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Earth’s wobbles may explain some extinctions, research finds

Oct. 11, 2006
Courtesy Nature
and World Science staff
Updated Oct. 14

Wob­bles in the Earth’s or­bit may ex­plain a puz­zling cy­cle of ex­tinc­tions in the fos­sil rec­ord, a study has found.

 Mam­ma­li­an and some other spe­cies tend to sur­vive for an av­er­age of 2.5 mil­lion years be­fore go­ing ex­tinct, said the re­search­ers. New spe­cies tend to then arise in place of the old. 

The so-called Cas­cante sec­tion of the Te­ruel Ba­sin in north­east­ern Spain. The rough­ly nine-million to 10-million-year-old rocks con­tain rec­ords of ge­o­log­i­cal changes as well as abun­dant ro­dent fos­sils. (Cred­it: Hem­mo Ab­els).

The scientists, Jan van Dam of Utrecht Uni­ver­si­ty in the Neth­er­lands and col­leagues, sur­veyed about 22 mil­lion years’ worth of fos­sil da­ta. 

They found that peaks of spe­cies “turn­o­ver”—bouts of ex­tinc­tion ac­com­pa­nied by rise of new spe­cies—seem to cor­re­s­pond to changes in or­bit that cool the plan­et.

The group stud­ied the fos­sil rec­ord of ro­dents in Spain, which they said pro­vides a de­tailed ac­count of when these spe­cies rose and fell. 

Writ­ing in the Oct. 12 is­sue of the re­search jour­nal Na­ture, they ar­gued that “turn­o­ver” rates showed a com­plex pat­tern con­sist­ing of two dif­fer­ent cy­cles. One, long­er, has peaks rough­ly eve­ry 2.5 mil­lion years; the sec­ond peaks eve­ry mil­lion years.

The tim­ing of these mir­rors os­cil­la­tions in the Earth’s be­hav­iour, they added. The 2.5-mil­lion-year peaks oc­cur when the Earth’s or­bit is clos­er to be­ing a per­fect cir­cle; the mil­lion-year highs come when our world is shift­ing its de­gree of tilt on its ax­is.

Both pro­cesses lead to ice-sheet ex­pan­sion, glob­al cool­ing and changed pre­cip­i­ta­tion pat­terns, the re­search­ers ar­gued. 

“The as­tro­nom­i­cal hy­poth­e­sis for turn­o­ver of­fers a plau­si­ble ex­pla­na­tion for the char­ac­ter­is­tic du­ra­tion… of the mean spe­cies lifes­pan in mam­mals, and may ex­plain si­m­i­lar du­ra­tions in oth­er bi­o­log­i­cal groups,” they wrote.

Van Dam wrote in an e­mail that the cy­cle would pre­dict that a next peak in “turn­o­ver” rates would be in about 600,000 to 800,000 years. But cli­mate changes in the past three mil­lion years, and an­i­mals’ ad­ap­ta­tion to those changes, make it un­cer­tain that the cy­cle will re­peat it­self, he added. In any case, the time to that next pre­dicted peak is far long­er than the time most sci­en­tists think it will take for hu­man-caused in­dus­trial emis­sions, if un­checked, to cause ca­tas­troph­ic glo­bal warm­ing.

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Wobbles in the Earth’s orbit may explain the puzzling regularity with which new mammalian and other species appear and vanish in the fossil record, a study has found. Mammalian species tend to survive for an average of 2.5 million years before being snuffed out, said the researchers, Jan van Dam of Utrecht University in the Netherlands and colleagues The team of palaeontologists and geologists surveyed about 22 million years’ worth of fossil data. They found that peaks of species “turnover”—periods in which many species go extinct, to be replaced by new ones—seem to correspond to changes in Earth’s orbit, which cool the planet. The group studied the fossil record of rodents in Spain, which they said provides a detailed account of when these species rose and fell. Writing in the Oct. 12 issue of the research journal Nature, they argued that “turnover” rates showed a complex pattern consisting of two different cycles. One, longer, has peaks roughly every 2.5 million years; the second peaks every million years. The timing of these mirrors oscillations in the Earth’s behaviour, they added. The 2.5-million-year peaks occur when the Earth’s orbit is closer to being a perfect circle, and the million-year peaks come when the Earth is shifting its degree of tilt on its axis. Both processes result in ice-sheet expansion, global cooling and changes to precipitation patterns, the researchers argued. “The astronomical hypothesis for turnover offers a plausible explanation for the characteristic duration… of the mean species lifespan in mammals, and may explain similar durations in other biological group,” they wrote. Although the researchers didn’t venture a prediction of when the next peak in “turnover” rates would be, extrapolation from the historical figures they provided suggested it would come in slightly under 600,000 years.