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See Inside Evolution: What Makes Us Human

Lucy's Baby [Preview]

An ancient infant skeleton yields new insight into how humans evolved to walk upright
A Hodgepodge Hominin

Experts may disagree over the functional significance of Selam's apelike skeletal characteristics, but they concur that different parts of the hominin body were undergoing selection at different times. A. afarensis is “a good example of mosaic evolution,” Johanson states. “You don't just magically flip some evolutionary switch somewhere and transmute a quadruped into an upright-walking bipedal human.” It looks like natural selection is selecting for bipedalism in the lower limbs and pelvis first, and things that are not really used in bipedal locomotion, such as arms and shoulders, change at a later stage, he says. “We're getting to know more and more about the sequence of changes” that produced a terrestrial biped from a tree-dwelling, apelike creature.

Analysis of Selam's skull hints at a similarly piecemeal metamorphosis. The shape of the hyoid—a delicate, rarely preserved bone that helps to anchor the tongue and the voice box—indicates that A. afarensis had air sacs in its throat, suggesting the species possessed an apelike voice box. Conversely, the child's brain shows a subtle sign of humanity. By studying the fossil's natural sandstone endocast, an impression of the braincase, Alemseged's team ascertained that Selam had attained only 65 to 88 percent of the adult brain size by the age of three. A chimp of comparable age, in contrast, has reached more than 90 percent of its adult brain size. This raises the tantalizing possibility that A. afarensis experienced a more humanlike pattern of brain growth.

More fossils are needed to discern whether the new skeleton is representative of A. afarensis infants, and scientists are doubtless eager to recover remains of other A. afarensis children of different ages—if they ever can—to see how they compare. But the little girl from Dikika still has more secrets to spill. “I think the impact of this specimen will be in its information of the growth and development of Australopithecus, not only for individual body parts but for rates of development among structures within one individual,” observes Carol V. Ward of the University of Missouri–Columbia.

Since the initial description of Selam, continued removal of the sandstone adhering to her remains has exposed all of the bones to some extent. Alemseged expects that he will eventually be able to reconstruct nearly the entire body of an A. afarensis three-year-old—and begin to understand what growing up australopithecine was all about.

MORE TO EXPLORE

New Look at Human Evolution. Special edition. Scientific American, Vol. 13, No. 2; 2003.

Becoming Human: Evolution and the Rise of Intelligence. Special edition. Scientific American, Vol. 16, No. 2; 2006.

Geological and Palaeontological Context of a Pliocene Juvenile Hominin at Dikika, Ethiopia. Jonathan G. Wynn et al. in Nature, Vol. 443, pages 332–336; September 21, 2006.

A Juvenile Early Hominin Skeleton from Dikika, Ethiopia. Zeresenay Alemseged et al. in Nature, Vol. 443, pages 296–301; September 21, 2006.

From Lucy to Language. Donald Johanson and Blake Edgar. Simon & Schuster, 2006.

Lucy's Legacy: The Quest for Human Origins. Donald C. Johanson and Kate Wong. Broadway, 2010.

Extended coverage of Lucy's baby—including more interviews, commentaries and photographs—can be found at www.ScientificAmerican.com/article.cfm?id=special-report-lucys-baby

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