By Brandon Keim EmailFebruary 20, 2009 | 4:00:00
Footxray
If you've ever wondered why humans don't have long, prehensile toes
that would turn our feet into extra hands, here's an answer: stubby
toes may be custom-made for running.
Biomechanical analysis shows that long toes require more energy and
generate more shock than short toes, making them one of many
adaptations that may have helped our savannah-dwelling ancestors chase
their prey.
"Longer toes require muscles to do more work, and exert stronger
forces to maintain stability, compared to shorter toes," said
University of Calgary anthropologist Campbell Rolian. "So long as we
were engaged in substantial amounts of running, natural selection
would favor individuals with shorter toes."
Most primates — including our closest relative, the chimpanzee — have
proportionately longer toes than humans. Our own are comparatively
dwarfish and two-dimensional, capable only of extending and flexing.
Most animals that run, however, also have extremely short toes. Some
species, such as cats and dogs, have paws composed almost entirely by
palms. This led Rolian's team to wonder if our foot's physiology could
be explained by running.
The importance of running to early Homo is, of course, conjectural.
But it does make sense: few other animals are capable of long-distance
running, and none can do so under a blazing sun. (Wolves and hyenas,
for example, require cold weather or nightfall for long-distance
hunting; otherwise they overheat.) Endurance running might have set
early humans apart from the pack.
According to study co-author and Harvard University anthropologist
Daniel Lieberman, many modern anatomical features make sense in the
context of savannah marathons. Achilles tendons act as springs to
store energy. Our hind limbs have extra-large joints. Our buttocks
muscles are perfect for stabilization, as are regions of the brain
uniquely sensitive to the physical pitching generated by the motion of
running.
Toes may belong to this class of adaptations.
"Humans are well-adapted for endurance running. That's much of what
makes the human body what it is," said Lieberman. "We're actually
terrible sprinters, but the world's best long-distance runners."
The long-runner hypothesis is not universally accepted. "Walking and
running use the same body parts," said University of Wisconsin
paleoanthropologist John Hawks, who was not involved in the study.
"It's hard to argue that these are specifically crafted for running" —
and, to be even more specific, for long-distance running.
However, Rolian's study, published recently in the Journal of
Experimental Biology, makes at least a plausible case for the
importance of toes for running. During the moment of propulsion, when
one foot is in the air and the other is on the ground, between one-
half and three-quarters of a body's weight falls squarely on the
forefoot.
"When you're walking, before you push off to start the next step, your
other foot has already hit the ground. You've transferred some of your
body weight," said Rolian. "Your toes have to do much more work in
running, to push you."
When his team analyzed force exerted by fifteen test subjects running
and walking on a pressure-sensitive surface, they found that
increasing toe length by just 20 percent produced a doubling of motor
force. This can be explained in terms known conversationally known
from the action of a see-saw: levering force is magnified by the
distance between pressure and a fulcrum.
Rolian also observed that longer toes require an additional energy
investment when "braking," or using them to guide the forward-falling
motion that underlies both running and walking.
The additional work required by long toes, and a resulting increase in
muscle stress and damage, likely made them a victim of natural
selection. The fossil record, though spotty, provides a fitting
narrative: the toes of great apes are longer than those of
Australopithecus — the first bipedal hominid — which in turn are
longer than the toes of Homo, the genus to which modern humans
belong.
Hawks notes that long-distance running is now extremely rare, and
"where it exists, it is supported by very sophisticated cultural
adaptations, including tracking, water storage and staged transport of
meat back to home bases. There is presently little or no evidence for
these cultural adaptations in early Homo."
But Lieberman points out that early Homo and its descendants clearly
ate large game, though the projectile technologies ostensibly
necessary to slay them were invented just several thousand years ago.
"How did our ancestors, those weak little primates, kill big animals?
The answer is that we chased them. We made them gallop. They can't
pant and gallop at the same time," said Lieberman. "We can run down a
gazelle not through speed, but through endurance."
Of course, in the modern world of grocery stores and restaurants, long-
distance running is a recreational activity, and hard-soled shoes
absorb much of the shock felt by a bare foot. Freed from ancient
evolutionary pressures, what will happen to our feet?
It's too soon to tell, and nothing at all may happen, but "that's
generally a question you could ask about many features of the human
anatomy," said Rolian. Because it isn't required to push off, he said,
"There's talk about whether the pinkie toe is eventually going to
disappear."
Citation: "Walking, running and the evolution of short toes in
humans." By Campbell Rolian, Daniel E. Lieberman, Joseph Hamill, John
W. Scott and William Werbel. Journal of Experimental Biology, Vol.
212, Issue 5. March 1, 2009.
http://www.archaeologynews.org/story.asp?ID=396952&Title=These%20Toes...
First published online February 13, 2009
Journal of Experimental Biology 212, 713-721 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.019885
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Walking, running and the evolution of short toes in humans
Campbell Rolian1,*,{dagger}, Daniel E. Lieberman1, Joseph Hamill2,
John W. Scott3 and William Werbel1
1 Department of Anthropology, Harvard University, Cambridge, MA 02138,
USA
2 Department of Kinesiology, University of Massachusetts, Amherst, MA
01003, USA
3 School of Medicine, Vanderbilt University, Nashville, TN 37232, USA
{dagger} Author for correspondence (e-mail: cprol...@ucalgary.ca)
Accepted 25 November 2008
The phalangeal portion of the forefoot is extremely short relative to
body mass in humans. This derived pedal proportion is thought to have
evolved in the context of committed bipedalism, but the benefits of
shorter toes for walking and/or running have not been tested
previously. Here, we propose a biomechanical model of toe function in
bipedal locomotion that suggests that shorter pedal phalanges improve
locomotor performance by decreasing digital flexor force production
and mechanical work, which might ultimately reduce the metabolic cost
of flexor force production during bipedal locomotion. We tested this
model using kinematic, force and plantar pressure data collected from
a human sample representing normal variation in toe length (N=25). The
effect of toe length on peak digital flexor forces, impulses and work
outputs was evaluated during barefoot walking and running using
partial correlations and multiple regression analysis, controlling for
the effects of body mass, whole-foot and phalangeal contact times and
toe-out angle. Our results suggest that there is no significant
increase in digital flexor output associated with longer toes in
walking. In running, however, multiple regression analyses based on
the sample suggest that increasing average relative toe length by as
little as 20% doubles peak digital flexor impulses and mechanical
work, probably also increasing the metabolic cost of generating these
forces. The increased mechanical cost associated with long toes in
running suggests that modern human forefoot proportions might have
been selected for in the context of the evolution of endurance running.
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