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A new the­ory about what trig­gered the loss of a num­ber of large car­ni­vores around two mil­lion years ago in East Africa has been launched recently at a sym­po­sium on human evo­lu­tion and cli­mate change.

On 19^th April, Lars Werdelin of the Swedish Museum of Nat­ural His­tory in Stock­holm pre­sented his hypoth­e­sis at the sym­po­sium which was hosted by Colum­bia University’s Lamont-​Doherty Earth Observatory.

In short it comes down to our ances­tors who started to use stone tools and devel­oped an omniv­o­rous diet that included sig­nif­i­cantly more meat than their pre­de­ces­sors con­sumed. So they became com­peti­tors for the 18 large car­ni­vores present about two mil­lions years ago. Because they were more flex­i­ble in their diet — as omni­vores they could sur­vive in times when prey was scarce, unlike the large preda­tors which relied on meat — and able to drive away the large preda­tor from its prey with their stone tools. And so our ances­tors drove about 12 large car­ni­vore species to extinc­tion in East Africa. This trig­gered cas­cades of ecosys­tem dis­rup­tion, because when top preda­tors dis­ap­pear the pop­u­la­tion size of their prey will increase fol­lowed by sev­eral other consequences.

Werdelin devel­oped this the­ory because he dis­cov­ered that two mil­lion years ago the num­ber of small car­ni­vore species that went extinct was sig­nif­i­cantly less than the num­ber of large car­ni­vore species that dis­ap­peared. Like­wise, the pat­tern of loss in the fos­sil records that Werdelin stud­ied bore no resem­blance to the pat­tern of cli­mate change related losses that mod­ern car­ni­vores have been expe­ri­enc­ing. This led to Werdelin’s con­clu­sion that cli­mate change was not the cause of the decline in large preda­tors. Yet cli­mate change prob­a­bly caused envi­ron­men­tal changes between two to three mil­lion years ago, forc­ing our ear­li­est ances­tors to leave the trees and onto the open savanna where they had to face the large carnivores.

Although Werdelin impressed his peers at the sym­po­sium, sev­eral ques­tions were raised, espe­cially about the tim­ing. When exactly did our ances­tors start to use stone tools, and when did the decline of large preda­tor species set in, as well as how did this coin­cide or not with the cli­mate change events? Fur­ther­more, quite a few assump­tions in Werdelin’s the­ory can be chal­lenged. Some work has to be done before this the­ory can be proven as being valid. Some more fos­sil data has to be stud­ied. Until then it is work in progress.

(Source: Sci­en­tific Amer­i­can, 25.04.2012; Wikipedia)