UKCOVER CMYK Cyan Magenta Yellow Black The proper study of mankind A survey of human evolution December 24th 2005 Republication, copying or redistribution by any means is expressly prohibited without the prior written permission of The Economist C B M R Y G K W C B M R Y G K W ChristmasChristmas survey special The Economist December 24th 2005 1 Human evolution The proper study of mankind New theories and techniques have revolutionised our understanding of humanity’s past and present, says Georey Carr EVEN hundred and forty centuries ago, give or take a few, lations from larger ones can accel- the skies darkened and the Earth caught a cold. Toba, a vol- erate evolutionary change, be- Also in this section cano in Sumatra, had exploded with the sort of eruptive cause a small population’s Sforce that convulses the planet only once every few mil- average characteristics are likely The long march of lion years. The skies stayed dark for six years, so much dust did the to dier from those of the larger everyman eruption throw into the atmosphere. It was a dismal time to be group from which it is drawn. Like It all started in Africa. alive and, if Stanley Ambrose of the University of Illinois is right, much evolutionary theory, this is Page 4 the chances were you would be dead soon. In particular, the just applied common sense. But population of one species, known to modern science as Homo sa- only recently has such common Meet the relatives piens, plummeted to perhaps 2,000 individuals. sense been applied systematically A large and diverse family. The proverbial Martian, looking at that darkened Earth, would to areas of anthropology that have Page 6 probably have given long odds against these peculiar apes mak- traditionally ignored it and some- If this is a man ing much impact on the future. True, they had mastered the art of times resisted it. The result, when Why it pays to be brainy. tool-making, but so had several of their contemporaries. True, too, combined with new techniques Page 7 their curious grunts allowed them to collaborate in surprisingly of genetic analysis, has been a sophisticated ways. But those advantages came at a huge price, for revolution in the understanding The concrete their brains were voracious consumers of energya mere 2% of of humanity’s past. savannah the body’s tissue absorbing 20% of its food intake. An interesting And anthropology is not the Evolution and the modern evolutionary experiment, then, but surely a blind alley. only human science to have been world. Page 9 This survey will attempt to explain why that mythical Martian infused with evolutionary theory. would have been wrong. It will ask how these apes not only sur- Psychology, too, is undergoing a Starchild vived but prospered, until the time came when one of them could makeover and the result is a sec- Evolution is still weave together strands of evidence from elds as disparate as ge- ond revolution, this time in the continuing. Page 11 ology and genetics, and conclude that his ancestors had gone understanding of humanity’s through a genetic bottleneck caused by a geological catastrophe. present. Such understanding has been of two types, which often Not all of his contemporaries agree with Dr Ambrose about get confused. One is the realisation that many human activities, Toba’s eect on humanity. The eruption certainly happened, but not all of them savoury, happen for exactly the same reasons as in there is less consensus about his suggestion that it helped form the other species. For example, altruistic behaviour towards relatives, basis for what are now known as humanity’s racial divisions, by indelity, rape and murder are all widespread in the animal king- breaking Homo sapiens into small groups whose random physical dom. All have their own evolutionary logic. No one argues that quirks were preserved in dierent places. The idea is not, how- they are anything other than evolutionarily driven in species ever, absurd. It is based on a piece of evolutionary theory called other than man. Yet it would be extraordinary if they were not so the founder eect, which shows how the isolation of small popu- driven in man, because it would mean that natural selection had 1 2 Christmas surveyspecial The Economist December 24th 2005 Human evolution 2 somehow contrived to wipe out their genetic underpinnings, speculation scarcely worthy of the name of theory, which only for them to re-emerge as culturally determined phenomena. seemed to change with every new discovery. Then, in the 1980s, a Understanding this shared evolutionary history with other geneticist called Allan Wilson decided to redene the meaning of species is important; much foolishness has owed from its denial. the word fossil. He did so in a way that instantly revealed an- But what is far more intriguing is the progress made in under- other 6 billion specimens, for Wilson’s method made a fossil out standing what makes humanity dierent from other species: of every human alive. friendship with non-relatives; the ability to conceive of what oth- ers are thinking, and act accordingly; the creation of an almost un- Living fossils imaginably diverse range of artefacts, some useful, some merely In retrospect, Wilson’s insight, like many of the best, is blindingly decorative; and perhaps most importantly, the use of language, obvious. He knew, as any biologist would, that an organism’s which allows collaboration on a scale denied to other creatures. DNA carries a record of its evolutionary past. In principle, looking There are, of course, gaps in both sets of explanations. And this at similarities and dierences in the DNA sequences of living or- eld of research being a self-examination, there are also many ganisms should allow a researcher to reconstruct the family tree controversies, not all driven by strictly scientic motives. But the linking those organisms. In practice, the sexual mixing that hap- outlines of a science of human evolution that can explain human- pens with each generation makes this tedious even with today’s ity’s success, and also its continuing failings, are now in place. It is DNA-analysis techniques. With those available in the 1980s it just a question of lling in the canvasor the cave wall. 7 would have been impossible. Wilson, however, realised he could cut through the problem by concentrating on an unusual type of DNA called mitochondrial DNA. Mitochondria are the parts of a cell that convert energy stored in sugar into a form that the rest of the cell can use. Most of a cell’s The long march of genes 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 their everyman own. Mitochondrial genomes are easy to study for three reasons. First, they are small, which makes them simple to analyse. Sec- It all started in Africa ond, mitochondria reproduce asexually, so any changes between the generations are caused by mutation rather than sexual mix- UT of Africa, always something new, wrote Caius Pli- ing. Third, in humans at least, mitochondria are inherited only " nius Secundus, a Roman polymath who helped to invent from the mother. the eld of natural history. Pliny wrote more truly than In 1987, Rebecca Cann, one of Wilson’s students, applied his Ohe could possibly have realised. For one ne day, some- insight to a series of specimens taken from people whose ances- where between 85,000 and 60,000 years before he penned those tors came from dierent parts of the world. By analysing the muta- words, something did put its foot over the line that modern geog- tional dierences that had accumulated since their mitochondria raphers draw to separate Africa from shared a common ancestor, she was Asia. And that somethingor, rather, able to construct a matriline (or, per- somebodydid indeed start some- haps more accurately, a matritree) con- thing new, namely the peopling of the necting them. world. The result was a revelation. Which- Writing the story of the spread of ever way you drew the tree (statistics humanity is one of the triumphs of not being an exact science, there was modern science, not least because the more than one solution), its root was in ink used to do it was so unexpected. Africa. Homo sapiens was thus un- Like students of other past life forms, veiled as an African species. But Dr researchers into humanity’s prehis- Cann went further. Using estimates of toric past started by looking in the how often mutations appear in mito- rocks. The rst fossilised human to be chondrial DNA (the so-called molecu- recognised as such was unearthed in lar clock), she and Wilson did some 1856 in the Neander Valley near Dus- matridendrochronology. The result seldorf in Germany. Neanderthal suggests that all the lines converge on man, as this skeleton and its kin be- the ovaries of a single woman who came known, is now seen as a cousin lived some 150,000 years ago. of modern humans rather than an an- There was much excited reporting cestor, and subsequent digging has re- at the time about the discovery and vealed a branching tree of humanity dating of this African Eve. She was whose root can be traced back more not, to be clear, the rst female Homo than 4m years (see next article). sapiens. Fossil evidence suggests the Searching for human fossils, species is at least 200,000 years old, though, is a frustrating exercise.