Walk, Run, Eat: The Evolution of Our Body

It can be difficult to visualize the stages that led from our chimp ancestors to the body that we see in the mirror. But Daniel E. Lieberman’s The Story of the Human Body: Evolution, Health, and Disease is a fine time machine. It took me back six million years to changes in feet, legs, arms and torso, all molded as our ancestors searched for food.

Reconstruction of sahelanthropus tchadens, who lived six to seven million years ago. (smithsonianscience.org)

Reconstruction of sahelanthropus tchadens, who lived six to seven million years ago. Not your average chimp. (smithsonianscience.org)

Human evolution can be said to have begun when one of our ancestors developed a feature that has been unique to us: We walk on two legs. That ability separated us from our cousin chimps between six and seven million years ago. We remain the only two-footed walking animal that doesn’t carry the feathers of a bird or the tail of a kangaroo.

Why walk? We began walking when the fruit that we ate became sparser; the African continent was cooling and the forests were shrinking. (I’ve conflated the species that Lieberman names to “us.”) Those who could stand upright and walk distances on two feet found not only more fruit but also edible stems and leaves. We were chimp-size, but as bi-pedal walkers our arms and hands became free for new uses.

intermediate human

A reconstruction of australopithicus bosei, “Nutcracker Man,” who lived two million years ago, discovered by Mary and Louis Leakey in 1959. Our intermediate stage. (Wikipedia)

The transition continued. By four million years ago, our anatomy had changed again. Foraging over distances fostered “more habitual and efficient long-distance walking.” Our feet acquired an arch that put a spring in our step and pushed the body forward. For stronger chewing, molars and jaws became much larger than ours today. These ancestors are nicknamed “Nutcracker Man.” We were still small but more upright, and still with relatively long arms and short legs.

The next stage was the Ice Age, two and a half million years ago. Foraging over larger areas required more calories, calories that meat could provide. Our ability to throw accurately brought down animals. Sharp stone tools cut up their flesh and made it chewable and digestible. We became taller, with arms and legs close to today’s proportions. We developed external noses to humidify the air that we inhaled during long walks. We began to run—far—with Achilles tendons for more spring and unique sweat glands and finer fur to stay cool.  As teeth and snouts shrank and brains grew, heads became rounder. Organized hunting and gathering became necessities. Generally, females gathered while males hunted. Unlike chimps, we shared food readily with extended families. Cooperation, coordination, and communication were means of survival. We—Homo erectus—became “significantly human.” 

homo erectus

Homo erectus reconstructed.
“Significantly human,” writes Lieberman. (Wikipedia)

Lieberman continues the story of our evolution into the present and discusses its relevance to disease. After millions of years of seeking food and storing its energy in our bodies whenever we could find it, today we eat more calories than we need while we burn off fewer calories than ever before. As a result, we suffer from “mismatch diseases” such as diabetes and other conditions like hardening of the arteries that our ancestors had no need to adapt to. We may treat the symptoms successfully, but we ignore the reality that, given evolution’s slow clock, we won’t be adapting to resist them any time soon.

march of progress

The original version of the “March of Progress,” from Time magazine in 1965. The details are out of date now and the notion of linear, progressive development has been criticized. But the image remains indelible.

6 thoughts on “Walk, Run, Eat: The Evolution of Our Body

  1. Fun post but I think Lieberman is mostly engaging in flights of fancy. There is a lot we know about evolution. Mechanisms are clearly defined and proven. Fossils provide steps where they are available. But the precise reasons for the steps (e.g. We became bipedal to free our hands in order to x) are just convincing stories we tell ourselves. We don’t know which story is right.

    Think about physics. Newton be gave us math that described how gravity worked and made up a story about forces acting at a distance. Then centuries later Einstein came along and factored in Tau to a few of Newton’s equations. Newton’s equations were still excellent approximations in most cases. But Newton’s story about masses acting at a distance on each other was entirely thrown out. We got a new story about mass shaping space and space moving masses.

    So the stories about how and why we evolved are generally just made up. We don’t know the exact why. We tell the stories because they stir our imaginations better than just stating, some set of selective pressures x{n} acted with intensities y(t){n} over time to alter gene frequencies p{n} until mutations that coded for bipedalism became universal among our ancestors. So, from a spiritual perspective, it is worth continuing to tell them as long as we don’t forget to doubt their veracity.

    • I don’t think anyone is claiming to explain *why* we evolved, nor do they argue that we became bipedal *in order to* free our hands. And although the stories themselves change as we learn more, there are a lot of scientists looking hard at the evidence (including old DNA) and challenging each other’s theories intensively–there is a lot of academic prestige and money riding on these stories. I don’t expect there will ever be a single “right” one, but the outline of early human development has spiritual value to me–it’s the process that brought us where we are. It’s our very factual Garden of Eden.

      Thanks for your own challenge to all this, though.


      • “Why walk? … Those who could stand upright and walk distances on two feet found not only more fruit but also edible stems and leaves.”

        Sorry, I didn’t mean to imply that anyone was implicating teleology in evolution. Anyone who reads evolutionary conjecture quickly becomes accustomed to the short hand for “behaviour x created an adaptive advantage for trait y,” being written “y evolved so we could x”. My point was more along the lines that stories like the one quoted above are conjecture at best. Fossils may provide evidence that we were eating more stems and leaves at some point in our evolution. Fossils may provide evidence that our limbs became more efficient for walking distances. Fossils may provide evidence that we could reach higher from the ground at some point in our evolution compared to some other point. But there simply isn’t any evidence that all of this came together to form the causal relationship implied in the quoted sentence above.

        I agree totally that there is a spiritual aspect to trying to imagine the evolutionary story unfolding. But the story is likely so convoluted so as to confound even the most vivid and creative imagination. Adaptive pressures fluctuate from generation to generation, year to year, season to season. Sometimes, they fluctuate faster than they can have an effect and gene frequencies perform what would appear to be random walks. Multiple pressures act at the same time, some contradicting each other, while others work together. Partially formed traits that started out as responses to one set of pressures, provide the opportunity to respond to other pressures in new ways. Temporarily isolated populations each develop different mutations which then encounter one another when barriers are overcome or disappear. In a million years, perhaps two hundred thousand generations live out their lives and each life has its own story but perhaps a few hundred meager fossils record only a few hints at the details of a few of those multitude of lives.

        In all of this, which traits were encouraged by which pressures is a mystery beyond our imagining. Trying to imagine them is fun and spiritually uplifting and therefore worth while. But despite the many scientists working to build evidence for this or that causal relationship, there will simply never be enough evidence to be certain of the true story. At the end of the day, there is also a spirituality to living in doubt as to the details. There is a deep awe that comes from recognizing that the story of even the evolution of any one trait in any one species out of the hundreds of millions that have lived on this Earth, is so complex as to beyond the grasp of even our well developed minds.

        • Thanks for the detailed response, Eric. I certainly agree about the complexities that you outline. And I agree about the spirituality of living in recognition of the complexity—— as well as, and overwhelmingly, the sheer number of years and lives that is beyond grasping.


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