A Short History of Nearly Everything-第98章

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　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’s　head　through　such　a　tightspace　it　must　be　born　while　its　brain　is　still　small—and　while　the　baby；　therefore；　is　stillhelpless。　this　means　long…term　infant　care；　which　in　turn　implies　solid　male–female　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。　“it　was　not　somuch　that　lucy　and　her　like　left　the　forests；”　john　gribbin　has　written；　“but　that　the　forestsleft　them。”

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’t　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。　“perhaps；”　suggests　matt　ridley；　“we　ate　them。”

conventionally；　the　homo　line　begins　with　homo　habilis；　a　creature　about　whom　we　knowalmost　nothing；　and　concludes　with　us；　homo　sapiens　（literally　“man　the　thinker”）。　inbetween；　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　（“handy　man”）　was　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—that　the　movement　out　of　the　forests　necessitatedcunning　new　strategies　that　fed　off　of　or　promoted　braininess—so　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。

“there　is　simply　no　pelling　reason　we　know　of　to　explain　why　human　brains　gotlarge；”　says　tattersall。　huge　brains　are　demanding　organs：　they　make　up　only　2　percent　of　thebody’s　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’t　touch　the　stuff。　it　wants　glucose　instead；　and　lots　of　it；　even　if　it　means　short…changing　other　organs。　as　guy　brown　notes：　“the　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。”　a　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—even　if　you　ran　it　backonly　a　relatively　short　way　to　the　dawn　of　hominids—the　chances　are　“quite　unlikely”　thatmodern　humans　or　anything　like　them　would　be　here　now。

“one　of　the　hardest　ideas　for　humans　to　accept；”　he　says；　“is　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。”　indeed；　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—each　species　of　hominid　carrying　the　baton　of　development　so　far；　then　handingit　on　to　a　younger；　fresher　runner。　now；　however；　it　seems　certain　that　many　of　these　earlyforms　followed　side　trails　that　didn’t　e　to　anything。

luckily　for　us；　one　did—a　group　of　tool　users；　which　seemed　to　arise　from　out　of　nowhereand　overlapped　with　the　shadowy　and　much　disputed　homo　habilis。　this　is　homo　erectus；　thespecies　discovered　by　eugène　dubois　in　java　in　1891。　depending　on　which　sources　youconsult；　it　existed　from　about　1。8　million　years　ago　to　possibly　as　recently　as　twenty　thousandor　so　years　ago。

according　to　the　java　man　authors；　homo　erectus　is　the　dividing　line：　everything　thatcame　before　him　was　apelike　in　character；　everything　that　came　after　was　humanlike。　homoerectus　was　the　first　to　hunt；　the　first　to　use　fire；　the　first　to　fashion　plex　tools；　the　first　toleave　evidence　of　campsites；　the　first　to　look　after　the　weak　and　frail。　pared　with　all　thathad　gone　before；　homo　erectus　was　extremely　human　in　form　as　well　as　behavior；　itsmembers　long…limbed　and　lean；　very　strong　（much　stronger　than　modern　humans）；　and　withthe　drive　and　intelligence　to　spread　successfully　over　huge　areas。　to　other　hominids；　homoerectus　must　have　seemed　terrifyingly　powerful；　fleet；　and　gifted。

erectus　was　“the　velociraptor　of　its　day；”　according　to　alan　walker　of　penn　stateuniversity　and　one　of　the　world’s　leading　authorities。　if　you　were　to　look　one　in　the　eyes；　itmight　appear　superficially　to　be　human；　but　“you　wouldn’t　connect。　you’d　be　prey。”

according　to　walker；　it　had　the　body　of　an　adult　human　but　the　brain　of　a　baby。

although　erectus　had　been　known　about　for　almost　a　century　it　was　known　only　fromscattered　fragments—not　enough　to　e　even　close　to　making　one　full　skeleton。　so　it　wasn’tuntil　an　extraordinary　discovery　in　africa　in　the　1980s　that　its　importance—or；　at　the　veryleast；　possible　importance—as　a　precursor　species　for　modern　humans　was　fully　appreciated。

the　remote　valley　of　lake　turkana　（formerly　lake　rudolf）　in　kenya　is　now　one　of　theworld’s　most　productive　sites　for　early　human　remains；　but　for　a　very　long　time　no　one　hadthought　to　look　there。　it　was　only　because　richard　leakey　was　on　a　fl