* The human body is awash with Henles. We have crypts of Henle in the eye, Henle’s ampulla in the uterus, Henle’s ligament in the abdomen, Henle’s tubules in the kidneys, and several more. All were discovered by a very busy, curiously uncelebrated German anatomist named Jakob Henle (1809–85).
10 ON THE MOVE: BIPEDALISM AND EXERCISE
Not less than two hours a day should be devoted to exercise and the weather should be little regarded. If the body be feeble, the mind will not be strong.
—THOMAS JEFFERSON
NO ONE KNOWS why we walk. Out of some 250 species of primates, we are the only ones that have elected to get up and move around exclusively on two legs. Some authorities think bipedalism is at least as important a defining characteristic of what it is to be human as our high-functioning brain.
Many theories have been proposed as to why our distant ancestors dropped out of trees and adopted an upright posture—to free their hands to carry babies and other objects; to gain a better line of sight across open ground; to be better able to throw projectiles—but the one certainty is that walking on two legs came at a price. Moving about in the open made our ancient forebears exceedingly vulnerable, for they were not formidable creatures, to say the least. The young and gracile protohuman famously known as Lucy, who lived in what is now Ethiopia some 3.2 million years ago and is often used as a model for early bipedalism, was only about three and a half feet tall and weighed just sixty pounds—hardly the sort of presence to intimidate a lion or cheetah.
It’s likely Lucy and her tribal kin had little choice but to take the risk of stepping out into the open. As climate change made their forest habitats shrink, they very probably needed to forage over larger and larger areas to survive, but they almost certainly scampered back to trees when they could. Even Lucy appears to have been only a partial convert to life at ground level. In 2016, anthropologists at the University of Texas concluded that Lucy died after falling out of a tree (or suffered a “vertical deceleration event,” as they put it, just a touch drily), the implication being that she spent a great deal of time in the canopy of trees and was probably as much at home up there as on the ground. Or at least she was until the last three or four seconds of her life.
Walking is a more skillful undertaking than we generally appreciate. By balancing on just two supports, we exist in permanent defiance of gravity. As toddlers amusingly demonstrate, walking is essentially a matter of hurling the body forward and letting the legs run to catch up. A pedestrian in motion has one foot or the other off the ground for as much as 90 percent of the time, and thus engages in constant unconscious adjustments of balance. In addition, our center of gravity is high—just above our waists—which adds to our innate tippiness.
In order to proceed from arboreal ape to upright modern human, we had to undertake some pretty profound changes to our anatomy. As noted earlier, our necks became longer and straighter and joined the skull more or less centrally rather than toward the rear as in other apes. We have a supple back that bends, outsized knees, and ingeniously angled thigh bones. You may think your legs drop straight down from your waist—they do in apes—but in fact the femur angles inward as it descends from pelvis to knee. This has the effect of moving our lower legs closer together, giving us a much smoother, more graceful gait. No ape can be trained to walk like a human. They are compelled by their bone structure to waddle, and to do so in a most inefficient way. A chimpanzee uses four times as much energy to move around at ground level as does a human.
To power our forward motion, we have a distinctively gigantic muscle in our buttocks, the gluteus maximus, and an Achilles tendon, something no ape has. We have arches in our feet (for springiness), a sinuous spine (to redistribute weight), and reconfigured pathways for our nerves and blood vessels—all made necessary, or at least advisable, by the evolutionary imperative of putting our head way above our feet. To keep from overheating when we exert ourselves, we became relatively hairless and developed abundant sweat glands.
Above all, we evolved a very different head from other primates. Our faces are flat and conspicuously snoutless. We have a high forehead to accommodate our more impressive brain. Cooking has left us with smaller teeth and a more delicate jaw. Inside, we have a short oral cavity and thus a shorter, more rounded tongue, and a larynx that sits lower in the throat. The changes to our upper anatomy left us by happy accident with vocal tracts uniquely able to make articulate speech. Walking and talking probably went hand in hand. If you are a little creature that hunts big creatures, being able to communicate is obviously an advantage.
At the back of your head is a modest ligament, not found on other apes, that instantly betrays what it is about us that allowed us to thrive as a species. It is the nuchal ligament, and it has just one job: to hold the head steady when running. And running—serious, dogged, long-distance running—is the one thing we do superlatively well.
We are not the speediest of creatures, as anyone who has ever chased a dog or cat or even an escaped hamster will know. The very fastest humans can run about twenty miles an hour, though only for short bursts. But put us up against an antelope or wildebeest on a hot day and allow us to trot after it, and we can run it into the ground. We perspire to keep cool, but quadrupedal mammals lose heat by respiration—by panting. If they can’t stop to collect themselves, they overheat and become helpless. Most large animals can’t run for more than about nine miles before they drop. That our ancestors could also organize themselves into hunting parties, to harry quarry from different sides or drive prey into confined spaces, made us all the more effective.
These anatomical changes were so monumental that they spawned an entirely new genus (the biological rank above species but below family) called Homo. Daniel Lieberman, of Harvard, stresses that the transformation was a two-stage process. First, we became walkers and climbers, but not runners. Then, gradually, we became walkers and runners, but no longer climbers. Running is not just a faster form of locomotion than walking but mechanically quite different. “Walking is a stilt-like gait and involves very different adaptations from running,” he says. Lucy was a walker and climber but lacked the physique for running. That came much later, after climate change turned much of Africa into open woodlands and grassy savanna, impelling our vegetarian ancestors to adjust their diets and become carnivores (or really omnivores).
All these changes, in lifestyle and anatomy, happened with exceeding slowness. Fossil evidence suggests that early hominins were walking by about 6 million years ago, but needed an additional 4 million years to acquire the capabilities for endurance running and, with it, persistence hunting. Then a further million and a half years had to pass before they gathered enough cerebral momentum to manufacture tipped spears. That’s a long time to wait for a full set of survival capabilities in a hostile, hungry world. Despite these deficiencies, our ancient forebears were successfully hunting large animals 1.9 million years ago.