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Pre-“Lucy” fossils reveal secrets

Oct. 1, 2009
Courtesy Science
and World Science staff

Sci­en­tists are re­port­ing that they have for the first time thor­oughly de­scribed Ar­di­p­ithe­cus ramidus, a spe­cies of ape-like hu­man an­ces­tor that lived 4.4 mil­lion years ago in what is now Ethi­o­pia. 

The anal­y­sis, con­sist­ing of 11 re­search pa­pers by var­i­ous groups, ap­pears in the Oct. 2 is­sue of the re­search jour­nal Sci­ence. The fos­sils in­clude a par­tial ske­l­e­ton of a fe­male, nick­named Ardi. 

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

The last com­mon an­ces­tor shared by hu­mans and chim­panzees is thought to have lived six or more mil­lion years ago. Though Ar­di­p­ithe­cus is not this com­mon an­ces­tor, it probably shared many of its char­ac­ter­is­tics, re­search­ers say. 

Ardip­ithe­cus is more than a mil­lion years old­er than a fa­mous par­tial ske­l­e­ton of Aus­tra­lo­pith­e­cus afa­ren­sis, nick­named Lu­cy. Un­til the dis­cov­ery of the Ar­di­p­ithe­cus re­mains, the fos­sil rec­ord con­tained scant ev­i­dence of oth­er ho­minids old­er than Aus­tra­lo­pith­e­cus. 

Through anal­y­sis of the bones, the re­search­ers have found that Ar­di­p­ithe­cus had a mix of “prim­i­tive” traits, shared with its pre­de­ces­sors, the pri­ma­tes of the so-called Mi­o­cene ep­och, and “derived” traits, which it shares ex­clu­sively with lat­er ho­minids.

Ar­di­p­ithe­cus takes us clos­er to the still-elusive last com­mon an­ces­tor, re­search­ers said; but many of its traits don’t ap­pear in mod­ern-day Af­ri­can apes, so the lat­ter probably have changed ex­ten­sively since the last shared an­ces­tor. Thus, they re­marked, to­day’s chimps and go­ril­las may be poor mod­els for the last com­mon an­ces­tor and for un­der­stand­ing our own ev­o­lu­tion since then.

“In Ar­di­p­ithe­cus we have an un­spe­cial­ized form that has­n’t evolved very far in the di­rec­tion of Aus­tra­lo­pith­e­cus. So when you go from head to toe, you’re see­ing a mo­sa­ic crea­ture, that is nei­ther chim­pan­zee, nor is it hu­man,” said Tim White of the Un­ivers­ity of Cal­i­for­nia Berke­ley, who is one of the lead au­thors of the re­search. 

“With such a com­plete skel­e­ton, and with so many oth­er in­di­vid­u­als of the same spe­cies at the same time ho­ri­zon, we can really un­der­stand the bi­ol­o­gy of this ho­minid,” said Ge­n Suwa of the Un­ivers­ity of To­kyo, a pa­le­oan­thro­po­l­o­gist and al­so lead au­thor of one of the papers. Ho­minids are an ev­o­lu­tion­ary group that in­cludes hu­mans and theif fos­sil an­ces­tors.

Reconstructed frontal view of "Ardi," based on re­search by C. Owen Love­joy of Kent State Uni­ver­sity in the jour­nal Sci­ence. (© J. H. Mat­ter­n­es)

“These ar­ti­cles con­tain an enor­mous amount of da­ta col­lect­ed and an­a­lyzed through a ma­jor in­terna­t­ional re­search ef­fort,” said Brooks Han­son, dep­u­ty ed­i­tor at Sci­ence.

“They throw open a win­dow in­to a pe­ri­od of hu­man ev­o­lu­tion we have known lit­tle about, when early ho­minids were es­tab­lish­ing them­selves in Af­ri­ca, soon af­ter di­verg­ing from the last an­ces­tor they shared with the Af­ri­can apes.”

White and coau­thors in­tro­duce their dis­cov­ery of over 110 Ar­di­p­ithe­cus spec­i­mens in­clud­ing a par­tial ske­l­e­ton with much of the skull, hands, feet, limbs and pel­vis. This in­di­vid­ual, Ardi, was a fe­male who weighed about 50 kilo­grams (110 pounds) and stood about 120 cen­time­ters (47 inches) tall. 

Un­til now, re­search­ers have gen­er­ally as­sumed that chimps, go­ril­las and oth­er mod­ern Af­ri­can apes re­tained many of the traits of the last an­ces­tor they shared with hu­mans – in oth­er words, this pre­sumed an­ces­tor was thought to be much more chim­pan­zee-like than hu­man-like. For ex­am­ple, it would have been adapted for swing­ing and hang­ing from tree branches, and per­haps walked on its knuck­les while on the ground. 

Ar­di­p­ithe­cus chal­lenges these as­sump­tions, re­search­ers said.

These ho­minids ap­pear to have lived in a wood­land en­vi­ron­ment, where they climbed on all fours along tree branches – as some of the Mi­o­cene pri­ma­tes did – and walked, up­right, on two legs, while on the ground, sci­en­tists ex­plained. They don’t seem to have been knuckle-walkers, or to have spent much time swing­ing and hang­ing from branches, es­pe­cially as chimps do.

Charles Dar­win, found­er of ev­o­lu­tion­ary the­o­ry, was “very wise on this mat­ter,” said White. “Dar­win said we have to be really care­ful. The only way we’re really go­ing to know what this last com­mon an­ces­tor looked like is to go and find it.”

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Scientists are reporting that they have for the first time thoroughly described Ardipithecus ramidus, a species of ape-like human ancestor that lived 4.4 million years ago in what is now Ethiopia. The analysis, consisting of 11 research papers by various groups, appears in the Oct. 2 issue of the research journal Science. The fossils include a partial skeleton of a female, nicknamed Ardi. The last common ancestor shared by humans and chimpanzees is thought to have lived six or more million years ago. Though Ardipithecus is not this last common ancestor, it probably shared many of its characteristics, researchers say. Ardipithecus is more than a million years older than a famous partial skeleton of Australopithecus afarensis, nicknamed Lucy. Until the discovery of the Ardipithecus remains, the fossil record contained scant evidence of other hominids older than Australopithecus. Through analysis of the bones, the researchers have found that Ardipithecus had a mix of “primitive” traits, shared with its predecessors, the primates of the so-called Miocene epoch, and “derived” traits, which it shares exclusively with later hominids. Ardipithecus takes us closer to the still-elusive last common ancestor, researchers said; but many of its traits don’t appear in modern-day African apes, so the latter probably have changed extensively since the last shared ancestor. This fact, they remarked, makes today’s chimps and gorillas poor models for the last common ancestor and for understanding our own evolution since then. “In Ardipithecus we have an unspecialized form that hasn’t evolved very far in the direction of Australopithecus. So when you go from head to toe, you’re seeing a mosaic creature, that is neither chimpanzee, nor is it human,” said Tim White of the University of California Berkeley, who is one of the lead authors of the research. “With such a complete skeleton, and with so many other individuals of the same species at the same time horizon, we can really understand the biology of this hominid,” said Gen Suwa of the University of Tokyo, Project paleoanthropologist and also a lead Science author. Hominids are an evolutionary group that includes humans and theif fossil ancestors. “These articles contain an enormous amount of data collected and analyzed through a major international research effort. They throw open a window into a period of human evolution we have known little about, when early hominids were establishing themselves in Africa, soon after diverging from the last ancestor they shared with the African apes,” said Brooks Hanson, deputy editor for physical sciences, at Science. White and coauthors introduce their discovery of over 110 Ardipithecus specimens including a partial skeleton with much of the skull, hands, feet, limbs and pelvis. This individual, Ardi, was a female who weighed about 50 kilograms (110 pounds) and stood about 120 centimeters (47 inches) tall. Until now, researchers have generally assumed that chimps, gorillas and other modern African apes retained many of the traits of the last ancestor they shared with humans – in other words, this presumed ancestor was thought to be much more chimpanzee-like than human-like. For example, it would have been adapted for swinging and hanging from tree branches, and perhaps walked on its knuckles while on the ground. Ardipithecus challenges these assumptions, researchers said. These hominids appear to have lived in a woodland environment, where they climbed on all fours along tree branches – as some of the Miocene primates did – and walked, upright, on two legs, while on the ground, scientists explained. They don’t seem to have been knuckle-walkers, or to have spent much time swinging and hanging from tree-branches, especially as chimps do. Overall, the findings suggest that hominids and African apes have each followed different evolutionary pathways, and we can no longer consider chimps as “proxies” for our last common ancestor. Charles Darwin, founder of evolutionary theory, was “very wise on this matter,” said White. “Darwin said we have to be really careful. The only way we’re really going to know what this last common ancestor looked like is to go and find it.”