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1. The Human Creature
A human is any member of the species Homo sapiens ("wise man"), the only living representative of the family Hominidae. The Hominidae, or hominids, are a group of upright-walking primates with relatively large brains (see page 33). So, all humans are hominids, although not all hominids could be called human. Next, all humans are primates. The mammalian order of Primates includes about 180 species of prosimians (such as lemurs, tarsiers, and lorises), monkeys, apes, and ourselves. Primates are unusual mammals, for they have evolved such distinctive traits as highly developed binocular vision (and a corresponding enclosed eye socket in the skull), mobile fingers and toes with flat nails instead of claws, and with sensitive pads at the tips, a shortened snout with a reduced sense of smell, and large brains relative to body size. If primates are unusual for mammals, humans are even more unusual for primates. We are essentially elaborated African apes. We share almost 99 percent of our genetic material -- the information that codes for our proteins, bones, brains, and bodies -- with chimpanzees. Yet, despite such similarities, there are significant genetic and pronounced physical differences between humans and apes. Clearly, that distinctive portion of our DNA must involve some regulatory genes that code for our unusual features. Humans also possess 46 chromosomes to an ape's 48, and our version of the extra ape genes has been lumped together on chromosome 2 perhaps in that process some crucial mutations arose, but we would have to sequence and study all human and ape DNA to solve this paradox. Our close genetic affinity to apes has prompted some authorities, notably Jared Diamond in his book The Third Chimpanzee, to argue that it is fallacious to separate humans and apes into two separate families (African apes constitute the family Pongidae). We walk upright on two limbs, and to accommodate such a strange posture we have developed a specialized pelvis, hip and leg muscles, and an S-shaped vertebral column. We have tiny canine teeth and flat faces with a protruding nose. Males have a pendulous penis, while in females the physical signs of ovulation are concealed, something that happens in no other primate. Humans are highly social animals, a trait we inherited from our primate past, but we have taken it to new extremes through the development of complex written and spoken language which enables us to communicate nuances of feeling as well as information, and a material culture that includes symbolic art. We are also called a moral animal. Besides that strange habit of walking upright, perhaps it is our inventiveness and our introspective nature that truly distinguish humans among the primates. Our species, Homo sapiens, was first described in 1758 by the Swedish botanist Carl von Linne;, whom we know better as Carolus Linnaeus. Most early descriptions concentrated on a very few traits, the most obvious being brain size. If we were to create a richer, more complete biological characterization of our species, many other traits would need to be included. Humans have a relatively long life span that begins with immaturity at birth and a prolonged infancy. Physical maturation is delayed during childhood, then occurs quickly during the adolescent growth spurt. We are polytypic in morphology and skin and hair color but genetically very homogeneous. Our behavior is marked by habitual tool use, communication through spoken and written language, and the symbolic representation of objects. We are culturally adapted to survive in a broad range of physical environments, climates, and temperatures. We are omnivorous and share food extensively with others, another mark of our social being. Our body is relatively hairless except for the head and face, axilla (arm pits), and pubic region. Skeletal features include a hand with an opposable thumb that endows us with a power grip and precise, fine hand movements relatively straight and slender limb bones a pelvis, lower limbs, and associated muscles specifically modified for bipedal locomotion an enlarged hallux, or big toe, in line with the rest of our toes rather than opposed to them and a foot with a weight-bearing arch to absorb the stresses of two-legged locomotion. This list and the aforementioned traits provide a hint of who we are biologically. Many other features of anatomy, behavior, and diversity in Homo sapiens, as well as in other hominids, provide the basis of content for this book. As we query the remains of our extinct relatives for clues to who we are and how we got that way, we will discover that we are much nearer to them than we think, even if separated by millions of years.
2. The Quest for Origins
Since at least the Upper Paleolithic, some 40,000 years ago, every human society has devised a creation myth to explain how humans came to be. The need to explain our origins is one of the universals of being human. Creation myths are based on cultural beliefs that have, in one manner or another, been adopted as legitimate explanation by a particular society. To a large extent, creation myths glorify the specialness of humans. In the broadest view, such myths undertake to explain our differences from all other creatures-our humanness. In contrast to cultural myths about human origins, the science of paleoanthropology, which also tries to construct a narrative about how humans came to be, is rooted in the scientific method. This method, based on objective observation and evaluation, is governed by a set of rules that permit the testing of hypotheses, and the results of such tests may lead to the rejection or modification of the original construct. The success of paleoanthropology rests on integrating two different fields: Darwinian evolutionary theory and the study of Earth's geological history (see page 21). Much like a detective story, the quest for clues to our origins is exhilarating and filled with surprises. The goal, however, is not to figure out "who done it" but to understand why and how: why we differ from our closest relatives, the African apes, and how we became a bipedal, large-brained, culturally dependent animal. We are the last species in the zoological family Hominidae (hominids, in the vernacular), and to understand something of our place in nature (see page III) we need to explore the lessons held by the past. As we learn more about our origins, it becomes apparent that although an ape ancestor became bipedal several million years ago, there was nothing in that development that ensured the eventual evolution of Homo sapiens. Bipedalism, a basic feature of hominids, did not make modern humans inevitable. Paleoan-thropological discoveries make it clear that the human family tree is nota single lineage in which one species succeeded another, leading relentlessly to the appearance of modern humans. Instead, the hominid fossil record suggests that our ancestry is better thought of as a bush, with the branches representing a number of bipedal species that evolved along different evolutionary lines. All of those species were successful, sometimes for long periods, and ail went extinct. At the probable time of a common ancestor for humans and African apes, 6 to 8 million years ago, there was no guarantee that humans would evolve. Yet we did evolve, and because we turned out to be inquisitive creatures with the ability to reflect on our past, we have done so avidly. Paleoanthropology in part plays to that inquisitive, exploratory part of our makeup. Expeditions to remote terrains feature prominently in paleoanthropology, and whereas living for months in a tent, usually under desert conditions, is not to everyone's liking, the pursuit of our origins can be enjoyed in other ways. Each new find, it seems, receives front-page coverage in newspapers, and magazines featuring reconstructions of out ancestors on their cover become the bestselling issues of the year. Hominid fossils touch a responsive chord in people everywhere, who seem to have an inherent drive to know their beginnings. We want to know what the fossils have to say to us. There seems to be a magic in the fossilized bones that transcends time. Specimens like Lucy, a 3.2 million-year-old partial skeleton from Hadar, Ethiopia, have become touchstones for discussing human origins. Although older human ancestors have now been discovered, Lucy, with her affectionate name, has become a benchmark by which people judge new hominid discoveries. Even though distant relatives like Lucy lived very different lifestyles from us modern humans, the message they bring, after millions of years of suspended animation, is important to us all. Ultimately our fascination with the study of human origins nourishes our need for exploration and for understanding both our uniqueness and our close link to the natural world. Today more than ever, people are thinking about the future of the universe and the survival of humankind. For many, the lessons we can learn from out past give us a better perspective on ourselves, our place in nature, and how we view our future.
3. Is Human Evolution Different?
For most of human evolution, cultural evolution played a fairly minor role and did not pick up speed until the Upper Paleolithic, 40,000 years ago. If we look back to the time of the australopithecines, some 4 million to I million years ago, it is obvious that culture had little or no influence on the lives of these creatures, who were constrained and directed by the same evolutionary pressures as the other organisms with which they shared their ecosystems. So, for most of the time during which hominids have existed, human evolution was no different from that of other organisms. Once our ancestors began to develop a dependence on culture for survival, however, a new layer was added to human evolution. According to Sherwood Washburn, professor emeritus at the University of California, Berkeley, there is a definite relationship between biology and culture that he terms "biocultural feedback. "Washburn suggests that the unique interplay of culture change and biological change could account for why humans have become so different. His basic premise is that as culture became more advantageous for the survival of our ancestors, natural selection favored the genes responsible for such behavior. Those genes that improved our capacity for culture would have had an adaptive advantage. The ultimate result of the interplay between genes and culture was a significant acceleration of human evolution, as manifested in, among other features, the growth of our brain and its mental capacity over the past 2 million years. Cultural and biological evolution contrast in a number of ways and are very different processes. Biological change, or evolution, is facilitated by the transmission of genetic information from one generation to a succeeding one by the configuration of DNA in genes. Cultural evolution is the passing on of information by behavioral means and involves the processes of teaching and learning. By these definitions, a bird might make a tool or a nest, but it does not learn how to do this it is born with the genetic endowment which, at the appropriate time, "turns on," and a nest is constructed. Although humans are genetically equipped with basic biological imperatives, our sophisticated cultural behavior must be learned by teaching, and, most important, this learning is associated with a symbolic mode of communication, usually language. Information transmitted by DNA involves passing that information from one individual to another, which can only be done at a single point (conception) in the life span of that individual. Cultural evolution, on the other hand, is not passive but active and incorporates lifelong teaching and learning. Further, any one individual can teach one or many and a single individual can learn from one or many. In cultural evolution, in contradistinction to biological evolution, where the information is stored as a DNA sequence, information can be memorized, written, videotaped, audiotaped, and transmitted using sound, pictures, and words. Culturally transmitted information is behaviorally very flexible and not restricted. A bird can sing a mating song beautifully, but a tenor can sing many romantic arias in several different languages with noticeably distinct levels of passionate commitment. The plasticity of learned cultural information is a true hallmark of being human, as is evidenced by the myriad societies around the world. Cultural behavior is passed on by communication and therefore can spread to many more individuals than a genetic novelty that is transmitted only in the DNA. Biologically based behavior requires an enormous number of generations to spread, whereas cultural innovations, especially with the information revolution, spread exceedingly quickly. Human evolution is an intriguing interplay of biological evolution and cultural evolution. And in the view of sociobiologists Charles Lumsden of the University of Toronto and the noted E. O. Wilson of Harvard University, who have dubbed this interaction "gene-culture coevolution," humans have been shaped through the synergetic interaction of genes and culture. In the final analysis, human evolution is different from the evolution of all other life on this planet. We are distinguished by our capacity for culture, which ultimately has biological roots.
4. The Science of Paleoanthropology
Paleoanthropology calls on a broadly conceived and strategically implemented multidisciplinary approach to discover and interpret the evidence for human evolution. It is the responsibility of the paleoanthropologist to coordinate activities in the field and in the laboratory, carefully integrating knowledge contributed by specialists in geology, biology, and the social sciences. The goal is to understand, as thoroughly as possible, the process by which we became human. The major coordination of a field project seeking to recover important clues to our ancestry is customarily under the leadership of an individual with a background in anthropology (the study of mankind). The paleoanthropologist (one who studies ancient humans) works closely with scientific colleagues to raise funding to support field projects, with a primary expectation being the recovery of the fossilized remains of our ancestors. The paleoanthropologist has the ultimate responsibility for overseeing the research from the planning stages, to actual fieldwork, to the recovery and in-depth laboratory study of particularly the hominid fossils, and finally to the publication of research results in the scientific literature. Once sites have been located (see page 25), an interdisciplinary team enters the field for weeks to months to undertake exploration and excavation focused on finding hominid remains. In the case of cave sites, the process of excavation is slow and tedious. In open-air sites such as those round in the Great Rift Valley in East Africa, exploration takes the form of foot survey, with teams of expedition members scouring the landscape in search of hominid fossils that have eroded from ancient geological strata. Hominids are not all that a survey team might fruitfully uncover, however. In the past, hominids were a rather small and insignificant component of paleocommunities.
1996 by Donald C. Johanson & Blake Edgar
Library of Congress subject headings for this publication: Human evolution, Fossil hominids, Australopithecines