Piecing together the puzzle of human origins and evolution is an interdisciplinary endeavor. The fields of genetics, paleontology, paleoanthropology, archaeology, cultural anthropology, primatology, animal behavior and ecology, anatomy, physiology, kinesiology, psychology, cognitive sciences, economics, and many others offer pieces of the puzzle.

The roots of humankind can be traced back to the beginnings of the universe because we are composed of atoms, to the dawn of life on earth because we are carbon-based, to the first fish to make a living on land because we are tetrapods, to the small mammals that survived the dinosaur-ending apocalypse because we suckle milk as babies and are furry (some more than others), to the apes that for some strange reason got good at teetering around on their hind legs, and finally to expert bipeds with big brains for mastering tool-making and language.

Certainly the entire history of the universe as it pertains to human origins and evolution cannot be covered in this volume. The main focus of human evolutionary studies (and the main focus here) is the roughly 6 million crucial years from the moment our lineage split from the ancestors we share with chimpanzees up until the first modern humans emerged in Africa, just before our individual and population histories began to diverge.

Although most technical terms are defined in the glossary, there are two that need special attention from the start. Hominins are modified African apes. They include living humans and all the extinct descen-dents of the last common ancestor with chimpanzees that are on the human lineage as opposed to the chimpanzee lineage. Fossil hominins are either our direct ancestors or are our evolutionary cousins located on different branches of the hominin tree from ours. Most hominins are bipedal and relatively large-brained, but the very earliest ones may not have been so, if those traits did not evolve immediately after the split from the common ancestor with chimpanzees. Humans are the only surviving hominin species and are distinct from other hominin species in our unique combination of complex material culture, social behavior, bodily characteristics, and intelligence. In this book, we will refer only to anatomically modern humans or Homo sapiens as "humans." These are people we would call "people" if we were to stand eye-to-eye with one of them. To put it another way, if we traveled back in a time machine to 200,000 years ago we would probably recognize the upright walking beings as human beings (even though they probably did not yet use language or symbolism like we do), but there is no guarantee that we would connect the same way with anything living before that, not necessarily even with Neanderthals who lived side by side with some human populations until 30,000 years ago. Although some scientists refer to everything within the human genus (Homo) as "humans," we will reserve that title for ourselves only.

It seems logical that if one is to study human origins there should be universal agreement as to what exactly makes a human "human." But if asked, "what defines humans or what makes them unique?" many popular answers lie within the intellectual and emotional realm: love, laughing, regret and loss, creativity, curiosity, hunger for learning, perception of things unseen, concept of the future, spirituality, suicide, and language. These are not the types of traits that are readily traced through prehistory with fossils, artifacts, or genetics. Comparative observations of other animals help us understand the nature of these human attributes, but the more we learn, from apes in particular, the more we find that very little of what defines humans is exclusively human.

The 2 percent of our genome that differs from chimpanzees sometimes seems too small to contain all of our differences. Physically, we have a larger brain with thick hair covering the skull and, in males, the face as well. Our body hair is drastically reduced and our naked skin contains many more sweat glands. Our hands are much more dexterous but our bodies are much weaker than chimpanzees'. We have large conspicuous sex organs and we walk on only two very long legs shod with rigid, sturdy feet. Much of the white of our eyes, which is whiter than other apes, is visible due to the almond shape of our eyelids. Behaviorally, we depend on highly complex vocalizations. We control fire. We graffiti nearly everything, including ourselves. On the whole, we peacefully gather in enormous groups containing mostly unrelated individuals. We exploit and selectively breed other species (and even members of our own, in arranged marriages). We are skillful hunters without fangs and claws and are capable of adapting to every sort of environment on earth.

In the following pages we track the origin and evolution of the traits that enabled humans to become "human." If these traits will someday be mapped in the genome, we will find that they are expressed or regulated by that less than 2 percent DNA difference that separates us from chimpanzees. As we learn more about our own genome and that of related animals, we are finding that most of what we consider unique to our species probably emerged recently and because most traits are complex and expressed through multiple genes, uniquely human traits are going to be a challenge to pinpoint at the molecular level, let alone trace through evolution.

By no means can the entire field of human origins fit into a single book, so the fundamentals and the highlights of the material evidence are included here. For anatomical vocabulary as well as the epochs, periods, and eras of geologic time, please consult Appendices A and B. Throughout the text "Mya" and "Kya" will be shorthand for millions of years ago and thousands of years ago. For additional, in-depth information, check the recommended resources in Appendix C, and the references in the bibliography for places to start your own quest for human origins.

Chapter 1 begins the book with a brief history of the search for human origins and human evolutionary studies. As a consequence of self-consciousness, humans have probably been curious about their origins since the first hominin developed the mental capacity to do so. But the modern study of human origins did not seriously begin until the middle of the 19th century when the first Neanderthals were found in Europe. Early in the history of the field, the controversy surrounding new discoveries was due to uncertainty as to whether or not humans evolved at all. Now that the old controversy has been conquered by science, controversy surrounding new discoveries merely refers to the details involved in their interpretation. Unless fossils come with identification tags, there will always be controversy about how to interpret them. For most every issue, however, there is a majority of scientists who agree on an interpretation which becomes the working paradigm unless a new discovery flips everything upside down again. This collegial debate over new finds is still, unfortunately, repackaged and then touted by antievolutionists as proof that there are flaws in evolutionary theory.

Because this book is centered on the processes of evolution, Chapter 2 outlines evolutionary theory and walks through Darwin's formulation of the theory of evolution by natural selection and also by sexual selection.

It focuses on how we understand evolution today with our current grasp of genetics. In this chapter we look to the evidence we glean from the living world around us for human origins and evolution, so methods of classification and forming family trees are discussed and then groups within the Order Primates, of which we are a member, are discussed.

Without time travel, one can never be truly certain what life was like before moving pictures were first recorded or before people began to write down history. Old bones and stones, however, provide an astonishing amount of information about the origins and evolution of humans. In Chapter 3, we dig down to bare bones of our search for evidence of human origins, into the fossil and archaeological records. We meet our ancestors and our close cousins, and we sort through the garbage they left behind. We discuss the fossils and artifacts that by chance got preserved, that by hard work and a little luck got discovered, and that by careful scientific scrutiny got interpreted as evidence for the nature of our origin and evolution. Our body's architecture and mechanics are only part of the vastly underappreciated knowledge researchers are compiling everyday on our evolution and place in nature. The establishment of geologic methods in the beginning of the chapter puts the fossils into perspective. Early fossil primates, monkeys, and apes are briefly toured and then once hominins are reached they are organized into subheadings by species. Only the general anatomical features are discussed, but keep in mind that scientists base species distinctions and anatomical interpretations on careful measurements and statistical analyses.

Because so much of evolutionary theory relies on modern genetics, Chapter 4 includes advances that the field of genetics has made in our understanding of human origins and evolution. Through DNA, human ancestry can be traced as far back as the origins of life and multicellu-lar organisms. With increasingly powerful biotechnology, the search for human origins is no longer simply based on dusty fossils and artifacts. Today, artifacts within the genome are just as important as those buried in the ground. In an age where molecular analysis is ever increasing in precision and scope, we can use it as a tool for tracking our evolution. Knowledge of genes and inheritance from Chapter 2 is used to explore the use of molecular clocks to determine lineage-splitting times in prehistory, the role ofmitochondrial DNA in determining our African ancestry, and also the evolution of some recent human adaptations. Although ancient DNA is not technically modern evidence, the techniques by which it is extracted and analyzed are modern, so it is included here.

In Chapter 5 we synthesize the evidence from living animal models, fossils, artifacts, and genetics in order to track the origins and evolution of some fundamental human traits. Consider this chapter a blueprint for building a human from an ape-like ancestor as opposed to building a chimpanzee from that same ancestor. The evolution of many, if not all, human traits are highly dependent on the evolution of others. For example, the adoption of habitual bipedalism allowed selection to act on the anatomy of newly freed hands that became even more adept at using and making increasingly elaborate tools. Therefore, one cannot adequately study bipedalism without understanding the archaeological record of stone tools.

Our ancestors were shaped by the same natural processes and evolutionary forces as all other earthly organisms. But despite worldwide dispersal, cultural complexity, the innovative modifications we make to our surroundings, and the great impact we have on the environment, are those same forces working on humans today? What spurred modern humans to spread across the world in the first place and how and when did world colonization occur? Will we have time to evolve much more or will we go extinct before we can find out? And if given the chance to do it all over again, would humans still evolve? These questions are asked in Chapter 6.

Evolutionary research is revolutionary. With additional discoveries from dig sites and from the use of new technologies, not only can hypotheses about human origins and evolution be tested and retested from many different scientific perspectives, but they are constantly tweaked to reflect the influx of new evidence. Tomorrow someone could find a new piece of the puzzle that either reinforces what most people already knew or that totally overturns their perspectives and forces them to rethink the whole thing. A new fossil species or a genetic breakthrough can change the way we think about our origins and evolution literally overnight.

If, hypothetically, someone tomorrow discovers a fossil hominin from 9 Mya that walked upright, then parts of this book will need to be rewritten (because as it stands now, the overwhelming evidence points to a hominin origin at about 6 Mya). But no matter the future of the science of human origins and evolution, the information contained in this volume serves as a basis for understanding why such a discovery would be significant and how it would impact the current understanding based on the overwhelming evidence from the fossil, archaeological, and genetic records.

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