by Kevin Reilly
Changing Surfaces . Anyone who has looked at the shape of Africa and South America on a map has seen how the two continents were once joined. Actually, various landmasses have come together and moved apart continually over the past half billion years. These landmasses have also drifted over the surface of the earth in ways that bear no resemblance to their current configuration.
By the beginning of the Mesozoic era, 250 million years ago, various landmasses around the globe had come together as a single global continent, the bulk of which lay in the Southern Hemisphere. Then it began to split apart. About 200 million years ago, a southern section including what is today Africa, India, Southeast Asia, Australia, and New Zealand broke off and drifted toward the South Pole. Then, around 150 million years ago, the western half split apart and drifted farther west, opening up an area that became the Atlantic Ocean.
Continental landmasses are not the only loose crusts sliding over the surface of the earth. Both lands and oceans sit on large plates that slide around the globe over a more fluid core. These plates sometimes collide, pushing up great mountain ranges, or slide next to each other, causing earthquakes. For example, the collision of India with the rest of Asia raised the Himalayan Mountains. Similarly, the Pacific plate pushed against North and South America, creating the Andes Mountains in South America and triggering earthquakes along the coast from Chile to Alaska.
Changes in Climate . Some of these sliding plates also affect climate. In general, the larger a continent, the colder it gets, especially in the interior. This is because large continental landmasses block the moderating warm air and water flows that circulate in the atmosphere and oceans. Near the poles or at high altitudes, such continents build up snow and permanent ice, or glaciers. Large glaciers make the atmosphere even drier and cooler since ice and snow absorb moisture and reflect sunlight away. At the other extreme, islands and small land areas are warmed by circulating air and currents. The many small landmasses of the late Mesozoic and early Cenozoic eras kept global temperatures quite balmy. Fifty million years ago, North Dakota sweltered under tropical forests.
Global temperatures turned colder about 33 million years ago. For most of the past 30 million years, icing and warming periods lasted about the same amount of time. But during the most recent 2.5 million years, ice ages have lasted longer, and warming periods have been much shorter. In the past million years, the warm interglacial periods lasted only about 20,000 years each before the ice returned. Since the last ice age ended about 12,000 years ago, we may be near the end of the current interglacial warming. This time, however, human behavior, especially our burning of fossil fuels—the swamp grasses and giant trees of the Paleozoic era (350 million years ago) turned into coal, gas, and oil—may be slowing or even reversing the natural process. Whether this current “global warming,” the first change caused by humans, delays the next ice age or makes the world permanently warmer remains to be seen.
Human Origins
The similarity of humans and monkeys is evident to anyone who visits a zoo. It is a staple of story and mythology in every society where humans have come into contact with them. The Indian Ramayana legend tells of the Princess Sita being carried off by monkeys. The Chinese story Monkey imagines a simian guide for an early Chinese Buddhist missionary. So Charles Darwin was hardly the first person to imagine that humans and monkeys were related.
Natural Selection . Darwin added the idea of descent to the recognition of similarity. His argument that humans and monkeys shared ancestors was part of a larger argument that all species changed or evolved. The importance of change was certainly a dominant idea in Darwin’s England of the mid-nineteenth century. At the same time that Darwin’s contemporaries were discovering fossils of extinct species in English stone, English stone masons were losing work to the new industrial workers. Transplanted farmers forged giant steel beams to carry coal-belching steam locomotives across what had only recently been (according to the poet) a “green and pleasant land.” In a world of wrenching mechanical change, Darwin thought that he had found the mechanisms for change in nature. He called them random mutation and natural selection. A species would randomly produce offspring with slight variations. Some of these variants would prove more resilient than others, and a rare one would initiate a new divergence, possibly becoming a new species. In some cases, the old variants would die off, and the new ones would replace them. Nature is a harsh and unpredictable task master; more than 99 percent of the species it produced are now extinct.
In the past 40 years, the new science of molecular biology, the study of the most basic elements—the DNA—of organisms, has given us the tools to go far beyond Darwin’s guesswork. Where earlier scientists debated—for instance, whether humans were more closely related to the African gorilla or the Asian orangutan, both large apes—molecular biologists have discovered that we are much closer to chimpanzees, with which we share 98.4 percent of our genetic DNA.
Molecular biology can measure not only nature’s similarities and differences more precisely but also change over time. The principle is that differences in DNA develop at a fixed rate over time so that the greater the differences in DNA between two organisms, the longer the two have grown apart. This has also deepened our understanding of human origins by helping us figure out just when our first human ancestors began their own branch on the family tree of primates. It turns out that a 98.4 percent similarity means that our human ancestors separated from the ancestors of chimpanzees 5 million to 6 million years ago.
Hominids Stand Tall . Our human ancestors are called hominids. While initially not very different from the other tailless chimpanzee-like animals of the time, they gradually developed the physical features we associate with modern humans: less hair, habitual erect posture, bipedalism (walking on two feet), legs longer than arms, flat face, smaller jaw and teeth, larger brains, and longer period of infant growth after birth, among others. Some of these changes had profound consequences for hominid development. Physical changes in the brain, lips, larynx, and tongue enabled the development of a capacity for speech and language. Walking upright led to hands that could carry, manipulate, and use tools. With language and tools came ideas and skills—cultural tricks for survival that meant less dependence on nature and that enabled each generation to give the next a leg up.
Hominids to Humans . Combined with DNA analysis, the fossil remains of the past 6 million years allow us to chart the transition of hominids to humans with some degree of certainty. Finding the particular hominid species that led to the first humans—and to nothing else—is more problematic, however. Scientists believe that this happened 5 million to 8 million years ago. Skeletons of hominids from shortly after this period, like the early bipedal Ardipithecus, may be our ancestors, but they could also be examples of a hominid that went extinct. These had the stature and brain size of modern chimpanzees. They lived in forests in East Africa, where their hooked big toe allowed them to swing from the trees, crawl on all fours. and possibly walk upright.4 From a slightly later period, 4 million to 2 million years ago, there are skeletal remains of the hominid Australopithecus from East Africa and South Africa. They are upright, apelike, three and a half to five feet tall, with a brain capacity of 400 to 500 cubic centimeters and limbs, skull, jaw, and teeth that combine ape and human features. They too went extinct.
A third phase of hominid evolution—and a more likely human ancestor—began with Homo erectus (also from East Africa, about 1.9 million years ago) with a brain size of 900 to 1,000 cubic centimeters and a height of five to six feet. Homo erectus appears to be the first hominid to travel outside of Africa, as fossil remains have been found in Europe, China, and Java. Homo erectus made stone tools, controlled fire, probably used hides for clothing, and may have had spoken language. Most scientists believe that they went extinct without contributing to the genes of modern humans.
Homo sapiens appeared in East Africa between 400,000 and 100,000 years ago, with a modern brain size of 1,400 cubic centimeter
s. They made tools of wood and bone as well as stone. The species was called “sapiens” (wise or thoughtful) because its members probably used language symbolically and expressed certain religious and aesthetic ideas. There is evidence, for instance, of burial, body painting, jewelry, carving, and cave painting.
Finally, about 150,000 years ago, humans whose skeletal remains suggest modern human physical features appeared, with brain capacities of 1,400 to 1,600 cubic centimeters. With more than a touch of bravado, scientists named this, our own species, Homo sapiens sapiens. (We’re so smart we have to say it twice.) For much of the past 100,000 years, these Homo sapiens sapiens were not alone. One of our cousins, called the Neanderthal, named after the German town where remains were first discovered, originated about 150,000 years ago and lived in North Africa, Europe, and Southwest Asia. Despite their bad press, Neanderthals had larger heads and brains (1,400 to 1,700 cubic centimeters) than we do and very muscular stout bodies. They buried their dead and survived the cold climate of northern Europe. Before they became extinct 28,000 years ago, recent DNA analysis shows that they contributed to our gene pool.5 The existence of another cousin, called Denisovan, has recently been discovered in Siberia. A small amount of its DNA can be found in people of New Guinea and the Pacific. In addition, the remains of possibly another human species, called Homo florensis, have recently been discovered on the island of Flores in Indonesia, where these people lived until at least 13,000 years ago, possibly much later. Their skeletons show a people who measured only about three feet tall and had heads only a third the size of modern humans. There is no evidence of their interbreeding with our ancestors.
Culture Trumps Nature . We have noted the increasing brain size in the history of hominid evolution. Larger brains, supported by thinner frames, allowed humans to advance more by thinking than by the exertion of brute force. But within any species, brain sizes were similar. Modern Homo sapiens sapiens did not differ significantly by hat size. And hat sizes had nothing to do with inventiveness. In the world of Homo sapiens sapiens, culture (what we learned) was far more important than nature (our biology) in determining what we could do. More than any other creatures of the earth, humans are products of culture; they are also its creators. Rabbits may breed more quickly, but their lives are very similar, generation after generation. Through culture, humans have made—and continue to remake—themselves. And they have been able to do so throughout the world in every environment.
Global Migration
Humans were not the first of Earth’s creatures to spread throughout the world and colonize every continent. They are not even the most numerous of the Earth’s approximately 30 million species. It is even possible that other global colonizers will outlast humans—cock-roaches, for example. But if that happens, humans will have only themselves to blame because in their brief span on the planet, humans have reshaped it to their every need.
Humans as Travelers . So far, we have been imagining a particular branch of hominids as they became human beings—and then went out to travel the world. But it might make more sense to see the process of becoming human as part of the process of walking and traveling. Walking meant upright posture, seeing where you are going, better vision and planning, and more things to do with the arms and hands. Traveling meant discovering, confronting, adapting, and inventing.
Most hominid species (probably all) originated in East Africa, but they did not stay there. They traveled throughout Africa and to Australia, to Europe and Asia, and there is evidence that they did this over and over again, learning new skills and ideas and in the process becoming what we mean by human.
Homo erectus was probably the first homi-nid to travel beyond Africa. A representative of the species left teeth in China almost 2 million years ago.6 Erectus may have traveled to Java by water or an ice-age land bridge as much as 1.8 million years ago. Later generations settled in southern Africa over a million years ago. About 800,000 years ago, new members of Homo erectus traveled to Europe, India, and China. Homo sapiens migrated out of Africa between 100,000 and 200,000 years ago, followed by our immediate ancestor, Homo sapiens sapiens, beginning less than 100,000 years ago.
What knowledge of clothing, sewing, fire, and cooking was prompted by their movement into the forests of northern Asia and Europe? What social skills, language, or communication ability answered the need to make camp in a new area, perhaps colder or wetter, with different animals as potential prey or predator and unrecognizable mushrooms that might cure or kill? What new scraper, spear point, or fishhook was invented to kill the mammoths of the northern Asian grasslands or the seals of the Bering Sea?
We cannot know the specific answers to these questions. We do know that these travelers became remarkably adept at colonizing and conquering new lands. We do not know if Homo sapiens sapiens were responsible for the extinction of other human species, like the Neanderthals or Homo florensis. Whether or not these or other early humans were annihilated by Homo sapiens sapiens, many animal species probably were. Humans were by no means the largest animals, but they used their brains to capture and kill with abandon. So devastating was the human contact with large mammals and birds that we can practically chart the migration of Homo sapiens sapiens by looking for the multiple extinctions of these creatures: 50.000 years ago in Australia and 14,400 years ago in northern Eurasia.
Between 18,000 and 15,000 years ago,7 Homo sapiens sapiens crossed the Bering Sea land bridge created by low ice-age ocean levels. They may have followed the path of small groups of earlier humans who settled in the Western Hemisphere much earlier; there is some evidence of human settlements in Chile 30.000 years ago and in South Carolina possibly 50,000 years ago. But the settlement at the end of the ice age, between 18,000 and 15,000 years ago, had a far greater ecological impact. They arrived in a world of huge elephant-like mastodons, woolly mammoths standing over 10 feet to the shoulder and weighing 13 tons, birds with the wing span of a small airplane, bears that weighed 1,500 pounds, giant bison, sloths, horses, camels, and lions. It was a world that makes our own look “zoologically impoverished,” the great naturalist Alfred Russell Wallace, Darwin’s collaborator, remarked. At some time before 13,000 years ago, these travelers perfected a stone spear point (called Clovis after its discovery in Clovis, New Mexico) that gave the new Americans a deadly advantage over the large mammals.
The resulting impact may have been a “megafaunal overkill,”8 rivaling the extinction of the dinosaur. Virtually every large animal species on the continent was hunted to extinction before a second human migration came by sea about 8,000 years ago.
The First Modern Humans
Homo sapiens sapiens, the colonizers of every continent but Antarctica over the past 100,000 years, were the first truly modern human beings with regard to the size of their brains, the height of their foreheads, and their general appearance. They were the first of our ancestors who, with the right haircut, diet, and clothes, would fail to surprise us if we saw them on the street or in the shower.
We used to think that the early ancestors of our species were late bloomers, that it took more than 100,000 years before these anatomically moderns became behavioral and thinking moderns. Without much evidence of Homo sapiens sapiens’ art or invention between the time of their appearance 100,000 to 200,000 years ago and the dramatic cave paintings created 30,000 years ago, archaeologists thought that the first half of our species’ existence was fairly uneventful. But no more.
Recent discoveries in sub-Saharan Africa from almost 100,000 years ago reveal an early propensity of our species for artistic expression and abstract thought. We find a wide range of highly specialized tools—scrapers, fishhooks, awls, and needles—for specific functions, and we find them in various shapes, sizes, and media—stone, wood, and bone. These people also carved their tools for aesthetic effect. We also find red ocher pigments often associated with burial, body decoration, and religion.9 In addition, recent excavations in South Africa uncovered a set of pierced beadlike shells that may
have been worn as jewelry 75,000 years ago.10
Human clothing may also date from this period. Research on the “molecular clock” of lice11 indicates that human body lice diverged from human head lice about 75,000 years ago. Since body lice live in clothing and most other mammals support only one kind of lice, the reasoning is that only a widespread human use of clothing would have precipitated such a successful genetic mutation.
As early as 40,000 years ago, people in modern-day Australia engraved thousands of circles on a high sandstone monolith and surrounding boulders. Early human burials date to more than 50,000 years ago; in caves in the Middle East, there are examples of children buried with deer antlers or the skull of a wild boar, indicating some religious or totemic identification of human and animal. All these efforts to beautify, plan, or give meaning suggest if not the origins of art and religion, then at least the beginnings of abstract thought and a fairly developed capacity for expression and communication.
We also see the beginning of cultural differences in this period. Tool kits, the set of tools a group employs, begin to vary from one area to another. They vary not only to serve different purposes—fishing or hunting the big game residing in the forest or grasslands—but also to reflect a local style or tradition. These cultural differences mean that culture was beginning to shape human behavior. Nature had moved to the back seat.