The Breakout
TWO MILLION YEARS AGO the australopithecines of Africa, their genes spreading among multiple species, still roamed the savanna forests and grasslands of Africa. They walked on hind legs, which set them apart from all other primates that had ever existed. Their heads were ape-like in shape and dentition. Their brains were no larger than those of the great apes who lived around them. Their populations were scattered and small, and at any time all might have plunged to extinction. Within another half million years, all in fact were gone.
That is, all except one. The australopithecine radiation had yielded a single survivor, whose descendants were destined not only to persist but to dominate the world. At first, these ancestors of modern humanity were no more assured of a future than their close relatives had been. By two million years before the present, the favored australopithecine line had begun the transition to the still-larger-brained Homo erectus. This species had a brain smaller than that of present-day Homo sapiens, but it was able to shape crude stone tools and use controlled fire at campsites. Its populations spread out of Africa, blanketing the land up into northeastern Asia and pushing south all the way to Indonesia. Homo erectus was adaptable to an unprecedented degree for a primate. Some of its populations survived in the cold winters of present-day northern China, and others in the steaming tropical climate of Java. Across its great range, paleontologists have excavated fragments of every part of the erectus skeleton and repeatedly pieced them together. And in two sedimentary layers near northern Kenya’s Lake Turkana, they discovered something as remarkable as skulls and thighbones: fossilized footprints. The impressions today have changed very little since a strolling Homo erectus, mud squishing between its toes, made them 1.5 million years ago.
Homo erectus, with a culture advanced well beyond that of its apish ancestors, and more adaptable to new and difficult environments, expanded its range to become the first cosmopolitan primate. It failed to reach only the isolated continents of Australia and the New World and the far-flung archipelagoes of the Pacific Ocean. Its great range buffered the species against early extinction. One of its genetic lines acquired potential immortality by evolving into Homo sapiens. The ancestral Homo erectus still lives. It is us.
At a far outlier of its range, Homo erectus produced a less fortunate offshoot. This was Homo floresiensis, a tiny, small-brained hominin that lived on Flores, a medium-sized island in the Lesser Sunda chain east of Java. Its fossil remains and stone tools date from 94,000 to only 13,000 years ago. At one meter in height and possessing a brain no larger than that of the African australopithecines, Flores man, also popularly known as the Hobbit, remains a tantalizing puzzle. It most likely originated as an extreme variant of Homo erectus, diverging during its isolation from the main Indonesian erectus populations. Its small size fits a loose rule of island biogeography: animal species isolated on islands and weighing less than twenty kilograms tend to evolve into relative giants (an example is the immense tortoises of the Galápagos), while those more than twenty kilograms tend to evolve into midgets (the dwarf deer of the Florida Keys). If its currently recognized status as a distinct hominin is correct, Homo floresiensis tells us a great deal about the vagaries of the evolutionary maze through which Homo erectus traveled to arrive at our own species. Its relatively recent extinction, following a long life, opens the possibility that it was erased, like our other sister species the Neanderthals, during the spread of all-conquering Homo sapiens around the world.
Homo sapiens, the successful descendant of Homo erectus, when viewed dispassionately is actually even more bizarre than the pygmy of Flores. Besides the bulging forehead, oversize brain, and long, tapering fingers, our species bears other striking biological features of the kind biological taxonomists call “diagnostic.” This means that in combination, some of our traits are unique among all animals:
• A productive language based on infinite permutations of arbitrarily invented words and symbols.
• Music, comprising a wide array of sounds, also in infinite permutations and played in individually chosen mood-creating patterns; but, most definitively, with a beat.
• Prolonged childhood, allowing extended learning periods under the guidance of adults.
• Anatomical concealment of female genitalia and the abandonment of advertisement of ovulation, both combined with continuous sexual activity. The latter promotes female-male bonding and biparental care, which are needed through the long period of helplessness in early childhood.
• Uniquely fast and substantial growth in the brain size during early development, increasing 3.3 times from birth to maturity.
• Relatively slender body form, small teeth, and weakened jaw muscles, indicative of an omnivorous diet.
• A digestive system specialized to eat foods that have been tenderized by cooking.
Approximately 700,000 years ago, populations of Homo erectus were evolving larger brains. By inference, they had acquired at least the rudiments of some of the diagnostic traits of Homo sapiens just cited. Yet in this early period skulls were still far from modern. Archaic Homo erectus possessed bulging brow ridges, more projecting faces, and less lateral expansion of the overall skull than were to be the case for modern Homo sapiens. By 200,000 years before the present, the African ancestors had come anatomically closer to contemporary humans. The populations also used more advanced stone tools and may have engaged in some form of burial practice. But their skulls were still relatively heavy in construction. Only around 60,000 years ago, when Homo sapiens broke out of Africa and began to spread around the world, did people acquire the complete skeletal dimensions of contemporary humanity.
The ancestors who achieved the breakout from Africa and conquered Earth were drawn from a diverse genetic mix. Throughout their evolutionary past, during hundreds of thousands of years, they had been hunter-gatherers. They lived in small bands, similar to present-day surviving bands composed of at least thirty and no more than a hundred or so individuals. These groups were sparsely distributed. Those closest to each other exchanged a small fraction of individuals each generation, most likely females. They diverged genetically enough that the entire ensemble of bands (the metapopulation, as biologists call such a collectivity) was far more variable than the indigenous humans destined to achieve the breakout.
That difference persists. It has long been known that Africans south of the Sahara are far more diverse genetically than native peoples in other parts of the world. The magnitude of this disparity became especially clear when in 2010 all of the protein-coding sequences of the genome were published for four Bushman hunter-gatherers (also known as the San or Khoisan) from different parts of the Kalahari, plus a Bantu from a neighboring agricultural tribe in southern Africa. Amazingly, despite the outward physical similarity among them, the four San proved to differ more from one another than an average European does from an average Asian.
It has not escaped the attention of human biologists and medical researchers that the genes of modern-day Africans are a treasure house for all humanity. They possess our species’ greatest reservoir of genetic diversity, of which further study will shed new light on the heredity of the human body and mind. Perhaps the time has come, in light of this and other advances in human genetics, to adopt a new ethic of racial and hereditary variation, one that places value on the whole of diversity rather than on the differences composing the diversity. It would give proper measure to our species’ genetic variation as an asset, prized for the adaptability it provides all of us during an increasingly uncertain future. Humanity is strengthened by a broad portfolio of genes that can generate new talents, additional resistance to diseases, and perhaps even new ways of seeing reality. For scientific as well as for moral reasons, we should learn to promote human biological diversity for its own sake instead of using it to justify prejudice and conflict.
The Homo sapiens populations that spread from Africa into the Middle East and beyond took long journeys of the kind routine for modern-day travelers. Generation upon generation, the bands slogged cautiously on foot into the strange lands that lay before them. The pattern they appeared to follow was to venture a few tens of miles, settle, increase in numbers, then divide into two or more bands, capable of moving on into new territory. Apparently the initial invaders pressed north in this manner along the Nile Valley to the Levant, then spread out north and east. Quite possibly the first pioneers into the corridor made up only one or a very few bands. Within a few thousand years their descendants became a net of loosely connected tribes cast up on nearly the whole of the Eurasian continent.
This scenario of slow initial advance by a very few followed by local population growth is supported by two lines of evidence assembled by independent groups of researchers during the past ten years. First is the great genetic diversity of present-day southern Africans, suggesting that only a small part of the whole African population participated in the breakout. Second, analyses and mathematical models made of the amount of genetic differences among living human populations suggest that the pioneers created a “serial founder effect,” with a few individuals moving out from an older, established population, then in turn serving as the source for the next emigration beyond. Eventually came multiple such spearheads radiating in many directions, and the human population coalesced.
Scientists have pieced together data from geology, genetics, and paleontology in order to envision more precisely how the out-of-Africa pattern began. Between 135,000 and 90,000 years ago, a period of aridity gripped tropical Africa far more extreme than any that had been experienced for tens of millennia previously. The result was the forced retreat of early humanity to a much smaller range and its fall to a perilously low level in population. Death by starvation and tribal conflict, both of which were to become routine in later historical times, must have been widespread in prehistory. The size of the total Homo sapiens population on the African continent descended into the thousands, and for a long while the future conqueror species risked complete extinction.
Then, finally, the great drought eased, and from 90,000 to 70,000 years ago tropical forests and savanna slowly expanded back to their previous ranges. Human populations grew and spread with them. At the same time, other parts of the continent became more arid, and the Middle East as well. With intermediate levels of rainfall prevailing throughout most of Africa, an especially favorable window of opportunity opened for the demographic expansion of pioneer populations out of the continent altogether. In particular, the interval was long enough to maintain a corridor of continuous habitable terrain up the Nile to Sinai and beyond, bisecting the arid land and allowing a northward sweep of colonizing humans. A second possible route was eastward, across the Bab el Mandeb Strait onto the southern Arabian Peninsula.
There followed the penetration of Homo sapiens into Europe by no later than 42,000 years before the present. Anatomically modern humans spread up the Danube River, entering the heartland of its sister human species the Neanderthals (Homo neanderthalensis). The latter populations had evolved in much earlier times from archaic human stock. Although genetically close to Homo sapiens, they were a distinct biological species, which on contact only rarely interbred with sapiens. Perhaps because the Neanderthals depended more on big game, they were poorly equipped to compete with skilled warriors who subsisted not only on big game but also on a wider variety of other animal and plant products. By 30,000 years before the present, Homo sapiens had entirely replaced them. Homo sapiens also replaced another species related to the Neanderthals, the recently discovered “Denisovans” of southern Siberia, known from remains in Denisova Cave in the Altai Mountains.
The remainder of the routes followed by the growing human populations, as best can be deduced by fossil and genetic evidence, extended outward into Asia and along the Indian Ocean coastline around 60,000 years ago. The colonists entered the Indian subcontinent and then the Malay Peninsula, while somehow making it across the straits to the Andaman Islands, where ancient aboriginal populations still exist. They apparently failed to reach the Nicobar Islands close by—where the genetic makeup of current inhabitants suggests a more recent Asian origin, 15,000 years before the present. The earliest human traces found to date in Indonesia, from the Niah Cave of Borneo, are 45,000 years old. The oldest from Australia, unearthed at Lake Mungo, date to 46,000 years. New Guinea was likely settled somewhat earlier. Major changes in the fauna of Australia, probably owing to predation and the use of burn-offs of low vegetation to drive game, give evidence that the date of the Australian incursion was at least 50,000 years before the present. The native people of New Guinea and Australia are thus truly aboriginals—direct descendants of the first modern humans to arrive in the same land they occupy today.
The question of exactly when anatomically modern Homo sapiens arrived in the New World, with its catastrophic impact on the virgin fauna and flora, has gripped the attention of anthropologists for many years. Like a photographic image in very slow developing fluid, the picture seems finally to be coming into focus. From genetic and archaeological studies across Siberia and the Americas, it now appears that a single Siberian population reached the Bering land bridge no sooner than 30,000 years ago, and possibly as recently as 22,000 years. In this period, the continental ice sheets had pulled enough water from the oceans to expose the Bering Land Bridge, while at the same time blocking entry into present-day Alaska. Around 16,500 years before the present, the retreat of the ice sheets cleared the way south, and a full-scale invasion through Alaska began. By 15,000 years before the present, as revealed by archaeological discoveries in both North and South America, the colonization of the Americas was well under way. It appears likely that the first populations dispersed along the recently deglaciated Pacific coastline, along land still exposed by the incomplete withdrawal of the ice sheets but nowadays mostly underwater.
FIGURE 9-1. The first colonists of a new continent. Early in the history of modern humanity (Homo sapiens), tribes began burial ceremonies, which were antecedents or accompaniments of primitive religious belief. This reconstruction is a burial by early Australian aboriginals at Mungo, southeastern Australia, at least forty thousand years ago. Red ocher powder is being poured on the body of the corpse. (© John Sibbick. From The Complete World of Human Evolution, by Chris Stringer and Peter Andrews [London: Thames & Hudson, 2005], p. 171.)
Approximately 3,000 years ago, the ancestors of the Polynesian people began colonizing the Pacific archipelagoes. Starting at Tonga and proceeding stepwise eastward with large canoes designed for long voyages, they reached, by AD 1200, the extreme reaches of Polynesia, a triangle formed by Hawaii, Easter Island, and New Zealand. With this achievement of the Polynesian voyagers, the human conquest of Earth was complete.