What, in evolutionary terms, propelled us to become human? The answer lies not in our forebears’ big brains or their facility with language but in their ability to walk on two feet. That remarkable factstanding and walking seem so mundaneonly starts the drama that Craig Stanford, codirector of the Jane Goodall Research Center, tells of our origins.
Today scientists are finding far more evidence than ever before about our beginnings. The discoveries are prompting dramatic reappraisals of common beliefs about our past. Throw out the simple idea that millions of years ago some apes moved to the African savanna, where they evolved into runty hominids who eventually metamorphosed into us. Dump that textbook image of an ape transforming into a human in five stages. Newly found remnants of two-legged “proto-humans” show that our ancestry is much richer and more convoluted. In no way can we still think of ourselves as standing on the top rung of an evolutionary ladder of excellence.
But what about our tremendous thinking powers? Our brains could have started to grow because, as our ancestors adapted to standing and walking upright, they became more successful at hunting ever larger animals. The meatier diet could have fueled the increase in brain size. And the switch to standing and walking tall may have allowed our forebears to develop language, let alone take over the entire world as their home.
Describing his - and others' - latest research and interpretations, Stanford offers a fresh, galvanizing take on what made us human.
|Publisher:||Houghton Mifflin Harcourt|
|Product dimensions:||5.50(w) x 8.25(h) x 0.56(d)|
About the Author
Craig Stanford is co-director of the Jane Goodall Primate Research Center and associate professor at the department of anthropology, University of Southern California. His previous books are: Significant Others: The Ape-Human Continuum and the Quest for Human Nature (Basic, 2001), The Hunting Apes: Meat-easting and the Origins of Human Behavior (Princeton U. Press,1999), and Chimpanzee and Red Colobus: The Ecology of Predator and Prey (Harvard U. Press, 1998).
Read an Excerpt
I remember vividly the first time that each of my three children took her or his first unassisted steps. My firstborn had been “cruising” for weekspulling herself up and walking while holding on to furniture, people, dogs, and anything else that she could grab. But at ten months she was ready to be a biped. She stepped away from my hands and walked several lock-legged goose steps into her mother’s arms. My daughter’s wide eyes showed her shock at the performance. We beamed, imagining that our parenting skills had something to do with teaching her this most natural of all uniquely human acts. Three years later we were living in a village in rural Mexico and obsessing about the diseases that our younger daughter was contracting by crawling in the dust. Then one day she stood up and toddled, and that was that. My son was a different story; I was in East Africa, having left on a month-long trip knowing I would likely miss the big event. Sure enough, shortly after I arrived in Uganda, I learned through the crackling static of a phone call that Adam, after much frustration at trying to carry a ball while crawling, had simply stood up and walked, the ball in his arms and an ear-to- ear grin of accomplishment on his face.
Few of us appreciate our history of becoming bipeds, perhaps because walking requires so little energy or thought. Most of us think that our exalted intellect or our ability to grasp with our thumbs is what sets us apart from the other primates. But all primates share the grasping thumb, and the difference between an ape’s brain and our own is not as great as people think. Some parts have undergone a critical reorganization, such as the speech centers, but a human brain is basically a ballooned version of a chimpanzee brain.
Our ability to stand and walk habitually on two feet, however, represents a fundamental change from the kind of creatures that our ancestors were. Bipedalism preceded the expansion of brain size by about five million years; it truly announced the dawn of humanity. Becoming bipedal made us human. Whenever a fossil human is discovered, the first piece of crucial information that everyone wants to know is “Did it walk upright?” The second question is “How will it change our family tree?” To an extent unappreciated by most of us, walking is sexy. It is the key part of a cascade of traits that evolved together in an intricate mosaic of ape and early human features. For instance, walking on two legs rather than four released our bodies from the constraints of the synchronized breathing gait that so many other animals, such as dogs and horses, live by. Once the lungs of our two-legged ancestors were freed, they could modulate their breathing in subtle ways that may have contributed to the evolution of speech. The connection between walking upright and speaking is one of many vivid examples of the jigsaw-puzzle evolution of our bodies.
Why we are bipedal is not simple to explain. In this book I show that the question really consists of two parts: what made our ancestors take their first steps, and what the evolutionary impetus was for those “toddlers” to become highly efficient marathon walkers and runners. The first steps were, according to the latest research, mere shuffles that helped our simian ancestors reach plant foods such as figs that were just beyond their grasp. Then these marginal bipeds found a light at the end of the endless search-for- energy tunnel: meat. The meat came in the form of both small game animals, which could be captured and eaten raw, and the carcasses of animals, both small and large, for which the Promethean humans fanned out in search of each day.
Meat eating provided a new and crucial source of protein, fat, and calories that may have enabled the evolution of bigger brains and helped our cognitive abilities to evolve. As meat eating became more important, our ancestors adopted new ways of life that resulted in a hominid that began to rule the planet. Being bipedal did not, contrary to popular conceptions, lead directly to brain expansion; the two events occurred millions of years apart in evolutionary time.
The traditional view of human origins goes something like this: Six million years ago an ape ancestor left the comfort and security of the African forests for life on the savanna. Its new home offered many opportunities for advancementincluding open country and a meat-rich diet that the old home lacked. The ape evolved a means of travel in which standing upright became not just a quick periscope but a way of life. Upright posture allowed our ancestors to carry tools, chunks of butchered carcass, and even babies. But the trade-off was enormous. Predators oof every shape and size, from leopards to saber-toothed cats, wandered the grass searching for prey all day and night. The new stance leffffft the emerging human without a means of rapidly escaping predators. The single advantage that allowed early humanity to survive, and turned the tide in favor of our lineage, was a rapidly expanding brain. Armed only with its wits, the runty little human eked out an existence for millions of years, eventually prospering and sending its progeny into the present as big-brained Homo sapiens.
The familiar image of our ancestors’ progression through stages of hunched-over, shuffling, apelike creatures into humans is an appealing one, but each of its elements is being called into question. The idea that we slowly evolved toward perfection is as wrong as it is entrenched. Animals don’t evolve toward anything; natural selection molds them generation by generation. At each stage the animal’s form must be efficiently designed so that it may succeed at eating, rearing offspring, and so on, or natural selection eliminates that animal’s genes from the next generation. We are not at the apex of life’s intricate evolution from our ape forebears, no matter what both popular and scientific accounts claim.
BIPEDS ARE BIZARRE
Standing on two feet is a bizarre posture and an even more bizarre way to walk. Of the more than two hundred species of primates on earth today, one is bipedal. Of more than 4,000 species of mammals, onethe same oneis fully bipedal when walking (a few oddities such as kangaroo rats and meerkats stand bipedally for a few moments at a time). If we include thousands more kinds of animalssuch as amphibians and reptileswalking on two feet emerges as the most unlikely way to get around. Kangaroos and birds such as ostriches and penguins are bipedalsort of. But they are built on an entirely different body plan and are not, strictly speaking, reliant only on their legs for transport. Even if we throw in all the extinct forms of terrestrial animal life, such as Tyrannosaurus rex and its kin, the percentage of bipeds is still remarkably small. And birds and dinosaurs differ markedly in their brand of upright posture. Most birds have stiff and relatively short tails. They maintain stability by having their center of mass far forward from the pelvis; this forward gravity center necessitates standing with the upper leg bone bent. Birds that have adopted flightlessness, like ostriches, generate their power stride by rotating the lower leg around the knee joint. Upright dinosaurs like Allosaurus or Velociraptor opted for a center of gravity near the pelvis and rotated their entire leg during striding.
The reason that upright posture and walking arose is the most fundamental question in human evolution. It begets critical puzzles, such as why bipedalism hasn’t evolved many more times, and whether the evolution of our unique posture and gait is connected to our massive brain and extraordinary intelligence.
In this book I emphasize that humankind is only a twig on an evolutionary bush rather than the top rung of an evolutionary ladder of excellence. The fossil record for the rise of bipedalism has just begun to tell us that even as protohumans diverged from the apes, bipedalism existed in a variety of forms. In 2000, for instance, researchers in Kenya announced the discovery of an early human fossil that they named Kenyanthropus platyops, which appears to have been a contemporary of other early human species, such as that to which the famous fossil human commonly known as Lucy belongs. Until that discovery, we believed that our family tree had only one trunk. An even more recent and controversial find, “Toumai,” is a primitive fossil from the Sahara Desert that some experts believe represents the earliest known member of the human family.
These are heady times in fossil hunting. We are learning that a wide variety of evolutionary experiments, using bipedalism as a recurring theme, took off about five million years ago. Most failed. One branching lineage survived to the present. Also, our early ancestors were not poor bipeds who evolved slowly into “good” two-legged walkers. Emerging evidence suggests that a menagerie of species existed with a variety of characteristics, and they did not form one linear progression from “primitive” to “advanced” bipeds. Our obsession with linear progress has led us severely astray in solving the riddle of why we became bipedal.
How we walk today comes from a cornucopia of evolutionary forces at work on our ancestors’ bodies. The modern architecture of the spine, pelvis, feet, and hands, and even nervous and circulatory systems, follows directly from the conversion from quadrupedalism to bipedalism. Other changes, not preserved in stone but equally important in our ability to stand and walk on two legs, took place in our behavior. Our apelike ancestors lived in the forest, climbed in trees, and ate fruits and leaves and occasionally meat. As the hominid emerged from the forest, it underwent changes in foraging strategies, diet, preferred habitat, and tool technologies. The hominid’s mating system and social life are unknown to us, although we can make some reasonable inferences. And from this ape ancestor came one with a cerebral volume that was only marginally larger but had an entirely new way of walking. No doubt, changes in social behavior contributed to cognitive changes. Tool technologies changed too, expanding the resources available to this population. Because all these new aspects of emerging humanity cascaded one upon the other, teasing out which caused which is difficult. The intricate puzzle of our humanity came into focus as new pieces were added to old ones, slowly changing our ancestors from one thing to another.
How we became bipedal is a chronicle of how we became human. As our way of moving about changed, so did our niche in the world, our perspective, and our prospects. This chronicle is also an argument for why we must move our view of the earliest stages of humanity from old-fashioned notions of progress and linearity into a more modern Darwinian sensibility. The debates in human evolution research are fierce, because the fossils are few and far between, and because their implications are far-reaching. I will try to convey a sense of the science and the scientific politics that drive the process of making and breaking theories, as well as recent research that has uncovered a variety of key pieces of human ancestry. The story at the heart of this book is truly an odyssey, made more fantastic because it actually occurred.
Copyright © 2003 by Craig Stanford. Reprinted by permission of Houghton Mifflin Company.
Table of Contents
CONTENTS Acknowledgments xi Preface: Baby Steps xv 1 A First Step 1 2 Knuckling Under 15 3 Heaven’s Gait? 38 4 The Extended Family 61 5 Everybody Loves Lucy 78 6 What Do You Stand For? 104 7 The Search for Meat 122 8 Better Bipeds 142 9 Sky Walkers 172 Bibliography and Further Reading 179 Index 194