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20
août
2008

Stone-Age graveyard reveals life in a “green Sahara”

Sci­en­tists in Ni­ger have found the Sa­hara De­sert’s larg­est known Stone-Age grave­yard, which of­fers an un­par­al­leled rec­ord of life when the re­gion was green, the Na­tional Ge­o­graph­ic So­ci­e­ty an­nounced last Thurs­day.

Archaeologists work on a grave­site es­ti­mated as 10,000 years old in Go­be­ro, Ni­ger. (Image cour­tesy Nat’l Geo­gra­phic So­ciety) Uni­ver­s­ity of Chi­ca­go pro­fes­sor and Na­tional Ge­o­graph­ic Ex­plor­er-in-Re­si­dence Paul Se­re­no, whose team first hap­pened on the site dur­ing a di­no­saur-hunt­ing ex­pe­di­tion, un­earthed the ce­me­tery, ac­cord­ing to the or­gan­iz­a­tion.

Dat­ing back 10,000 years and called Gob­ero af­ter the Tua­reg name for the ar­ea, the site was brim­ming with skele­tons of hu­mans and an­i­mals in­clud­ing large fish and croc­o­diles, re­search­ers said.

Gob­ero is hid­den away with­in Ni­ger’s for­bid­ding Ténéré Des­ert, known to lo­cal Tua­reg no­mads as a “des­ert with­in a des­ert.” The Ténéré is the set­ting of other dra­ma­tic sci­en­ti­fic find­ings in­clud­ing the 500-toothed, plant-eat­ing di­no­saur Ni­ger­saur­us and the enor­mous ex­tinct croc­o­dil­ia Sar­co­suchus, al­so known as Su­per­Croc.

The dis­cov­ery of the lake­side grave­yard—said to rep­re­sent two suc­ces­sive hu­man popula­t­ions di­vid­ed by more than 1,000 years—is re­ported in the Sep­tem­ber is­sue of Na­tional Ge­o­graph­ic mag­a­zine and the Aug. 14 is­sue of the re­search jour­nal PLoS One.

As they ex­plored the site, re­search­ers said, they tip­toed among doz­ens of fos­sil­ized hu­man skele­tons laid bare on the sur­face of an an­cient dune field by the hot Sa­har­an wind. Jaw­bones still clenched nearly full sets of teeth ; a ti­ny hand reached up through the sand, its fin­ger bones in­tact. On the sur­face lay har­poon points, pot­sherds, beads and stone tools. The site was pris­tine, ap­par­ently nev­er vis­ited.

“Ev­ery­where you turned, there were bones be­long­ing to an­i­mals that don’t live in the de­sert,” said Sereno. “I real­ized we were in the green Sa­hara.”

Two sea­sons of ex­cava­t­ion sup­ported by the so­ci­e­ty even­tu­ally re­vealed some 200 graves clearly be­long­ing to two suc­ces­sive lake­side popula­t­ions, sci­en­tists said. The old­er group, de­ter­mined to be Kif­fian, were hunters of wild game who left ev­i­dence that they al­so speared huge perch with har­poons when they col­o­nized the green Sa­hara dur­ing its wet­test per­i­od be­tween 10,000 and 8,000 years ago. Their tall stat­ure, some­times reach­ing well over 6 feet, was not im­me­di­ately ap­par­ent from their tightly bound bur­i­al po­si­tions.

The more re­cent popula­t­ion was the Tene­r­ian, a more lightly built peo­ple who ap­peared to have had a di­verse econ­o­my of hunt­ing, fish­ing and cat­tle herd­ing, ac­cord­ing to the re­search team. They lived dur­ing the lat­ter part of the green Sa­hara, about 7,000 to 4,500 years ago. Their one-of-a-kind bur­i­als of­ten in­clud­ed jew­el­ry or rit­u­al pos­es—a girl wear­ing an upper-arm brace­let carved from a hip­po tusk, for ex­am­ple, and a stun­ning tri­ple bur­i­al con­tain­ing a wom­an and two chil­dren in a poign­ant em­brace.

“At first glance, it’s hard to im­ag­ine two more bi­o­log­ic­ally dis­tinct groups of peo­ple bur­y­ing their dead in the same place,” said team mem­ber Chris Sto­janowski, a bioar­chae­o­lo­g­ist from Ar­i­zo­na State Uni­ver­s­ity. “The big­gest mys­tery is how they seemed to have done this with­out dis­turb­ing a sin­gle grave.”

Al­though the Sa­hara has long been the world’s larg­est des­ert, a faint wob­ble in Earth’s or­bit and oth­er fac­tors oc­cur­ring some 12,000 years ago caused Af­ri­ca’s sea­son­al mon­soons to shift slightly north, bring­ing new rains to the Sa­hara. From Egypt in the east to Mau­ri­ta­nia in the west, lakes with lush mar­gins dot­ted the form­erly parched land­scape, draw­ing an­i­mals, fish and even­tu­ally peo­ple. Sep­a­rat­ing these two popula­t­ions was an ar­id in­ter­val per­haps as long as a mil­len­ni­um that be­gan about 8,000 years ago, when the lake disap­peared and the site was aban­doned.

Dat­ing the sun-bleached bones of fos­sil hu­mans in the Sa­hara has was very hard, re­search­ers said. Us­ing a new tech­nique, the team re­ported it had ob­tained nearly 80 so-called ra­dio­car­bon dates from Gob­ero bones and teeth. Ra­dio­car­bon dat­ing is a meth­od of es­ti­mat­ing the age of bi­o­log­ical ma­te­ri­al based on changes in its con­tent of ra­dio­ac­t­ive car­bon.

Ar­chae­o­lo­g­ist El­e­na Garcea of the Uni­ver­s­ity of Cas­si­no in Italy helped iden­ti­fy the site’s poorly known cul­tures so well-pre­served at the site. Garcea, an ex­pert in an­cient pot­tery who has spent nearly three dec­ades dig­ging at Stone Age sites in north­ern Af­ri­ca, trav­eled with Sereno in 2005 to the site. She re­calls stand­ing there amazed, gaz­ing at far more hu­man skele­tons than she had seen in all her pre­vi­ous digs com­bined.

She quickly homed in on two dis­tinct types of pot­tery, one that bore a pointil­lis­tic pat­tern linked with the Tene­r­ian and anoth­er that had wavy lines and zigzags. “These are Kif­fian,” a puz­zled Garcea told Sereno. “What is so amaz­ing is that the peo­ple who made these two types of pots lived in the same place more than a thou­sand years apart.”

Over the next three weeks Sereno, Garcea and their team of five ex­cavators made a de­tailed map of the site. They ex­humed eight bur­i­als and col­lect­ed scores of ar­ti­facts from both cul­tures. In a dry lake bed near­by, they found doz­ens of Kif­fian fish hooks and har­poons carved from an­i­mal bone as well as skele­tal re­mains of mas­sive Nile perch, croc­o­dile and hip­po.

A year lat­er, a sec­ond round of ex­cava­t­ion turned up more rid­dles, re­search­ers said. An adult Tene­r­ian male was bur­ied with his skull rest­ing on part of a clay ves­sel ; anoth­er adult male was in­terred seated on the shell of a mud tur­tle.

One bur­i­al, how­ev­er, brought 2006 ac­ti­vity at the site to a stand­still : Ly­ing on her side, the ske­l­e­ton of a pe­tite Tene­r­ian wom­an emerged from the sand, fac­ing the skele­tons of two young chil­dren ; their slen­der arms reached to­ward her and their hands were clasped in an ev­er­last­ing em­brace. Sam­ples tak­en from un­der the skele­tons con­tained pol­len clus­ters—taken as ev­i­dence the peo­ple had been laid out on a bed of flow­ers. The team em­ployed a range of new tech­niques to pre­serve this re­mark­a­ble bur­i­al ex­actly as it had been for more than 5,000 years.

Bioar­chae­o­lo­g­ist Sto­janowski an­a­lyzed doz­ens of in­di­vid­u­als’ bones and teeth for clues to the two popula­t­ions. “This in­di­vid­ual, for ex­am­ple, had huge leg mus­cles,” he said of ridges on the thigh bone of a Kif­fian ma­le, “which sug­gests he was eat­ing a lot of pro­tein and had an ac­tive, stren­u­ous lifestyle. The Kif­fian ap­pear to have been fairly healthy—it would be dif­fi­cult to grow a body that tall and mus­cu­lar with­out suf­fi­cient nu­tri­tion.” In con­trast, the fe­mur ridge of a Tene­r­ian male was barely per­cep­ti­ble. “This man’s life was less rig­or­ous, per­haps tak­ing smaller fish and game with more ad­vanced hunt­ing tech­nolo­gies,” Sto­janowski said.

Anal­y­sis of mea­sure­ments on Kif­fian skulls links them to skulls found across north­ern Af­ri­ca, some as old as 16,000 years, Sto­janowski said. The Tene­r­ian, how­ev­er, are not closely linked to these an­cient popula­t­ions. The team is con­tin­u­ing to an­a­lyze Gob­ero bones for more clues to the peo­ple’s health and di­et. A large-scale re­turn ex­pe­di­tion is planned to the site to fur­ther ex­plore the two popula­t­ions that coped with ex­treme cli­mate change.

Courtesy National Geographic Society and World Science staff



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