Encyclopedia of Evolution.pdf - Online Reading Center

Skeleton of “Lucy,” the Australopithecus afarensis discovered by Donald
Johanson and associates. It is one of the most complete australopithecine
skeletons that has been found, and it demonstrated that upright posture
preceded increase in brain size in human evolution. (Courtesy of Science
VU/Visuals Unlimited)
and the pelvis was suitable for walking. The feet, however,
appear to have been only partly adapted to upright walking:
the big toe was still at a noticeable angle. This suggests
that Australopithecus lived mostly on the ground, walking
upright, but frequently scrambled back into the trees when
danger threatened. The relatively long arms and short legs
of A. afarensis further suggest a mixture between arboreal
australopithecines
and ground life. It even suggests the possibility that upright
posture began as an adaptation for shinnying up trees from
brief visits to the ground and later allowed full adaptation
to ground life.
The primarily upright locomotion of A. afarensis and/
or related australopithecines was confirmed by famous fossilized
footprints found by anthropologist Mary Leakey (see
Leakey, Mary) at Laetoli in Tanzania (see photo below).
At about three million years of age, the footprints were
undoubtedly produced by A. afarensis or a closely related
species. Their date is fairly certain, as the footprints were
imbedded in volcanic dust, on which radiometric dating
can be used.
Trail of hominin footprints in volcanic ash. This 00-foot ( 0-m) trail was
discovered by Mary Leakey’s expedition at Laetoli, Tanzania, in . The
footprints were probably made by Australopithecus afarensis individuals
. million years ago. They demonstrate that hominins had already
acquired the upright, free-striding gait of modern humans. The footprints
have well-developed arches and the big toe does not diverge noticeably.
They are of two adults with possibly a third set belonging to a child who
walked in the footsteps of one of the adults. (Courtesy of John Reader/
Photo Researchers, Inc.)