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Feature

Upright Characters
What Made Us Human

Nina Jablonski

Those of us who study human evolution often find ourselves pondering a dilemma. On the one hand, we recognize that humans share myriad anatomical and behavioral characteristics with other animals, particularly with other large primates. But we also know that humans exhibit many characteristics and behaviors that easily distinguish us from even our closest primate relatives. Gorillas run and charge on two legs for short distances, but they would never make it as linebackers. Chimps can fashion simple tools from twigs, but they can't make a spear or a cooking pot. Vervet monkeys have different warning calls for approaching snakes, leopards, and birds of prey, but they can't tell each other what they did yesterday. There was even a recent report of a male chimp wearing a "necklace" made of the fur of the colobine monkey he had just killed, but his closet was otherwise bare.

Over the years, many of the evolutionary Rubicons that were seen by scientists and philosophers as separating humans from our closest relatives-the ability to make and use tools, hunt live prey, use language, and have a concept of self-have all been crossed by other species. In their place is the recognition that, over the course of our recent evolution, we have developed more elaborate expressions of many behaviors that we share with other species. It's not that we invariably display something absolutely new that other species don't have, but that compared to our closest ape relatives, we have the Ferrari, not the wheel barrow.

There is one very distinctive attribute of hominids (humans and their immediate ancestors) that got us going on the Ferrari track. When a new attribute makes possible a cascade of anatomical and behavioral changes, it is referred to as a key innovation. Such innovations often bring about a diversifying explosion of evolution-an adaptive radiation-in their wake. In the case of the human lineage, the key innovation was the evolution of habitual bipedalism, the ability to routinely stand and walk on two legs. Once established in the human lineage, bipedalism made possible altogether new ways for humans to interact with the environment and with each other. If you had to use your hands to walk and support your body weight, you wouldn't accomplish much of anything that we would call human. But with hands and arms freed of such responsibilities, a world of gesture, manipulation, and the manufacture and transport of articles opens before your eyes.

The evolution of bipedalism was a process of singular importance, but it is still not fully understood because the fossil record does not preserve the behaviors that led to the adoption of habitual two-leggedness. So, the solution to perhaps the biggest question in human evolution --What caused us to stand up?--has required years of scientific detective work, including meticulous comparative anatomical study of living and fossil primates and exhaustive exploration of indirect evidence.

We now know two things with absolute certainty. The first is that the shift to habitual bipedalism was not a fine-tuning but a major evolutionary change. It required reshaping of our skeleton and muscles and extensive rewiring of the parts of the nervous system that control movement. We also know that such major changes usually occur in nature because they afford a major advantage to individuals in terms of the number of healthy offspring they can leave behind. The essence of Darwin's concept of natural selection is that success in the natural world means reproductive success.

So what kind of special advantages did early bipedalism afford? Was it that habitual two-leggedness made possible more efficient feeding and carrying, or the making and wielding of better tools? Did becoming taller allow us to see approaching predators more clearly? Was bipedalism, from a physiological point of view, a more metabolically efficient mode of locomotion or one which reduced the heat stress experienced by hungry apes crossing the sunny savanna in search of food? All these possibilities have been suggested in recent years, but none has proven terribly convincing as a first cause, even though all of these factors probably reinforced bipedalism once it started to evolve.

Over the last eight years, my husband, George Chaplin, and I have been developing a theory for the origin of bipedalism that we feel is a stronger evolutionary explanation, because it speaks directly to the question of reproductive success. First of all, we looked at our monkey and ape relatives and determined under what conditions they engage in bipedal behaviors. All of them are able to adopt two-legged positions for short periods of time, to reach up into shrubs and short trees to gather food, and to scan their environments for approaching predators. But one set of behaviors would seem to have a disproportionately great effect on their reproductive success: bipedal threat displays.

Gorillas and chimpanzees -our closest relatives-use bipedal displays to resolve disputes over food or mating rights. Such displays are not long in duration, but they are large in effect. They generally involve one individual rising to its full height, often with hair on end, charging toward another individual or a small group. These displays are intended to intimidate, not injure. The other individual sometimes counters with a similar display, or, alternatively, immediately presents an appeasement gesture. In either case, each display sequence results in a subtle shift in interpersonal politics: the initiator has scored evolutionary points because he (or she) has successfully contested for an important resource.

In gorilla and chimpanzee societies, bipedal threat displays are used by males and females in a variety of contexts, but they are most intense when used between two males to determine who will have the right to have sex with a receptive female. Any animal that can establish and maintain his access to fertile females with a dramatic, upright display, rather than a physical battle, can expect an immediate and direct effect on his reproductive success.

It is not difficult to imagine how bipedalism, in the context of social control and directly tied to reproductive success, once in place, became a widespread behavior. One of the most important aspects of bipedal displays is that they are highly effective means of resolving disputes without risk of injury and death. We have recently used computerized demographic modeling to show that early pre-hominid populations that reduced physical violence and death through the use of bipedal displays would have grown at a faster rate than those that didn't. Threat displays are intended to look intimidating; they symbolize the potential physical threat. Our most distinctive attribute was thus born of bluff and display to avoid physical harm.

Imagine yourself as an early proto-biped. If you are female, you are probably about one meter tall and weigh about 30 kilograms; if male, about 35 centimeters taller and 15 kilograms heavier. You live in an open woodland where you still find it easy to take shelter by climbing trees when danger threatens. Now that you're spending more time on two feet, you can start engaging in other bipedal behaviors more often. It's possible to carry food from one place to another, even to gather food for others. It's possible to use your hands more extensively to communicate, whether in play, hunting, or other contexts. Steadily balanced on two feet, you can throw more accurately. And with hands freed of locomotor responsibilities, you are able to gather and shape new materials-plant fibers, wood, bone--to make tools. Like your chimp relatives, you use those same hands to groom and comfort and tickle, as you cultivate long-lasting social bonds with your relatives. Could it be, as primatologist Robin Dunbar has written, that language itself grew out of such a scenario of physical proximity and the need to communicate with increasing numbers of other individuals? This was, perhaps, the most exciting time in human evolution.

The fossil record attests to the fact that, for hominids, bipedalism was the evolutionary equivalent of winning the lottery. It opened up possibilities for ways of life that previous apes never could have exploited. Between four million and 2.5 million years ago, we see a fantastic multiplication of hominids--eight or more species-onto the open landscapes of Africa. Paleoanthropologists argue about the exact number of different species, but they all agree that several distinct kinds of bipeds existed and that they appear to have pursued slightly different lifestyles. Further, there is good evidence that in eastern Africa, at least two distinct forms lived side by side fewer than two million years ago. They included the so-called robust australopithecines, Paranthropus aethiopicus and Paranthropus boisei, and the earliest known species of our own genus, Homo rudolfensis. We don't know how these pioneering hominids divvied up available resources, but studies of tooth size and wear suggest that Paranthropus species subsisted on a largely vegetarian diet supplemented by scavenged meat, while early Homo ate larger amounts of meat, possibly obtained by hunting.

These new denizens of the African woodlands and savannas all sported brains larger than those of the earliest bipeds, brains that were both creating and responding to the challenges of new environments. And we know that by about 1.6 million years ago, members of the species Homo ergaster, epitomized by the now-famous "Nariokotome boy," were highly active, striding bipeds, close to modern stature and capable of long-distance travel. Their larger brains and higher activity levels created new opportunities, but also raised new biological problems.

One of the greatest of these was keeping cool. The brain is very temperature-sensitive, and keeping such a large organ (about 750 grams worth at that time) and the rest of the body cool was a big challenge. There are many inferences about early human physiology that we must make indirectly, and inferences about cooling mechanisms are a good example. We know that all primates lose heat through sweating, but that most nonhuman primates have relatively low densities of sweat glands in their skin. Humans solved the problem primarily by increasing the number of sweat glands, so increasing the potential for evaporation and essentially making themselves into portable swamp coolers. This almost certainly meant that, at the same time, the surface of the body lost most of its covering of hair.

The evolution of humanness may have started with bipedalism, but it was propelled by the increased use and manufacture of tools. Scientists, eager for "hard" data to support inferences about early tool use, have tended to downplay the probable importance of soft tools in early hominid economies. Evidence of transport and manufacture of simple stone tools extends back almost 2.5 million years, but these durable and reusable items were certainly preceded by tools made out of softer plant or animal materials. After all, we know that chimpanzees today in eastern Africa make use of termite "fishing sticks," and some in western Africa use large rocks as anvils to crack nuts. So, it is safe to assume that tools of some kind-twigs, leaves, unfashioned stones-were used by our last common ancestor with chimpanzees, and that tools became more common, diverse, and refined during the earliest phases of human history.

Stone tools, however, hail a new age in hominid technology. At sites throughout eastern Africa dating to between 2.3 and 1.7 million years ago, we find stone artifacts and cobbles that the late archeologist, Glynn Isaac, once referred to as "Stone Age visiting cards." These artifacts had been carried, sometimes over distances exceeding 20 kilometers, from areas where suitable raw materials were available to places where they could be used. Such evidence indicates not only technological sophistication, but also reveals geographic knowledge and foresight by our ancient relatives. Individuals collecting and transporting such items had a clear notion of undertaking purposeful acts at some future time. They could navigate over a complex landscape, retrieve a specific resource from a specific place, return to a home base, and then proceed to make something they had been thinking about. We take such things for granted, but thinking about the sheer brain power required-the number of memory feats alone-empowers those distant ancestors in our eyes.

Careful inspection of stone tools also reveals a remarkable record of past decision-making processes. As the archeologist John Gowlett has pointed out, a hand axe such as that used by our Homo erectus ancestors 1.5 million years ago, preserves a literal blow-by-blow account of its manufacture and, indirectly, of the mental processes necessary to make it. These tools were made by individuals capable of maintaining a plan in their heads, a procedural template, in Gowlett's words, that allowed them to hold to a design while maintaining enough mental flexibility to adjust to the requirements of the individual tool. The considerable mental concentration and the physical coordination this involved would have again required subtle modifications of the brain.

Consider the social context in which tool-making occurred. Tool-making was a deliberate, skilled act that was learned from others. Although an individual may have been able to learn how to make simple stone tools by observation alone, it seems reasonable that some form of language would have facilitated the process. New evidence, marshaled by Richard Kay and colleagues, indicates that Homo erectus, a proficient manufacturer of hand axes beginning over 1.5 million years ago, may have been capable of a primitive form of articulate speech. Kay deduced this from a comparative study of the size of the hypoglossal canal at the base of the skull. This canal transmits the hypoglossal nerve into the head, where it then provides the motor nerve supply to the tongue.

The tongues of modern humans are large and very finely coordinated muscles capable of rapid and precise movements. Air exhaled from our lungs passes through the vocal cords and then into the oral cavity, where the positions of the tongue and lips dictate the precise nature of the sound that is generated. Most letters in the English alphabet cannot be spoken without using the tongue. The word "dentists," for instance, requires six tongue movements. The hypoglossal nerve in Homo erectus is relatively larger than it is in earlier australopithecine hominids, but it is still significantly smaller than it is in modern humans. Although there is now spirited debate about the validity of these observations, the evidence is tantalizing indeed.

Combining this with other information we have on the shape of the oral cavity and the size and configuration of the brain in Homo erectus, leads us to deduce, on anatomical evidence alone, that some form of articulate speech and language was possible among these early tool-makers. Clearly, neither tool-making nor language appeared overnight; rather they are elements of humanness that became more diverse and refined over the course of at least two million years of evolution.

What else would we consider essential elements of the human condition? Certainly most people would say that we must include symbolic culture and art. It is in this arena that our own species, Homo sapiens, has excelled. The cultural badges of sophistication in our own society-paintings, sculpture, music-are unique to us having emerged mostly within the last 30,000 years.

Not that we have a monopoly on symbolic culture. Although "hard" evidence is scant, we know that Homo neanderthalensis, the Neandertals of the late Ice Age of Europe and the Middle East, deliberately buried their dead. Now, this doesn't mean a funeral at Westminster Abbey, but it demonstrates that these people used forethought to place another individual in a grave, perhaps to insure the safe journey of the deceased to another world, or to protect the recently departed from being scavenged, or from contaminating the living. Whether Neandertals buried their dead for spiritual or prosaic reasons, there is no doubt that the survivors envisioned and speculated upon a future. Several lines of anatomical evidence indicate that Neandertals were capable of essentially modern articulate speech, further strengthening the case that these Ice Age hunters, too, enjoyed a symbolic culture, not as rich as our own, but magnificent in its own right.

Some years ago, my Ph.D. advisor remarked that as far as human evolution was concerned, "everything was boringly predictable after we evolved bipedalism." I wouldn't go quite that far, but there is no question that the key innovation of bipedalism paved the way for the intensification and diversification of culture in our lineage. There aren't many unique characteristics left for humans to boast about, but without bipedalism, the ape that used sophisticated tools, language, and symbols to write this article would have never become human.

Nina Jablonski is Irvine Curator and Chair of Anthropolgy at the California Academy of Sciences. She specializes in primate evolution.

Summer 1999

Vol. 52:3