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December 24, 2012

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Fluctuating environment may have spurred human intelligence

Dec. 24, 2012
Courtesy of Penn State University
and World Science staff

A string of rap­id en­vi­ron­men­tal changes in East Af­ri­ca around two mil­lion years ago may have spurred the growth of early hu­man in­tel­li­gence, sci­en­tists pro­pose.

“The land­scape early hu­mans were in­hab­it­ing tran­si­tioned rap­idly back and forth be­tween a closed wood­land and an open grass­land about five to six times dur­ing a pe­ri­od of 200,000 years,” said Clay­ton Mag­ill, grad­u­ate stu­dent in geo­sciences at Penn State Uni­vers­ity who co-authored a re­port on the sub­ject. The work is pub­lished in this week’s on­line edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.

Re­search­ers ex­am­ined lake sed­i­ments from Oldu­vai Gorge in north­ern Tan­za­nia, look­ing for bio­mark­er­s—fos­sil mo­le­cules—from an­cient trees and grass­es. (Cred­it: Gail Ash­ley)
“These changes hap­pened very ab­rupt­ly, with each tran­si­tion oc­cur­ring over hun­dreds to just a few thou­sand years,” Mag­ill added.

Sci­en­tists’ pre­vail­ing view has been that hu­man ev­o­lu­tion in this era was linked to a long, steady en­vi­ron­men­tal change, or one big change in cli­mate, said geo­sci­en­tist Kath­er­ine Free­man of the uni­vers­ity.

“There is a view this time in Af­ri­ca was the ‘Great Dry­ing,’ when the en­vi­ronment slowly dried out over three mil­lion years,” she said. “But our da­ta show that it was not a grand pro­gres­sion to­wards dry; the en­vi­ronment was highly vari­able.”

Many an­thro­po­l­o­gists be­lieve vari­abil­ity of ex­pe­ri­ence can trig­ger cog­ni­tive de­vel­op­ment.

“Early hu­mans went from hav­ing trees avail­a­ble to hav­ing only grasses avail­a­ble in just 10 to 100 genera­t­ions, and their di­ets would have had to change in re­spon­se,” Mag­ill said. “Changes in food avail­abil­ity, food type, or the way you get food can trig­ger ev­o­lu­tion­ary mech­a­nisms to deal with those changes. The re­sult can be in­creased brain size and cog­ni­tion, changes in lo­co­mo­tion and even so­cial changes—how you inte­ract with oth­ers in a group,” he ex­plained.

“Our da­ta are con­sist­ent with these hy­pothe­ses,” he added. “The en­vi­ronment changed dra­mat­ic­ally over a short time, and this vari­abil­ity co­in­cides with an im­por­tant pe­ri­od in our hu­man ev­o­lu­tion when the ge­nus Ho­mo was first es­tab­lished and when there was first ev­i­dence of tool use.” Ho­mo is a line­age that in­cludes hu­­mans and their closer, extinct rela­tives.

The re­search­ers ex­am­ined lake sed­i­ments from Oldu­vai Gorge in north­ern Tan­za­nia. They sam­pled or­gan­ic ma­te­ri­al that long ago was washed or blown in­to the lake from the sur­round­ing plants, mi­crobes and oth­er or­gan­isms. They fo­cused on “biomark­ers”—pre­served mo­le­cules from an­cient or­gan­isms—from the waxy coat­ing on plant leaves. “We looked at leaf waxes be­cause they’re tough, they sur­vive well in the sed­i­ment,” said Free­man.

Using tech­niques known as gas chro­ma­tog­ra­phy and mass spec­trom­e­try, the sci­en­tists meas­ured the rel­a­tive abun­dances of dif­fer­ent waxes. They al­so meas­ured their var­y­ing lev­els of iso­topes, or vari­ants, of car­bon. With the da­ta they re­con­struct­ed the types of vegeta­t­ion pre­s­ent in the ar­ea at dif­fer­ent times.

The sci­en­tists fur­ther in­ves­t­i­gated what might have caused the rap­id changes they iden­ti­fied.

“The or­bit of the Earth around the sun slowly changes with time,” said Free­man. “These changes were tied to the lo­cal cli­mate at Oldu­vai Gorge through changes in the mon­soon sys­tem in Af­ri­ca. Slight changes in the amount of sun­shine changed the in­tens­ity of at­mos­pher­ic cir­cula­t­ion and the supply of wa­ter. The rain pat­terns that drive the plant pat­terns fol­low this mon­soon cir­cula­t­ion. We found a cor­rela­t­ion be­tween changes in the en­vi­ronment and plan­e­tary move­ment.”

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A series of rapid environmental changes in East Africa around two million years ago may be have spurred the growth of early human intelligence, scientists propose. “The landscape early humans were inhabiting transitioned rapidly back and forth between a closed woodland and an open grassland about five to six times during a period of 200,000 years,” said Clayton Magill, graduate student in geosciences at Penn State University who co-authored a report on the subject. The work is published in this week’s online edition of the research journal PNAS. “These changes happened very abruptly, with each transition occurring over hundreds to just a few thousand years,” Magill added. Scientists’ prevailing view has been that human evolution in this era was linked to a long, steady environmental change, or one big change in climate, said geoscientist Katherine Freeman of the university. “There is a view this time in Africa was the ‘Great Drying,’ when the environment slowly dried out over 3 million years,” she said. “But our data show that it was not a grand progression towards dry; the environment was highly variable.” According to Magill, many anthropologists believe that variability of experience can trigger cognitive development. “Early humans went from having trees available to having only grasses available in just 10 to 100 generations, and their diets would have had to change in response,” he said. “Changes in food availability, food type, or the way you get food can trigger evolutionary mechanisms to deal with those changes. The result can be increased brain size and cognition, changes in locomotion and even social changes—how you interact with others in a group,” he explained. “Our data are consistent with these hypotheses… the environment changed dramatically over a short time, and this variability coincides with an important period in our human evolution when the genus Homo was first established and when there was first evidence of tool use.” The researchers examined lake sediments from Olduvai Gorge in northern Tanzania. They sampled organic material that long ago was washed or blown into the lake from the surrounding vegetation, microbes and other organisms. They focused on “biomarkers”—fossil molecules from ancient organisms—from the waxy coating on plant leaves. “We looked at leaf waxes because they’re tough, they survive well in the sediment,” said Freeman. Used techniques known as gas chromatography and mass spectrometry, the scientists measured the relative abundances of different waxes. They also measured their varying levels of isotopes, or variants, of carbon. With the data they reconstructed the types of vegetation present in the area at different times. The scientists further investigated what might have caused the rapid changes they identified. “The orbit of the Earth around the sun slowly changes with time,” said Freeman. “These changes were tied to the local climate at Olduvai Gorge through changes in the monsoon system in Africa. Slight changes in the amount of sunshine changed the intensity of atmospheric circulation and the supply of water. The rain patterns that drive the plant patterns follow this monsoon circulation. We found a correlation between changes in the environment and planetary movement.”