Before the Dawn: Recovering the Lost History of Our Ancestors

by Nicholas Wade

  Pääbo regards the dating of the gene as compatible with Klein’s argument that modern language evolved very recently and was probably the spur to the human behavioral changes seen in the archaeological record 50,000 years ago.

  Societies with two kinds of people, of greatly differing language abilities, may have existed during the evolution of language. As each new variant gene arose, conferring some improvement in language ability, the carriers of the gene would leave more descendants. When the last of these genes—perhaps FOXP2—swept through the ancestral human population, the modern faculty of language was attained.

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  EDEN

  We can see, that in the rudest state of society, the individuals who were the most sagacious, who invented and used the best weapons or traps, and who were best able to defend themselves, would rear the greatest number of offspring. The tribes, which included the largest number of men thus endowed, would increase in number and supplant other tribes. . . . As a tribe increases and is victorious, it is often still further increased by the absorption of other tribes. The stature and strength of the men of a tribe are likewise of some importance for its success, and these depend in part on the nature and amount of the food which can be obtained. All that we know about savages, or may infer from their traditions and from old monuments, the history of which is quite forgotten by the present inhabitants, show that from the remotest times successful tribes have supplanted other tribes.

  CHARLES DARWIN, THE DESCENT OF MAN

  WITH THE DEVELOPMENT OF LANGUAGE, the process of human evolution in Africa reached a decisive stage. After 5 million years, the human lineage that split off from apes had emerged into a people quite similar in their form and faculties to those who live today.

  This people, which can be called the ancestral human population, was probably the first to have possessed fully modern speech, and the last from which all people on earth are descended. Since it dispersed so quickly after its formation, it may have endured for only a few thousand years.

  Not only did the ancestral population probably have a fleeting existence, it seems to have survived by the narrowest of margins. It lived sometime between 50,000 and 100,000 years ago, probably nearer to the 50,000-year mark. Between 60,000 and 40,000 years ago much of Africa was depopulated, and only in East Africa can archaeologists detect a human presence.63 The reason may have been a long period of dry climate that shrank the forests and dried out the savannas. The ancestral population itself, geneticists estimate, shrank to as few as 5,000 people.

  From this village-sized population, the world was peopled. And since people in societies around the world behave in much the same way, the principal elements of human nature must already have been present in the ancestral human population before its dispersal into Africa and the world beyond.

  It would be of the greatest interest to know everything about the ancestral human population—its way of life, its social structure, the roles of men and women, its religion, the language that its members spoke. Not a trace of these first people has yet been found by archaeologists. Yet despite the total lack of direct evidence, a surprising amount can now be inferred about the ancestral human population.

  Geneticists can estimate how large the population was and, by identifying its closest descendants, can point to where in Africa the ancestral population may have lived. They can even say something about the language the ancestral people spoke. And by analyzing the behaviors common to societies around the world, particularly the hunter-gatherers who seem closest to the ancestral people, anthropologists can describe how the ancestral population probably lived and what its people were like.

  The Genealogies of Eve and Adam

  Because everyone in the world is descended from the ancestral population, geneticists can infer some of its properties by analyzing the DNA of living people, and then working backward.

  Two parts of the human genome are particularly useful for this purpose. One is the Y chromosome, the only chromosome possessed by men alone. The other is known as mitochondrial DNA. These are the only two parts of the genome that escape the shuffling of genetic material between generations. The shuffling, an evolutionary mechanism for generating diversity rapidly at each generation, means that almost all other parts of the human genome have a pedigree that is at present too complex to untangle. d

  Unlike most pairs of chromosomes, the X and Y do not exchange segments of DNA between generations (except at their very tips). This is to ensure that the Y’s most important gene, the one that makes a person male, never gets shuffled into the X chromosome. The Y chromosome is therefore passed down essentially unchanged from father to son, generation after generation. Mitochondrial DNA escapes shuffling through a different process. Mitochondria, cellular components that generate chemical energy, are former bacteria that were enslaved long ago by animal cells. They live in the main body of the cell, outside the nucleus that holds the chromosomes. When the sperm fuses with the egg, all the sperm’s mitochondria are destroyed, leaving the fertilized egg equipped with only the mother’s mitochondria. Because of this arrangement, mitochondria are bequeathed unchanged from mother to child (and a man’s mitochondria are not passed on to his children).64

  In addition to their exempt status, the Y chromosome and mitochondrial DNA each have a special and surprising property of uniqueness. All men in the world today carry the same Y chromosome, and both men and women carry the same mitochondria. All of today’s Y chromosomes were in herited from the same, single source, a Y chromosome carried by an individual male who belonged to, or lived slightly before, the ancestral human population. The same is true of mitochondrial DNA; everyone carries the same mitochondrial DNA because all are copies of the same original, the mitochondrial DNA belonging to a single woman.

  The metaphor is hard to avoid—this is Adam’s Y chromosome, and Eve’s mitochondrial DNA. The ancestral human population, of course, included many Adams and Eves, indeed about 2,500 of each if the geneticists’ calculations are to be believed. So how did it come about that just one man bequeathed his Y chromosome to the whole world and one woman her mitochondria?

  It’s a curious fact of genetics that one version of a gene, especially in small populations, can displace all the other existing versions of the same gene in just a few generations, through a purely random process called genetic drift. Consider how this might work among surnames, which are passed on from father to son just like Y chromosomes. Suppose a hundred families are living on an island, each with a different surname. In the first generation, many of those families will have only daughters or no children at all. So in just one generation, all those families’ surnames (and accompanying Y chromosomes) will go extinct. Assuming no new male settlers arrive on the island, the same unavoidable winnowing will happen each generation until only one surname (and Y chromosome) is left.

  This is what has happened with the human Y chromosome. Every Y chromosome that exists today is a copy of the same original, carried by a single individual in the ancestral human population. The Y chromosomes of all the other Adams have perished at some point along the way when their owners had no sons.

  But despite all being copies of the same original, Y chromosomes are not identical. Over the generations, mutations—the switch of one of the four kinds of DNA units for another—have built up on the Y. The mutations are harmless but serve the invaluable purpose for geneticists of assigning the owners of Y chromosomes to different male lineages. The reason is that once a man has acquired a novel mutation in his Y chromosome, all his sons will carry that mutation, and no one else will. If one of the sons has a second mutation, all of his descendants will carry the two mutations. Each new muta tion thus creates a fork on the family tree—between those who carry it and those who don’t—and stands at the head of all the lineages beneath it.

  FIGURE 4.1. THE UNIVERSAL HUMAN Y CHROMOSOME.

  Only men carryaYchromosome. In each generation, some men have no children or only daughters, reducing the number of Y chro
mosomes in the population, until only one remains. This is why all men in the world carry a Y chromosome inherited from a single individual—the Adam of the Y chromosome—who lived in the ancestral human population. The same is true of mitochondrial DNA and the mitochondrial Eve.

  By looking at the most informative of the mutations on the Y chromosome, geneticists can assign every man to one lineage or another. Since there is only one Y chromosome, all these lineages or branches eventually coalesce to a single trunk, the Y chromosome of the original “Adam.”

  Mutations get incorporated into the Y chromosome at a fairly steady rate, which enables geneticists to put a date on each branch point by counting the number of mutations down a lineage. And the lineages can be assigned not only a date but a geographical location. This is because human populations were expanding across the globe at the time the mutations of interest occurred but then, to a remarkable extent, people lived and bred in the same place they were born. So geneticists can impose the Y chromosome tree across the map of the world, assigning each of its forks and lineages to specific geographical regions.

  Of particular help in defining the ancestral human population is the lineage of men that left Africa. A few men inside Africa, and all men outside it, carry a Y chromosome mutation known as M168. This means that modern humans left Africa sometime shortly after the M168 mutation occurred. Based on the mutation-counting method, one recent estimate is that M168 occurred 44,000 years ago.65 Genetic dates, however, generally come with a wide range of possible error. This one, say Peter Underhill and colleagues at Stanford University, could range anywhere between 39,000 and 89,000 years ago. The root of the Y chromosome tree dates to 59,000 years ago, though this too has a wide range of possibilities, from 40,000 to 140,000 years ago. Still, a date around 59,000 years ago seems a reasonable estimate for the time when the Y chromosomal Adam walked the earth. This date fits well with a date of 50,000 years ago for the ancestral human population, because genes tend to have slightly deeper ancestries than do populations.

  Estimating the Ancestral Population Size

  The Y chromosome is just a small part of the human genome. But it seems likely to represent human population history well enough, not least because its story is corroborated by mitochondrial DNA. Mitochondrial DNA can be used to construct genealogies of women just as the Y chromosome generates lineages of men.

  FIGURE 4.2. THE Y CHROMOSOME FAMILY TREE

  AND ITS GEOGRAPHICAL DISTRIBUTION.

  Although all men carry the same Y chromosome, mutations have gradually built up on it. The mutations allow men to be assigned to different lineages, depending on which set of mutations they carry. Because the mutations accumulated while the ancestral people were spreading through the world, different lineages of men are found in different regions of the world.

  All male lineages outside sub-Saharan Africa carry the Y chromosome mutation known as M168. Men who carry the M173 mutation may have been the first modern humans to enter Europe 45,000 years ago, founding what archaeologists call the Aurignacian culture. Bearers of M170 are thought to have brought the Gravettian culture that succeeded the Aurignacian 28,000 years ago. The M242 mutation occurred just before the first humans crossed the Bering land bridge from Siberia to the Americas.

  The women’s lineages, like the men’s, have all turned out to be branches from a single root, the mitochondrial DNA possessed by a single woman who lived in or before the ancestral human population. The mitochondrial Eve appears to have lived considerably earlier than the Y chromosomal Adam—about 150,000 years ago—but that may reflect the difficulty of dating mitochondrial DNA, which gathers mutations more rapidly than does the Y chromosome.

  The mitochondrial genealogy of humankind has three main branches, known as L1, L2 and L3. L1 and L2 are confined to Africans who live south of the Sahara. The L3 branch gave rise to a lineage known as M, and it was the descendants of M who left Africa.

  The Y and mitochondrial data can be made to yield another vital piece of information—the “effective” size of the ancestral population. The effective population is a statistical concept inferred by population geneticists from the amount of variation seen in samples of DNA. It is a large fraction of the real population size—for humans, often considered to be about half.66 The effective size of the ancestral human population has long been estimated to have been around 10,000 individuals, but recent calculations, which mitigate a confounding factor in the earlier estimates, suggest the actual number may have been even smaller. An estimate based on the Y chromosome suggests an effective population size of just 1,000 men of reproductive age.67 Assuming the same number of women, this implies an “effective population” of 2,000, which is equivalent to a census-size population of a mere 4,000 individuals, or say 5,000 in round numbers.

  The first two branches of the Y chromosome genealogy, whose bearers are found only in Africa, have many sub-branches. This suggests the ancestral human population soon became quite spread out and diverse. There are other hints in the pattern of mutation that many Y chromosome lineages that once existed are now extinct. The ancestral population, in other words, may have suffered several calamities with widespread loss of male life.68

  Because foragers lived in groups, generally of 150 people or so who may have liked to trade with neighboring groups, a population as small as 4,000 or 5,000 people is unlikely to have been distributed over the whole continent of Africa. It would probably have had a much smaller range—“perhaps the size of Swaziland or Rhode Island” according to one estimate.69 The smaller the area, the more possible that at one time a single language was spoken. Archaeologists have not yet located this ancestral homeland. Given that its inhabitants would have been hunters and gatherers, they may have left little sign of their presence.

  FIGURE 4.3. THE MITOCHONDRIAL DNA FAMILY TREE

  AND ITS GEOGRAPHICAL DISTRIBUTION.

  All men and women carry the same mitochondrial DNA, derived from a mitochondrial Eve. Like the Y chromosome, this single version of mitochondrial DNA has over time collected mutations which can be used to assign women to lineages (mitochondrial DNA is inherited only through the female line).

  All women in sub-Saharan Africa belong to one of the first three branches of the mitochondrial DNA tree, known as L1, L2 and L3. All women outside Africa belong to M or N, the two daughter lineages of L3. Women in the western part of the Eurasian continent are all daughters of N; those of the eastern part descend from M or N. The daughter lineages A, B, C, D and X reached the Americas.

  But geneticists have figured out where in Africa this ancient homeland may have been located. The clues lie not just in genes but also in tongues, specifically the click languages of Africa and the San people who speak them.

  The Click Language Echo from the Mother Tongue

  The Dutch settlers who first arrived in southern Africa in 1652 found the country inhabited by two groups of click language speakers—cattle herders, whom they called Hottentots, and foraging peoples whom they referred to as either bushmen or San, a Hottentot word for “original settlers.” The Hottentots called themselves Khoi-Khoi but are now known as Khwe. From these two words, Khoi and San, is derived the word Khoisan, which is used to describe the linguistic family of click languages.

  The southern San of the Cape were largely driven to extinction by the Dutch settlers. Anthropologists have studied the northern San, who live in a large area from southern Angola to Botswana that includes the Kalahari desert. Until the 1970s, when settlement became widespread, many of the San lived as hunters and gatherers, one of the few remaining peoples to follow this ancient way of life. The main language of the northern San is !Kung, a name that seems to have been invented by German missionaries and means “they” in the Angola !Kung dialect.70 These northern San are often referred to as the !Kung or the !Kung San. The “!” represents one of the many click sounds in their language.

  To add to the confusion, anthropologists have recently started to refer to the !Kung San by their name for
themselves, the Ju|’hoansi, which means “the Real People.” The Real People’s name for both Europeans and non-San Africans is !ohm, a category that includes predators and other inedible beasts.

  The “|” in Ju|’hoansi designates one of the click sounds that are used as extra consonants in click languages. There are 5 kinds of click made by sucking air in, and a larger number made by expelling it. The “|” is an in-coming dental click, made by sucking the tongue in smartly from the upper front teeth, like the “tsk, tsk” sound used to indicate disapproval to children. The sound systems of Khoisan are said to be among the most complex in the world.71

  About 30 different click languages are still spoken in southern Africa. They fall into three groups that, apart from their clicks, bear little evident relationship to each other. Speakers of another two click languages, known as Hadza and Sandawe, live far away in Tanzania. Hadza and Sandawe are both isolates, meaning they have no known relationship to each other or to the !Kung language of the San.

  Despite the fact that many of the click languages apparently have nothing in common save their clicks, Joseph Greenberg, the great classifier of the world’s languages, assigned them all to a single family, known as Khoisan. Linguists grumbled that it was illogical to define a group of unrelated languages as a family, but went along with the idea because no one knew what else to do with the click languages. Greenberg is at present reviled by most historical linguists, but his classification of the Khoisan languages seems a stroke of genius in light of a surprising new link that has now emerged among them.