To run without a tail

Born To Run
Biomechanical research reveals a surprising key to the survival of
our species: Humans are built to outrun nearly every other animal on
the planet over long distances
By Ingfei Chen
DISCOVER Vol. 27 No. 05 | May 2006

Late one night over beers in the Welsh hamlet of Llanwrtyd Wells, an
innkeeper got into an argument with a foxhunter about who could run
faster, man or horse. The innkeeper insisted that over many miles, a
human runner would have greater stamina, and prevail. Thus was born a
tradition: Every year since 1980, Llanwrtyd Wells has hosted the Man
Versus Horse Marathon, which pits hundreds of runners against dozens
of horses with riders. On two legs or four, contestants take on 22
miles of challenging trails laced across a dazzling green
countryside. They trot through fragrant pine forests, scramble up
mountainous rock-strewn sheep trails, cross rolling moorlands, and
ford rivers.


In June 2004, for the first time ever, the human won. The innkeeper
was delighted and so were University of Utah biologist Dennis Bramble
and Harvard University paleoanthropologist Daniel Lieberman. That
summer the two scientists were putting the finishing touches on a
theory with a new view on how conditions millions of years ago molded
the way humans move today. The standard explanation among physical
anthropologists has long been that early hominids left life in the
trees to forage on the open savanna and that walking upright was the
key to surviving in that new environment. Bramble and Lieberman do
not dispute this general theory, but they have identified a suite of
traits in the human anatomy that add a dramatic twist to the story line.


The traits appear to be specifically adapted for running�and for
jogging for long distances. So Bramble and Lieberman were not at all
surprised that a man won the Man Versus Horse Marathon. It fits their
hypothesis. Unlike many mammals, not to mention primates, people are
astonishingly successful endurance runners, "and I don't think it's
just a fluke," Lieberman says. He and Bramble argue that not only can
humans outlast horses, but over long distances and under the right
conditions, they can also outrun just about any other animal on the
planet�including dogs, wolves, hyenas, and antelope, the other great
endurance runners. From our abundant sweat glands to our Achilles
tendons, from our big knee joints to our muscular glutei maximi,
human bodies are beautifully tuned running machines. "We're loaded
top to bottom with all these features, many of which don't have any
role in walking," Lieberman says. Our anatomy suggests that running
down prey was once a way of life that ensured hominid survival
millions of years ago on the African savanna.

Although Bramble has studied locomotion in animals ranging from
tortoises to jackrabbits for 40 years, he was first tipped off to the
hypothesis that humans were born to run by one of his students, David
Carrier. In the 1970s, Carrier was assisting with Bramble's studies
of how dogs, horses, and people regulate breathing while running. A
marathoner himself, Carrier began to wonder about the role of
endurance running in human evolution. People, he noted, can shed heat
quickly not by panting, like most animals, but by perspiring through
millions of sweat glands. A lack of fur also helps dissipate heat
more quickly.


Other researchers have proposed that such features emerged because
our ancestors had to cope with the sun as they moved from a shady
forest habitat to the scorching savanna. Carrier suspected that these
traits were more relevant for handling physical exertion. The human
body generates six times more heat when sprinting at top speed than
when sitting in the sun. Most animals, humans included, must stop
trotting when they overheat, or they die.

(In one legendary experiment, Harvard biologists stuck a rectal
thermometer into a cheetah, put the cat on a treadmill, and found
that it refused to move once its temperature hit 105 degrees
Fahrenheit, even though it was loping well below its top speed.)
Controlling body temperature, Carrier once wrote, "is critical for
animals that run for extended periods." Given that humans excel at
releasing heat and distance running, he speculated that we were built
to run far and wide.

"I didn't buy it at all," Bramble says. Like most of his peers,
Bramble's first reaction to Carrier's hypothesis was that "humans are
pitifully slow." From the perspective of a vertebrate morphologist,
humans lack one of the most obvious features of animals adapted for
serious speed: a tail. In creatures that cover ground bipedally, such
as kangaroos, kangaroo rats, and roadrunners, "the tail is the major
balance organ," Bramble says. "In the whole history of vertebrates on
Earth�the whole history humans are the only striding biped that's a
runner that's tailless."

Still, Bramble eventually came to realize that people turn in
remarkable performances. He once filmed a horse cantering, with
Carrier running alongside at the same pace. The movie showed that
Carrier's legs were churning more slowly than the horse's, which
meant that the student's strides had to be spanning more distance per
step than the horse's.


Although Carrier moved on to other research, Bramble grew convinced
that his student had discovered something. During a visit to Harvard
in 1991, Bramble encountered Daniel Lieberman, then an anthropology
Ph.D. student, making a pig trot on a treadmill. To glean insights
into how bones grow�and thus to better interpret fossilized human
jaws and skulls�the student wanted to see whether the repeated impact
of running would spur a thickening of the pig's skull. "You know,"
Bramble said, "that pig's not holding its head still." He went on to
explain that adept runners like horses, dogs, and rabbits keep their
noggins remarkably steady as they lope, thanks to an obscure bit of
anatomy called the nuchal ligament. It's a tendonlike band that links
the head to the spine. People, he said, have a version of this band.

Rummaging through a collection of replicas of fossilized primate
bones in a nearby lab, Bramble pointed out that the nuchal ligament
leaves a trace�a delicate ridge�where it attaches at the base of the
human skull. Then the scientists noticed the ridge in a pitted,
yellowed skull of our 2-million-year-old relative Homo erectus�but
not in older hominids known as australopithecines, who walked the
earth as far back as 4.4 million years ago. "Holy moley!" Lieberman
thought. "There's something going on here, and what's more, we might
be able to study it in the fossil record."

"Once the idea is in your head, then you start thinking about things
differently," Lieberman says. A short 41-year-old with a receding
hairline, a slight paunch, and disarming dimples, Lieberman doesn't
look athletic, but he has been a jogger since his teens. I joined him
for his morning run with his dog, Vashti, a border collie mix, whom
he easily proved he could outlast. Lieberman says it's wrong to
assume, as many do, that running is like walking. The two motions are
strikingly different. He demonstrates that during walking his heel
hits the ground first, the leg straightens, and then the body vaults
over it.

"Your center of gravity, which is basically near your belt buckle,
r-i-i-i-ses" he takes a slow-motion step forward with his right leg
and pauses, now up on the ball of his right foot�"so that it's over
your leg." The body has now stored potential energy. The arch of the
foot stiffens, and Lieberman pushes off against it. As he tips
forward, potential energy converts to kinetic energy, and he swings
his left foot ahead to complete the stride.

But in running, he says, the legs become springs. You land on and
squash the entire arch and bend your knee. So initially the body's
center of gravity falls. "You go down and then you go up," Lieberman
says. Kinetic energy from the crash landing is stored in the many
stretchy tendons of the arch and the leg, most notably the huge
Achilles tendon connecting calf muscles to the heel bone. Like rubber
bands, the tendons extend and then recoil boing! to launch you onto
the next step.

"So why do we have all these tendons in our legs?" Lieberman asks.
"You don't evolve big tendons unless you're a runner." Kangaroos,
antelope, and other serious animal runners all have a great set of
springs, which do nothing for walking. So our tendons can't be
explained as being necessary for walking.

Part by part, Bramble and Lieberman have reinterpreted the hominid
physique by juxtaposing bits of fossil evidence with what's known
about the physiology and biomechanics of jogging. Although much of
the anatomy that lets us lope is the same equipment that humans first
evolved for walking, the researchers say many of our physical traits
seem tailor-made for sustained running.

To test their ideas, they conduct biomechanical studies in
Lieberman's lab Room 53 in Harvard's small redbrick Peabody Museum.
The space looks more like a meld between a sports medicine clinic and
an untidy engineering workshop than an anthropologist's sanctum of
precious old bones. In addition to a gray-and-black running
treadmill, there are wall shelves and counters cluttered with boxes
of cotton applicators and latex gloves, containers of antiseptic, a
small toolbox, tangles of electric cords, and plastic models of human
parts, including a gigantic ear.

In one recent experiment, a volunteer named Jeff, dressed only in
dark Lycra shorts and white socks, looked like a guinea pig trapped
in a bad sci-fi flick. To track the electrical activity of key
muscles, Lieberman and a postdoc, David Raichlen, had taped circular
force-detecting pads to the bottoms of Jeff's feet and carefully
rigged other parts of him with electrodes. Wires from those sensors
ran through a small preamplifier box strapped to his lower back and
then to a nearby computer.

To capture an image of his movement, they attached little silvery
gray reflective balls onto Jeff's shoulders, hips, knees, and other
joints. Three infrared cameras would track the balls' motion and
record a stick-figure animation of him as he moved. Finally, to
measure forces acting on his skull, the researchers mounted an inch-
long accelerometer and gyroscope onto a small round tin and tied it
all on top of Jeff's head with a black mesh do-rag that knotted
securely under his chin.

Then Lieberman and Raichlen put Jeff on the treadmill and started it
up. "Focus on the gazelle on the savanna," Lieberman instructed over
the hum of the machine, indicating a big black X on a sheet of paper
taped to a shelf straight ahead. The researchers gradually cranked up
the pace until Jeff was pounding along at a hard run, sweaty and
breathless. "Your gluteus is getting a serious workout," Lieberman
said cheerfully.


The goal of the exercise was to understand how joggers stabilize
their heads and torsos part of the distinctive human balancing act
that puzzled Bramble years ago. Without the balancing help of a tail,
how do we avoid falling over when we run? The butt, it turns out, is
crucial right up there with the chin among traits that make us
uniquely human. Chimps and other primates have little buns. Our own
rear ends are huge; the upper part of the gluteus maximus is greatly
expanded. Although few scholars have studied its role in running, the
butt is, according to Bramble, "basically a substitute for a tail."


The treadmill studies support that idea. A few months later,
Lieberman showed the results from Jeff and 15 other joggers, partly
summarized in a conference presentation ("Why Is Our Gluteus So
Maximus?"). Sitting at his computer in shorts and flip-flops, Raich
len pulled up Jeff's data: a rainbow-colored series of 16
synchronized electrical recordings from all the sensors. Looking like
an EKG signal, the electromyograph, or EMG, reading from the gluteus
maximus, in red, showed little activity when Jeff strolled. But once
he broke into a jog, boom the red line tightly zigzagged. The faster
the pace, the bigger the spikes, like an earthquake signal on a
seismograph.

What that shows, says Lieberman, is that the butt isn't much involved
during walking. In running, however, the body leans forward so that
each time the leading foot strikes the ground, the trunk wants to
topple forward. The gluteus maximus prevents that: It fires just
before the foot slams into the floor, creating a braking action that
keeps the torso from falling down.

Meanwhile, the way we pump our arms back and forth in a trot helps
steady us too. And based on their experiments, the researchers
suspect that the motions of our shoulders and arms actually help
counterbalance the head, preventing it from pitching forward on each
landing. Simultaneously, with each heel strike, certain shoulder
muscles contract and put tension on the nuchal ligament, pulling up
the skull and keeping it level.

Our long neck is also important for running, Bramble says, because it
allows the shoulders to twist freely of the head as we gaze forward.
Chimps, in contrast, have hulking muscles anchoring the skull to the
shoulders, which appear permanently shrugged an orientation ideal for
reaching overhead to dangle from tree branches. Controversial fossil
evidence hints that australopithecines also had chimplike shoulders.
But by the time of 2-million-year-old Homo erectus, says Bramble,
hominids had lowered their shoulders, losing the thick, muscular
connections to the head.

These running features, the researchers argue, are unmistakably
obvious once you look for them. What's really hard to pin down, they
admit, is when these adaptations emerged. How do you figure out when
the first human butt appeared on the savanna? Muscles, tendons, and
sweat glands don't fossilize, and old bones can't reveal precisely
how their owners moved. Still, between biomechanical studies and bone
analyses, it's possible for researchers to infer whether a fossil
hominid was a jogger.

For example, most scientists reasoned that the 3.2-million-year-old
hominid Lucy, with her chimplike build, couldn't have been a good
endurance runner. She was squat, with short legs, a wide waist, long
arms, and long, curving fingers and toes that suggest a tree-climbing
lifestyle. Although researchers disagree on Lucy's gait while walking
upright, nobody thinks she could have strolled like a human. Yet more
than a million years later, Homo erectus roamed Africa with a much
longer, leggier build, sporting a dramatically different set of
physical changes that made it harder to climb trees but easier to
jog, Bramble says. "All the running equipment's already there."

What biomechanics and paleontology studies cannot reveal is why these
transitional hominid types forsook life among the boughs to become
earthbound marathon runners. Archaeological studies at hominid sites
offer one strong clue animal bones. About 2.6 million years ago, our
forebears started eating meat and marrow, rich sources of protein and
fat that perhaps eventually fueled the growth of larger brains.
Bramble and Lieberman find it conceivable that endurance running
helped hunters pursue prey to exhaustion.

Back in the 1980s, Carrier had read ethnographers' accounts of
indigenous peoples who chased deer, antelope, and kangaroos to
exhaustion under the scorching sun. The Tarahumara of the mountainous
desert of northwestern Mexico, for example, were legendary runners.
But by modern times, their running tradition had turned to sport: Men
wearing simple tire-tread sandals bound with leather thongs compete
in a 24-hour footrace that involves kicking a ball over about 100
miles of mountainous road. So Carrier, a triathlete in college, took
it upon himself to prove his case. He and his younger brother, Scott,
went to the desert in Utah and Wyoming to chase pronghorn antelope.
The beasts ditched them every time. The sleek, bouncy animals would
join up with others, and soon the men would be huffing after a dozen
of them. "You wouldn't know which were the animals you started with,"
Carrier says.

For direct evidence of endurance hunting, Bramble and Lieberman point
to the observations of Louis Liebenberg, author of The Art of
Tracking: The Origin of Science, who has spent time on the
traditional hunts of the Bushmen hunter-gatherers in the central
Kalahari Desert in Botswana. Liebenberg ran with them when they
chased down kudu antelope on two occasions. For eight other hunts he
trailed them in his Land Cruiser, sometimes with a GPS device. The
men attempted to run prey to exhaustion only when temperatures neared
100 degrees F, says Liebenberg. Three men would gulp a lot of water
and head out together. Two initially did the hard work of tracking
and pursuing over the arid grassland and woodland terrain, while the
other held back. Eventually, the leaders dropped behind, leaving the
third man to hound and spear the antelope when it reached its limit.
"The animal will either just completely collapse, or it will actually
slow down to a point where it just stands there . . . with sort of
glazed-over eyes," Liebenberg says. "Essentially, you're pushing the
animal to overheat." The hunters would then walk home with the meat,
enough to share in small portions with the tribe.

During a chase, Liebenberg noted that the men maintained speeds of
around 4 to 6 miles per hour, for anywhere from two to six and a half
hours, and traversed up to 22 miles of terrain. These stats fall well
within the performance range of the world's fastest competitive
marathoners, who set a pace of roughly 12 miles an hour to cover 26
miles, albeit under far less harsh conditions.

Although Liebenberg's observations support the runner-as-hunter
hypothesis, Bramble and Lieberman think early Homo would more likely
have first run to scavenge prey killed by other carnivores�a strategy
the Hadza people of East Africa are known to use. When leopards or
hyenas bring an animal down, the hunters "can spot these fresh kills
at a distance from the vultures circling above," Bramble says. A
carcass is an ephemeral treasure, picked clean within hours, so the
Hadza quickly head off running, chase away the carnivores, and take
what's left.

Of course, no one knows whether scavenging reaped enough caloric and
nutritional returns to make it worthwhile for our forebears. But
Bramble and Lieberman feel that the collective evidence, fielded from
so many different angles, makes a compelling case for the running
hypothesis. Even ordinary studies of human physiology, for example,
suggest that humans are so adapted for intense physical activity that
a sedentary lifestyle spawns modern-day scourges like diabetes and
heart disease. Additional support could come from the chimpanzee
genome, which may allow researchers to clock when the genes for slow-
twitch muscle fibers�crucial for running long distances and plentiful
in people but not chimps�diverged in the common evolutionary history
of humans and apes. Other clues could come from tracing the genes
involved in our abundant sweat glands and loss of body hair.

Meanwhile, other researchers are looking for holes in the argument.
Functional morphologist Brigitte Demes, at the State University of
New York at Stony Brook, notes that the gluteus maximus is absolutely
essential for rising from a squatting posture at rest or during
foraging, so it might not have evolved just for running. Stony Brook
anatomist Jack Stern, famed for analyses of how Lucy walked, says
it's a tough call to classify the Achilles tendon as an adaptation