• Abstract and Figures
• Public Full-text

LUCY’S BABY AN EXTRAORDINARY NEW HUMAN FOSSIL RENEWS DEBATE OVER THE EVOLUTION OF UPRIGHT WALKING BY KATE WONG he arid badlands of Ethiopia’s remote Afar brow and the small size of the canine teeth, among Tregion have long been a favorite hunting ground other humanlike characteristics. Further evaluation, for paleoanthropologist. Many hominins—the however, would have to wait until the fossil was group that included all the creatures in the human cleaned—a painstaking process in which the line since it branched away from that of the cementlike matrix is removed from the bone almost chimps—once called it home. The area is perhaps grain by grain with dental tools. best known for having yielded “Lucy,” the 3.2–mil- lion–year–old skeleton of a human ancestor known It took Alemseged five years to expose key ele- as Australopithecus afarensis. Now researchers ments of the child’s anatomy; many more bones have unveiled another incredible A. afarensis speci- remain obscured by the sediment. Still, the find has men from a site called Dikika, just four kilometers already surrendered precious insights into a species from where Lucy turned up. But unlike Lucy, who that most researchers believe gave rise to our own was well into adulthood by the time she died, the genus, Homo. Alemseged and his colleagues new fossil is that of an infant, one who lived 3.3 described the fossil and its geologic and paleonto- million years ago (and yet has nonetheless been logical context in two papers published in the dubbed “Lucy’s baby”). September 21 Nature. And at a press conference held in Ethiopia to announce the discovery, they No other hominin skeleton of such antiquity— christened the child Selam—“peace” in several including Lucy—is as complete as this one. Ethiopian languages—in hopes of encouraging har- Moreover, as the earliest juvenile hominin ever mony among the warring tribes of Afar. found, the Dikika child provides an unprecedented opportunity to study growth processes in our The skeleton, judged to be that of a three-year- ancient relatives. “If Lucy was the greatest fossil old girl, consists of a virtually complete skull, the discovery of the 20th century,” says Donald C. entire torso, and parts of the arms and legs. Even the Johanson of Arizona State University, who kneecaps—no larger than macadamia nuts—are unearthed the famed fossil in 1974, “then this baby preserved. Many of the bones are still in articula- is the greatest find of the 21st thus far.” tion. Hominin fossils this complete are incredibly rare, and ones of infants are rarer still because their bones are that much more fragile. Indeed, the next BUNDLE OF JOY oldest skeleton of a juvenile that is comparably intact is a Neandertal baby dating to around 50,000 IT WAS THE AFTERNOON of December 10, years ago. 2000, when fossil hunters led by Zeresenay Alemseged, now at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, WALKING VS.CLIMBING spotted the specimen. Only part of its tiny face was visible; most of the rest of the skeleton was THE EXCEPTIONAL PRESERVATION of Selam, entombed in a melon-size block of sandstone. But as well as that of other animals found at the site, “right away it was clear it was a hominin,” indicates to team geologist Jonathan G. Wynn Alemseged recollects, noting the smoothness of the of the University of South Florida that her “Lucy’s Baby,” Kate Wong. Scientific American. December 2006, pp. 78–85. 1 2 Lucy’s Baby body was buried shortly after death by a flood imaging, the team was able to glimpse her semicir- event. Whether she perished in the flood or before it cular canal system, which is important for maintain- is unknown. ing balance. The researchers determined that Selam’s semicircular canals are similar to those of Although she was only three when she died, African apes and A. africanus. This, they suggest, Selam already possessed the distinctive characteris- could indicate that A. afarensis was not as fast and tics of her species. Her projecting snout and narrow agile on two legs as we modern humans are. It could nasal bones, for example, readily distinguish her also mean that A. afarensis was limited in its ability from another ancient youngster, the so-called Taung to decouple the movements of its head and torso, a child from South Africa, who was a member of the feat that seems to play a key role in endurance run- closely related A. africanus species. And her lower ning in our own species. jaw resembles mandibles from Hadar, the site where Lucy and a number of other A. afarensis individuals The conclusion that A. afarensis was a bipedal were found. creature with an upper body at least partly adapted for life in the trees echoes what Jack T. Stern, Jr., of Selam also exhibits the same mash-up of traits Stony Brook University and his colleagues wrote in her postcranial skeleton that has long vexed sci- years ago in their reports on Lucy and her contem- entists interested in how A. afarensis moved around poraries. “I was happy to see that this paper sug- the landscape. Scholars agree that A. afarensis was gests I might have been right,” Stern comments. a creature that got around capably on two legs. But Johanson agrees that the case for a partly arboreal A. starting in the 1980s, a debate erupted over whether afarensis is stronger than it once was. “Early on I the species was also adapted for life in the trees. The was a staunch advocate of strict terrestrial bipedal- argument centered on the observation that whereas ism in afarensis,” he remarks. But taking more the species has clear adaptations to bipedal walking recent findings into consideration, Johanson says, in its lower body, its upper body contains a number “it’s not out of the realm of possibility that they of primitive traits better suited to an arboreal exis- were still exploiting some of the arboreal habitats tence, such as long, curved fingers for grasping tree for getting off the ground at night and sleeping up branches. One camp held that A. afarensis had made there or going back to familiar food sources.” a full transition to terrestrial life and that the tree- friendly features of the upper body were just evolu- A combination of walking and climbing would tionary baggage handed down from an arboreal fit neatly with the picture that is emerging from ancestor. The other side contended that if A. afaren- studies of the environments of early hominins, sis had retained those traits for hundreds of thou- including Selam. Today Dikika is an expanse of sands of years, then tree climbing must have still dusty hills dotted with only the occasional tree or formed an important part of its locomotor reper- shrub. But 3.3 million years ago, it was a well- toire. watered delta flanked by forests, with some grass- lands nearby. “In this context, it is not surprising to Like her conspecifics, Selam has legs built for have an ‘ape’ that spends time in the trees and on the walking and fingers built for climbing. But she also ground,” comments project member René Bobe of brings new data to the controversy in the form of the University of Georgia. two shoulder blades, or scapulae—bones previous- ly unknown for this species. According to Not everyone is persuaded by the arboreal Alemseged, her scapulae look most like those of a argument. C. Owen Lovejoy of Kent State gorilla. The upward-facing shoulder socket is par- University disputes the claim that Selam’s scapula ticularly apelike, contrasting sharply with the later- looks like a gorilla’s. “It’s primitive, but it’s really ally facing socket modern humans have. This orien- more humanlike than gorillalike,” he remarks. tation, Alemseged says, may have facilitated raising Lovejoy, a leading proponent of the idea that A. the hands above the head—something primates do afarensis was a dedicated biped, maintains that the when they climb. (Although gorillas do not forelimb features that are typically held up as indi- climb as adults, they do spend time in the trees as cators that A. afarensis spent time in the trees only youngsters.) provide “evidence that the animal has an arboreal history.” The discovery of the famed Laetoli foot- Further hints of arboreal tendencies reside in prints in 1978 closed the debate, he states. The trail the baby’s inner ear. Using computed tomographic did not show a prehensile big toe, without which, Lucy’s Baby 3 Lovejoy says, A. afarensis simply could not move Alemseged’s team ascertained that Selam had about effectively in the trees. 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 A HODGEPODGE HOMININ adult brain size. This raises the tantalizing possibil- ity that A. afarensis experienced a more humanlike EXPERTS MAY DISAGREE over the functional pattern of brain growth. significance of Selam’s apelike skeletal characteris- tics, but they concur that different parts of the More fossils are needed to discern whether the hominin body were undergoing selection at differ- new skeleton is representative of A. afarensis ent times. A. afarensis is “a good example of mosa- infants, and scientists are doubtless eager to recover ic evolution,” Johanson states. “You don’t just mag- remains of other A. afarensis children of different ically flip some evolutionary switch somewhere and ages—if they ever can—to see how they compare.

Load more

  3

Copy Link