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MARGINALIA

You’ll Never Guess Who Walked In!

Ardi redefines the branch between apes and hominins

Pat Shipman

The best thing about paleontology is the surprises.

No matter how carefully you have analyzed the fossils, no matter how insightful your understanding of the links between anatomical form and function, Mother Nature always comes up with something totally unpredicted.

Surprises certainly have been sprung by, and on, the international team of paleoanthropologists and paleontologists that looks for fossils in the remote Aramis region of Ethiopia where the Afar people live. The team is co-led by Berhane Asfaw of the Rift Valley Research Service in Addis Ababa, Ethiopia; Yonas Beyene of the Ministry of Youth, Sports and Culture in Ethiopia;the late J. Desmond Clark, formerly of the University of California, Berkeley; Giday Woldegabriel of the Los Alamos National Laboratory; and Tim White of the University of California, Berkeley. With a nice touch of delicacy, White refers to Clark as “inspiring but no longer making decisions” about the project.

Partial skeleton of “Ardi,” from the species Ardipithecus ramidusClick to Enlarge ImageOn October 2, 2009, the team published in Science their analyses of a hominin (member of the human lineage) called Ardipithecus ramidus. The best representative of the species is a partial female skeleton nicknamed Ardi; she is 4.4 million years old and is certainly astonishing and noteworthy. There are parts of at least 35 other individuals in the collection, in addition to thousands of specimens of plants, invertebrates, fish and assorted nonprimate mammals from the same location.

Ardi is the oldest partial skeleton in the hominin lineage at present, but she doesn’t represent a new species. Ardipithecus ramidus has been known since the first research paper about it was published in Nature in 1994 under what might be considered an “alias”: Australopithecus ramidus.

The naming of Ardi has had some singular twists and turns. The genus into which the type specimen of Ardipithecus was initially classified was Australopithecus, meaning “southern ape,” a term created in 1925 by Raymond Dart to accommodate a face, jaw and endocast (a natural cast of the braincase) of a young juvenile hominin found in South Africa. The “Taung baby,” as it was nicknamed, was the oldest (about 2.5 million years) and first member of the human lineage to be found in Africa at that point. Skepticism from the scientific establishment at the time of discovery was rife. The hoaxed Piltdown skull, with its large brain (variously estimated between 1,000 and 1,500 cubic centimeters) and seeming antiquity, had been announced in 1911–12 and was still believed to be genuine. Thus the small brain (about 340 cubic centimeters) of the Taung baby was so unexpected that many anthropologists dismissed the fossil as merely an immature, if ancient, ape. Surely our large brain was a fundamental human characteristic extending back to the beginning of our lineage! “Surely”—but not in fact.

More than 25 years passed before the anthropological community realized what Dart had recognized from his fossil: Our ancestors had humanlike teeth long before they had a large, humanlike brain. Instead of brain size being the fundamental adaptation that separated our earliest ancestors from the apes, it was our teeth and, later discoveries showed, our bipedal locomotion. How surprising—and disappointing to those who had prided themselves on their big brains.

Since 1925, many different species of Australopithecus have been described and named, including Australopithecus afarensis, the species to which the famous partial skeleton Lucy belongs. All are bipedal hominins, although not necessarily directly ancestral to modern humans. Some apparently ate fruit, whereas others had a more fibrous diet. Some were larger, some smaller, and most species showed a striking difference in body size between males and females (a trait called sexual dimorphism). Some probably made stone tools. They lived in both East and South Africa and may have been more widely spread across the continent between 4.2 and 1.2 million years ago.




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