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Old­est evi­dence of split between Old World mon­keys and apes – 25 mil­lion years old

pub­lished 18 May 2013 | mod­i­fied 05 April 2014

Rukwapithecus NsungwepithecusTwo fos­sil dis­cov­er­ies from the East African Rift reveal new infor­ma­tion about the evo­lu­tion of pri­mates, accord­ing to a study pub­lished online on 15 May in Nature led by Ohio Uni­ver­sity scientists.

The team’s find­ings doc­u­ment the old­est fos­sils of two major groups of pri­mates: the group that today includes apes and humans (homi­noids), and the group that includes Old World mon­keys such as baboons and macaques (cer­co­p­ithe­coids). Geo­log­i­cal analy­ses of the study site indi­cate that the finds are 25 mil­lion years old, sig­nif­i­cantly older than fos­sils pre­vi­ously doc­u­mented for either of the two groups.

Both pri­mates are new to sci­ence, and were col­lected from a sin­gle fos­sil site in the Rukwa Rift Basin of Tan­za­nia. Ruk­wap­ithe­cus flea­glei is an early homi­noid rep­re­sented by a mandible pre­serv­ing sev­eral teeth. Nsung­wep­ithe­cus gun­nelli is an early cer­co­p­ithe­coid rep­re­sented by a tooth and jaw frag­ment. The pri­mates lived dur­ing the Oligocene epoch, which lasted from 34 to 23 mil­lion years ago. For the first time, the study doc­u­ments that the two lin­eages were already evolv­ing sep­a­rately dur­ing this geo­log­i­cal period.

“The late Oligocene is among the least sam­pled inter­vals in pri­mate evo­lu­tion­ary his­tory, and the Rukwa field area pro­vides a first glimpse of the ani­mals that were alive at that time from Africa south of the equa­tor,” said Nancy Stevens, an asso­ciate pro­fes­sor of pale­on­tol­ogy in Ohio University’s Her­itage Col­lege of Osteo­pathic Med­i­cine who leads the palaeon­to­log­i­cal team.

Doc­u­ment­ing the early evo­lu­tion­ary his­tory of these groups has been elu­sive, as there are few fossil-​bearing deposits of the appro­pri­ate age, Stevens explained. Using an approach that dated mul­ti­ple min­er­als con­tained within the rocks, team geol­o­gists could deter­mine a pre­cise age for the specimens.

“The rift set­ting pro­vides an advan­tage in that it pre­serves dat­a­ble mate­ri­als together with these impor­tant pri­mate fos­sils,” said lead geol­o­gist Eric Roberts of James Cook Uni­ver­sity in Australia.

Prior to these finds, the old­est fos­sil rep­re­sen­ta­tives of the homi­noid and cer­co­p­ithe­coid lin­eages were recorded from the early Miocene, at sites dat­ing mil­lions of years younger.

The new dis­cov­er­ies are par­tic­u­larly impor­tant for help­ing to rec­on­cile a long-​standing dis­agree­ment between diver­gence time esti­mates derived from analy­ses of DNA sequences from liv­ing pri­mates and those sug­gested by the pri­mate fos­sil record, Stevens said. Stud­ies of clock-​like muta­tions in pri­mate DNA have indi­cated that the split between apes and Old World mon­keys occurred between 30 mil­lion and 25 mil­lion years ago.

Fos­sils from the Rukwa Rift Basin in south­west­ern Tan­za­nia pro­vide the first real test of the hypoth­e­sis that these groups [apes and Old World mon­keys] diverged so early, by reveal­ing a novel glimpse into this late Oligocene ter­res­trial ecosystem
Nancy Stevens, Her­itage Col­lege of Osteo­pathic Med­i­cine, Ohio University »

The new fos­sils are the first pri­mate dis­cov­er­ies from this pre­cise loca­tion within the Rukwa deposits, and two of only a hand­ful of known pri­mate species from the entire late Oligocene, glob­ally. The sci­en­tists scanned the spec­i­mens in a MicroCT scan­ner, allow­ing them to cre­ate detailed 3-​dimensional recon­struc­tions of the ancient spec­i­mens that were used for com­par­isons with other fossils.

Look back mil­lions of years at ancient fos­sils in this video (Credit: Ohio University/​NSF):

“This is another great exam­ple that under­scores how mod­ern imag­ing and com­pu­ta­tional approaches allow us to address more refined ques­tions about ver­te­brate evo­lu­tion­ary his­tory,” said Patrick O’Connor, co-​author and pro­fes­sor of anatomy in Ohio University’s Her­itage Col­lege of Osteo­pathic Medicine.

In addi­tion to the new pri­mates, Rukwa field sites have pro­duced sev­eral other fos­sil ver­te­brate and inver­te­brate species new to sci­ence. The late Oligocene inter­val is inter­est­ing because it pro­vides a final snap­shot of the unique species inhab­it­ing Africa prior to large-​scale fau­nal exchange with Eura­sia that occurred later in the Ceno­zoic Era, Stevens said.

A key aspect of the Rukwa Rift Basin project is the inter­dis­ci­pli­nary nature of the research team, with pale­on­tol­o­gists and geol­o­gists work­ing together to recon­struct ver­te­brate evo­lu­tion­ary his­tory in the con­text of the devel­op­ing East African Rift System.

“Since its incep­tion this project has employed a mul­ti­fac­eted approach for address­ing a series of large-​scale bio­log­i­cal and geo­log­i­cal ques­tions cen­tered on the East African Rift Sys­tem in Tan­za­nia,” O’Connor said.

The team’s research, funded by the U.S. National Sci­ence Foun­da­tion (NSF), the Leakey Foun­da­tion and the National Geo­graphic Soci­ety, under­scores the inte­gra­tion of palaeon­to­log­i­cal and geo­log­i­cal approaches that are essen­tial for address­ing com­plex issues in ver­te­brate evo­lu­tion­ary his­tory, the sci­en­tists noted.

(Source: Ohio Uni­ver­sity Research media release, 15.05.2013; NSF press release, 15.05.2013)

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