Human beings are actually rather peculiar mammals, with an unusual bipedal posture that makes them different from not only quadrupedal mammals, but also from our most closely related primate relatives. It doesn't seem to be a particularly efficient mode of locomotion, either; we can't sprint as fast as four-legged animals of the same size, and our running has a higher energetic cost. So why are people built the way they are? What advantage did it give us during our evolution?

One possibility is that our posture is a reasonable compromise, a way to derive a relatively efficient terrestrial mode from an ape-like body. We evolved this way because it allows us to walk long distances. Another possibility described in a new paper by Bramble and Lieberman is that our posture is an adaptation for high-performance endurance running, and that really we're a species of lopers, joggers, and marathon runners.

The top speed for a human sprinters is about 10.2 m s^-1 for less than 15 seconds, while horses and greyhounds can hit 15-20 m s^-1 for several minutes—in other words, we're pathetic sprinters. Endurance running (ER) is different, though. ER involves sustained running over long distances and long times, and is carried out aerobically; that is, we only burn oxygen during the run as rapidly as our respiratory system can deliver it to the tissues. We actually seem to be able to hold our own in this activity. Human ER speeds fall between 2.3 m s^-1 (I must be somewhere around there) and 6.5 m s^-1 (for an Olympic class marathoner), with typical speeds for a moderately fit jogger of 3.2-4.2 m s^-1. In comparison, the trotting speed of a horse is about 3.1 m s^-1, and once they hit 4.4 m s^-1, they break into an anaerobic gallop. Over long distances, the average speed sustained by a horse is about 5.8 m s^-1—which means that a well-trained, conditioned human being can keep up with or even outrun a horse if the race is sustained long enough. This range of speeds is illustrated diagrammatically below.

Range of speeds for human ER and sprinting, and minimum trot (Tm), preferred trot (Tp), trot-gallop transition (T-G), preferred gallop (Gp), and maximum sustained gallop (Gms) for ponies, and predicted for quadrupeds of 65 and 500 kg. Also indicated is Gld, the optimal long distance (approx. 20 km), daytime galloping speed for horses. Note that quadrupeds sprint at speeds above Gms.

What this is saying is that we can't compete with these other animals at the high end, in the short range sprint. We don't even have a gait comparable to the quadrupedal gallop (the orange bars), which is an efficient medium range running rate. What we have done, though, is pushed that long-range, aerobic gait, the blue bars, to a greater speed than quadrupeds can match.

The paper then goes into the details of precisely how we accomplish that. They examine features of human anatomy and physiology that contribute to four broad parameters of running performance: energetics, skeletal strength, stabilization, and thermoregulation.

Energetics refers to those features that economize energy use during the activity. For instance, we have built-in 'springs' in leg tendons and skeletal features like the bony arch of the foot that store the energy of elastic recoil. We maximize energy use by increasing stride length rather than rate, so long legs are a benefit. One cost of long legs is that we're swinging a lot of weight, so reduction of foot mass is another advantage to runners.

Running is a relatively high-impact activity, sending shock waves up through the skeletal system every time a foot hits the ground. We lower joint stress by increasing the surface area of joint surfaces and by using those springy shock absorbers, our feet.

Just walking bipedally is a precarious exercise, and running amplifies the problem. These big heads bobbing on the end of a stalk have to be stabilized, both by reducing mass and by structures such as our nuchal ligament along the cervical vertebrae. Alternately swinging massive legs back and forth generates a substantial amount of torque, which is opposed by swinging the upper body to compensate—our relatively narrow waists are an adaptation to allow greater upper body mobility.

As we all know, sustained jogging is a great way to overheat, so we have many thermoregulatory adaptations: extensive sweat glands, reduced body hair, intricate cranial circulation, and elongate morphology. The authors mention that mouth-breathing during strenuous activity, which increases the rate of respiratory ventilation, is another feature humans exhibit which is unusual for an ape.

Many of these adaptations are manifest in the skeleton, and so we have a record of their appearance in our evolutionary history. The table below lists a series of these features, along with their functional role, whether they assist in walking (W), running (R), or both, with more advantage to running (R>W).

Now look at these diagrams of a few primates that illustrate these features. At the top left is us, and top right is Homo erectus; despite the smaller brainpan, H. erectus has virtually all of the features for endurance running that we have. The lower left image is a chimpanzee, which can run short distances at a rapid sprint, but has no endurance at all. Compare that to Australopithecus afarensis at the lower right—it's intermediate between our chimp cousins and ourselves.

Anatomical comparisons of human, chimpanzee, H. erectus and A. afarensis. a, c, Anterior and posterior views of human, enumerating features related to endurance running listed in previous table. b, d, Anterior and posterior views of chimpanzee. Labelled muscles connect the head and neck to the pectoral girdle and are reduced or absent in humans. e, Reconstruction of H. erectus based primarily on KNM-WT 15000; f, reconstruction of A. afarensis based primarily on AL-288.

In the two middle pictures, you'll also notice a key difference between us and chimpanzees; we have a massively bunched gluteus maximus, while the chimp has a mere strap of muscle. That muscle is used relatively little in walking, but is crucial at higher running speeds. A firm plump butt is actually a cursorial adaptation.

There are still some major evolutionary questions: did walking evolve first, or was it concurrent with the evolution of endurance running? What behaviors drove this feature—pursuing prey to exhaustion, rapid exploitaton of carrion, tracking injured prey, or just getting early man close enough to use projectile weapons? And what were the consequences?

Additional research will help to clarify and test when and how ER capabilities evolved in humans, and to examine more thoroughly their implications for human evolution. For example, it is known that major increases in encephalization occurred only after the appearance of early Homo. The hypothesis that ER evolved in Homo for scavenging or even hunting therefore suggests that ER may have made possible a diet rich in fats and proteins thought to account for the unique human combination of large bodies, small guts, big brains and small teeth. Today, ER is primarily a form of exercise and recreation, but its roots may be as ancient as the origin of the human genus, and its demands a major contributing factor to the human body form.

These physical adaptations to a walking/running lifestyle came first, and our big brains may be a consequent side effect.

Bramble DM, Lieberman DE (2004) Endurance running and the evolution of Homo. Nature 432:345-352.