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
Walking vs. Climbing

The exceptional preservation of Selam, as well as that of other animals found at the site, indicates to team geologist Jonathan G. Wynn of the University of South Florida that her body was buried shortly after death by a flood event. Whether she perished in the flood or before it is unknown.

Although she was only three when she died, Selam already possessed the distinctive characteristics of her species. Her projecting snout and narrow nasal bones, for example, readily distinguish her from another ancient youngster, the so-called Taung child from South Africa, who was a member of the closely related Australopithecus africanus species. And her lower jaw resembles mandibles from Hadar, the site where Lucy and a number of other A. afarensis individuals were found.

Selam also exhibits the same mash-up of traits in her postcranial skeleton that has long irked scientists interested in how A. afarensis moved around the landscape. Scholars agree that A. afarensis was a creature that got around capably on two legs. But starting in the 1980s, a debate erupted over whether the species was also adapted for life in the trees. The argument centered on the observation that whereas the species has clear adaptations to bipedal walking in its lower body, its upper body contains a number of primitive traits better suited to an arboreal existence, such as long, curved fingers for grasping tree branches. One camp held that A. afarensis had made a full transition to terrestrial life and that the tree-friendly features of the upper body were just evolutionary baggage handed down from an arboreal ancestor. The other side contended that if A. afarensis had retained those traits for hundreds of thousands of years, then tree climbing must have still formed an important part of its locomotor repertoire.

Like her conspecifics, Selam has legs built for walking and fingers built for climbing. But she also brings new data to the controversy in the form of two shoulder blades, or scapulae—bones previously unknown for this species. According to Alemseged, her scapulae look most like those of a gorilla. The upward-facing shoulder socket is particularly apelike, contrasting sharply with the laterally facing socket modern humans have. This orientation, Alemseged points out, may have facilitated raising the hands above the head—something primates do when they climb. (Although gorillas do not climb as adults, they do spend time in the trees as youngsters.)

Further hints of arboreal tendencies reside in the baby's inner ear. Using computed tomographic imaging, the team was able to glimpse her semicircular canal system, which is important for maintaining balance. The researchers determined that Selam's semicircular canals are similar to those of African apes and A. africanus. This, they suggest, could indicate that A. afarensis was not as fast and agile on two legs as we modern humans are. It could also mean that A. afarensis was limited in its ability to decouple the movements of its head and torso, a feat that seems to play a key role in endurance running in our own species.

The conclusion that A. afarensis was a bipedal creature with an upper body at least partly adapted for life in the trees echoes what Jack T. Stern, Jr., of Stony Brook University and his colleagues wrote years ago in their reports on Lucy and her contemporaries. “I was happy to see that this paper suggests I might have been right,” Stern comments. Johanson agrees that the case for a partly arboreal A. afarensis is stronger than it once was. “Early on I was a staunch advocate of strict terrestrial bipedalism in afarensis,” he remarks. But taking more recent findings into consideration, Johanson says, “it's not out of the realm of possibility that they were still exploiting some of the arboreal habitats for getting off the ground at night and sleeping up there or going back to familiar food sources.”

A combination of walking and climbing would fit neatly with the picture that is emerging from studies of the environments of early hominins, including Selam. Today Dikika is an expanse of dusty hills dotted with only the occasional tree or shrub. But 3.3 million years ago, it was a well-watered delta flanked by forests, with some grasslands nearby. “In this context, it is not surprising to have an ‘ape’ that spends time in the trees and on the ground,” comments project member René Bobe, now at George Washington University.

Not everyone is persuaded by the arboreal argument. C. Owen Lovejoy of Kent State University disputes the claim that Selam's scapula looks like a gorilla's. “It's primitive, but it's really more humanlike than gorillalike,” he remarks. Lovejoy, a leading proponent of the idea that A. afarensis was a dedicated biped, maintains that the forelimb features that are typically held up as indicators that A. afarensis spent time in the trees only provide “evidence that the animal has an arboreal history.” The discovery of the famed Laetoli footprints in 1978 closed the debate, he states. The trail did not show a prehensile big toe, without which, Lovejoy says, A. afarensis simply could not move about effectively in the trees.

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