Walk, Run, Eat: The Evolution of Our Body

Visualizing the evolution of our bodies from our chimp ancestors to what we see in the mirror does not come easily. 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 our feet, legs, arms, head 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 tchadensis, 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 is still 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 Australopithecus 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, still with relatively long arms and short legs.

Next 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 to make chewable and digestible. We grew taller, with arms and legs close to today’s proportions. We developed external noses that humidify the air 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” like diabetes and conditions like hardening of the arteries that our ancestors never worried about. We may treat the symptoms successfully, but given evolution’s slow clock, we won’t be adapting to resist them any time soon.

But we are always walking. We walked our way into becoming human, we walked our way around the world and into history, we speak of journeys, progress, protest marches. There is little else we do that is more essentially us.

march of progress

The original version of the “March of Progress,” from Time magazine in 1965. The details are out of date now but the image remains indelible.
(Wikipedia)

Who Were Homo Sapiens’ Parents?

Let’s imagine for a moment that you don’t know who your parents were. No records of them have been found yet. And because you don’t know your parents, you also don’t know for sure who your grandparents, great-grandparents or other direct ancestors were. But by some fluke you do know about some of your great-aunts and great-uncles, though none are alive now.

So you know about your general ancestry, where you came from, how your ancestors lived. But the family tree is complicated and you can’t be sure who your direct relatives were. You may be the offshoot of one of these aunts or uncle for all you know, you may be the result of a one-night fling or other scandalous pairing, or maybe your parents and grandparents just haven’t shown up in the records…yet.

Such a situation is where we stand with our species as a whole. We were “born” as a species when our bodies reached their present proportions about 195,000 years ago. Here’s a basic  version of the family tree around us and just preceding us, with some species omitted. There isn’t complete agreement on it, and it keeps changing as new bones and DNA samples come in.

  • Homo habilis (“handy man”) might be viewed as our great-great-grandfather (the masculine here will stand for both genders). He was good with tools and lived in Africa from 2.5 million years ago to 1.4 mya.
  • His descendant or cousin, Homo ergaster (“working man,” even better with tools) lived at about the same time,  1.9 to 1.4 mya.
  • homo_heidelbergensis-wikipedia-com

    Homo heidelbergensis (wikipedia)

    One of H. ergaster’s descendants was our grand uncle, Homo erectus, the first to stand tall and erect. Overlapping with our origins and a dominant presence in our past. H. erectus lived a long life not only in Africa but in Asia as well until 70,000 years ago. He used fire and he cooked. He lived in small, organized bands of families. He was thought to be our parent for a while but today the connection looks shaky.

  • The strong contender for our immediate ancestor at present is Homo heidelbergensis, an offshoot of the handy man H. habilis. H. heidelbergensis lived about the same time that we appeared.
  • Some H. heidelbergensis migrated into Europe where they evolved into the Neanderthals, our genealogical brothers or cousins. When we H. sapiens later migrated out of Africa, some of us lived near H. neanderthalensis, interbred with them (today almost all of us have a little Neanderthal in our genes), and survived them.

It’s a stunning story, all the more so because where we connect to it is still uncertain. The traits that we recognize as us—the abilities to walk and run, the skillful eye-hand coordination, the smarts to keep track of who to trust and who not to, our abilities both to exchange gossip and to discuss philosophy—all appeared step by step through such interesting early versions of us. And imagine being Homo erectus, with curiosity about how to chip a slightly sharper edge on your cutting stone or the skill to try out slightly different sounds as you talk to others. Or hearing the rumor that a group of men that look a little different than you, men who seem more organized and who carry longer spears, were appearing in the next valley.