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Riddle of the Bones

Intro | How did they move? | What did they look like? | Are they all the same species? | When did they live?

How did they move?

Of all primates living today, only we humans walk fully upright. But Lucy and other fossil finds reveal that more than 3 million years ago, a relatively small-brained, ape-faced human ancestor walked steadily on two feet.

To get a picture of how Lucy's species, Australopithecus afarensis, moved, scientists compare fossils to the bones of modern humans, as well as to the anatomy of "knuckle-walking" primates like chimpanzees. They also have another remarkable clue: actual footprints preserved in the volcanic ash of Laetoli.

Like wet sand on a beach, the volcanic ash of Laetoli captured the traces of two individuals strolling side by side. One set of prints is clearly larger than the other. Perhaps these individuals were a parent and child, or a large male and his diminutive mate. The details of the prints are so fine that scientists can even tell that they walked at a leisurely pace.

As they stepped, their feet fell close together. Like humans today, they maintained good balance because their center of gravity moved forward in a straight line. By contrast, when a chimpanzee tries to walk upright, its feet stay wide apart, its center of gravity shifts side-to-side, and it awkwardly teeters for only a short time.

Lucy and other members of her species could walk well because their hip and knee joints were more like humans' than like chimps'. The very first fossils of this species -- found by paleoanthropologist Donald Johanson at Hadar in 1973 -- were the parts of a knee joint. Like a human knee, it could "lock," allowing its owner to stand straight-legged. The knees of chimps and other living primates, by contrast, always stay slightly bent.

As an isolated piece of evidence, this knee couldn't tell much about its owner. But it helped entice Johanson back to Hadar the following year, when he discovered Lucy. Lucy's pelvic, hip, and leg bones offered more proof. As in a human, Lucy's thighbones (femurs) angle in from the hip toward the knee. Her shinbones (tibiae) then descend straight to the ground, allowing her to walk with feet close together. Some scientists suggest that Lucy's long arms may have helped her balance while bipedal -- much like the pole of a tightrope walker.

With dangling arms and long, slightly curved hands and feet, Lucy and other members of her species may have also been good climbers. Scrambling up trees could protect them from predators and help them reach choice foods. Some scientists point to these "tree-dwelling" traits to argue that A. afarensis was not fully bipedal. The debate among anthropologists may continue, but the reputation of A. afarensis as "the ape that walked upright" makes it a celebrity species in the story of human evolution.


Details of the fossil evidence

Lucy: pelvis
Lucy's pelvis hints that she walked upright on two legs. When her crushed remains were carefully reconstructed by anthropologist C. Owen Lovejoy, her pelvis looked much like a modern woman's. Broad, fan-shaped bones on either side, called the iliac blades, curve to form a pelvic basin. This basin helped support her internal organs and entire upper body as she moved.

arm bone

Lucy: arm bones
With dangling, powerful arms, Lucy was likely a good climber, and she may have spent some time in trees. Her upper arm bones are long and sturdy relative to those of modern humans.

pair of left and right prints

Laetoli Footprints: prints
Whoever left these prints undoubtedly moved forward on two feet, likely with a smooth, stable gait. The left and right prints lie close together -- as they would for a human, but not a sideward-swaying chimp. Fine details also reveal a human-like step: weight fell first on the heel, then on the outside of the foot, and finally on the ball and big toe for the push-off to the next step.

First Family: heel bones
The broad heels of this creature could withstand the pressure of walking upright. Like human heels, they are filled with shock-absorbing "spongy" bone, rather than the more solid bone found in the heels of other apes.

various toe bones

First Family: toe bones
Toe bones found among the First Family are long compared to those of humans, but they don't curve forward toward the heel as they do in modern tree-climbing primates.

-> Find out how what they looked like

Intro | How did they move? | What did they look like? | Are they all the same species? | When did they live?

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