Leonardo's Foot

by Carol Ann Rinzler

  Forty years later, the ape/early hominin C-curve and the human S-curve fit nicely into Rudolph Zallinger’s well-known and often-imitated drawing, “Ascent of Man” in Early Man (Time Life, 1965), a simple, but memorable picture of how our ancestors slowly stood up to become the human race—or at least half the human race. In 2008, an illustrator for the magazine New Scientist tweaked the image slightly for an article about evolution “myths and misconceptions,” substituting a female figure for the man standing latest in the evolutionary line.

  Back in 1924, Dart was delighted with his Christmas day discovery (“I doubt if there was any parent prouder of his offspring”), which he named Australopithecus africanus, from the Latin australis meaning southern and the Greek pithekos meaning ape. A swift two months later, he published his findings in the February 1925 issue of the journal Nature. The Taung Child, he wrote, was more human-like than ape-like, and the position of the foramen magnum showed that he was a biped, walking upright with arms and hands free in front. In other words, the Taung Child, seemed an excellent candidate for the title of a missing link between apes and humans, one that would co-incidentally prove that humans appeared first in Africa.

  It was a genuine Eureka! moment, and Dart waited expectantly for someone—or many someones—to shout an equally genuine, Huzzah!

  But no one cheered.

  Strike one was Dart’s assumption that the Taung Child’s small brain and semi-upright posture showed that the foot took precedence over the brain in moving us on our way to being human. The early twentieth century anthropology establishment, including the legendary Leakeys—Louis (1903–1972), Mary (1913–1996), Richard (1944– )—believed the opposite to be true, that the prime mover was our evolving brain, which fostered our increasing skill with tools.

  The second strike was where Dart found his fossil. True, in The Descent of Man (1871) Charles Robert Darwin (1809–1882) had suggested that hominins appeared first in Africa, his evidence being the presence of chimpanzees and gorillas, the two species most like us. But the company line, endorsed by the leading European anthropologists as well as by the British anatomist Arthur Keith (1866–1955), a Pooh-Bah equal in stature to Grafton Elliot Smith, was that the ancestors of human beings, each with a large brain and a sloping ape-like face, had arisen in Europe. Others, such as Louis Leakey, voted for East Asia. Such small difference aside, all were sure that Dart had simply mistaken the skull of a juvenile ape for that of a hominin. Their argument was not unreasonable. Juveniles do often lack the facial characteristics of adult chimpanzees or gorillas.

  Strike three? There was no bundle of similar skulls to back up Dart’s claim; no more specimens had surfaced. Tuang was off the beaten anthropological path, and faced with a virtually universal lack of support, Dart and his students abandoned the excavation.

  Like so many prophets, Dart was about to spend a lot of time without honor in his own land of anthropology.

  Eventually, other biped hominins with chimpanzee-size brains did appear, such as the ape–man skull discovered by Mary Leakey in 1959 in Tanzania. But the woman who drove Dart’s argument home was not Leakey, it was “Lucy,” the small-brain, biped fossilized skeleton discovered near a lake in Ethiopia on November 24, 1974. Lucy was significant not only because she bolstered the theory of early bipedalism in Africa, but also because her fossil re-ignited an anthropological argument about whether hominins stood up first in the savannah or by a lake.

  The “Aquatic Ape Theory” was introduced by marine biologist Alister Clavering Hardy (1896–1985) in an address to the British Sub Aqua-Club in 1960. Hardy’s evidence was salty human sweat, which he believed showed that early hominins lived in a watery environment. Had they been walking on four legs, he explained, they would have drowned—or at least been seriously uncomfortable when entering the water in search of food. Hardy’s idea of a waterside human predecessor turned out to be even less popular than Dart’s presentation of the Taung Child as a missing link. Several more biped fossils, including Lucy’s, were found near bodies of water, but the tale of the watery ape remains an anthropological outlier.

  As for Raymond Dart and his foot-before-brain thesis, as Dennis O’Neil of California’s Palomar College has written, “It is now clear that upright bodies and bipedal locomotion long preceded the large human brain. The early 20th century speculation that our ancestors would be large brained apes proved to be incorrect. We attained the full human form of bipedalism by about 2.5 million years ago, if not earlier. However, the size of our brain continued to increase in a punctuated evolutionary pattern.”

  Happily, although people still argue over where exactly Australopithecus fits or even whether he and she belong as an ancestor of Homo sapiens, Raymond Dart lived long enough to see his theories about bipedalism versus the big brain vindicated. Today the skull of the Taung Child sits in a box in the fossil room of the Institute for Human Evolution at the medical school of the University of Witwatersrand in Johannesburg, where both Dart and his discovery found a home.

  Determining destiny

  In 1912, in his provocatively titled essay, “On the Universal Tendency to Debasement in the Sphere of Love,” Sigmund Freud (1856–1939) wrote that “Anatomy is destiny” (Die Anatomie ist das Schicksal), thus igniting a fire whose flames he fanned with his later observation that the “great question that has never been answered, and which I have not yet been able to answer, despite my thirty years of research into the feminine soul, is ‘What does a woman want?’”

  His annoying befuddlement aside, Freud did have a point. The experience of being male is different from the experience of being female. So is the experience of being tall as opposed to being short or being right-handed rather than left. To side-step the gender issue and make the sentence more universal, all you need do is change Freud’s comment from “Anatomy is destiny” to “Anatomy is perception.”

  Certainly, the anatomy that enabled us to stand up altered our perception of the world around us while changing both our about-to-be human body and our about-to-be bigger brain.

  Consider, for example, the effect of our new and more adventurous daily diet.

  As the forest receded and the savannah spread across the land, the evolving hominin’s pantry expanded exponentially. Some believe that apes and very early man subsisted only on a high carbohydrate plant-based diet— not so. Our primate ancestors were familiar with animal protein. They had always consumed insects, which are plentiful in any forest. As Marvin Harris (1927–2001), an American anthropologist with a special interest in the history of food, often explained, people and animals, prehistoric as well as modern, tend to eat what’s most easily at hand. Harris’ metaphor for nutritional supply and demand was paper money. Suppose, he posits in Good to Eat: Riddles of Food and Culture (1986), that you live in a forest with twenty-dollar bills (think chickens) and one-dollar bills (think insects) clipped to the upper branches of the trees. Your first instinct is to reach for the twenties, but if there are only a few twenty-dollar bills and zillions of one-dollar bills that changes the equation. The one-dollar bills—in this case, the insects—win every time. And why not? Nutritionally speaking, bugs are prize worthy. In the raw, a 100 gram (3.5 ounce) serving of large grasshoppers provides twenty-one grams of protein and six grams of fat, the same amount of protein and twice the calorie/energy rich fat in a similar amount of raw chicken meat, minus the skin. As for the aesthetics, if lobster is your dish, please note that grasshoppers and lobsters both have a long skinny body with multiple legs. Both are good sources of protein and dietary fat. The only difference is culture, which often yields to appetite, intellectual as well as culinary, especially if you live in a tree and do not even have to leave home to pluck dinner off the leaves or bark.

  Standing up makes a difference. With hands free, we can now hunt and kill and carry relatively easily. For Raymond Dart, this meant that the Taung Child and his relatives were hunters and probably violent killers, a belief then shared by many if not most of his contempo
raries. “Man emerged from the anthropoid background for one reason only, because he was a killer,” Robert Ardrey (1908–1980) wrote in African Genesis (1961). “Long ago, perhaps many millions of years ago, a line of killer apes branched off from the non-aggressive primate background. For reasons of environmental necessity, the line adopted the predatory way. For reasons of predatory necessity the line advanced. We learned to stand erect in the first place as a necessity of the hunting life. We learned to run in our pursuit of game across the yellowing African savannah. Our hands freed for the mauling and the hauling, we had no further use for a snout; and so it retreated. And lacking fighting teeth or claws, we took recourse by necessity to the weapon. A rock, a stick, a heavy bone—to our ancestral killer ape it meant the margin of survival.”

  Eventually, the explanation for the hominins’ more meaty diet shifted from man as bloody killer to man as sneaky thief. As Robert J. Blumenschine, director of the Center for Human Evolutionary Studies at Rutgers University, has explained, wild cats and other hunting animals are likely to consume less than one-fifth of the bodies of their kill. That means early hominins could scavenge profitably and even subsidize a prehistoric food bank, leaving some scraps for other passing hungry creatures.

  Either way, captured or stolen, the new higher protein diet changed our bodies, beginning with the face.

  The architecture of the biped body

  Nature coddles no one, but she does protect her children, particularly the boys, by giving them weapons with which to defend their own progeny. Primate babies are born with undeveloped nervous systems, helpless as day old kittens. A chimpanzee infant clings to his mother for at least a year, a gorilla baby for three. Luckily, the adults in the family have a well-developed arms system: impressive canine teeth, up to four times larger in the male than in the female.

  These teeth are not just for taking food apart; they are also for taking enemies apart, which makes you wonder whether all those toothy chimps in films and on TV are smiling or warning us to turn and run. In an interview with Scientific American in 1999, Knox College (Galesburg, Illinois) psychologist Frank McAndrew, an expert in facial expression, said it could go either way. “In the primate threat, the lips are curled back and the teeth are apart—you are ready to bite,” he said. “But if the teeth are pressed together and the lips are relaxed, then clearly you are not prepared to do any damage.” As we evolved from ape to human being, our facial expressions remained remarkably similar and seemingly hardwired. You don’t have to learn how to smile or bare your teeth in threat, McAndrew says. “Kids who are born blind never see anybody smile, but they show the same kinds of smiles under the same situations as sighted people.”

  The normal modern adult ape or human mouth has thirty-two teeth: eight incisors (the big teeth in the middle of the upper and lower jaw), four canines (the pointed teeth on either side of the incisors), eight premolars (the teeth between the canines and the molars), eight molars (the broad teeth in the back of the mouth). The ape jaw has room for another four molars; the smaller human jaw often does not. These “wisdom” teeth, so-called because they are the last permanent teeth to erupt, usually in the late teens as an adolescent becomes an adult and theoretically acquires wisdom, are usually extracted.

  The dental arch is the term used to describe how teeth are set into the jaw. An ape’s dental arch is a rectangle with one open side; the human dental arch is a rounded arc, narrower in front, wider in the back. Obviously, these arrangements affect the shape of the face, which is why fossil skulls are so valuable in showing how we went from ape to Homo sapiens.

  The front of an ape’s skull, his face, is prognathic (from the Greek words pro meaning before and gnathos meaning jaw). His large jaws extend beyond the anterior cranial fossa, the forehead and front top of the skull that encloses the frontal lobes of the brain. Our face is orthognathic (from the Greek word orthos meaning straight). Despite our projecting nose and chin, the human face is classified as “flat.” The difference is due to the size of the canine teeth and how they are set into the jaw.

  When Raymond Dart cleaned the Taung Child’s face, he found canines smaller than an ape’s, but larger than a human’s. The jaws were similarly intermediate, projecting further than a human’s, but less than a classic ape’s. And the markings (patterns of wear) on the teeth suggested that the child had crushed rather than ground his food.

  Apes grind very hard foods, such as nuts, whole, shell included, by moving the jaws from side to side like a mill grinding grain. Early hominins, like modern humans, crushed their meals of meat and other soft foods, exerting pressure simply by bringing their jaws together, bottom closing under top. Together, the smaller canines, the less protrusive jaw, the tooth markings, and the position of the Taung Child’s foramen magnum pointing to a semi-upright posture, strengthened Dart’s conviction that Australopithecus africanus was an intermediate species between apes and humans, more human in their dental arrangement but without the human chin.

  All mammals have a lower jaw, the mandible, that begins as two separate bones, one left, one right, separated by a fiber and cartilage joint called a symphysis, Greek for growing together. In many animals, such as goats and sheep, the bones remain separate, allowing the two sides to flex independently, in a rolling grinding motion that often seems comical to humans (think of watching a goat eat). Some mammals, such as camels and hippos and horses, have mandibles whose bones fuse at about the time that their teeth appear; in apes and humans this occurs within the first year of life. But of all the roughly 5,000 species of mammals on earth, only one—us—has a mandible re-enforced by a bar of bone that forms a protruding bump, the mental (from the Latin mentum meaning chin) protuberance, at the center.

  Explanations for the chin abound. Perhaps it stabilizes the lower jaw, reducing stress on the symphysis. Or it may increase the force with which we chew our food. Or it may enhance our ability to speak clearly. Or it may be sexually appealing; the male chin is more rounded and more prominent than the female, which is more delicate and pointy. Unfortunately, intriguing though these assumptions may be, there is a counter argument for each and every one. Goats, cows, and horses munch away on hard grains and apes chew through nut shells without a chin. As for speaking clearly, so long as you can open and close your mandible and move your tongue, you can make yourself understood. And that sex thing? Maybe. Maybe not. Either way, our chin remains an evolutionary mystery.

  Not so the rest of our body, which, over the millennia, has clearly evolved to accommodate an upright posture.

  Begin with the shoulders. Like an ape’s shoulder joints, ours are still flexible; although as a very long list of baseball pitchers can tell you, the muscle-and-tendon rotator cuff around the shoulder is exquisitely sensitive to stress. Bipedalism has made some muscles once used to climb and swing redundant. For example, most of us no longer have a subclavius muscle stretching from the first (top) rib to the collarbone to stabilize the back and neck while walking on four legs. Some higher apes and humans no longer have one or more palmaris muscles running from the elbow to the wrist to strengthen the arm and flex the wrist and hand. Those who do have scored an anatomical bonus: Should they tear another, more important muscle, their surgeon can clip out the palmaris and use it to reconstruct the injured tissue.

  Even without extra muscles, our arms can still support our weight in push- or pull-ups, but the humerus, the long bone in our upper arm, is not as robust as an ape’s. The radius and ulna bones in our forearm are straighter, but not stronger than his, which are fused into one bone. Unlike an ape, whose arms are longer than his relatively short legs, our arms are shorter than our relatively long legs.

  At the ends of the arms, our hands are smaller. Apes often swing themselves along. We walk, so while our fingers still grasp and hold efficiently, they are no longer curved like an ape’s, nor as long as they were when our ancestors moved from tree to tree through the forest canopy.

  Next up, or rather, down: our ribs. All apes have
ribs in their necks; fewer than one in one hundred of us still do. About one in eight of us has an extra set of ribs, thirteen pairs like the chimps and gorillas rather than the standard twelve pairs for humans arranged in a rib cage whose shape was influenced by our stand-up diet. Chimps and gorillas do not have a waist because their rib cage, which looks like a bell—narrow at the top, wider at the bottom where it meets flaring hip bones—has no room for a waist separating ribs from hips. Our rib cage is shaped like a barrel with a clear separation, a waist between ribs and hips, a testament to our bipedal catch-all diet. Penn State paleoanthropologist Allan Walker, a member of the Richard Leakey team, which in 1984 discovered the skeleton of Turkana Boy, another African biped, wrote that the narrow-to-wide ape rib cage was designed to accommodate the long herbivore intestinal tube whereas the omnivore human intestines fit nicely into the shorter barrel shape cage.

  Pulling nutrients out of a plant-based diet high in insoluble dietary fiber requires a long, multi-tasking intestinal tract. The higher the proportion of insoluble fiber, the more complicated the digestive apparatus will be. Herbivores such as cows that live on plants such as hay and grass use salivary enzymes to start the process of breaking the food apart into its nutritional components, but they still need more than one stomach to complete the job on that insoluble fiber. Animals that feed on softer fruits and vegetables have shorter intestinal tubes and only one stomach. Food from animals is even easier to digest, so carnivores such as the big cats get by with one stomach and an even shorter intestinal tract. We omnivorous human beings cannot digest insoluble dietary fiber, but our relatively short gut efficiently processes nutrients from both animal and soft plant foods.