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GENETICS & GENESIS

It has often and confidently been asserted, that man’s origin can never be known: but ignorance more frequently begets confidence than does knowledge: it is those who know little, and not those who know much, who so positively assert that this or that problem will never be solved by science.

CHARLES DARWIN, THE DESCENT OF MAN

 

 

 

TRAVEL BACK INTO THE HUMAN PAST, and the historical evidence is plentiful enough for the first couple of hundred years, then rapidly diminishes. At the 5,000-year mark written records disappear altogether, yielding to the wordless witness of archaeological sites. Going farther back, even these become increasingly rare over the next 10,000 years, fading almost to nothing by 15,000 years ago, the date of the first human settlements. Before that time, people lived a nomadic existence based on hunting and gathering. They built nothing and left behind almost nothing of permanence, save a few stone tools and the remarkable painted caves of Europe.

Travel on back for another 35,000 years and you will have reached the 50,000-year mark, the time when the ancestral human population was still confined to its homeland somewhere in northeast Africa but had begun to show the first signs of modern behavior. If this is the point at which the modern human story begins, then written records exist for just the last 10% of it; 90% of human history seems irretrievably lost.

Keep traveling back in time to the earliest starting point in the human narrative, the period 5 million years ago when the ape-like creatures at the head of the human line of descent split from those at the head of the chimpanzee line of descent. The only physical evidence from throughout this period, which saw the evolution from ape to human form, is a handful of battered skulls and a few stone tools.

No deep understanding, it might seem, could ever be gained of these two vanished periods, the 5 million years of human evolution and the 45,000 years of prehistory. But in the past few years an extraordinary new archive has become available to those who study human evolution, human nature and history. It is the record encoded in the DNA of the human genome and in the versions of it carried by the world’s population. Geneticists have long contributed to the study of the human past but are doing so with particular success since the full sequence of DNA units in the genome was determined in 2003.

Why should the human genome, specifically shaped for survival in the present, have so much to say about the past? As the repository of hereditary information that is in constant flux, the genome is like a document under ceaseless revision. Its mechanism of change is such that it retains evidence about its previous drafts and these, though not easy to interpret, provide a record that stretches deep into the past. The genome can therefore be interrogated at many different time levels. It can supply answers that reach back more than 50,000 years to the genetic Adam, a man whose Y chromosome is carried by all men now alive. Or it can be queried about the events of a mere couple of centuries ago, such as whether Thomas Jefferson, the third president of the United States, had a secret family with his slave mistress Sally Hemings.

From Adam to Jefferson, the genome is helping researchers create a new and far more detailed picture of human evolution, human nature and history. From the great darkness, a surprisingly full narrative is emerging. This new narrative of the human past rests on a solid foundation laid by paleoanthropologists, archaeologists, anthropologists and many other specialists. It can be called new in the sense that genetic information now contributes to each of these traditional disciplines and is beginning to draw them together.

This book describes those aspects of human evolution, nature and prehistory that have been illumined by genetic discoveries of the last few years. Readers who do not follow these fields closely may be surprised at the richness of the information in the new narrative. There exists no video of how apes slowly morphed into people, but a sequence of the salient events can for the most part be reconstructed. There is no map that records the dispersal of the new humans from their ancestral homeland, but researchers can now follow the path they took out of Africa and their migrations through the world outside. It’s even possible to reconstruct some of the social institutions that emerged as people made the transition from a nomadic way of life, based on hunting and gathering, to today’s complex societies.

Information from the genome has helped tell paleoanthropologists when humans lost their body hair and when they gained the power of speech. It has clarified for archaeologists their long quandary as to whether Neanderthals and modern humans peacefully interbred with each other or fought until the Neanderthals’ extinction. It has furnished anthropology with information about human adaptation to cultural practices like cattle-herding and cannibalism. The cascade of DNA data is even benefiting historical linguistics, though indirectly, as biologists apply the tree-building methods developed for gene genealogies to reconstructing the evolution of language.

On the critical question of the ancestral human population of 50,000 years ago, the last group from which everyone alive today is descended, the techniques of paleoanthropology and archaeology are powerless to say anything about a people that has vanished without trace. But geneticists, by rummaging around in the genome’s rich attic, can fill in all kinds of unexpected detail. They can estimate how large the ancestral population was. They can say where in Africa it probably lived. They can put a date, though a rough one, on when language emerged. They can even infer, in one instance, what the first language sounded like.

The First Tailored Clothes

Few findings better illustrate geneticists’ ability to cast light into surprising corners of the human past than a recent estimate of the date that people first sewed their own clothes. Early humans may have used loose animal skins for millions of years, worn perhaps like a cape against the cold, but fabricated garments were a more recent invention. Archaeologists have never been able to determine when clothes were first worn because both the materials and the bone needles used to sew them are highly perishable.

In the fall of 1999, Mark Stoneking’s son came home from school in Leipzig, Germany, with a note warning that a classmate had a case of head lice. Stoneking, an American researcher at the Max Planck Institute for Evolutionary Anthropology, read it as carefully as would any anxious parent. But as a geneticist long interested in human origins, his attention was drawn to a reassurance in the school note that lice cannot survive longer than 24 hours away from the warmth of the human body. “I thought if that was true, then lice must have been spread around the world by human migrations,” he says. Stoneking figured that if he could prove this were so, he would have discovered an independent confirmation of the migration pattern implied by human DNA. But after a few hours of library research, he realized that the lice might hold in their DNA an even more interesting fact—the date when humans first wore clothing.

The compilers of the book of Genesis were so exercised by the question of human nakedness that they included not just one but two accounts of how people came to seek modesty in clothing. In the first, Adam and Eve sewed themselves aprons of fig leaves after realizing their state of undress. In the second, the Creator himself tailored the errant pair coats of skins before expelling them into the world beyond Eden.¹ Neither account gives due weight to the other interested party in the story of human clothing, the louse. Once, after all, in the days when human forebears were fully covered with hair just like any other ape or monkey, the louse must have ranged freely from head to toe.

When humans lost their body hair, the louse’s domain shrank, confining it to the lonely island of hair that tufts absurdly from the human head. But it patiently bided its time and many millennia later, when people started to wear clothes, the head louse seized the chance to regain its lost territory by evolving a new variety, the body louse, that could live in clothing. The head and body louse closely resemble each other except the body louse is larger and has claws specialized for grasping material, not shafts of hair. Stoneking realized that he could date the invention of clothing if he could only figure out from variations in lice DNA the time at which the body louse began to evolve from the head louse.

He collected head and body lice from the citizens of 12 countries around the world, from Ethiopia to Ecuador to New Guinea. He analyzed all the variations in a small segment of each head and body louse’s genetic material and arranged each population’s lice in a family tree. Knowing the rate at which variations accumulate on DNA over the centuries, he could then calculate the dates of the various forks or branch points in the tree.

The branch point at which the body louse first evolved from the head louse turned out to be around 72,000 years ago, give or take several thousand years either way.² Assuming the body louse evolved almost immediately after its new niche was available to it, then people first addressed their nakedness only in the most recent stage of their evolutionary history. It was about this time, or a few thousand years later, that people perfected language and broke out of Africa to colonize the rest of the world. It seems they had decided to get dressed for the occasion.

From Adam to Jefferson

Genetics, with its fresh new insights into the human past, ranges across many other academic territories. At least seven traditional disciplines bear on the human past. Paleoanthropologists, the students of fossil human remains, have reconstructed the major steps by which the human lineage branched off from apes 5 million years ago and, by 100,000 years ago, had morphed into humans who were anatomically though not behaviorally similar to people of today. Archaeologists have picked up the story from there, establishing the foundation of dates and basic facts on which other specialists seek to reconstruct various aspects of past human behavior. Population geneticists have tracked the migration of human populations around the world. Their early analyses were based on differences in human proteins but the emphasis has now switched to DNA, a more convenient and informative source.

Historical linguists have traced back the family tree of human languages, reconstructing vanished tongues such as proto-Indo-European, the inferred ancestral tongue of many languages spoken in Europe, Iran and India. Primatologists, after many years of patient observation, have gained a deep understanding of how chimpanzee and bonobo societies work. This achievement provides insights into the social organization of the primates from which both chimps and people evolved, since chimps may closely resemble those ancient ancestors. Social anthropologists, through the study of surviving hunter-gatherers and other primitive societies, have laid the basis for reconstructing the evolution of human social structures. Evolutionary psychologists seek to identify the tasks that evolution has designed the mind to perform. In two other fields closely related to evolutionary psychology, those of human behavioral ecology and evolutionary anthropology, researchers explore ways of applying the principles of evolutionary biology to human societies. From these three subjects have emerged many sharp insights into how the search for reproductive advantage shapes people’s choices in marriage, parenting and the allocation of their resources.

Researchers in each of these seven disciplines have helped delineate the distant human past, often by ingenious interpretation of fragmentary evidence. The seven traditional disciplines are increasingly being aided by an eighth, that of evolutionary biology, the body of theory on which evolutionary psychology seeks to draw. Many specialists have assumed that evolutionary change works so slowly that its effect on the recent human past, if any, can safely be ignored. But it was only lack of knowledge that made it seem evolution’s hand had been stayed. As is now evident from analyses of DNA, human genes have continued to evolve until the present day. Like everything else in biology, the human past and present are incomprehensible except in the light of evolution.

The human genome is a new source of data that enriches all the disciplines concerned with the human past. It furnishes two quite different types of information, one to do with genes, the other with genealogies.

New versions of existing genes often arise in the course of evolution, and become more common in a population because they confer some advantage. These new versions carry distinctive properties that allow geneticists to estimate their approximate age. So when a gene is found that concerns some major feature of human evolution, like the FOXP2 gene, which is involved in language, or the melanocortin receptor gene which influences skin color, geneticists can often set dates on the window of time in which the feature evolved.

A second kind of information in the genome allows ancestries to be traced, usually through a special part of the genome like the Y chromosome, which is passed down essentially unchanged from one generation to another. Every few generations a mutation—the random conversion of one DNA unit to another—occurs on the Y, with the result that all descendants of the man in whom the mutation occurred will also carry it. All men can be assigned to different lineages, based on the particular pattern of mutations they carry on their Y chromosome. These patterns allow many inferences to be drawn about human migrations because the lineages for the most part are confined to the specific geographical regions where their owners first settled.

The human genome thus records a vast span of the human past and enriches the findings of traditional disciplines. Following are the principal themes, explored in the pages that follow, to which DNA has added new insights:

• There is a clear continuity between the ape world of 5 million years ago and the human world that emerged from it. The thread is most visible at the level of DNA: the genomes of humans and chimpanzees are 99% identical. It is evident enough in the physical resemblance between the two species. But perhaps the most interesting level of continuity is between the social institutions of the ape and human worlds.

The apes ancestral to both chimpanzees and humans probably lived as small bands of related individuals who defended a home territory, often with lethal attacks against neighbors. They had separate male and female hierarchies and most infants were sired by the society’s dominant male or his allies. The emerging human line was also territorial but in time developed a new social structure based on pair bonding, a stable relationship between a male and one or more females. This critical shift would have given all males a chance of reproduction and hence a stronger interest in the group’s welfare, making human societies larger and more cohesive.

• A principal force in the shaping of human evolution has been the nature of human society. After splitting from the apes, those in the human line of descent evolved upright stature and developed dark skin in place of the ape’s body hair. But the most significant change—a steady increase in brain size—probably evolved in response to the most critical aspect of the environment, the society in which an individual lived. Judging whom to trust, forming alliances, keeping score of favors given and received—all were necessities made easier by greater cognitive ability. By 50,000 years ago, the social benefits of more efficient communication had prompted the evolution of a novel ability possessed by no other social species, the faculty of language.

• The human physical form was attained first, followed by continued evolution of human behavior. Anatomically modern humans, people whose physical remains resemble the skeletons of people today, became common 100,000 years ago. But they showed no sign of the advanced behaviors that emerged 50,000 years later, probably made possible by the evolution of language. With this new faculty and the greater social cohesion it provided, the first behaviorally modern humans were able to break out of Africa and displace the archaic humans like the Neanderthals who had left Africa many thousands of years previously.

• Most of human prehistory occurred in, and was shaped by, the last ice age. The first modern humans to leave Africa probably crossed over the Red Sea at its southern end and into Arabia. Reaching India, the population went separate ways. One group traveled along the coasts of southeast Asia, arriving in Australia some 46,000 years ago. Another explored the land route northwest from India, reaching Europe and slowly evicting the Neanderthals from their ancient homeland. The expansion into the cold northern latitudes of Eurasia required technical innovation and probably genetic adaptations too. Then a climatic catastrophe, the return of the glaciers 20,000 years ago, emptied Europe and Siberia of people. Descendants of the survivors spread north again several thousand years later as the Pleistocene ice age drew to a close. Some of these new northerners, the Siberians in the eastern half of Eurasia, contrived the first domestication, that of the dog, and discovered the land bridge that then joined Siberia to Alaska and the Americas.

• The adaptations for three principal social institutions, warfare, religion and trade, had evolved by 50,000 years ago. The ancestral human population, the first to possess the power of fully articulate modern speech, may have numbered only 5,000 people, confined to a homeland in northeast Africa. These ancestral people, though less cognitively advanced than people today, possessed all the distinctive features of human nature and had developed, at least in rudimentary form, the institutions that are found in societies throughout the world. These may have included warfare centered round a defense of territory, religious ceremony as a means of social cohesion, and an instinct for reciprocity that governed social relations within the group and trade with those outside it.

• The ancestral people had a major limitation to overcome: they were too aggressive to live in settled communities. Early human societies lived as small bands of hunter-gatherers, their existence dominated by incessant warfare. For 35,000 years after leaving the ancestral homeland, these nomads were unable to settle down. Only gradually did humans evolve to become less aggressive. The tempo of warfare eased and a more gracile, or delicately boned, human form evolved in populations throughout the world. In the Near East, around 15,000 years ago, people at last accomplished a decisive social transition, the founding of the first settled communities. In place of the hunter-gatherers’ egalitarianism and lack of possessions, people in settled societies developed a new social structure with elites, specialization of roles, and ownership of property. Human groups started for the first time to produce storable surpluses of food and other products, which led to more complex societies and to increased trade between groups.

• Human evolution did not halt in the distant past but has continued to the present day. The ancestral human population of 50,000 years ago differed greatly from the anatomically modern humans of 100,000 years ago, and people today have had just as long to evolve away from the ancestral population. The human genome bears many marks of recent evolution, prompted by adaptations to events such as cultural changes or new diseases.

More visible evidence of recent evolution is the existence of human races. After the dispersal of the ancestral population from Africa 50,000 years ago, human evolution continued independently in each continent. The populations of the world’s major geographical regions bred for many thousand years in substantial isolation from each other and started to develop distinctive features, a genetic differentiation which is the basis for today’s races. But these separate evolutionary paths were to some extent parallel as people in different continents responded to the same challenges. Gracilization occurred worldwide. Lactose tolerance, the genetic ability to digest lactose in adulthood, evolved among cattle-herding people in Europe 5,000 years ago but also among pastoral peoples of Africa and the Middle East.

• People probably once spoke a single language from which all contemporary languages are derived. Just as the ancestral population, after its dispersal, diverged into different races and ethnic groups, the ancestral tongue split into a growing family of different languages. Some of these languages expanded under the influence of factors such as warfare or agriculture, so that certain language families, like Indo-European, are now spoken over large areas while others, like many in South America or New Guinea, have a range of a few miles. Because language splits follow population splits, the genealogy of human languages must mirror, to some extent, the tree of descent of human populations; some biologists hope that the genealogy of human languages can be reconstructed far into the past, perhaps even near to its root, the mother tongue of the ancestral human population.

• The human genome contains excellent records of the recent past, providing a parallel history to the written record. The genome evolves so fast that whenever any community starts to breed in isolation, whether for reasons of religion, geography or language, within a few centuries its genetics assume a distinctive signature. DNA sheds a novel light on the history of peoples such as Jews, Icelanders and the inhabitants of the British Isles. It records the genetic impact of male dynasties like those of the Mongols and the Manchus. And for those who know to ask the right questions, it retains the secret family history of individuals such as Thomas Jefferson.

The compilers of the book of Genesis did their best, from available myths and legends, to frame a coherent account of human origins. They sought to address such questions as why people speak so many languages, suffer pain in childbirth and wear clothes to conceal their nakedness. Human origins can now be explained in another way. Given that so little has been preserved of the distant human past, it is remarkable how much is now being retrieved. Many of the findings described here have been made in the last few years. Though the frontiers of science are turbulent, throwing up many claims that require revision in light of further evidence, the flood of new findings described in these pages includes many unmistakable advances. The biological framework of human origins and nature is beginning to emerge with surprising clarity as the record of past evolutionary change now streams forth from the sequence of the human genome. In the long search to understand ourselves, our obscure origins, our strange and contradictory nature, and the fragmentation of the once united human family into different races and warring cultures speaking thousands of different languages, we can begin at last to comprehend the long darkness before the dawn.