As the world’s most famous human ancestor, the 1-meter-tall primate nicknamed Lucy has made headlines ever since she was discovered at Hadar in the badlands of Ethiopia in 1974. Every aspect of her life story has been scrutinized, from the way she walked to how svelte her figure was. Now, a new controversy is swirling about how she died 3.2 million years ago. A provocative analysis of fractures riddling her bones suggests that she toppled from a tall tree and hit the ground so hard that she smashed many of her bones.
The study offers new insight into the mysterious death of one of paleoanthropology’s most iconic individuals, and the scientists involved say it may give clues to how much time her species, Australopithecus afarensis, still spent in the trees. Although Lucy had a brain and body the size of a large chimp—and probably slept in a tree nest—she walked fully upright and her species may already have lost its agility in trees as it adapted to life on the ground. But other paleoanthropologists call the work a “just-so” story, saying that the fractures are likely due to routine crushing of bones long after death.
The debate is the result of a study published online today in Nature by paleoanthropologist John Kappelman and his colleagues at the University of Texas (UT), Austin. The team scanned Lucy’s partial skeleton, which preserves 40% of her bones, when she was touring the United States in 2006. Using the high resolution x-ray computed tomography (CT) scanner at UT, they produced 35,000 cross-sectional CT “slices” of Lucy’s bones, which they used to explore how forces were distributed across the bones, and so reconstruct how Lucy moved.
But as Kappelman looked closely at the bones in 3D models, he noted a striking pattern in the way the head of the upper right arm bone, or humerus, had collapsed as it was shoved into the bone’s long shaft. When he checked the clinical literature, the break looked like a dead ringer for a so-called “four-part proximal humeral fracture”—a characteristic shoulder bone fracture in which the shoulder blade smashes like an anvil into the head of the humerus, driving it forcefully into the long bone. Today, such horrific breaks happen when people fall from a great height with arms outstretched, or brace their hands on the dashboard in a car accident.
Kappelman used his laser printer to produce 3D models of Lucy’s broken humerus and showed them to orthopedic specialists: Nine out of nine agreed that she endured the distinctive four-part compound fracture. Kappelman also observed similar but less severe fractures on her left shoulder, right ankle, knee, pelvis and first rib, which he considers the “hallmark of severe trauma.” His team calculated that the forces that fractured Lucy’s upper arm were equal to a fall from a height of about 13.7 meters—as high as a four-story building or the top of a tall tree, such as a mature acacia tree—at a velocity of about 59 kilometers per hour.
The sharp, clean edges of the breaks show no evidence of healing, suggesting that they occurred just before Lucy died. And Kappelman argues that tiny fragments or slivers remaining around the broken bones show that the fractures were not caused by being tossed by floodwaters or stomped in trampling, for example, because the tiny bits would have been dispersed, he says. He also notes no crush marks from animals, and says that flooding usually tears apart the skeleton.
The last part of the puzzle was figuring out how Lucy could have fallen so hard. The most likely explanation is that she fell from a tall tree, as chimpanzees occasionally do. Although Lucy walked upright, she probably still slept in a tree nest at night or climbed trees to escape predators. “It may well have been the case that adaptations that permitted her to live more efficiently on the ground compromised her ability to move safely in the trees—and may have predisposed her kind to more falls,” Kappelman says.
This is a “plausible scenario for the demise of Lucy,” says paleoanthropologist William Jungers of the State University of New York at Stony Brook, who reviewed the paper for Nature. “The detailed, comprehensive analysis of her fracture pattern compared to the extensive human clinical literature on skeletal trauma … is especially compelling.”
But other paleoanthropologists who have studied Lucy’s bones are highly skeptical. There are simply too many fractures to be the result of even a great fall, says biological anthropologist Owen Lovejoy at Kent State University in Ohio, who did a postdoc in orthopedic biomechanics. Lucy’s ribs, for example, are fractured in so many places that “you couldn’t do that with a shotgun blast,” much less a fall, Lovejoy says.
Instead, the pattern of breakage looks just like fractures on the bones of all sorts of animals found at Hadar and beyond. Most breaks on fossilized bones are “the result of geological processes well after death,” says paleoanthropologist Tim White of the University of California, Berkeley, such as movement in water or soil, tectonic forces, pressure of overlying sediments, and weathering or trampling of fossils as they erode out of sandstone. “These authors make no effort to test the alternative hypothesis that these cracks and other breaks were made during the process of fossilization and erosion,” he wrote in an email. “The damage to Lucy’s bones is completely run of the mill,” agrees paleoanthropologist William Kimbel of Arizona State University’s (ASU’s) Institute of Human Origins in Tempe, who has studied hundreds of specimens of Lucy’s species, A. afarensis.
“Terrestrial animals like antelopes and gazelles, elephants and rhinos and giraffes—all these bones show very similar fracture and breakage patterns as Lucy,” adds paleoanthropologist Donald Johanson of ASU, co-discoverer of Lucy. “You can be sure they didn’t fall out of trees.” But he’s optimistic the study will stimulate further research to explore the cause of different patterns of fractures in primates’ bones before and after death. “At the moment, this is a just-so story that you can’t verify or falsify. But it could stimulate additional research.”