A Short History of Nearly Everything-第102章

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ke　anydna　found　on　earth　now；　strongly　indicating　that　there　was　no　genetic　connection　betweenneandertals　and　modern　humans。　now　this　really　was　a　blow　to　multiregionalism。

1one　possibility　is　that　neandertals　and　cro…magnons　had　different　numbers　of　chromosomes；　a　plicationthat　monly　arises　when　species　that　are　close　but　not　quite　identical　conjoin。　in　the　equine　world；　forexample；　horses　have　64　chromosomes　and　donkeys　62。　mate　the　two　and　you　get　an　offspring　with　areproductively　useless　number　of　chromosomes；　63。　you　have；　in　short；　a　sterile　mule。

then　in　late　2000　nature　and　other　publications　reported　on　a　swedish　study　of　themitochondrial　dna　of　fifty…three　people；　which　suggested　that　all　modern　humans　emergedfrom　africa　within　the　past　100；000　years　and　came　from　a　breeding　stock　of　no　more　than10；000　individuals。　soon　afterward；　eric　lander；　director　of　the　whiteheadinstitute/massachusetts　institute　of　technology　center　for　genome　research；　announced　thatmodern　europeans；　and　perhaps　people　farther　afield；　are　descended　from　“no　more　than　afew　hundred　africans　who　left　their　homeland　as　recently　as　25；000　years　ago。”

as　we　have　noted　elsewhere　in　the　book；　modern　human　beings　show　remarkably　littlegenetic　variability—“there’s　more　diversity　in　one　social　group　of　fifty…five　chimps　than　inthe　entire　human　population；”　as　one　authority　has　put　it—and　this　would　explain　why。

because　we　are　recently　descended　from　a　small　founding　population；　there　hasn’t　been　timeenough　or　people　enough　to　provide　a　source　of　great　variability。　it　seemed　a　pretty　severeblow　to　multiregionalism。　“after　this；”　a　penn　state　academic　told　the　washington　post；“people　won’t　be　too　concerned　about　the　multiregional　theory；　which　has　very　littleevidence。”

but　all　of　this　overlooked　the　more　or　less　infinite　capacity　for　surprise　offered　by　theancient　mungo　people　of　western　new　south　wales。　in　early　2001；　thorne　and　his　colleaguesat　the　australian　national　university　reported　that　they　had　recovered　dna　from　the　oldest　ofthe　mungo　specimens—now　dated　at　62；000　years—and　that　this　dna　proved　to　be“genetically　distinct。”

the　mungo　man；　according　to　these　findings；　was　anatomically　modern—just　like　you　andme—but　carried　an　extinct　genetic　lineage。　his　mitochondrial　dna　is　no　longer　found　inliving　humans；　as　it　should　be　if；　like　all　other　modern　people；　he　was　descended　from　peoplewho　left　africa　in　the　recent　past。

“it　turned　everything　upside　down　again；”　says　thorne　with　undisguised　delight。

then　other　even　more　curious　anomalies　began　to　turn　up。　rosalind　harding；　a　populationgeneticist　at　the　institute　of　biological　anthropology　in　oxford；　while　studying　betaglobingenes　in　modern　people；　found　two　variants　that　are　mon　among　asians　and　theindigenous　people　of　australia；　but　hardly　exist　in　africa。　the　variant　genes；　she　is　certain；arose　more　than　200；000　years　ago　not　in　africa；　but　in　east　asia—long　before　modern　homosapiens　reached　the　region。　the　only　way　to　account　for　them　is　to　say　that　ancestors　ofpeople　now　living　in　asia　included　archaic　hominids—java　man　and　the　like。　interestingly；this　same　variant　gene—the　java　man　gene；　so　to　speak—turns　up　in　modern　populations　inoxfordshire。

confused；　i　went　to　see　harding　at　the　institute；　which　inhabits　an　old　brick　villa　onbanbury　road　in　oxford；　in　more　or　less　the　neighborhood　where　bill　clinton　spent　hisstudent　days。　harding　is　a　small　and　chirpy　australian；　from　brisbane　originally；　with　the　rareknack　for　being　amused　and　earnest　at　the　same　time。

“don’t　know；”　she　said　at　once；　grinning；　when　i　asked　her　how　people　in　oxfordshireharbored　sequences　of　betaglobin　that　shouldn’t　be　there。　“on　the　whole；”　she　went　on　moresomberly；　“the　genetic　record　supports　the　out…of…africa　hypothesis。　but　then　you　find　theseanomalous　clusters；　which　most　geneticists　prefer　not　to　talk　about。　there’s　huge　amounts　ofinformation　that　would　be　available　to　us　if　only　we　could　understand　it；　but　we　don’t　yet。

we’ve　barely　begun。”　she　refused　to　be　drawn　out　on　what　the　existence　of　asian…origingenes　in　oxfordshire　tells　us　other　than　that　the　situation　is　clearly　plicated。　“all　we　cansay　at　this　stage　is　that　it　is　very　untidy　and　we　don’t　really　know　why。”

at　the　time　of　our　meeting；　in　early　2002；　another　oxford　scientist　named　bryan　sykes　hadjust　produced　a　popular　book　called　the　seven　daughters　of　eve　in　which；　using　studies　ofmitochondrial　dna；　he　had　claimed　to　be　able　to　trace　nearly　all　living　europeans　back　to　afounding　population　of　just　seven　women—the　daughters　of　eve　of　the　title—who　livedbetween　10；000　and　45；000　years　ago　in　the　time　known　to　science　as　the　paleolithic。　to　eachof　these　women　sykes　had　given　a　name—ursula；　xenia；　jasmine；　and　so　on—and　even　adetailed　personal　history。　（“ursula　was　her　mother’s　second　child。　the　first　had　been　taken　bya　leopard　when　he　was　only　two。　。　。　。”）when　i　asked　harding　about　the　book；　she　smiled　broadly　but　carefully；　as　if　not　quitecertain　where　to　go　with　her　answer。　“well；　i　suppose　you　must　give　him　some　credit　forhelping　to　popularize　a　difficult　subject；”　she　said　and　paused　thoughtfully。　“and　thereremains　the　remote　possibility　that　he’s　right。”　she　laughed；　then　went　on　more　intently：

“data　from　any　single　gene　cannot　really　tell　you　anything　so　definitive。　if　you　follow　themitochondrial　dna　backwards；　it　will　take　you　to　a　certain　place—to　an　ursula　or　tara　orwhatever。　but　if　you　take　any　other　bit　of　dna；　any　gene　at　all；　and　traceit　back；　it　will　takeyou　someplace　else　altogether。”

it　was　a　little；　i　gathered；　like　following　a　road　randomly　out　of　london　and　finding　thateventually　it　ends　at　john　o’groats；　and　concluding　from　this　that　anyone　in　london　musttherefore　have　e　from　the　north　of　scotland。　they　might　have　e　from　there；　of　course；but　equally　they　could　have　arrived　from　any　of　hundreds　of　other　places。　in　this　sense；according　to　harding；　every　gene　is　a　different　highway；　and　we　have　only　barely　begun　tomap　the　routes。　“no　single　gene　is　ever　going　to　tell　you　the　whole　story；”　she　said。

so　genetic　studies　aren’t　to　be　trusted？

“oh　you　can　trust　the　studies　well　enough；　generally　speaking。　what　you　can’t　trust　are　thesweeping　conclusions　that　people　often　attach　to　them。”

she　thinks　out…of…africa　is　“probably　95　percent　correct；”　but　adds：　“i　think　both　sides　havedone　a　bit　of　a　disservice　to　science　by　insisting　that　it　must　be　one　thing　or　the　other。　thingsare　likely　to　turn　out　to　be　not　so　straightforward　as　either　camp　would　have　you　believe。　theevidence　is　clearly　starting　to　suggest　that　there　were　multiple　migrations　and　dispersals　indifferent　parts　of　the　world　going　in　all　kinds　of　directions　and　generally　mixing　up　the　genepool。　that’s　never　going　to　be　easy　to　sort　out。”

just　at　this　time；　there　were　also　a　number　of　reports　questioning　the　reliability　of　claimsconcerning　the　recovery　of　very　ancient　dna。　an　academic　writing　in　nature　had　noted　howa　paleontologist；　asked　by　a　colleague　whether　he　thought　an　old　skull　was　varnished　or　not；had　licked　its　top　and　announced　that　it　was。　“in　the　process；”　noted　the　nature　article；　“largeamounts　of　modern　human　dna　would　have　been　transferred　to　the　skull；”　rendering　ituseless　for　future　study。　i　asked　harding　about　this。　“oh；　it　would　almost　certainly　have　beencontaminated　already；”　she　said。　“just　handling　a　bone　will　contaminate　it。　breathing　on　itwill　contaminate　it。　most　of　the　water　in　our　labs　will　contaminate　it。　we　are　all　swimming　inforeign　dna。　in　order　to　get　a　reliably　clean　specimen　you　have　to　excavate　it　in　sterileconditions　and　do　the　tests　on　it　at　the　site。　it　is　the　trickiest　thing　in　the　world　not　tocontaminate　a　specimen。”

so　should　such　claims　be　treated　dubiously？　i　asked。

harding　nodded　solemnly。　“very；”　she　said。

if　you　wish　to　understand　at　once　why　we　know　as　little　as　we　do　about　human　origins；　ihave　the　place　for　you。　it　is　to　be　found　a　little　beyond　the　edge　of　the　blue　ngong　hills　inkenya；　to　the　south　and　west　of　nairobi。　drive　out　of　the　city　on　the　main　highway　touganda；　and　there　es　a　moment　of　startling　glory　when　the　ground　falls　away　and　you　arepresented　with　a　hang　glider’s　view　of　boundless；　pale　green　african　plain。

this　is　the　great　rift　valley；　which　arcs　across　three　thousand　miles　of　east　africa；marking　the　tectonic　rupture　that　is　setting　africa　adrift　from　asia。　here；　perhaps　forty　milesout　of　nairobi；　along　the　baking　valley　floor；　is　an　ancient　site　called　olorgesailie；　which　oncestood　beside　a　large　and　pleasant　lake。　in　1919；　long　after　the　lake　had　vanished；　a　geologistnamed　j。　w。　gregory　was　scouting　the　area　for　mineral　prospects　when　he　came　across　astretch　of　open　ground　littered　with　anomalous　dark　stones　that　had　clearly　been　shaped　byhuman　hand。　he　had　found　one　of　the　great　sites　of　acheulean　tool　manufacture　that　iantattersall　had　told　me　about。

unexpectedly　in　the　autumn　of　2002　i　found　myself　a　visitor　to　this　extraordinary　site。　iwas　in　kenya　for　another　purpose　altogether；　visiting　some　projects　run　by　the　charity　careinternational；　but　my　hosts；　knowing　of　my　interest　in　huma