The pelvis of Ardipithecus shows the hominid could both walk upright and climb trees
UPRIGHT CLIMBER An artist’s reconstruction shows 4.4-million-year-old Ardipithecus ramidus
in action as both a climber and a two-legged walker. This early hominid
combined humanlike, nearly straight-legged walking with powerful,
apelike climbing abilities, a new study finds. Mauricio Anton/Science Source
A famous 4.4-million-year-old member of the human evolutionary family
was hip enough to evolve an upright gait without losing any
tree-climbing prowess.
The pelvis from a partial Ardipithecus ramidus skeleton nicknamed Ardi (SN: 1/16/10, p. 22)
bears evidence of an efficient, humanlike walk combined with plenty of
hip power for apelike climbing, says a team led by biological
anthropologists Elaine Kozma and Herman Pontzer of City University of
New York. Although researchers have often assumed that the evolution of
walking in hominids required at least a partial sacrifice of climbing
abilities, Ardi avoided that trade-off, the scientists report the week of April 2 in the Proceedings of the National Academy of Sciences.
“Ardi
evolved a solution to an upright stance, with powerful hips for
climbing that could fully extend while walking, that we don’t see in
apes or humans today,” says Pontzer, who is also affiliated with CUNY’s
Hunter College. Ardi’s hip arrangement doesn’t appear in two later
fossil hominids, including the famous partial skeleton known as Lucy, a
3.2-million-year-old Australopithecus afarensis.
Ardi’s lower pelvis is longer than that of humans, which led some researchers to argue that Ardipithecus
mainly climbed in trees and walked slowly with bent knees and hips, or
perhaps not at all. But the new study shows it “would not have impeded
its ability to walk upright in a humanlike fashion,” says
paleoanthropologist Carol Ward of the University of Missouri in
Columbia.
Hip trip
A digital reconstruction of 4.4-million-year-old Ardi’s pelvis, in
gray, fits inside 3.2-million-year-old Lucy’s reconstructed pelvis, in
yellow. Unlike Lucy, Ardi’s upper pelvis was oriented behind her
relatively long lower pelvis, enabling both a straight-legged gait and
powerful, apelike climbing, a new study suggests. Gen Suwa/The University Museum, Univ. of Tokyo
Unlike other hominids and living apes, Ardi’s upper pelvis is
positioned behind the lower pelvis, enabling a straight-legged gait,
Pontzer and his colleagues find. An evolutionary reorienting of the
pelvis in that way enabled back muscles to support an upright spine,
Ward suggests.
A relatively large gluteus maximus works with hamstring muscles to
push humans into a straight-legged stance. Ardi may have had a small
rear-end muscle for her size, making a forward-positioned lower pelvis
especially critical for walking, Pontzer says.
Using previous data from present-day humans, chimps and monkeys,
Pontzer’s group documented a relationship between the shape and
orientation of the lower pelvis and the energy available for a range of
motions involved in walking and climbing. They used those findings to
examine fossil pelvises of Ardi, Lucy and a 2.5-million-year-old Australopithecus africanus. No other fossil hominids from that long ago included a pelvis complete enough for analysis.
The researchers also evaluated a nearly 18-million-year-old fossil pelvis from an African ape, Ekembo nyanzae.
A. afarensis and A. africanus displayed pelvic
arrangements for upright walking, but not for Ardi’s apelike climbing
power. In particular, the lower pelvis of the two Australopithecus species was nearly as short as the walking-specialized lower pelvis of people today. E. nyanzae’s
pelvis was specialized for climbing, as in modern apes and monkeys. Its
long, straight pelvis enabled walking with bent hips and knees.
The new study coincides with previous evidence that Ardi’s lower back
was flexible enough to support straight-legged walking, says
paleoanthropologist Owen Lovejoy of Kent State University in Ohio.
Lovejoy, who led an initial investigation of Ardi’s lower-body bones, has long contended that ancient hominids had a humanlike gait (SN: 7/17/10, p. 5).
“A. afarensis and A. africanus walked much like we do, and for the most part that goes for Ardi as well,” Lovejoy says.
Ardi’s unusual mix of walking and climbing abilities spurred the
evolution of hominid bodies geared toward minimizing lower-limb
injuries, Lovejoy proposes. Ardi’s long lower pelvis and apelike,
opposable big toe were replaced in Lucy’s kind by a short lower pelvis connected to smaller hamstring muscles, a humanlike big toe and a fully developed arch (SN: 3/12/11, p. 8).
Those changes made climbing harder for A. afarensis,
but stabilized its upright stance, helping to prevent foot injuries and
hamstring tears when stopping suddenly or accelerating quickly, Lovejoy
says.
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