people weremarginal creatures. Bones from peri-ods prior to the invention of agricul-ture are therefore excedingly rare. Theresulting data vacuum was lled byspeculation scarcely worthy of the name of theory, whichseemed to change with every new discovery. Then, in the 1980s, ageneticist called Allan Wilson decided to redene the meaning ofthe word fossil. He did so in a way that instantly revealed an-other 6 billion specimens, for Wilson’s method made a fossil outof every human alive. Living fossils In retrospect, Wilson’s insight, like many of the best, is blindinglyobvious. He knew, as any biologist would, that an organism’s DNA carries a record of its evolutionary past. In principle, lookingat similarities and dierences in the DNA sequences of living or-ganisms should allow a researcher to reconstruct the family treelinking those organisms. In practice, the sexual mixing that hap-pens with each generation makes this tedious even with today’s DNA -analysis techniques. With those available in the 1980s itwould have been impossible. Wilson, however, realised he couldcut through the problem by concentrating on an unusual type of DNA called mitochondrial DNA . Mitochondria are the parts of a cell that convert energy storedin sugar into a form that the rest of the cell can use. Most of a cell’sgenes are in its nucleus, but mitochondria, which are the descen-dants of bacteria that linked up with one of humanity’s unicellu-lar ancestors some 2 billion years ago, retain a few genes of theirown. Mitochondrial genomes are easy to study for three reasons.First, they are small, which makes them simple to analyse. Sec-ond, mitochondria reproduce asexually, so any changes betweenthe generations are caused by mutation rather than sexual mix-ing. Third, in humans at least, mitochondria are inherited onlyfrom the mother. In 1987, Rebecca Cann, one of Wilson’s students, applied hisinsight to a series of specimens taken from people whose ances-tors came from dierent parts of the world. By analysing the muta-tional dierences that had accumulated since their mitochondriashared a common ancestor, she wasable to construct a matriline (or, per-haps more accurately, a matritree) con-necting them.The result was a revelation. Which-ever way you drew the tree (statisticsnot being an exact science, there wasmore than one solution), its root was inAfrica. Homo sapiens was thus un-veiled as an African species. But DrCann went further. Using estimates ofhow often mutations appear in mito-chondrial DNA (the so-called molecu-lar clock), she and Wilson did somematridendrochronology. The resultsuggests that all the lines converge onthe ovaries of a single woman wholived some 150,000 years ago.There was much excited reportingat the time about the discovery anddating of this African Eve. She wasnot, to be clear, the rst female Homosapiens . Fossil evidence suggests thespecies is at least 200,000 years old,and may be older than that. And youcan now do a similar trick for the patri-line using part of the male ( Y ) chromo-some in the cell nucleus, because thispasses only from father to son. Unfor-tunately for romantics, the most recent   Christmas survey   12  The Economist December24th2005 Christmas special 3 Humanevolution common ancestor of the Y -chromosome is a lot more recent thanits mitochondrial equivalent. African Adam was born 60,000-90,000 years ago, and so could not have met African Eve. Never-theless, these two pieces of DNA as they have weaved their waysdown the generations have lled in, in surprising detail, the high-ways and byways of human migration across the face of theplanet. Sons of Adam, daughters of Eve Detail, however is not the same as consensus, and there are twoschools of thought about how people left Africa in the rst place.Appropriately, some of their main protagonists are at the rivalEnglish universities of Oxford and Cambridge. The Oxfordschool, championed by Stephen Oppenheimer, believes that thedescendants of a single emigration some 85,000 years ago, acrossthe strait of Bab el Mandeb at the southern end of the Red Sea, areresponsible for populating the rest of the world. The Cambridgeschool, championed by Robert Foley and Marta Mirazón Lahr,agrees that there was, indeed, a migration across this strait, thoughprobably nearer to 60,000 years ago. However, it argues thatmany non-Africans are the descendants of at least one subse-quent exodus.Both schools agree that the Bab el Mandebites spread rapidlyalong the coast of southern Arabia and thence along the southcoast of Asia to Australia, though Dr Oppenheimer has them turn-ing inland, too, once they crossed the strait of Hormuz. But it is indescribing what happened next that the two versions really partcompany, for it is here that the descendants of the Oxford migra-tion run into the eruption of Toba.That Toba devastated South and South-East Asia is not indoubt. Thick layers of ash from the eruption have been found asfar aeld as northern Pakistan. The question is whether therewere people in Asia at the time. One of the most important piecesof evidence for Dr Oppenheimer’s version of events is some stonetools in the ash layer in Malaysia, which he thinks were made by Homo sapiens . Molecular clocks have a regrettable margin of er-ror, but radioactive dating is a lot more accurate. If he is right,modern humans must have left Africa before the eruption. Thetools might, however, have been crafted by an earlier species ofhuman that lived there before Homo sapiens .For Dr Oppenheimer, the eruption was a crucial event, divid-ing the nascent human population of Asia into two disconnectedparts, which then recolonised the intermediate ground. In theCambridge version, Homo sapiens was still conned to Africa74,000 years ago, and wouldmerely have suered the equiv-alent of a nuclear winter, notan ash-fall of up to ve metresthough Dr Ambrose and hiscolleagues think even thatwould have done the popula-tion no good. The Cambridge version isfar more gentle. The descen-dants of its subsequent exodusexpanded north-eastwardsinto central Asia, and thencescattered north, south, east andwestthough in a spirit ofopen-mindedness, Sacha Jones, a research student in DrFoley’s department, is lookingin the ash layer in India to seewhat she can nd there.Which version is correct should eventually be determined bythe Genographic Project, a huge DNA -sampling study organisedby Spencer Wells, a geneticist, at the behest of America’s NationalGeographic Society and IBM . But both already have a lot in com-mon. Both, for example, agree that the Americas seem to havebeen colonised by at least two groups. The Cambridge school,though, argues that one of these is derived ultimately from the rstBab el Mandeb crossing while the other is descended from thelater migrants. Both also agree that Europe received two waves of migration.The ancestors of the bulk of modern Europeans came via centralAsia about 35,000 years ago, though some people in the Balkansand other parts of southern Europe trace their lines back to an ear-lier migration from the Middle East. But the spread of agriculturefrom its Middle Eastern cradle into the farthest reaches of Europedoes not, as some researchers once thought, seem to have been ac-companied by a mass movement of Middle Eastern farmers.The coming together of two groups of humans can be seen inmodern India, too. In the south of the subcontinent, people have Y -chromosomes derived almost exclusively from what the Cam-bridge school would interpret as being northern folk (and the Ox-ford school as the western survivors of Toba). However, morethan 20% of their mitochondria arrived in Asia with the rst mi-gration from Africa (or, according to taste, clung on along thesouth-eastern fringes of the ash plume).That discovery speaks volumes about what happened whenthe two groups met. It suggests that many modern south Indiansare descended from southern-fringe women, but few from south-ern-fringe menimplying a comprehensive conquest of thesoutherners by the northerners, who won extra southern wives.This observation, in turn, helps explain why Y -chromosomeAdam lived so much more recently than mitochondrial Eve. Dis-placement by conquest is one example of a more general phe-nomenonthat the number of ospring sired by individual malesis more variable than the number born by individual females.This means that more males than females end up with no o-spring at all. Male gene lines therefore die out faster than femaleones, so those remaining are more likely, statistically, to convergein the recent past.Successful male gene lines, though, can be very successful in-deed. Students of animal behaviour refer to the top male in agroup as the alpha. Such dominant animals keep the others un-der control and father a large proportion, if not all, of the group’sospring. One of the curiosities of modern life is that voters tend African srcins Over 160,000 years ago120,000 years ago85,000 years ago75,000 years ago65,000 years ago40,000 years ago20-30,000 years ago40,000 years ago12,500 years ago15-19,000 years ago25,000 years ago22-25,000 years ago   A S I AAFRICAAUSTRALASIAEUROPENORTHAMERICASOUTHAMERICA PACIFIC OCEAN  ARCTIC OCEAN  INDIAN OCEAN NORTH ATLANTIC  SOUTH ATLANTIC  506035451540 Possible migration patterns of early humans: Populating the Earth  Y-chromosome route markersThousands of years agoSource: The Genographic Project  00 Christmas survey 
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