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鈥渓ucy and her kind did not loote in anything like the modern human fashion；鈥潯nsiststattersall。 鈥渙nly when these hominids had to travel between arboreal habitats would they findthemselves walking bipedally； 鈥榝orced鈥櫋o do so by their own anatomies。鈥潯ohanson doesn鈥檛accept this。 鈥渓ucy鈥檚 hips and the muscular arrangement of her pelvis；鈥潯e has written； 鈥渨ouldhave made it as hard for her to climb trees as it is for modern humans。鈥�
matters grew murkier still in 2001 and 2002 when four exceptional new specimens werefound。 one； discovered by meave leakey of the famous fossil…hunting family at laketurkana in kenya and called kenyanthropus platyops （鈥渒enyan flat…face鈥潱弧s from about thesame time as lucy and raises the possibility that it was our ancestor and lucy was anunsuccessful side branch。 also found in 2001 were ardipithecus ramidus kadabba； dated atbetween 5。2 million and 5。8 million years old； and orrorin tugenensis； thought to be 6 millionyears old； making it the oldest hominid yet found鈥攂ut only for a brief while。 in the summerof 2002 a french team working in the djurab desert of chad （an area that had never beforeyielded ancient bones） found a hominid almost 7 million years old； which they labeledsahelanthropus tchadensis。 （some critics believe that it was not human； but an early ape andtherefore should be called sahelpithecus。） all these were early creatures and quite primitivebut they walked upright； and they were doing so far earlier than previously thought。
bipedalism is a demanding and risky strategy。 it means refashioning the pelvis into a fullload…bearing instrument。 to preserve the required strength； the birth canal must beparatively narrow。 this has two very significant immediate consequences and one longer…term one。 first； it means a lot of pain for any birthing mother and a greatly increased dangerof fatality to mother and baby both。 moreover to get the baby鈥檚 head through such a tightspace it must be born while its brain is still small鈥攁nd while the baby； therefore； is stillhelpless。 this means long…term infant care； which in turn implies solid male鈥揻emale bonding。
all this is problematic enough when you are the intellectual master of the planet； but whenyou are a small； vulnerable australopithecine； with a brain about the size of an orange；3therisk must have been enormous。
3absolute brain size does not tell you everything…or possibly sometimes even much。 elephants and whales bothhave brains larger than ours； but you wouldnt have much trouble outwitting them in contract negotiations。 it isrelative size that matters； a point that is often overlooked。 as gould notes； a。 africanus had a brain of only 450cubic centimeters； smaller than that of a gorilla。 but a typical africanus male weighed less than a hundredpounds； and a female much less still； whereas gorillas can easily top out at 600 pounds （gould pp。 181…83）。
so why did lucy and her kind e down from the trees and out of the forests？ probablythey had no choice。 the slow rise of the isthmus of panama had cut the flow of waters fromthe pacific into the atlantic； diverting warming currents away from the arctic and leading tothe onset of an exceedingly sharp ice age in northern latitudes。 in africa； this would haveproduced seasonal drying and cooling； gradually turning jungle into savanna。 鈥渋t was not somuch that lucy and her like left the forests；鈥潯ohn gribbin has written； 鈥渂ut that the forestsleft them。鈥�
but stepping out onto the open savanna also clearly left the early hominids much moreexposed。 an upright hominid could see better； but could also be seen better。 even now as aspecies； we are almost preposterously vulnerable in the wild。 nearly every large animal youcan care to name is stronger； faster； and toothier than us。 faced with attack； modern humanshave only two advantages。 we have a good brain； with which we can devise strategies； andwe have hands with which we can fling or brandish hurtful objects。 we are the only creaturethat can harm at a distance。 we can thus afford to be physically vulnerable。
all the elements would appear to have been in place for the rapid evolution of a potentbrain； and yet that seems not to have happened。 for over three million years； lucy and herfellow australopithecines scarcely changed at all。 their brain didn鈥檛 grow and there is no signthat they used even the simplest tools。 what is stranger still is that we now know that forabout a million years they lived alongside other early hominids who did use tools； yet theaustralopithecines never took advantage of this useful technology that was all around them。
at one point between three and two million years ago； it appears there may have been asmany as six hominid types coexisting in africa。 only one； however； was fated to last： homo；which emerged from the mists beginning about two million years ago。 no one knows quitewhat the relationship was between australopithecines and homo； but what is known is thatthey coexisted for something over a million years before all the australopithecines； robust andgracile alike； vanished mysteriously； and possibly abruptly； over a million years ago。 no oneknows why they disappeared。 鈥減erhaps；鈥潯uggests matt ridley； 鈥渨e ate them。鈥�
conventionally； the homo line begins with homo habilis； a creature about whom we knowalmost nothing； and concludes with us； homo sapiens （literally 鈥渕an the thinker鈥潱！nbetween； and depending on which opinions you value； there have been half a dozen otherhomo species： homo ergaster； homo neanderthalensis； homo rudolfensis； homoheidelbergensis； homo erectus； and homo antecessor。
homo habilis （鈥渉andy man鈥潱as named by louis leakey and colleagues in 1964 and wasso called because it was the first hominid to use tools； albeit very simple ones。 it was a fairlyprimitive creature； much more chimpanzee than human； but its brain was about 50 percentlarger than that of lucy in gross terms and not much less large proportionally； so it was theeinstein of its day。 no persuasive reason has ever been adduced for why hominid brainssuddenly began to grow two million years ago。 for a long time it was assumed that big brainsand upright walking were directly related鈥攖hat the movement out of the forests necessitatedcunning new strategies that fed off of or promoted braininess鈥攕o it was something of asurprise； after the repeated discoveries of so many bipedal dullards； to realize that there wasno apparent connection between them at all。
鈥渢here is simply no pelling reason we know of to explain why human brains gotlarge；鈥潯ays tattersall。 huge brains are demanding organs： they make up only 2 percent of thebody鈥檚 mass； but devour 20 percent of its energy。 they are also paratively picky in what they use as fuel。 if you never ate another morsel of fat； your brain would not plainbecause it won鈥檛 touch the stuff。 it wants glucose instead； and lots of it； even if it means short…changing other organs。 as guy brown notes： 鈥渢he body is in constant danger of beingdepleted by a greedy brain； but cannot afford to let the brain go hungry as that would rapidlylead to death。鈥潯 big brain needs more food and more food means increased risk。
tattersall thinks the rise of a big brain may simply have been an evolutionary accident。 hebelieves with stephen jay gould that if you replayed the tape of life鈥攅ven if you ran it backonly a relatively short way to the dawn of hominids鈥攖he chances are 鈥渜uite unlikely鈥潯hatmodern humans or anything like them would be here now。
鈥渙ne of the hardest ideas for humans to accept；鈥潯e says； 鈥渋s that we are not theculmination of anything。 there is nothing inevitable about our being here。 it is part of ourvanity as humans that we tend to think of evolution as a process that； in effect； wasprogrammed to produce us。 even anthropologists tended to think this way right up until the1970s。鈥潯ndeed； as recently as 1991； in the popular textbook the stages of evolution； c。
loring brace stuck doggedly to the linear concept； acknowledging just one evolutionary deadend； the robust australopithecines。 everything else represented a straightforwardprogression鈥攅ach species of hominid carrying the baton of development so