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.
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