Jeremy DeSilva’s new book "First Steps” is a book anyone interested in science would enjoy, particularly those of us curious about locomotion and anatomy.  DeSilva is an anthropologist, who as a young man, became curious as to why humans walk on two, rather than four, legs. In doing so, he realized that the many things he wondered about are all connected, and at the root of it all is the unusual way we move. He discovered that our bipedal locomotion was a gateway to many of the traits that make us human.

Part I of his book investigates what the fossil record tells us about the origin of upright walking in the human lineage. Part II explains how it was a prerequisite for changes that define our species, from our large brains to the way we parent our children—and how those changes allowed us to expand from our ancestral African homeland to populate the Earth. Part III explores how the anatomical changes required for efficient upright walking affect the lives of humans today, from our first steps as babies to the aches and pains we experience as we age. The conclusion examines how our species managed to survive and thrive despite the many downsides of walking on two, rather than four, legs.

Humans are unique

Humans are weird and certainly unique. As primatologist John Napier wrote in 1967, “Human walking is a unique activity during which the body, step by step, teeters on the edge of catastrophe.”  There are other different features as well.  Although we are mammals, we have comparatively little body hair. While other animals communicate, we talk. Other animals pant, but we sweat. We have exceptionally large brains for our body size and have developed complex cultures. But, perhaps oddest of all, humans navigate the world perched on fully extended hind limbs.

How we walk

Researchers often describe bipedal walking as a “controlled fall.” When we lift a leg, gravity takes over and pulls us forward and down. Of course, we don’t want to fall on our faces, so we catch ourselves by extending our leg forward and planting our foot on the ground. At that point, our bodies are physically lower than they were at the start of our journey, so we need to raise ourselves upward again. The calf muscles in our legs contract and raise our center of mass. We then lift the other leg, swing it forward, and fall again.

Speed

According to DeSilva’s book, Usain Bolt set the men’s world record in the one-hundred-meter dash at 9.58 seconds. Between the sixty- and eighty-meter mark, he maintained a peak speed of nearly twenty-eight miles per hour for about 1.5 seconds. But by the standards of other mammals in the animal kingdom, this human speed demon is pathetically slow-footed. Cheetahs, the fastest land mammals, exceed sixty miles per hour. Cheetahs do not typically hunt humans, but lions and leopards, who occasionally do, top out at fifty-five miles per hour. Even their prey, including zebras and antelopes, can flee snapping jaws at fifty to fifty-five miles per hour. In other words, the predator-prey arms race in Africa currently stands at no less than fifty miles per hour. That’s how fast most predators run, and how fast most prey try to escape. Except for us.  Despite our slow speed, in a lifetime, the average, nondisabled person will take about 150 million steps—enough to circle the Earth three times.

Downsides

Our flaws illuminate not only our evolutionary past but also our present and future. One of the major problems is our upright posture.  It was adapted from a body plan that had mammals walking on all fours.  This tinkering aided our early ancestors as standing on our own two feet promoted tool use,  enhanced intelligence and allowed us to carry weapons.  On the other hand our backbone has since adapted somewhat to the awkward change, the lower vertebrae have grown bigger to cope with the increased vertical pressure, and our spine is curved a bit to keep us from toppling over. These fixes have resulted into a number of orthopedic problems, particularly as we age.  Bones lose minerals as we continue to grow older, the demineralization makes bones susceptible to fracture and osteoporosis.  Spinal disks can slip, rupture or bulge. Muscles continue to lose mass and tone and weakness contributes to lower back pain.  Veins in the legs are more prone to varicosity, they become enlarged and twisted.  Valves that should snap shut between heartbeats malfunction, causing blood to pool.  Severe varicose veins can lead to swelling, pain and on rare occasions, life threatening blood clots.  Knee joints in particular, are somewhat fragile and complicated, cartilage is worn away with repetitive use, causing bones to grind against each other.

Final thoughts

Bipedalism also makes our gait somewhat unstable. Sometimes our graceful “controlled fall” is not controlled at all. According to the U.S. Centers for Disease Control and Prevention, more than 35,000 Americans die annually from falling—nearly the same number who die in car accidents. In contrast, when is the last time you saw a four-legged animal—a squirrel, dog, or cat—trip and fall?

 Max Sherman is a medical writer and pharmacist retired from the medical device industry.  His new book “Science Snippets” is available from Amazon and other book sellers. It contains a number of previously published columns.  He can be reached by email at  [email protected].