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September 18, 2007

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Cold probably didn’t end Neanderthals: study

Sept. 12, 2007
Courtesy Nature
and World Science staff

What caused the de­mise of Ne­an­der­thal peo­ple, around 28,000 years ago in Eu­rope? Among the lead­ing the­o­ries, one is that mod­ern hu­mans out-competed or killed off this stocky breed of hu­mans. Another is that a sud­den pe­ri­od of bru­tal cold wiped them out.

But a new study con­cludes that the lat­ter is un­like­ly.

Ne­an­der­thal (left) and mod­ern hu­man (right) skele­tons. (Pho­to: K. Mow­bray; Re­con­struc­tion: G. Saw­yer and B. Ma­ley, © Ian Tat­ter­sall)

Con­fu­sion over the is­sue per­sists partly be­cause of prob­lems as­cer­tain­ing when things ac­tu­ally hap­pened. The usu­al way to do this is through a meth­od called car­bon dat­ing, but this has pit­falls.

All liv­ing things use the el­e­ment car­bon—mostly in the form of an iso­tope, or var­i­ant, called car­bon-12. But a frac­tion of car­bon in the en­vi­ron­ment is an iso­tope des­ig­nat­ed car­bon-14 that is ra­di­o­ac­tive, or un­sta­ble.

When an an­i­mal dies, a cer­tain pro­por­tion of the car­bon in it is car­bon-14, which then grad­u­ally dis­in­te­grates. By meas­ur­ing the amount of car­bon-14 in a fos­sil, re­search­ers can gauge when the crea­ture died—as long as they have an idea what the orig­i­nal amount was. This can be es­ti­mat­ed up to a point, but not ex­act­ly. 

The un­cer­tainties arise be­cause the amount of car­bon-14 in the en­vi­ron­ment varies some­what over time. The am­bigu­i­ties are es­pe­cially marked for the times around when the Ne­an­der­thals died off.

In the new stu­dy, Chro­nis Tzedakis of the Un­ivers­ity of Leeds, U.K. and col­leagues got around the prob­lem by not both­er­ing to mea­sure time in real years. They in­stead found they could re­late the car­bon da­ta di­rectly to records of past cli­mate ob­tain­able through an­cient deep-sea sed­i­ments drilled from the sites in Venezue­la’s Ca­ri­aco Ba­sin. This would let them link par­ti­cu­lar fos­sils to the world cli­mate around their time.

Us­ing this meth­od, the team in­ves­t­i­gated three pro­posed dates for the end of the Ne­an­der­thals and found that the old­est two co­in­cide with no ex­treme cli­mate. The youngest, and most contro­versial, oc­curred just be­fore an ex­pan­sion of ice sheets; but this was a sev­er­al thousand-year long, grad­u­al tran­si­tion, not an ab­rupt cold snap that would ex­plain a sud­den ex­tinction, they ar­gued. The find­ings ap­pear in the Sept. 13 is­sue of the re­search jour­nal Na­ture.

“Our find­ings suggest that there was no single cli­ma­tic event that caused the ex­tinc­tion of the Nean­der­thals,” said Ka­te­rina Har­vati of the Max Planck In­sti­tute for Ev­o­lu­tion­ary An­thro­pol­o­gy in Leip­zig, Ger­ma­ny. At most, cold was just a con­tri­but­ing fac­tor in their de­mise, she added.

* * *

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What caused the demise of Neanderthal people, around 28,000 years ago in Europe? Perhaps the two most popular theories are that modern humans out-competed or killed off this stocky breed of humans; or that a sudden period of brutal cold wiped them out. But a new study has found that the latter is unlikely. Confusion over the issue persists partly because of problems ascertaining when things actually happened. The usual method is through a technology called carbon dating, but this has pitfalls. All living things use the element carbon—mostly in the form of an isotope, or variant, called carbon-12. But a fraction of carbon in the environment is an isotope designated carbon-14 which is radioactive, that is, unstable. When an animal dies, a certain fraction of the carbon within it is carbon-14, which then gradually disintegrates. By measuring the amount of carbon-14 in a fossil, researchers can gauge when the creature died—as long as they have an idea what the original amount was. This can be estimated up to a point, but not exactly. The uncertainties arise becuase the amount of carbon-14 in the environment varies somewhat over time. The ambiguities are especially marked around the Neanderthals went extinct. In the new study, Chronis Tzedakis of the University of Leeds, U.K. and colleagues got around the problem by ignoring actual chronology completely. They instead found they could relate the carbon data directly to records of past climate obtainable through ancient deep-sea sediments drilled from the sites in Venezuela’s Cariaco Basin. Using this method, the team investigated three proposed dates for the end of the Neanderthals and found that the oldest two do not coincide with any extreme climate events. The youngest, and most controversial, occured just before an expansion of ice sheets; but this was a several thousand-year long, gradual transition, not an abrupt cold snap that would explain a sudden extinction, they argued. The findings appear in the Sept. 13 issue of the research journal Nature.