"Long before it's in the papers"
February 17, 2011

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Recent “human ancestor” finds under question

Feb. 17, 2011
Courtesy of New York University
and World Science staff

Two sci­en­tists are ques­tion­ing claims that sev­er­al prom­i­nent fos­sil finds from the last dec­ade are re­mains of hu­man an­ces­tors.

It seems at least as like­ly, said one of the skep­tics, New York Uni­vers­ity bi­o­log­i­cal an­thro­po­l­o­gist Ter­ry Har­ri­son, that the bones are “fos­sil apes sit­u­at­ed close to the an­ces­try of the liv­ing great ape and hu­mans.”

“Don't get me wrong, these are all im­por­tant finds,” added his re­search part­ner Ber­nard Wood of George Wash­ing­ton Uni­vers­ity in Wash­ing­ton, D.C. The pair de­tail their doubts in a pa­per in the cur­rent is­sue of the re­search jour­nal Na­ture. They fo­cus on three fos­sil spe­cies, four to sev­en mil­lion years old, dubbed Or­rorin, Sa­helan­thro­pus and Ardi­pi­the­cus. The lat­ter, com­monly called Ardi, turned up in Ethi­o­pia and was very dif­fer­ent from what many re­search­ers had ex­pected for an early hu­man an­ces­tor, Wood and Harr­i­son main­tain. 

They claim that the orig­i­nal re­ports al­so failed to prop­erly con­sid­er that even dis­tantly re­lat­ed spe­cies can share iden­ti­cal char­ac­ter­is­tics. For ex­am­ple, they say, small ca­nine teeth in Ardi­pi­the­cus and Sa­helan­thro­pus may be the most con­vinc­ing ev­i­dence that they are hu­man an­ces­tors—yet even small ca­nines evolved in­de­pend­ently in sev­er­al ape lin­eages, probably due to si­m­i­lar shifts in di­et.

Sci­en­tists gen­er­ally be­lieve the hu­man line­age branched off from that of the chim­pan­zee six to eight mil­lion years ago. While it's easy to tell bones of those two spe­cies apart to­day, it's harder to dis­tin­guish them when they're clos­er to their com­mon an­ces­tor, as is the case with those three fos­sil spe­cies, Wood and Har­ri­son note.

They al­so cau­tion that his­to­ry has shown how un­crit­i­cal re­li­ance on a few si­m­i­lar­i­ties be­tween fos­sil apes and hu­mans can lead to er­rors. For in­stance, Ra­ma­pi­the­cus, a spe­cies of fos­sil ape from south Asia, was mis­tak­enly called an early hu­man an­ces­tor in the 1960s and 1970s, but lat­er found to be a close rel­a­tive of the orang­u­tan. And Ore­o­pi­the­cus bam­bolii, a fos­sil ape from It­a­ly, shares many si­m­i­lar­i­ties with early hu­man an­ces­tors, in­clud­ing skele­tal fea­tures sug­gest­ing it may have walked up­right. How­ev­er, the au­thors say, enough is known of its anat­o­my to show that it's dis­tantly re­lat­ed to hu­mans and ac­quired “hu­man-like” traits in­de­pend­ently.

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Homepage image: Artist's representa­tion of a fe­male Ar­d­i­pe­th­i­cus ra­mi­dus. (© J. H. Mat­ter­n­es)

Two scientists are questioning claims that several prominent fossil finds from the last decade are remains of human ancestors. It seems at least as likely, said one of the skeptics, New York University biological anthropologist Terry Harrison, that the bones are “fossil apes situated close to the ancestry of the living great ape and humans.“ “Don't get me wrong, these are all important finds,“ added his research partner Bernard Wood of George Washington University in Washington, D.C. The pair details their doubts in a paper in the current issue of the research journal Nature. They focus on three fossil species, four to seven million years old, dubbed Orrorin, Sahelanthropus and Ardipithecus. The latter, commonly called Ardi, turned up in Ethiopia and was radically different from what many researchers had expected for an early human ancestor. Woods and Harrison argue that the original reports also failed to properly consider that even distantly related species can share identical characteristics. For example, small canine teeth in Ardipithecus and Sahelanthropus may be the most convincing evidence that they are human ancestors, Wood and Harrison maintain—yet small canines also evolved independently in several ape lineages, probably due to similar shifts in diet. Scientists generally believe the human lineage branched off from that of the chimpanzee six to eight million years ago. While it's easy to tell bones of those two species apart today, it's harder to distinguish them when they're closer to their common ancestor, as is the case with those three fossil species, Wood and Harrison said. They also caution that history has shown how uncritical reliance on a few similarities between fossil apes and humans can lead to errors. For instance, Ramapithecus, a species of fossil ape from south Asia, was mistakenly called an early human ancestor in the 1960s and 1970s, but later found to be a close relative of the orangutan. And Oreopithecus bambolii, a fossil ape from Italy, shares many similarities with early human ancestors, including skeletal features suggesting it may have walked upright. However, the authors say, enough is known of its anatomy to show that it's distantly related to humans and acquired many “human-like“ features independently.