156 CHAPTER 7 | The First Bipeds
a photo that shows the chimpanzee style
of climbing clearly.
You might wonder, yeah so what?
Isn’t it obvious that chimpanzees climb
this way?
Well, it wasn’t so obvious which fea-
tures of the ankle might adapt chimpan-
zees to this style of climbing. By watching
the chimpanzees (and other apes) DeSilva
was able to determine the average amount
(and range) of dorsiflexion and inversion
of the feet while climbing, and could also
assess the extent to which dorsiflexion
is accomplished at the ankle joint (as
opposed to the midfoot). In this case,
the observations were pretty obvious—
chimpanzees were habitually flexing their
ankles in ways that would damage a hu-
man ankle. Then, by examining the bony
limits on human ankle flexibility, DeSilva
showed that fossil hominins shared the
same constraints on ankle
movement as recent people.
They couldn’t have climbed
like chimpanzees.Human Climbing
I would say that the ankle-
joint observations match
the rest of the skeleton. It
seems pretty obvious that
Australopithecus afarensis
and later hominids couldn’t
possibly have climbed
in the chimpanzee-like
manner described in
DeSilva’s paper, because
the hominins’ arms were
too short. If a logger tried to
climb with his arms instead
of a strap, even spikes on
his feet would be relatively
ineffective holding him
up. Dorsiflexion would be
hopeless—the normal com-
ponent of force against the
tree trunk would be insuffi-
cient to prevent slipping.
Humans who aren’t
loggers use a different
strategy to climb vertical
tree trunks—they put a
large fraction of the surface
area of their legs directly
in contact with the trunk.
Wrapping legs around and
pressing them together
gives the necessary friction
to hold the body up.
If you’re like me, you’ll
remember this climb-
ing strategy ruefully from
gym class, where “ropeclimbing” is the lowest common denom-
inator of fitness tests. The sad fact is
that many otherwise-normal humans fall
on the wrong side of the line between
mass and muscle power. Straining my
groin muscles to the max, I still could
never pull my way up a rope.
There’s nothing magical about get-
ting a human to climb. Ladders, after
all, are relatively easy for the large frac-
tion of the population who can’t climb
a rope or tree trunk. The trick with
a ladder is that friction is organized
in a more effective way for our ankle
mechanics and arm length. But you
don’t need to schlep a ladder, if you can
manage a little extra arm strength and a
low enough body mass.Early Hominin Climbing
Australopithecines were light in mass,
and from what we can tell, they hadstrong arms. So they had what it takes for
humans today to climb trees effectively—
not like chimpanzees, but like humans.
Up to A. afarensis, every early hominin
we know about lived in an environment
that was at least partially wooded.
In his comments about the paper,
DeSilva hypothesizes a trade-off be-
tween climbing ability and effective
bipedality, so that early hominins could
not have effectively adapted to both.
I don’t think a chimpanzee-like ankle
would have been any use with arms
as short as australopithecines’. So I
don’t see the necessity of a trade-off
in ankle morphology. A. afarensis—
long before any evidence of stone tool
manufacture—had very non-apelike
arms, hands and thumbs.
But there’s one significant ques-
tion that DeSilva omits discussing—the
footbones of a South African australo-
pithecine: StW 573. Clarke and Tobiasb
describe the foot of StW
573 as having a big toe
that is abducted (sticks
out) from the foot, in-
termediate between the
chimpanzee and human
condition. They conclude:[W]e now have the
best available evi-
dence that the earliest
South African aus-
tralopithecine, while
bipedal, was equipped
to include arboreal,
climbing activities in
its locomotor reper-
toire. Its foot has de-
parted to only a small
degree from that of
the chimpanzee. It is
becoming clear that
Australopithecus was
not an obligate terres-
trial biped, but rather
a facultative biped and
climber. (p. 524)DeSilva studied the
talus (an ankle bone),
not the toe. StW 573
has a talus, and although
it is not in DeSilva’s
sample, it probably would
place very close to the
other hominins in his
comparison. Even Clarke
and Tobias described its
talus as humanlike—
their argument for an
intermediate form was
based mostly on the toe.CONTINUED© Kristen Mosher/Danita Delimont. All rights reserved.
The amount of dorsiflexion in a chimpanzee’s foot allows it to climb
trees with the feet in a position that is impossible for humans.
Comparisons like this between living species allow paleoanthropologists
to reconstruct the pattern of locomotion in fossil groups.