To run is human.

to anyone who occasionally suspects that their urge to run long on the road or trail is not merely a cherished routine but a deeply rooted biological need, a review article published recently in Nature will be of interest, and perhaps come as no surprise. Scientists at the University of Utah and at Harvard propose that the human form was shaped by our species' unique ability--and need--to run long distances.

While anthropologists have long credited the need to walk long distances with influence over our physiological evolution, the current article is among the first to focus specifically on distance running. Previously, the role of running was minimized in human evolution, primarily because humans make poor sprinters compared to other mammals. It turns out, however, that once adjustments are made for body mass, we compete rather well in distance with many social carnivores. Even the most casual joggers can regularly run 10K distances, and hundreds of thousands of people complete marathons each year. The authors write that these abilities are comparable to specialized running mammals like horses and hyenas, which live in open habitats. And endurance running is entirely unique to humans among primates. The fact that such distances are unheard of among apes and chimps makes it all the more remarkable: humans run long distances very well, despite a primate ancestry.

For approximately three million years, hominids such as Australopithecus walked upright without resembling humans, so, the authors ask, what other activity suddenly transformed the hominid body? They then compared human and chimpanzee musculoskeletal features, as well as those that could be determined from the fossil record, with an eye toward their relevance for running-specific demands: energy, strength, stabilization and thermoregulation.

Though some of the traits are certainly useful for trekking, several evolutionary adaptations cannot be explained solely by the need for walking long distances. The spring-like tendons that developed in humans are not nearly as apparent in early hominids and apes. These include all the usual suspects prone to injury in runners: the Achilles tendon, the iliotibial band, the plantar arch and the peroneus longus muscle, which runs down the outer shin to the sole of the foot. Walking does not rely on a spring mechanism, but rather on a pendulum motion. The development of these muscles and tendons, therefore, cannot be explained through walking alone.

Similarly, stride length is a key component of increasing speed without increasing aerobic requirement. Humans have long legs--much longer than Australopithecus, as determined by fossil femur length. We also have more compact feet and shorter toes, which help to further reduce energy expenditure during running. Redistributing 3.6 kg of limb mass from the ankles to the hip, for example, reduces the metabolic cost of slow running by 15%. A narrower waist, too, helps counteract the inefficient side-bending movements induced by long legs during running. This is not a factor during walking.

Humans appear to have a skeletal strategy to lower running-induced joint stress: distinctly larger surface areas than early bipeds in the femoral head, knee and certain lumbar joints. In addition, the authors argue that sweating, hairlessness and cranial cooling systems found in the human species, though certainly useful for walking, are essential for endurance running in hot climes.

Large buttocks, a distinctly human feature among primates, are strongly indicative of a running adaptation, as they are not used at all to stabilize the trunk in walking. Additionally, certain adaptations in the forearm, shoulders, neck and head also help stabilize the trunk and conserve energy in distance running--though these are not of much use in walking--and do so at the expense of climbing skills. This latter realization suggests that running in and of itself helped early Homo become the first fully terrestrial hominoid.

Why did we run? The authors posit that before the development of long-distance hunting tools like the bow and arrow (which emerged only 40,000 years ago), plus competition to secure animal carcasses scattered across great distances, we needed to travel far on foot very quickly, much like the scavenging hyenas of both then and today.

It's fascinating to consider that our human appearance today is partially due to the demands of ancient, necessary endurance running. In turn, the idea that running is embedded in our ancestry, when coupled with evidence of our unique aptitude for it, is a refreshing one that just might help kick start that New Year's fitness regimen we've promised ourselves after a string of holidays and their accompanying winter slack. Who wouldn't do well to remember that, for humans, running is quite possibly the most natural thing in the world?

(Nature, 2004, Vol. 432, Nov. 18, pp. 345-352)