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Bird attacks influenced human evolution, researchers say

Aug. 29, 2006
By Holly Wagner/Ohio State University 
and World Science staff

Pre­his­tor­ic birds of prey may have tar­geted our an­ces­tors for meals so of­ten that the threat of them helped drive hu­man ev­o­lu­tion, re­searchers say based on a study.

A skull of a Di­an­a mon­key. The hole to the right of the na­sal cav­i­ty was like­ly in­flicted by an ea­gle, re­searchers say. (Pho­to by Jo Mc­Culty, Ohi­o State Uni­ver­si­ty)

An­a­ly­z­ing hun­d­reds of mo­d­ern mon­key bones ga­th­ered be­low Af­ri­can ea­g­les’ nests, the sci­en­t­ists found the birds are a se­vere me­nace to some of our pri­mate cou­sins. 

They al­so con­c­lu­d­ed that the re­sult­ing bone in­ju­ries are sus­pi­cious­ly ve­ry like those on the skull of an ape-like child of hu­man an­ces­tors, found de­cades ago. It ap­pa­r­ent­ly has clawed-out eye­sock­ets, they sa­id.

“It seems that rap­tors have been a se­lec­tive force in pri­ma­te ev­o­lu­tion for a long time,” said W. Scott Mc­Graw of Ohi­o State uni­ver­si­ty, the stu­dy’s lead au­thor. 

“Be­fore this stu­dy I thought that ea­g­les would­n’t con­t­ri­b­ute that much to the mor­tal­i­ty rate of pri­mates in the fo­r­est. I could­n’t have been more wrong.”

The ide­a that birds ate ear­ly hu­mans is­n’t new. A 1995 stu­dy sug­gested that the pre­his­tor­ic 3½-year-old “Taung Child,” whose skull turned up in a South Af­ri­can ca­ve in 1924, might have been a vic­tim of such an in­ci­dent. But sci­en­tists saw the ev­i­dence as in­con­clu­sive.

McGraw, though, ar­gued that punc­ture marks on mon­key skulls close­ly re­sem­ble those found on the Taung skull, of the spe­cies Aus­t­ra­lo­pi­the­cus af­ri­ca­nus. “Ea­gles leave ve­ry dis­tinc­tive beak and tal­on punc­tures around the face and in the eye sock­et­s,” he ar­gued. “The skull of the Taung child has these same kinds of punc­ture mark­s.”

An African crowned ea­gle. (Pho­to by N. My­burg, cour­te­sy Ro­berts Mul­ti­me­dia Birds)

“This fos­sil is pro­b­a­bly the most writ­ten-a­bout, stu­d­ied and han­dled ho­m­i­nid skull ev­er,” he added. “But al­most no one had real­ly bo­th­ered to look at skulls dis­carded from ea­gle nests” to ce­ment the case against birds.

Such re­mains of­fer the best way to learn about ea­gles’ prey, Mc­Graw ar­gued.

“Ea­gles are am­bush pre­da­tors—they go in for the kill quick­ly,” he said. “So the chance of ac­tu­al­ly see­ing an ea­gle at­tack a mon­key is ex­treme­ly slim.” 

However, rap­tors, or birds of prey, “are kind en­ough to leave all the bones a­round af­ter­wards. That means we can work back­wards and con­s­truct a prey pro­file based on what’s left over.”

His team col­lect­ed some 1,200 an­i­mal bones dis­carded from 16 nests of Af­ri­can crowned ea­gles at the Ivo­ry Coast’s Tai Rainfor­est. The birds, about as big as Amer­i­can Bald Ea­gles, weigh 10 to 12 pounds (4.5 to 5.5 kg) as adults.

Slight­ly more than half of the bones, 669, came from pri­mates, the re­searchers found. The stu­dy is to ap­pear in the Oc­to­ber is­sue of the Amer­i­can Jour­nal of Phys­i­cal An­thro­pol­o­gy.

Most of the bones came from smaller mon­keys, but some origina­ted from ones weigh­ing up to 24 pounds (11 kg), they added. The majority were from mangabeys, the for­est’s larg­est mon­key. It seems the ea­gle “specif­i­cally tar­gets these large, rel­a­tively rare mon­keys,” Mc­Graw said. 

The find­ing sug­gests birds of this size could suc­cess­ful­ly take on a young hom­i­nid, he added; ar­chae­ol­o­gists estima­te that the Taung tod­dler weighed around 26 pounds (12 kg). 

“Many peo­ple thought that an ea­gle of this size would­n’t have enough strength to lift a pri­ma­te the size of the Taung child,” Mc­Graw said. “That’s a non-is­sue, be­cause ea­gles don’t hunt and proc­ess their kills that way. They typ­i­cal­ly dis­mem­ber their prey ve­ry quick­ly, and then take pieces of the car­cass back to the nest.”

Prehistoric birds of prey may have targeted our ancestors for meals so often that the threat of them helped drive human evolution, researchers say based on a new study. Analyzing hundreds of modern monkey bones gathered beneath eagles’ nests in the Ivory Coast, the scientists found that birds are a severe threat to some primates in the region. They also concluded that the bone injuries are very suspiciously like those on the skull of an ape-like child of human ancestors, found decades ago. The skull apparently has clawed-out eyeballs, they said. “It seems that raptors have been a selective force in primate evolution for a long time,” said W. Scott McGraw of Ohio State University, the study’s lead author. “Before this study I thought that eagles wouldn’t contribute that much to the mortality rate of primates in the forest. “I couldn’t have been more wrong.” The idea that birds ate early humans isn’t new. A 1995 study suggested that the prehistoric 3½-year-old “Taung Child,” whose skull turned up in a South African cave in 1924, might have been a victim of such an incident. But scientists saw the evidence as inconclusive. McGraw, though, argued that puncture marks on monkey skulls closely resemble those found on the Taung skull. “Eagles leave very distinctive beak and talon punctures around the face and in the eye sockets,” he argued. “The skull of the Taung child has these same kinds of puncture marks.” “This fossil is probably the most written-about, studied and handled hominid skull ever,” he added. “But almost no one had really bothered to look at skulls discarded from eagle nests” to cement the case against birds in the death. The best way to learn about an eagle’s prey is to gather remains that are in or near the raptor’s nest, McGraw argued. “Eagles are ambush predators – they go in for the kill quickly,” he said. “So the chance of actually seeing an eagle attack a monkey is extremely slim,” he continued. “Yet raptors are kind enough to leave all the bones around afterwards. That means we can work backwards and construct a prey profile based on what’s left over.” Over three years the researchers collected some 1,200 animal bones discarded from 16 nests of African crowned eagles at the Ivory Coast’s Tai Rainforest. The birds are about as big as American Bald Eagles and weigh 10 to 12 pounds (4.5 to 5.5 kg) as adults. Slightly more than half of the bones, 669, came from primates, the researchers found. The study is to appear in the October issue of the American Journal of Physical Anthropology. Most of the bones came from smaller monkeys, but some originated from ones weighing up to 24 pounds (11 kg), they added. Most were from mangabeys, the forest’s largest monkey. “It appears that the crowned hawk eagle specifically targets these large, relatively rare monkeys,” McGraw said. The finding suggests birds of this size could successfully take on a young hominid, he added; archaeologists estimate that the Taung toddler weighed around 26 pounds (12 kg). “Many people thought that an eagle of this size wouldn’t have enough strength to lift a primate the size of the Taung child,” McGraw said. “That’s a non-issue, because eagles don’t hunt and process their kills that way. They typically dismember their prey very quickly, and then take pieces of the carcass back to the nest.”