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The proper study of mankindA survey ofhuman evolutionDecember 24th 2005

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SEVEN 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 isjust 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 mankindNew theories and techniques have revolutionised our understanding ofhumanity’s past and present, says Geo�rey Carr

Human evolution

Christmas special The Economist December 24th 2005 1

Also in this section

Christmas survey

The long march ofeverymanIt all started in Africa. Page 4

Meet the relativesA large and diverse family. Page 6

If this is a manWhy it pays to be brainy. Page 7

The concretesavannahEvolution and the modernworld. Page 9

StarchildEvolution is stillcontinuing. Page 11

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somehow contrived to wipe out their genetic underpinnings,only for them to re-emerge as 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 isjust a question of �lling in the canvas�or the cave wall. 7

The long march ofeverymanIt all started in Africa

OUT 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 by

speculation 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 fossilsIn retrospect, Wilson’s insight, like many of the best, is blindinglyobvious. He knew, as any biologist would, that an organism’sDNA 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’sDNA-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 ofDNA 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 mitochondria

shared 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

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Human evolution

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 EveDetail, 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 byHomo 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 Africa

74,000 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, SachaJones, 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 haveY-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 originsOver 160,000 years ago

120,000years ago

85,000years ago

75,000years ago

65,000years ago

40,000years ago

20-30,000years ago

40,000years ago

12,500years ago

15-19,000years ago

25,000 years ago

22-25,000years ago

A S I A

AFRICA

AUSTRALASIA

EUROPE

NORTH

AMERICA

SOUTH

AMERICA

P A C I F I C

O C E A N

A R C T I C O C E A N

I N D I A N

O C E A N

N O R T H

A T L A N T I C

S O U T H

A T L A N T I C

50

60

35

451540

Possible migration patterns ofearly humans:

Populating the Earth

Y-chromosome route markers

Thousands of years agoSource: The Genographic Project

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Human evolution

4 Christmas special The Economist December 24th 2005

Meet the relatives

A large and diverse family

WHEN Homo sapiens emerged as a species, he was notalone. The world he entered was already peopled bygiants and dwarfs, elves, trolls and pixies�in otherwords, creatures that looked humanlike, but were not

the genuine article. Or, at least, not as genuine as Homo sapienshas come to believe himself to be.

Like the story of Homo sapiens himself, the story of the wholehuman family begins in Africa. About 4.5m years ago, probably inresponse to a drying of the climate that caused forest cover in thatcontinent to shrink, one species of great ape found itself pushedout into the savannah, an ecological niche not normally occupiedby apes. Over the next 300,000 years these apes evolved anupright stance. No one know for sure why, but one plausible ex-planation, advanced by Peter Wheeler of John Moores Universityin Liverpool, is that standing upright reduces exposure to sunlight.To an animal adapted to the forest’s shade, the remorseless noon-day sun of the savannah would have been a threat. Dr Wheeler’s

calculations suggest that walk-ing upright decreases exposureat noon by a third comparedwith going on all fours, sinceless of the body’s surface facesthe overhead sun. Humanity,in the form of Australopithecusanamensis, had arrived.

Australopithecines of va-rious species lasted for over 3myears. But half-way throughthat period something interest-ing happened. One of them be-gat a species known to sciencevariously as Homo rudolfensisand Homo habilis. All modern great apes make tools out of sticksand leaves to help them earn their living, and there is no reason tobelieve that this was not true of australopithecines. But, aided byhands that no longer needed to double as part-time feet, Homo ha-bilis began to exploit a new and potent material that needs bothprecision and strength to work�stone. This provided its immedi-ate descendants with a powerful technology, but also gave its dis-tant descendants in human palaeontology laboratories an addi-tional way of tracing their ancestry, for stone tools often survivewhere bones do not.

Homo habilis’s successor species, Homo erectus, did not be-stride the globe in the way that his eventual descendant Homo sa-piens did, but he certainly stuck his nose out of Africa. Indeed, the�rst fossil erectus discovered was in Java, in 1891, and the secondone, several decades later, turned up in China, near Beijing. It wasnot until 1960 that erectus bones were found in Africa.

Homo erectus is a frustrating species. His tools are found allover the southern half of Eurasia, as well as in Africa. But Chinaand Java aside, his bones are scarce outside Africa. There are twoskullcaps from Georgia and half of one from India. He did, how-ever, leave lots of descendants.

Naming fossils is a game that beautifully illustrates Henry Kis-singer’s witticism about academic disputes being so bitter be-cause the stakes are so low. The best de�nition of a species that bi-ologists have been able to come up with is �a group of creatures

capable of fertile interbreeding, giventhe chance�, which clearly makes ithard to determine what species a par-ticular fossil belongs to. Researcherstherefore have to fall back on the physi-cal characteristics of the bones they�nd. That allows endless scope for argu-ment between so-called splitters, whoseem to want to give a new name to ev-ery skull discovered, and lumpers, wholike to be as inclusive as possible.

Some splitters, for example, arguethat the African version of Homo erec-tus should be called Homo ergaster.Whatever the niceties, it is clear that by500,000 years ago, if not before, Homoerectus was breaking up into anatomi-cally di�erent populations. Splitterswould like to turn the Georgia fossils, anearly twig of the erectus tree, into Homogeorgicus. There is also Homo rhodesien-sis, found in southern Africa, Homo hei-delbergensis from Europe, and a wholedrawer’s-worth of specimens known to

History manHumanity’s phylogenetic tree

Years before present, m

Source: Smithsonian Institution *Homo helmei

4.5 4.0 3.03.5 2.02.5 1.0 0.51.5 Present

? ?

Australopithecusanamensis

Australopithecusafarensis

Australopithecusgarhi

Homo habilis

Homo rudolfensis

Homoergaster

Homo erectus

Homofloresiensis

Homoheidelbergensis

Homosapiens

Homoneanderthalensis

Australopithecusafricanus

*

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to elect alpha males to high o�ce, and then a�ect surprise whenthey behave like alphas outside politics too. But in the days whenalphas had to �ght rather than scheme their way to the top, theytended to enjoy the sexual spoils more openly. And there werefew males more alpha in their behaviour than Genghis Khan, aman reported to have had about 500 wives and concubines, notto mention the sexual opportunities that come with conquest. It isprobably no coincidence, therefore, that one man in every 12 ofthose who live within the frontiers of what was once the Mongolempire (and, indeed, one in 200 of all men alive today) have astretch of DNA on their Y-chromosomes that dates back to thetime and birthplace of the great Khan. 7

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Human evolution

If this is a man

Why it pays to be brainy

THANKS to Dr Cann and her successors, the story of howHomo sapiens spread throughout the world is gettingclearer by the day. But why did it happen? What was it thatgave the species its edge, and where did it come from? Here,

the picture blurs. Until recently, it was common to speak of an Upper Palaeo-

lithic revolution in human a�airs�what Jared Diamond, of theUniversity of California at Los Angeles, called the Great Leap For-ward. Around 40,000 years ago, so the argument ran, humanityunderwent a mental step-change. The main evidence for this wasthe luxuriant cave art that appeared in Europe shortly after thistime. Palaeopsychologists see this art as evidence that the artistscould manipulate abstract mental symbols�and so they surelycould. But it is a false conclusion (though it was widely drawn be-fore Dr Cann’s work) that this mental power actually evolved inEurope. Since all humans can paint (some, admittedly, better thanothers), the mental ability to do so, if not the actual technique,must have emerged in Africa before the �rst emigrants left. In-deed, evidence of early artistic leanings in that continent has nowturned up in the form of drilled beads made of shells and coral,and�more controversially�of stones that have abstract patternsscratched on to them and bear traces of pigment.

That certainly pushes the revolution back a few tens of millen-nia. The oldest beads seem to date from 75,000 years ago, and aninspired piece of lateral thinking suggests that clothing appearedat about the same time. Mark Stoneking and his colleagues at theMax Planck Institute for Evolutionary Anthropology in Leipzig,Germany, applied the molecular-clock technique to human lice.They showed that head lice and body lice diverged 75,000 yearsago. Since body lice live in clothing, and most other species ofmammal support only one species of louse, the inference is thatbody lice evolved at the same time as clothes.

That is an interesting coincidence, and some think it doubly in-teresting that it coincides with the eruption of Toba. It may be evi-dence of a shift of thought patterns of the sort that the Upper Pa-laeolithic revolutionaries propose. On the other hand, there arealso signs of intellectual shifts predating this period. SallyMcBrearty, of George Washington University, and Alison Brooks,of the University of Connecticut, have identi�ed 14 traits, frommaking stone blades to painting images, which they think repre-sent important conceptual advances. Ten of them, including �sh-ing, mining, engaging in long-distance trade and making bonetools, as well as painting and making beads, seem to be unique tomodern Homo sapiens. However, four, including grinding pig-ments (for what purpose remains unknown, but probably bodypainting), stretch back into the debatable past of Homo helmei.

Given the fragmentary nature of the evidence from Africa,which has not been explored with the same sort of archaeological�ne-tooth comb as Europe, the speed of the emergence of modernbehaviour is still debatable. One thing, however, that clearlyplayed no part in distinguishing Homo sapiens from his hominidcontemporaries was a bigger brain.

Modern people do, indeed, have exceedingly large brains,measuring about 1,300 cm3. Other mammals that weigh roughly

some as Homo helmei and to others as archaic Homo sapiens.How little is really known, though, was thrown into sharp re-

lief by the announcement just over a year ago that yet anotherspecies, Homo �oresiensis, had been found. It was discovered onJava’s nearish neighbour island, Flores. Finding a new species ofhuman is always exciting, but what is particularly intriguingabout Homo �oresiensis is how small it was�barely a metre tallwhen fully grown. Perhaps inevitably, though to the disgust of itsdiscoverers, Homo �oresiensis became known to journalists as thehobbit, after J.R.R. Tolkien’s �ctional humanoid. Homo neander-thalensis, the descendant of Homo heidelbergensis, by contrast,was if not a giant then at least a troll. Though he stood �ve or tencentimetres shorter than a modern European Homo sapiens, thethickness of his bones suggests he was a lot heavier.

Both Homo neanderthalensis and Homo �oresiensis were cer-tainly around when Homo sapiens left Africa�whichever versionof that story turns out to be the correct one. There may also havebeen some lingering populations of other hominid species. Thatraises the intriguing question of what happened when these resi-dents met the sapiens wave.

Some researchers believe there was interbreeding, echoing theideas of an older school of palaeoanthropology called multire-gionalism. The multiregionalists thought either that pre-sapienshominids were all a vast, interbreeding species that graduallyevolved into sapiens everywhere, or, against all Darwinian logic,that Homo sapiens arose independently in several places by someunknown process of parallel evolution.

As recently as 2002, Alan Templeton, then at the University ofWashington at St Louis, claimed to have found a number of ge-netic trees whose roots were 400,000-800,000 years old, and yetwhich included non-Africans. That, if con�rmed, would supportmultiregionalism. Meanwhile, John Relethford, of the State Uni-versity of New York’s campus at Oneonta, has criticised the con-clusions of studies on mitochondrial DNA extracted from thebones of Neanderthals. This does not resemble DNA from anyknown modern humans, which led the authors of the work toconclude there was no interbreeding. Dr Relethford points outthat Neanderthal DNA brought into the sapiens population by in-terbreeding could subsequently have been lost by chance in thelottery of who does and who does not reproduce. Similar lossesare known to have happened in Australia, where mitochondrialDNA from human fossils is absent from modern Australians.

Most students of the �eld, though, think there was no inter-breeding, full stop. Either Homo sapiens persecuted his cousinsinto extinction or, with his superior technology, he outhunted,outgathered and outbred them. The next question is where thattechnology�or, rather, the brainpower to invent and make it�came from. 7

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6 Christmas special The Economist December 24th 2005

the same as human beings�sheep, for example�have brains withan average volume of 180cm3. In general, there is a well-estab-lished relationship between body size and brain size that peoplevery much do not �t. But as Dr Oppenheimer shows (see chart 2),most of this brain expansion happened early in human evolu-tionary history, in Homo habilis and Homo erectus. The brains ofmodern people are only about 6% larger than those of their imme-diate African predecessors. Perhaps more surprisingly, they aresmaller than those of Neanderthals. There is no doubt that thisearly brain growth set the scene for what subsequently happenedto Homo sapiens, but it does not explain the whole story, other-wise Homo erectus would have built cities and �own to the moon.

Flying to the moon may, in fact, be an apt analogy. Just as aspace rocket needs several stages to lift it into orbit, so the growthof human intelligence was probably a multi-stage process, witheach booster having its own cause or causes. What those causeswere, and when they operated, remains a matter of vehement ac-ademic dispute. But there are several plausible hypotheses.

The most obvious idea�that being clever helps people to sur-vive by learning about their surroundings and being able to solvepractical problems�is actually the least favoured explanation, atleast as the cause of the Great Leap Forward. But it was probablyhow intelligence got going in the pre-human primate past, andthus represented the �rst stage of the rocket.

Many primates, monkeys in particular, are fruit-eaters. Eatingfruit is mentally taxing in two ways. The �rst is that fruiting treesare patchily distributed in both space and time (though in thetropics, where almost all monkeys live, there are always trees infruit somewhere). An individual tree will provide a bonanza, butyou have to �nd it at the right moment. Animals with a goodmemory for which trees are where, and when they last came intofruit, are likely to do better than those who rely on chance. Also,fruit (which are a rare example of something that actually wantsto be eaten, so that the seeds inside will be scattered) signal totheir consumers when they are ready to munch by changing col-our. It is probably no coincidence, therefore, that primates havebetter colour vision than most other mammals. But that, too, isheavy on the brain. The size of the visual cortex in a monkey brainhelps to explain why monkeyshave larger brains than theirweight seems to warrant.

The intelligence rocket’ssecond stage was almost cer-tainly a way of dealing withthe groups that fruit-eatingbrought into existence. Be-cause trees in the tropics comeinto fruit at random, an animalneeds a lot of fruit trees in itsrange if it is to avoid starving.Such a large range is di�cult fora lone animal to defend. Onthe other hand, a tree in fruitcan feed a whole troop. Forboth these reasons, fruit-eatingprimates tend to live in groups.

But if you have to live in agroup, you might as well makethe most of it. That meansavoiding con�ict with your ri-vals and collaborating withyour friends�which, in turn,means keeping track of yourfellow critters to know who is

your enemy and who your ally. That, in turn, demands a lot ofbrain power.

One of the leading proponents of this sort of explanation forintelligent minds is Robin Dunbar, of Liverpool University in Eng-land. A few years ago, he showed that the size of a primate’s brain,adjusted for the size of its body, is directly related to the size ofgroup it lives in. (Subsequent work has shown that the same rela-tionship holds true for other social mammals, such as wolves andtheir kin.) Humans, with the biggest brain/body ratio of all, tendto live in groups of about 150. That is the size of a clan of hunter-gathers. Although the members of such a clan meet only fromtime to time, since individual families forage separately, they allagree on who they are. Indeed, as Dr Dunbar and several other re-searchers have noticed, many organisations in the modern world,such as villages and infantry companies, are about this size.

Living in collaborative groups certainly brings advantages, andthose may well o�set the expense of growing and maintaining alarge brain. But even more advantage can be gained if an animalcan manipulate the behaviour of others, a phenomenon dubbedMachiavellian intelligence by Andrew Whiten and RichardByrne, of the University of St Andrews in Scotland.

Size isn’t everythingMonkeys and apes manage this to a certain extent. They seem tohave a limited �theory of mind��the ability to work out what oth-ers are thinking�which is an obvious prerequisite for thewould-be simian politician. They also engage in behaviourwhich, to the cynical human zoologist, looks suspiciously like ly-ing. But it is those two words, �cynical� and �suspiciously�, thatgive the game away. For it is humans themselves, with their abil-ity to ponder not only what others are thinking, but also whatthose others are thinking about them, who are the past masters ofsuch manipulation.

And it is here that the question of language enters the equa-tion. Truly Machiavellian manipulation is impossible without it.And despite claims for talking chimpanzees, parrots and dol-phins, real language�the sort with complex grammar and syn-tax�is unique to Homo sapiens.

Dr Dunbar’s hypothesis isthat language arose as a substi-tute for the physical groomingthat other group-living pri-mates use to maintain bondsof friendship. Conversa-tion�or gossip, as he refers toit�certainly does seem to havethe same bond-forming role asgrooming. And, crucially forthe theory, groups rather thanjust pairs can �groom� eachother this way. Dr Dunbar seesthe 150-strong group size ofHomo sapiens as both a conse-quence and a cause of verbalgrooming, with large groupsstimulating the emergence oflanguage, and language thenpermitting the emergence oflarger groups still. Language,therefore, is the result of a pro-cess of positive feedback.

Once established, it can bedeployed for secondary pur-poses. Furthering the Machia-

Paranthropus sp.

Bigger, yes. Better, maybeHominid brain-size evolution, cm3

Rapidbrain

growth inAfrica

Steady braingrowth in linesnot ancestral to

Homo sapiens

Brain-sizereduction in

Homo sapiensline

Source: “Out of Eden”, by Stephen Oppenheimer

0

500Logscale

600

700

800

900

1,000

1,200

1,400

1,600

2.5 2.0 1.5 1.0 0.5

Years before present, m

0.2 0.1

H.habilis/H.rudolfensis

African:H.erectus/

H.ergaster/H.rhodesiensis

Asian erectus

European:H.heidelbergensis &H.neanderthalensis

ArchaicH.sapiens/ H.helmei

Anatomicallymodern humans:

H.sapiens

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The Economist December 24th 2005 Christmas special 7

Human evolution

The concrete savannah

Evolution and the modern world

THE eruption of Toba marked the beginning rather than theend of hostilities between Homo sapiens and the climate.Views di�er about whether the eruption was the trigger,but it is clear that an ice age started shortly afterwards.

Though the species spread throughout Asia, Australia and Europe(the populating of the Americas is believed by most researchers tohave happened after the ice began to retreat, although not every-body agrees), it was constrained by ecological circumstances inmuch the same way as any other animal. The world’s population10,000 years ago was probably about 5m�a long way from theimperial 6-billion-strong species that bestrides the globe today.

The killer application that led to humanity’s rise is easy toidentify. It is agriculture. When the glaciers began to melt and theclimate to improve, several groups learned how to grow crops anddomesticate animals. Once they had done that, there was no go-

vellian ends outlined by DrWhiten and Dr Byrnewould be one such pur-pose, and this woulddrive other feedbackloops as people evolvemore and more elaboratetheories of mind in orderto manipulate and avoidmanipulation. But languagewould also promote collabora-tive activities such as trade and theconstruction of sophisticated arte-facts by allowing specialisation anddivision of labour.

Not everyone agrees with the de-tails of this thesis, but the idea thatthe evolution of mental powers suchas language has been driven by two-way feedback loops rather than one-way responses to the environment isa powerful one. Terrence Deacon, aresearcher at the University of Cali-fornia at Berkeley, for instance, thinksthat language evolved in a feedbackloop with the complex culture that itallowed humans to create. Changesin culture alter and complicate theenvironment. Natural selectioncauses evolutionary changes thatgive people the means to exploit their new, more complex circum-stances. That makes the cultural environment still more compli-cated. And so on. Dr Deacon believes this process has driven thecapacity for abstract thought that accounts for much of what is re-ferred to as intelligence. He sees it building up gradually in earlyhominids, and then taking o� spectacularly in Homo sapiens.

The peacock mindPerhaps the most intriguing hypothesis about the last stage of themental-evolution rocket, though, is an idea dreamed up by Geof-frey Miller, of the University of New Mexico. He thinks that thehuman mind is like a peacock’s tail, a luxuriant demonstration ofits owner’s genetic �tness.

At �rst sight this idea seems extraordinary, but closer examina-tion suggests it is disturbingly plausible. Lots of features displayedby animals are there to show o� to the opposite sex. Again, this in-volves a feedback loop. As the feature becomes more pro-nounced, the judge becomes more demanding until the cost to thedisplayer balances the average reproductive bene�t.

Frequently, only one sex (usually the male) does the showingo�. That makes the sexually selected feature obvious, because it isabsent in the other sex. Dr Miller, though, argues that biologistshave underplayed the extent to which females show o� to males,particularly in species such as songbirds where the male plays abig part in raising the young, and so needs to be choosy aboutwhom he sets up home with. Like male birds, male humans areheavily involved in childrearing, so if the mind is an organ forshowing o�, both sexes would be expected to possess it�and beattracted by it�in more or less equal measure.

Dr Miller suggests that many human mental attributesevolved this way�rather too many, according to some of his crit-ics, who think that he has taken an interesting idea to implausibleextremes. But sexual selection does provide a satisfying explana-tion for such otherwise perplexing activities as painting, carving,

singing and dancing. On thesurface, all of these things

look like useless dissipa-tions of energy. All, how-ever, serve to demon-strate physical andmental prowess in ways

that are easy to see andhard to fake�precisely the

properties, in fact, that arecharacteristic of sexually se-

lected features. Indeed, a little intro-spection may suggest to the reader thathe or she has, from time to time, donesome of these things to show o� to a de-sirable sexual partner.

Crucially, language, too, may havebeen driven by sexual selection. Nodoubt Machiavelli played his part: rhet-oric is a powerful political skill. But se-duction relies on language as well, andencourages some of the most �oridspeech of all. Nor, in Dr Miller’s view ofthe world, is the ability to make usefulthings exempt from sexual selection.Well-made artefacts as much as artfuldecorations indicate good hand-eye co-ordination and imagination.

Whether Dr Miller’s mental peacocktails have an underlying unity is un-

clear. It could be the ability to process symbols; or it could be thatseveral di�erent abilities have evolved independently under asingle evolutionary pressure�the scrutiny of the opposite sex. Orit could be that sexual selection is not the reason after all, or atleast not the main part of it. But it provides a plausible explana-tion for modern humanity’s failure to interbreed with its Nean-derthal contemporaries, whether or not such unions would havebeen fertile: they just didn’t fancy them. 7

Christmas survey

ing back. Agriculture enabled man to shape his environment in away no species had done before.

In truth, agriculture turned out to be a Faustian bargain. Bothmodern and fossil evidence suggests that hunter-gatherers ledlonger, healthier and more leisured lives than did farmers untilless than a century ago. But farmers have numbers on their side.And numbers beget numbers, which in turn beget cities. The pathfrom Catalhoyuk in Anatolia, the oldest known town, to thestreets of Manhattan is but a short one, and the lives of people to-day, no matter how urbane and civilised, are shaped in large mea-sure by the necessities of their evolutionary past.

That fact has, however, only recently begun to be widely recog-nised. For many years, psychology, like anthropology, operated ina strange intellectual vacuum. Psychologists did not deny man’sevolutionary past, but they did not truly acknowledge it, either.Many in the �eld seemed to feel that humanity had somehowtranscended evolution. Indeed, those of a Marxist inclinationmore or less required that to be true. How else could people beperfectible? Dissenters were usually treated with disdain. But, atabout the time that Dr Cann was publishing the work that wouldexpose the fallacy of multiregionalism, a group who dubbedthemselves �evolutionary psychologists� began to stick theirheads above the academic parapets.

Eve’s psycheStudying the behaviour of humans is more di�cult than studyingthat of other animals, for two reasons. One is that the studentscome from the same species as the studied, which both reducestheir objectivity and causes them to take certain things forgranted, or even fail to notice them altogether. The other is that hu-man culture is, indeed, far more complex than the cultures ofother species. There is nothing wrong with studying that culture,of course. It is endlessly fascinating. But it is wrong to assume thatit is the cause of human nature, rather than a consequence; that isakin to mistaking the decorative �nishes of a build-ing for the underlying civil engineering. The aim ofevolutionary psychology is to try to detect the Dar-winian fabric through the cultural decoration, byasking basic questions.

Many of those questions, naturally, address sen-sitive issues of sex and violence�another reasonevolutionary psychologists are not universally pop-ular. David Buss, of the University of Texas, demon-strated experimentally what most people know in-tuitively�that women value high status in a mate ina way that men do not. Helen Fisher, of Rutgers Uni-versity, has dissected the evolutionary factorsthat cause marriages to succeed or fail. Shethinks, for example, that the tendency offemales to prefer high-status mates is atodds with the increasing economic in-dependence of women in the mod-ern world. Laura Betzig, of the Uni-versity of Michigan, put anexplicitly Darwinian spin on thetendency of powerful men to accu-mulate harems.

Randy Thornhill, of the Univer-sity of New Mexico, has shown thatphysical beauty is far from being inthe eye of the beholder. In fact, thosefeatures rated beautiful, most notably bo-dily symmetry, are good predictors ofhealthy, desirable attributes such as strong

immune systems�in other words, aesthetic sensibilities haveevolutionary roots.

Karl Grammer, of the Ludwig Bolzmann Institute of UrbanEthology, in Vienna, has shown that body odour, too, is correlatedwith symmetry and linked to immunological strength. Dr Thorn-hill, meanwhile, has raised quite a few hackles by arguing that apropensity to rape is an evolved characteristic of men rather thana pathology. Even murder has not escaped the attention of theevolutionary psychologists. Martin Daly and Margo Wilson, ofMcMaster University in Hamilton, Ontario, showed that adultsare far more likely to kill their stepchildren than their biologicalchildren�a fact that had escaped both police forces and sociolo-gists around the world. They then dared to propose a Darwinianexplanation for this, namely that step-parents have no direct inter-est (in the evolutionary sense) in the welfare of stepchildren.

However, something similar to this list of human behavioursthat have yielded to evolutionary psychology could be found inmany species. Indeed, it was often comparisons with other spe-cies that sparked the investigations in the �rst place. The males ofmany other species gather harems, but females rarely do so; fe-male swallows prefer their mates to have symmetrical tails andthey are also more faithful to high-status males; both male lionsand male baboons kill the infants of females in groups they havejust taken over; and so on. Where evolutionary explanations ofbehaviour become really interesting is when they home in onwhat is unique to humanity.

Playing games with the truthOne uniquely human characteristic is the playing of games withformal rules. Evolutionary psychology has not yet sought to ex-plain this, but it has exploited it extensively to develop and test itsideas. In their di�erent ways, the games devised by Leda Cos-mides and John Tooby, of the University of California at SantaBarbara, and Robert Axelrod, of the University of Michigan, have

underpinned that part of evolutionary psychologydevoted to uniquely human behaviour. For not allgames are about competition. Many also requiretrust, a sense of justice and sometimes self-denial.

Cases of animals apparently making sacri�ces,occasionally of their own lives, to help others arenot rare in nature, but at �rst sight they are surpris-ing. What is in it for the sacri�cer? The usual answer,worked out in the 1960s by William Hamilton, isthat the bene�ciary is a relative whose reproductiveoutput serves to carry genes found in the sacri�cerinto the next generation, albeit at one remove.Translated into human terms, this is good old-fash-

ioned nepotism. In a few species, though�man-kind being the most obvious�people will

make sacri�ces for non-relatives, or�friends�. The assumption is that the fa-vour will be paid back at some time in thefuture. The question is, how can the sac-ri�cer be sure that will happen?

Dr Axelrod used a branch of mathscalled game theory to come up with atleast part of the answer. He showedmathematically that as long as you canrecognise and remember your fellow

creatures, it makes sense to follow theproverb �fool me once, shame on you; fool

me twice, shame on me� and trust them pro-vided they don’t cheat you. (Sometimes in sci-

ence it is necessary to prove the obvious before1

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The Economist December 24th 2005 Christmas special 9

Human evolution

you go on to the less obvious.) Dr Cosmides and Dr Tooby used adi�erent sort of game to show that humans are thus, as Dr Axel-rod’s model suggests they should be, acutely sensitive to unfairtreatment. They did this by presenting some problems of formallogic to their experimental subjects as a card game. When the pro-blems were presented using cards with letters and numbers onopposite faces, and the subjects had to work out which cardsneeded to be turned over to yield the required answers, peoplefound them hard to do and more often than not got them wrong.However, when the problems were presented in a form that re-quired the subjects to decide whether people were being treatedfairly or not, they found them really easy. The researchers’ conclu-sion is that humans are hard-wired not for logic but for detectinginjustice.

Trust, and the detection and punishment of injustice, lie at theheart of human society. They are so important that people will ac-tually harm their own short-term interests to punish those they re-gard as behaving unfairly. Another game, for example, involvestwo people dividing a sum of money ($100, say). One makes thedivision and the other accepts or rejects it. If it is rejected, neitherplayer gets any money. On the face of it, even a 99:1 divisionshould be accepted, since the second player will be one dollar bet-ter o�. In practice, though, few people will accept less than a 70:30split. They will prefer to punish the divider’s greed rather thantake a small bene�t themselves.

This makes no sense in a one-o� transaction, but makes everysense if the two participants are likely to deal with each otherrepeatedly. And that, before the agricultural population boom(and also, for the most part, after it) was the normal state of a�airs.The people an individual dealt with routinely would have beenthe members of his circle of 150. Strangers would have been ad-mitted to this circle only after prolonged vetting. Such bonds oftrust, described by Matt Ridley, a science writer, as �the origins ofvirtue� in his book of that name, underlie the exchanges of goodsand services that are the basis of economics. They may also,though, underlie another sensitive subject that social scientists donot like biologists treading on: race.

Robert Kurzban, a colleague of Dr Cosmides and Dr Tooby,took the racial bull by the horns by reversing the old saw aboutbeauty. Dr Thornhill’s work overturned the folk wisdom thatbeauty is in the beholder’s eye by showing that universal stan-dards of beauty have evolved, and there are good reasons forthem. Dr Kurzban, by contrast, thinks he has shown that racereally does exist only in the eye�or, rather, the mind�of the be-holder, not the biology of the person being beheld, and does sofor good Darwinian reasons.

First impressions countDr Kurzban observes that the three criteria on which people rou-tinely, and often prejudicially, assess each other are sex, age andrace. Judgments based on sex and age make Darwinian sense, be-cause people have evolved in a context where these things matter.But until long-distance transport was invented, few people wouldhave come across members of other races. Dr Kurzban believesthat perceptions of racial di�erence are caused by the overstimu-lation of what might be called an �otherness detector� in the hu-man mind. This is there to sort genuine strangers, who will need towork hard to prove they are trustworthy, from those who aremerely unfamiliar members of the clan. It will latch on to any-thing unusual and obvious�and there is little that is more obvi-ous than skin colour. But other things, such as an odd accent, willdo equally well. Indeed, Dr Dunbar thinks that the speed withwhich accents evolve demonstrates that they are used in preciselythis sort of way.

If Dr Kurzban is right (and experiments he has done suggestthat assessments of allegiance are easily �rebadged� away fromskin colour by recognisable tokens such as coloured T-shirts, asany sports fan could probably have told him), it explains whyrace-perception is such a powerful social force, even though ge-neticists have failed to �nd anything in humans that would passmuster as geographical races in any other species. In fact, one ofthe striking things about Homo sapiens compared with, say, thechimpanzee is the genetic uniformity of the species. The only �ra-cial� di�erence that has a well-established function is skin colour.This balances the need to protect the skin from damage by ultra-violet light (which requires melanin, the pigment that makes skindark) and the need to make vitamin D (which results from the ac-tion of sunlight on a chemical in the skin). This explains dark,opaque skins in the tropics and light, transparent ones nearer thepoles. The test is that dark-skinned arctic dwellers, such as the In-uit of North America, have diets rich in vitamin D, and so do notneed to make it internally. As to other physical di�erences, theymay be the result of founder e�ects, as described by Dr Ambrose,or possibly of sexual selection, which can sometimes pick up andamplify arbitrary features.

Darwinian thinking can lead in other unexpected directions,too. Pursue Dr Buss’s observation about women preferring high-status males to its logical conclusion, and you have a plausible ex-planation for the open-endedness of economic growth. Psycholo-gists of a non-evolutionary bent sometimes profess themselvespuzzled by the fact that once societies leave penury behind (thecited income level varies, but $10,000 per person per year seemsabout the mark), they do not seem to get happier as they get richer.

That may be because incomes above a certain level are asmuch about status as about material well-being. Particularly ifyou are a man, status buys the best mates, and frequently more ofthem. But status is always relative. It does not matter how muchyou earn if the rest of your clan earn more. People (and men, inparticular) are always looking for ways to enhance their status�and a good income is an excellent way of doing so. Aristotle Onas-sis, a man who knew a thing or two about both wealth andwomen, once said: �If women didn’t exist, all the money in theworld would have no meaning.� Perhaps the founding father ofeconomics is not really Adam Smith, who merely explained howto get rich, but Charles Darwin, who helped to explain why. 7

Starchild

Evolution is still continuing

WHAT, then, of the future? Sitting in the comfort of theconcrete savannah, has humanity stopped evolving?

To help answer that question, it is instructive tolook at a paper published earlier this year by Gregory

Cochran. Dr Cochran, a scientist who, in the tradition of Darwinhimself, works independently of an academic institution, lookedat the unusual neurological illnesses commonly su�ered by Ash-kenazi Jews. Traditional wisdom has it that these diseases, whichare caused by faulty genes, are a consequence of inbreeding in asmall, closed population. The fact that they persist is said to showthat human evolution has stopped in our ever more mollycod-dled and medicalised world. Dr Cochran begged not only to dif-

Christmas survey

fer, but to draw precisely theopposite conclusion. He seesthese diseases as evidence ofvery recent evolution.

Until a century or two ago,the Ashkenazim�the Jews ofEurope�were often restricted bylocal laws to professions such asbanking, which happened to requirehigh intelligence. This is the sort of cultur-ally created pressure that might drive one ofDr Deacon’s feedback loops for mental abilities(though it must be said that Dr Deacon himself issceptical about this example). Dr Cochran, however,suspects that this is exactly what happened. He thinksthe changes in the brain brought about by the genes inquestion will be shown to enhance intelligence when onlyone copy of a given disease gene is present (you generallyneed two copies, one from each parent, to su�er the adversesymptoms). Indeed, in the case of Gaucher’s disease, whichis not necessarily lethal, there is evidence that su�erers aremore intelligent than average. If Ashkenazi Jews need to bemore intelligent than others, such genes will spread, even ifthey sometimes cause disease.

The fact is, you can’t stop evolution.Those who argue the opposite, pointing tothe survival thanks to modern medicine ofpeople who would previously have died,are guilty of more than just gross insensitiv-ity. They have tumbled into an intellectualpitfall that has claimed many victims sinceDarwin �rst published his theory. Evolutionis not about progress. It is about adaptation.Sometimes adaptation and progress are thesame. Sometimes they are the opposite. (Aska tapeworm, which has �degenerated� intoa mere egg-laying machine by a very successful process of adapta-tion.) If a mutation provides a better adaptation, as Dr Cochranthinks these disease genes did in �nanciers, it will spread. Giventhe changes that humanity has created in its own habitat, it seemsunlikely that natural selection has come to a halt. If Dr Deacon isright, it may even be accelerating as cultural change speeds up, al-

though the current rapidgrowth in the human popula-tion will disguise that for awhile, because selection worksbest in a static population.

The next big thingEvolution, then, has not stopped.

Indeed, it might be about to get an ar-ti�cial helping hand in the form of ge-

netic engineering. For the fallacy of evolu-tionary progress has deep psychological

roots, and those roots lie in Dr Miller’s peacock-tail version of events. The ultimate driver of sexual

selection is the need to produce o�spring who will bebetter than the competition, and will thus be selected by

desirable sexual partners. Parents know what traits are re-quired. They include high intelligence and a handful ofphysical characteristics, some of which are universal andsome of which vary according to race. That is why, oncethe idea of eliminating disease genes has been aired, ev-ery popular discussion on genetic engineering and clon-ing seems to get bogged down in intelligence, height and

(in the West) fair hair and blue eyes.This search for genetic perfection has

an old and dishonourable history, ofcourse, starting with the eugenic move-ment of the 19th century and ending inthe Nazi concentration camps of the 20th,where millions of the confrères of DrCochran’s subjects were sent to theirdeaths. With luck, the self-knowledge thatunderstanding humanity’s evolutionbrings will help avert such perversions inthe future. And if genetic engineering canbe done in a way that does not harm the

recipient, it would not make sense to ban it in a liberal society. Butthe impulse behind it will not go away because, progressive ornot, it is certainly adaptive. Theodosius Dobzhansky, one of thefounders of genetics, once said that �nothing in biology makessense except in the light of evolution�. And that is true even of hu-manity’s desire to take control of the process itself. 7

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10 Christmas special The Economist December 24th 2005

Future surveys

Countries and regions

Germany February 11th 2006

Chicago and the American heartland

March 18th 2006

China March 25th 2006

South Africa April 8th 2006

Poland May 6th 2006

Business, �nance and economics and ideas

Corporate organisation January 21st 2006

Wealth and philanthropy February 25th 2006

New media April 22nd 2006

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Acknowledgment and sourcesThe author would like to acknowledge thehelp of the numerous researchers named inthe text. The following books and websitemay be of interest to readers who wish tolearn more about the subject.

�Out of Eden�, by Stephen Oppenheimer(Constable and Robinson, paperback)�The Journey of Man�, by Spencer Wells(Penguin, paperback) �From Lucy to Language�, by Donald Johanson and Blake Edgar (Weidenfeld and Nicolson)�Grooming, Gossip and the Evolution ofLanguage�, by Robin Dunbar (Faber andFaber, paperback)�The Symbolic Species�, by TerrenceDeacon (W.W. Norton)�The Mating Mind�, by Geo�rey Miller(William Heinemann)�The Origins of Virtue�, by Matt Ridley(Penguin, paperback)

http://www.nationalgeographic.com/genographic

Christmas survey