If the universe had begun 13 years ago, then, at this moment . . .
The Earth would have existed for about	5 years
Large organisms with many cells would have existed for about	7 months
The asteroids that killed off the dinosaurs would have landed	3 weeks ago
Hominids would have existed for just	3 days
Our own species, Homo sapiens, would have existed for	53 minutes
Agricultural societies would have existed for	5 minutes
The entire recorded history of civilization would have existed for	3 minutes
Modern industrial societies would have existed for	6 seconds

Source: David Christian, “World History in Context,” Journal of World History, December 2003, 440.

The American Museum of Natural History in New York City recently installed an exhibit of universe history that begins with a light-show simulation of the big bang. After standing in the big bang, the visitor follows a long ramp that spirals down two floors. At the end is a plaque containing a line the width of a hair that represents 30,000 years of human history—a metaphor I cannot forget.

As I was writing this story of the world, several friends suggested that I let each page of my book represent a certain number of years. That thought occurred to them before they did the calculations, however; to represent just the 4.6 billion years of Earth history with, say, 300 pages, each page would have to represent 15 million years. And Homo sapiens would not enter until the last third of the last line. Most of the book would contain blank pages representing unknowable time—not a good marketing strategy.

Setting aside the earliest two-thirds of time, consider just the duration of Earth’s existence. For a linear metaphor, imagine laying out a string the length of thirty-one and a quarter football fields (3,125 yards). This would represent the 4.5 billion years since Earth began. The divergence of human development from that of the great apes, at about 5 million years ago, would occur at three and a half yards from the end of the string. The leap from hominids to Homo sapiens would occur about five inches from the end, while the time since agriculture began would be represented by a quarter of an inch.

Another simple way to represent Earth time is to condense it to the scale of time most familiar to us—a twenty-four-hour day. If we imagine the age of the Earth as a single day beginning at midnight; then the first single-celled organism would appear at about 4 A.M. and the first sea plant not until the evening at about 8:30 P.M. Plants and animals would get to land at about 10 P.M., with dinosaurs appearing just before 11 P.M. Dinosaurs would disappear at twenty-one minutes before midnight; humans would appear less than two minutes before midnight, and agriculture and cities just a few seconds before 12 P.M.²

However one represents it, the bare reality is that human history constitutes the tiniest fraction of planetary time—not to mention universe time.

From Divergence to Homo erectus

There is no exact moment at which human beings began to appear; the boundary between humans and apes is not a fixed point. Five to 7 million years ago some mutation occurred in an ape ancestor and survived, and from that single mutation other single mutations kept occurring in the branch called hominids, the bipedal apes. The mutations that bestowed advantage were preserved. These changes eventually led to modern Homo sapiens.

These genetic changes took place repeatedly in the same place—eastern Africa. For at least 3 million years human development occurred only in Africa; hominids did not live anywhere else, although apes lived in Europe and Asia as well. Sometime between 1 and 1.8 million years ago, a group of hominids we call Homo erectus left Africa and began to spread out over the rest of the Earth. Later, about 100,000 to 200,000 years ago, another group of people, by then evolved into Homo sapiens, left eastern Africa to inhabit the Earth, while the previous group of Homo erectus, as it had evolved in various places, became extinct. This is the big picture, as best as it can be constructed at the present time, with possibly another migration occurring between them.³

Why did human evolution happen in eastern Africa? What are the characteristics of this continent that it alone could cradle the development of human beings?

Eastern Africa is tropical; our lack of hairiness indicates that we evolved from tropical animals. To become humans, tropical apes came down from trees to live on grasslands; we are creatures of grasslands, not forests. The geography that could mold human development is found in the Great Rift Valley of eastern Africa, as described in the previous chapter.

People who visit the Great Rift Valley, the Olduvai Gorge, or the Ngorongoro Crater in Tanzania are usually deeply moved by the beauty of the place and the recognition of their ancestral homeland. There on the edge of the Serengeti Plain one can still see the abundance of animal and bird life that provided sustenance as humans emerged from apes. The walls of the gorge, the wooded areas, and the open plains provided shelter, enclosure, and food for the hunter-gatherers who crouched beside them.

The Great Rift Valley is formed by a rift in the African continental plate; the eastern piece of Africa will one day split off from the rest of the continent and drift away in the Indian Ocean, eventually to crash into India, China, Japan, or who knows where. The rift begins at the Red Sea in Ethiopia and extends down through Kenya, Tanzania, and Mozambique, with branches of it extending into Zaire and Zambia. The equator runs right through the middle of the length of the rift, at Mount Kilimanjaro in Tanzania. The flat coastal plains rise to an interior plateau between 1,200 and 1,400 feet (365-1,220 meters) above sea level. These highlands maintain the temperature range that is physiologically most natural for humans, on either side of 80 degrees Fahrenheit (Fig. 3.2).

The landscape in the Rift Valley was a tropical mixture of woodlands and grasslands, or savannas, with occasional mountain ranges. During the wet months lush grasses, trees, and flowering plants yielded fruit. During the dry months the plateau dried up, lightning ignited fires, and rebirth came again with the rains. The savanna provided a nursery with a comfortable temperature, replete with fruit, nuts, and game.

There were fluctuations, however. Earthquakes and ever-changing rainfall patterns created jigsaws of local environments. When Earth entered a glacial period, the savanna became cooler and drier, with more grassland. During interglacial periods, the savanna was hotter and wetter, with more rain forest.

Climate is currently considered to be a key factor in evolutionary change. Apes changing to humans had to adapt to immense climatic swings. If climate had not changed as it did, if gene pools in particular places had not been exposed to particular pressures, especially coolness and dryness in the tropics, then our species probably would not have appeared as it did.

Earth entered its present condition of alternating glacial and interglacial periods only about 2 million years ago. The first ice sheets arose in Antarctica about 35 million years ago, after Earth took the previous 65 million years to cool off about 15 degrees Fahrenheit. Apparently in the last 2 million years the planet has gotten into a temperature range in which colder and hotter cycles can easily tip back and forth.

3.2 The Great Rift Valley of Eastern Africa

Over the last million years there have been about ten ice ages, or glacial periods, at intervals of roughly 100,000 years. The last one, called the Great Ice Age, began about 90,000 years ago and reached its glacial maximum about 20,000 years ago. In the last 10,000 warm years the temperature has been on average 1.8 to 5.4 degrees Fahrenheit warmer than it was during the preceding glacial periods, with periods of cooling.

What causes these fluctuations? They seem to be the result of tiny changes in the tilt of the Earth’s axis, in its elliptical (oval) orbit about the sun, and in its wobble on its axis. Each of these has its own pattern—41,000 years for the tilt to vary from 21.39 degrees to 24.36 degrees and back, 95,800 years for the orbit to go from nearly circular to more elliptical back to nearly circular, and 26,000 years for the axis to work through one complete cone of wobble (precession). The effects of these three kinds of alteration are superimposed, sometimes intensifying each other, sometimes canceling each other out.

Other agents also cause fluctuations in climate—earthquakes, volcanoes, continental drift, changes in carbon percentages in the atmosphere, and hits of meteors and asteroids—not to mention that the magnetism of the poles sometimes reverses with a random pattern averaging every half million years or so. There have been 282 flips in the last 10 million years, known by the magnetism of the rocks at the bottom of the sea. The last reversal took place about 780,000 years ago, when Homo erectus was still learning to make stone tools. Scientists have currently noted that the strength of Earth’s current magnetic field has waned 10 to 15 percent, and the deterioration seems to be accelerating, causing a debate over whether a field reversal, which typically takes 5,000 to 7,000 years, has begun.

From about 6 million years ago, through extremes of climatic change, the lineage of bipedal apes developed slowly and erratically. As many as twenty species of them once existed; now we are the only one left. The fossil evidence for this development is sketchy, fragile, and confusing. Many species coexisted simultaneously. Paleoanthropologists agree that a clear-cut family chart cannot yet be constructed and maybe never can. By comparing the genome of humans with that of chimpanzees, scientists have identified a partial list of the genes that make people human. They include genes for hearing and speech, genes that wire the developing brain, genes for detecting odors and for shaping bones.

Specialists call the oldest group of bipedal apes Australopithicus, or southern ape. These creatures stood about 3 to 5 feet (1 to 2.6 meters) tall, with a head size like a chimpanzee. The oldest bones, dated about 4.4 million years ago, were found in 1992 at Afar, Ethiopia. The best-known Australopithicus is Lucy, whose less-than-half-complete skeleton was found in 1974 near Hadar, Ethiopia. She was named for the Beatles’ song, “Lucy in the Sky with Diamonds,” since the crews of excavators had this song playing as they worked. The bones at Hadar, parts of at least thirteen people, date from 3.2 million years ago.

Lucy was a north African ape that walked upright. She stood 3.5 feet tall, weighed less than 66 pounds (30 kilograms), was 19-21 years old, and had a pelvis like a modern woman but the face of a chimp. Her bones settled a long-standing controversy among anthropologists: Which did the human lineage develop first, large brains or bipedalism? The answer found in Lucy’s bones is bipedalism. Lucy’s skeleton showed that some great apes came down from trees, keeping their arboreal rotating arms and shoulders, and developed upright posture before their brains began to expand.

Another haunting image from the mists of our human development is a set of footprints found at Laetoli, Tanzania, in the late 1970s by excavators led by Mary Leakey. The footprints are those of two early bipeds who appeared to be walking across a field of ash from an erupting volcano. Their feet sank deep into the ash, which was damp from a light shower of rain. As the ash dried, the lime in it hardened. More ash fell into the prints, preserving them to be uncovered 3.6 million years later. What a find for humankind!

How did early great apes develop bipedalism? Specialists theorize that as apes in eastern Africa grew larger, they needed more food, which grew harder to obtain in trees, as forests were changing into grassland savannas. Probably the apes came down to look for food, then carried it back to their group. Being able to stand upright conferred the advantage of seeing farther, of carrying food and babies, and of freeing the arms and hands for other tasks. As legs became stronger and heavier, the body’s center of gravity shifted downward, making an upright posture easier to maintain. Minor improvements may have begun to operate as a self-reinforcing system.

Several species of Australopithicus endured simultaneously until maybe half a million years ago, definitely complicating the lives of paleoanthropologists trying to sort this all out. Meanwhile, other species developed, and by about 2.5 million years ago the lineage Homo had appeared as a small-boned, larger brained ape. By 2 million years ago, Homo habilus, or handy ape person, had appeared. In these ape people, standing about four feet high, the brain had started its expansion, from 300 to 400 cubic centimeters in chimpanzees to 600 to 800 cubic centimeters in habilus. As hands were freed from walking and swinging, they began to work at creating stone tools, which helped to develop brains. Eyes strained to see farther, which also developed brains. Bigger-brained males must have chosen females with wider pelvises. Bigger-brained babies were born earlier than usual in the pregnancy in order to make it out through the birth canal, and they required prolonged care, calling for more adult interaction. The advantage of big brains showed up in the production of the first stone tools and in increased cooperation, although the ability to talk was still in the future. The hand-eye-brain-cooperation self-reinforcing loop had begun.

Handy ape people may have been the first tropical daytime hunters, or at least scavengers, possibly obtaining about 10 percent of their calories from meat. The whole question of how much meat early ape people ate is frought with controversy. Since the way mammals live is largely shaped by what they eat, this is a crucial matter for discussion. There is little real evidence, however—only inferences from structures of teeth.

By about 1.8 million years ago, along came Homo erectus (erect people), someone taller, as high as 5 feet 6 inches (1.67 meters), and bigger brained (900-1,100 cubic centimeters). Since the average modern brain size is 1,350 cubic centimeters, it seems to be time to omit the word “ape” and call this creature a person.

Homo erectus begins to seem quite familiar. They created wooden spears and chipped stone into beautiful hand axes. They may have hunted big game, which requires precision tools and elaborate social cooperation, although they probably had only the most rudimentary speech. Perhaps 20 percent of their calories came from meat. They established home bases and cared for dependent infants. They probably made the critical transition from the male and female hierarchies of chimp society to the conjugal bonds of modern men and women.

Erect people also learned to be unafraid of flames, probably instigating one of humanity ’s great leaps—the use of fire. They learned to conserve the embers of burning tree stumps, which had been lit by lightning, to produce their own fires. The advantages of this risky behavior proved enormously worthwhile. People could frighten off predators, set fires to drive prey into traps, cook and eat a wider range of food, preserve food for long periods, light up dark caves, and stay warm in colder weather. Bigger brains were really beginning to pay off.

Indeed, some argue that the preparing, cooking, and sociable eating of food are so central to the human experience that the culinary arts may have been a central component of how we became human. They certainly allowed people to eat more items and receive more nutrition from them. Cooking meat rather than eating it raw at the site of the hunt may explain both the relative lack of difference in size between males and females (as females got more to eat) and the tendency of couples to stay together longer than most other primates. Estimates of when people began using fire vary from 2 million to 300,000 years ago.⁴

With fire, erect people took another first step: some of them moved out of the comfort of warm Africa for the first time, taking their fire with them to protect them against the cold. This likely happened about 1.2 million to 700,000 years ago, during a warm, wet period when the Sahara Desert had enough rainfall to cross safely. Erect people probably crossed at the land bridge where Africa joins Asia at what is now Saudi Arabia. This movement should not be thought of as a migration but simply as small bands of hunter-gatherers moving in their quest for food. Eventually erect people moved into the Near East, Europe, parts of northern Asia, and tropical southern and southeastern Asia. They could not inhabit extremely cold places, like most of northern Eurasia. Erect people did not reach Australia and the Americas. The whole human world contained probably a few tens of thousands of people. But people, like other animals, undertook globe-trotting; the lineage Homo is as peripatetic as any other. Traveling at 10 miles (16.09 kilometers) a year, it takes less than 2,500 years to walk around the Earth. During the era of Homo erectus the saber-toothed tiger became extinct. Were people already having an effect on their environment?

A possible alternative scenario has people leaving Africa about 1.8 million years ago, then developing into H. erectus in Asia, and returning to Africa. The reality must have been a highly complex process of much population movement over time, with all kinds of local expansions and contractions.

Descendants of Homo erectus

The descendants of H. erectus can be grouped by three different locales: Neanderthals in Europe and the Mediterranean, H. erectus in eastern Asia, and H. sapiens arising somewhere in eastern or southern Africa. The official classification does not reflect this clearly, because Neanderthals were earlier thought to be a subgroup of Homo sapiens. Hence they were called Homo sapiens neanderthalensis, the designation they retain, even though they have been shown not to belong to the species of Homo sapiens. True Homo sapiens are officially called Homo sapiens sapiens to distinguish them from Homo sapiens neanderthalensis. For brevity I am using the terms Neanderthals and Homo sapiens.

Neanderthals are in the fossil record from about 130,000 to 28,000 years ago, originating before the start of the last ice age, about 90,000 years ago. They were the first humans to adapt successfully to life on the edge of an ice age world. More bones of Neanderthals have been found than of any other hominid group, including some thirty nearly complete skeletons. They are named for the Neander Valley skeleton found near Düsseldorf, Germany, in 1856, even though there had been earlier finds of this type.

The adaptation of Neanderthals to cold can be seen in their skeletons. Their bones are shorter and more compact than modern human bones, indicating a bulky, squat physique with heavy muscles and barrel chests in men, women, and children. Males were about five feet 6 inches (1.7 meters) tall and weighed about 155 pounds (70 kilograms), while females were close to 5 feet 2 inches (1.6 meters) and about 120 pounds (54 kilograms). Some features of the hip area indicate they did not walk exactly as we do. Their brains were at least as large as our own, although shaped differently. Their skulls were long and low, like those of earlier humans, with a notable ridge above the eyes and a massive nasal opening, larger than in any humans before or since.

As toolmakers, Neanderthals did not change their designs over tens of thousands of years. From stone they made borers, scrapers, points, knives, and hand axes. They could hunt woolly mammoths, musk oxen, wolves, cave bears, wild horses, and reindeer, and lived mainly on a diet of hunted animal meat. They used wood but never perceived the possibilities in bone, antler, or ivory. There is no evidence of ornamentation until the very end of their existence, and no cave drawings.

Neanderthals definitely used fire. They scraped hides for clothing and shelter. They buried their dead—the first humans known to have done so. Bodies are found associated with tools, but there is no pattern of other burial goods or any clue to the possibility of ceremonies. Some skeletons show marks of illness or injury that occurred some time before death, so there must have been social care for disabled individuals.

What capacity for language the Neanderthals enjoyed is a topic for dispute. Anatomical reconstructions suggest that the larynx was positioned differently from that of modern humans, which must have limited the sounds Neanderthals could produce. Presumably their limited spoken language was extended by more gestures and facial and body language than we are accustomed to.

Modern geneticists have discovered that bones contain cells that do not vaporize immediately at death. Sometimes bits of DNA can be extracted from long-dead animals, so long as they are not dead too long. If a thousand years have passed since death, the chance of success in extracting DNA is about 70 percent. Yet in 1997 geneticists managed to extract a short sequence of DNA from Neanderthal bones from 30,000 years ago. The DNA suggested that Neanderthals were very different from a large selection of modern people and could not have been our ancestors. Neanderthals are now seen as a specialized form of Homo erectus adapted to extreme cold. (No DNA sample has yet been found for Homo sapiens of the same vintage.)⁵

As we shall see, by the time Homo sapiens finally arrived in Europe from its origins in southeastern Africa, Neanderthals were wiped out. It seems that the kind of people who evolved in Europe (e.g., Neanderthals) proved less fit than the kind who evolved in Africa. This fact could not be unraveled or assimilated by European minds until well into the 1960s and 1970s. Before that time, racist thinking, combined with the lack of excavations in Africa and poor dating techniques, prevented recognition of what now seems to be the accurate story line.

One example must suffice. In 1912 a find was announced of a skull of a large-brained hominid that had been located in the Piltdown gravels of Sussex, England. Piltdown man was taken in the European and American scientific communities as proof that the first large-brained human ancestor had evolved in England. The Piltdown skull became the standard against which other skeletal evidence was judged and found wanting.

Forty years after Piltdown man was found, it was proved to be a fake—an ingenious combination of fragments of a modern human skull and an orangutan jawbone, both doctored to look ancient. The perpetrator of this hoax has never been identified; the suspects include the anatomist who first commented on the remains, the amateur archaeologist who found them, the museum curator with a grudge against the archaeologist, and even Sir Arthur Conan Doyle, the creator of Sherlock Holmes and a friend of the archaeologist. This joke calls into question the credibility of the entire scientific enterprise. In the end, however, European archaeologists were able to defend their creed by unmasking this fraud, though it took them forty years to do so.⁶

In eastern Asia, Homo erectus was the first human species to arrive; there, erect people developed distinctive adaptations to the forest environments of tropical and temperate Asia. Forests rather than grasslands meant that people had to keep moving in order to find fruits and nuts. Instead of stone for their tools, they used bamboo and wood, raw materials that are not preserved in ancient sites. These forest cultures flourished and evolved slowly over hundreds of thousands of years, quite independently, it seems, of changes in humankind in Africa and Europe. Homo erectus seems to have lasted several hundred thousand years longer in Asia than in Europe and Africa. In the unforgettable words of linguist Derek Bickerton, H. erectus in northern China “sat for 0.3 million years in drafty, smoky caves of Zhoukoudian, cooking bats over smoldering embers and waiting for the caves to fill up with their own garbage.” ⁷

Homo sapiens Inhabits the World

So we come, at last, to us, modern human beings. Once again we find ourselves somewhere in eastern Africa where, sometime between 250,000 and 130,000 years ago, African descendants of Homo erectus mutated one more time into a fitter species, Homo sapiens, the last speciation event so far in the human line.

Homo sapiens was tall and thin, not robust like the Neanderthals. Homo sapiens had no brow ridges and a larger, higher forehead and cranium. Even though its brain capacity was smaller than the Neanderthals’, it was shaped quite differently.

Homo sapiens seems to have been the first group of hominids to develop fully articulate speech. Full fluency enabled Homo sapiens to develop syntax and the distinctive human qualities of abstract, rational, symbolic thought. The hand-eye-brain-speech reinforcing loop was in full gear.

Since there are only two anatomical ways to study the evolutionary development of human speech, our knowledge of it is one of the least developed areas of the human story. One can study the area of the brain that controls speech, called Broca’s area, whose size and shape can be deduced from endocasts taken of the inside of skulls, or one can study the development of the larynx and pharynx from the bones of the throat.

Broca’s area of the brain appears also to control precise hand movements. Speaking requires precise movements of the tongue, analogous to those of the hand. Research has shown that people who have suffered brain damage that impairs their ability to produce and understand language also cannot perform sequences of precise hand movements. Autistic children who learn sign language can sometimes become able to speak. Theorists believe that as early people developed fine finger movements, they also developed the area of the brain that enabled them to sequence words and develop syntax. Gesture moves in a continuum into speech.

Peculiar to humans is the position of the larynx. In most other animals it sits high in the throat and serves as a valve to separate the air flowing to the lungs and the liquid flowing to the esophagus. Other animals can drink and breathe simultaneously; we cannot. Our larynx has repositioned itself halfway down our throats, as shown by the position of the Adam’s apple in the adult male. This leaves a space at the back of the nose and the top of the throat that serves as a sound chamber, giving us a resonance that no other species has. This resonance, combined with the extreme dexterity of our tongue and lips, gives us a verbal fluency that is comparable to our amazing manual dexterity. Children reenact the evolutionary history of the larynx; as they grow, it drops from the top of the throat to its full human position by about fourteen years of age.⁸

Whether the location of the larynx was fully low-slung in Neanderthals is the subject of much debate. Most experts believe that in them the larynx was in an intermediate position, about like that of an eight-year-old child. All agree that by the time the noticeable takeoff in human culture occurred, at about 30,000 years ago, the larynx was in its modern position, and humans had mastered speaking.

Over time, Homo erectus in Africa gradually turned into Homo sapiens. Using their superior brains and language abilities, they gradually eclipsed other human species in Africa and achieved a population of maybe 50,000 by about 100,000 years ago. Some of them managed to move—in a short window of opportunity provided by Earth’s climate—out of the tropical savannas into the area of the eastern Mediterranean that is now Israel, Palestine, Syria, and Lebanon. Then, about 90,000 years ago, Earth tipped back into a cold glacial period in which the Sahara Desert rapidly dried up, preventing further human crossing until warmer, wetter times.

Homo sapiens does not seem to have reached Europe until about 60,000 to 40,000 years ago, even though they were in the eastern Mediterranean by 90,000 years ago. This poses the question: Why did it take so long for them to reach Europe?

Speculation has it that Homo sapiens needed time to adapt to weather colder than that in the warm savannas. People stayed in the eastern Mediterranean learning the skills required by the coldness—creating better clothing and shelter, and developing better hunting techniques since fruit and nuts were scarcer. During a short period of warming weather, 50,000 to 40,000 years ago, Homo sapiens moved into southern Europe, (ice sheets still covered northern Europe). When cold weather returned, these Homo sapiens, now known as Cro-Magnon people, adapted, not through physical changes as Neanderthals did, but by using advanced cultural skills.

This poses a fascinating question: What were the relations between Neanderthals and Homo sapiens? We know that they interacted in the Middle East and in central and western Europe, but we can only imagine how. Since they are classified as subspecies of the same species, earlier experts assumed that they could mate together. But modern geneticists doubt that Homo sapiens mixed its genes with Neanderthals. There may have been warfare between the two groups. Or there may simply have been a different mortality rate, due to skills in adapting. Supposing a difference of 1 percent in mortality rate, Neanderthals might have become extinct in thirty generations, within a mere thousand years, or a millennium. However it happened, by 32,000 to 34,000 years ago, Cro-Magnon people had become the sole hominids of Europe.

As some groups of Homo sapiens spread westward around the Mediterranean Sea into southern Europe, presumably other groups moved eastward into southeastern Asia. As yet there is no fossil evidence to substantiate this. We do not know when Homo sapiens colonized southeastern Asia, the Indonesian Islands, and the continent known to geologists as Sahul—a combination of New Guinea, Australia, and the shelf between them, now flooded but then above water as frozen glaciers reduced the sea level.

The colonization of Sahul by early people constitutes the first instance of seafaring by human beings. At the height of glaciation 20,000 years ago, the distance between the mainland and Sahul was about 62 miles (100 kilometers) of open water. It is known that people crossed this water earlier than this, with less glaciation and higher sea levels, when the distance on open sea must have exceeded those 62 miles.

Which people achieved this amazing feat—descendants of Homo erectus or Homo sapiens arriving from Africa? No one knows. It seems likely that the initial settlement was the result of both accidental and deliberate voyaging by tiny numbers of people in bamboo rafts. These trips, lasting at least seven days, must have taken place over thousands of years. Hunter-gatherers are known to have been living throughout much of New Guinea and Australia at least by 40,000 years ago; possibly they were there as early as 50,000 to 60,000 years ago. These early settlers in New Guinea and Australia were the first people to build boats capable of traveling over considerable stretches of open sea.

The settlement of central Asia, China, and Siberia was a complex process, not yet fully understood. Recent dental research has shown connections between teeth in northeastern Asia and those in southern China, rather than to those in either Europe or southeastern Asia. The differences in teeth are so striking that experts believe that the people of northeastern Asia were a group of Homo sapiens separate from those in southeastern Asia or Europe. These northeastern Asians may have eventually crossed over into the Americas and completed the peopling of the world.

During the Great Ice Age, the sea level fell far below modern levels. Land existed between Siberia and Alaska, a now-vanished continent called Beringia. This land bridge was at its maximum extent at the most intense glaciation about 50,000 years ago and again at 20,000 years ago. When the melting of the glaciers commenced, the rising seas began to cover Beringia, submerging it completely by 12,000 years ago. Any time between 90,000 and 12,000 years ago, it would have been possible for Ice Age hunter-gatherers to cross Beringia to the Americas without using any watercraft.

But when and how did they? Most experts agree on Beringia as the route. The earliest unequivocal archaeological evidence of settlement comes from the Clovis site in New Mexico, dated at 13,600 years ago. There are hints of earlier settlement, possibly back to 30,000 years ago.

However it happened, the settlement of the Americas was the culminating development in the slow expansion of modern humans from Africa—first into other tropical and temperate climates, then into the near glacial environments across the north of Eurasia, then to the new continents. By 11,000 years ago, hunting and gathering people occupied every corner of the Americas. Like their relatives around the world, they were inventive and ingenious, as they had to be to survive. They adapted to local environments in highly diverse ways, leading to the dazzling array of cultures that greeted people from Europe who arrived 12,000 years later.

The final steps in occupying the last inhabited islands of the world were taken by Polynesians, who sailed to Tonga and Samoa about 3,000 years ago; to the Marquesas, Rapa Nui (Easter Island), and Hawaii by 1,500 years ago; and to New Zealand about 1,200 years ago. People from Indonesia settled Madagascar about 1,200 years ago (Fig. 3.3).

In its expansion from Africa, Homo sapiens remained one species around the world. Even though it has been 100,000 to 200,000 years since modern people left Africa, Homo sapiens has not broken up into separate species, unlike chimpanzees, who broke into two distinct subspecies about 2 million years ago, separated by the Congo River. There really has not been enough time, genetically speaking, for humans to split. In addition, human populations have maintained contact with each other, over vast distances of time and space. The Great Ice Age must have assisted in this—the glaciers froze so much water that sea levels remained low and continents were connected, enabling at least some members of Homo sapiens to roam freely, mating back and forth at the edges of separate populations. Perhaps it was early members of trekking clubs who kept our species intact.

In summary, full human beings, the descendants of tropical arboreal apes, first made their appearance in eastern Africa between 100,000 and 190,000 years ago. They moved out from Africa to inhabit the Earth and survived some of its harshest climates to become the dominant life-form in every land they entered. In retrospect, it is astonishing how recent the current human is—only 100,000 to 200,000 years old—when our immediate ancestor, Homo erectus, took 1.4 million years to turn into Homo sapiens, not to mention the 3 to 4 million years it took before that, back to the common ancestor, for our divergence from apes. Indeed our species may still be in its childhood, if we have anything like the few million years that most species enjoy.

3.3 Human Migrations

(Source: David Christian, 2004, Maps of Time: An Introduction to Big History, Berkeley, CA: University of California Press, 193.)

From the story of how humans became the current dominant life-form, two main ideas seem worth emphasizing. One is how humans are inherently part of all life. We are vitally connected to the deep rhythms of the Earth and to all of its living forms. Our religions, psychologies, and philosophies have tended, for several centuries at least, to obscure and downplay our biological connections to Earth, as do our urban living arrangements, but in recent years awareness of our connection to all of life has been increasing among Western people. People close to the Earth, of course, have never lost this awareness.

Another main idea is that conditions on our Earth over time are never the same. Even though on a daily basis Earth seems quite permanent, nothing is further from the truth. A combination of forces on Earth produce extraordinary complexity and unpredictability, and forces that seem to be acting smoothly may result in sudden changes. Looking at the big picture, we see that we live in the gaps between what we consider catastrophes. We are learning to live with the awareness of these long-term changes while at the same time making the assumption of everyday permanence that is appropriate for conducting our short-term lives.

After three chapters, our story can be summarized thus: Our universe began 13.7 billion years ago as a speck of incomprehensible energy, which burst into expansion that continues still. After sufficient cooling, matter appeared as hydrogen and helium and formed stars, in which heavier atoms were created. Some stars exploded as supernovas, scattering the heavier elements, which formed new star systems, including our solar system and Earth. With the help of energy sources, such as ultraviolet rays and lightning, the chemical building blocks of life developed on Earth, leading eventually to the first living cell 3.5 to 4 billion years ago. This cell divided and multiplied, and life has been evolving from it ever since. A mutation about 6 million years ago started the development of chimpanzees toward humans, who appeared as a species a mere 200,000 to 100,000 years ago, dominated other human types by 30,000 years ago, and inhabited the planet by 13,000 years ago (Fig. 3.4).

Unanswered Questions

1. Where and when did Homo sapiens first appear?

In explaining the origins of archaic humans, scientists agree that Homo erectus evolved in Africa and spread out from there about a million years ago. Until recently scientists were still divided into two camps about the origin of modern humans (Homo sapiens). One camp argues that our most recent ancestors evolved independently and in parallel in different parts of the world. This hypothesis is called the Candelabra theory—each branch of human development is like a branch of a candelabra. The other camp, now in the majority, presents the story as it was developed in this chapter, that modern humans evolved in Africa and radiated out from there to the rest of the world. This hypothesis is called Noah’s Ark (we were once all in one boat), Out of Africa, or Garden of Eden.

According to the Candelabra theory, modern humans emerged in many places and diverged genetically at least 700,000 years ago and probably earlier. Noah’s Ark theorists argue that modern humans emerged in Africa 100,000 to 200,000 years ago, then spread out, with a much more recent genetic divergence. The Candelabra theory enjoyed popularity when most of the fossil specimens came from Europe, the Near East, and Asia. When fossils began to be found in Africa in the 1970s, many scientists shifted to the Noah’s Ark theory. Most of the recent evidence of various types points to a late divergence in Africa, but it is not yet fully conclusive.⁹

The two theories have very different implications for the anatomical diversity in modern geographic populations. Noah’s Ark theorists say that differences in skin color, hair form, and build are superficial, recent adapta-tions to different environments. Candelabra theorists say these genetic differences go back a million years.

3.4 The Big Picture

(Source: W. J. Howard, 1991, Life’s Beginnings, Coos Bay: Coast Publishing, 84-85.)

2. How can the findings of religion and science be reconciled?

Some people in the Judeo-Christian world, as do people in other religious traditions, reject the findings of science and continue to believe that God created the world as it is just a few thousand years ago. These people are known as “creationists,” and their position is called “young earth creationism.” There are other creationist positions. “Old earth creationists” accept modern geology and astrophysics, but reject the findings of biology, specifically evolution. Other creationists accept some of evolution, but not the continuity between extremely different kinds of creatures, specifically that between humans and apes. Many Africans, being familiar with apes, believed that people were descended from them, but that idea was not acceptable to Christianity or to Islam.

According to Gallup polls, in 1997, 44 percent of Americans believed that God created humans pretty much in their present form in the last 10,000 years, while only 10 percent affirmed evolution without God’s participation. The rest held that God guided the evolutionary process in some way.¹⁰ Most Americans try to synthesize evolution with the existence of a personal god.

Many leading scientists, such as Brian Goodwin, Richard Lewontin, and Richard Dawkins, do not believe this is possible. They see no progress or direction in evolution, but rather a set of improvisations and haphazard events, an unbridled creativity of life, a dance exploring the space of possibilities.

Other scientists work out of their religious background and/or address themselves to religious people. Examples of such books are Brian Swimme and Thomas Berry, The Universe Story: From the Primordial Flaring Forth to the Ecozoic Era; Ursula Goodenough, The Sacred Depths of Nature; Fritjóf Capra and David Steindl-Rast, Belonging to the Universe: Explorations on the Frontiers of Science and Spirituality; and Edward O. Wilson, The Creation: An Appeal to Save Life on Earth. A religious philosopher looking at the whole story from a naturalist point of view is Loyal Rue, Everybody’s Story: Wising Up to the Epic of Evolution.