A Troublesome Inheritance Read online

  Evolution and Speciation

  Races are a way station on the path through which evolution generates new species. The environment keeps changing, and organisms will perish unless they adapt. In the course of adaptation, different variations of a species will emerge in conditions where the species faces different challenges. These variations, or races, are fluid, not fixed. If the selective pressure that brought them into being should disappear, they will merge back into the general gene pool. Or, if a race should cease to interbreed with its neighbors through the emergence of some barrier to reproduction, it may eventually become a separate species.

  People have not been granted an exemption from this process. If human differentiation were to continue at the same pace as that of the past 50,000 years, one or more of today’s races might in the distant future develop into a different species. But the forces of differentiation seem now to have reversed course due to increased migration, travel and intermarriage.

  Races develop within a species and easily merge back into it. All human races, so far as is known, have the same set of genes. But each gene comes in a set of different flavors or alternative forms, known to geneticists as alleles. One might suppose that races differ in having different alleles of various genes. But, though a handful of such racially defining alleles do exist, the basis of race rests largely on something even slighter, a difference in the relative commonness, or frequency, of alleles, a situation discussed further in the next chapter.

  The frequency of each allele of a gene changes from one generation to the next, depending on the chance of which parent’s allele is inherited and whether the allele is favored by natural selection. Races are therefore quite dynamic, because the allele frequencies on which they depend are shifting all the time. A good description is provided by the historian Winthrop Jordan in his history of the historical origins of racism in the United States. “It is now clear,” he writes, “that mankind is a single biological species; that races are neither discrete nor stable units but rather that they are plastic, changing, integral parts of a whole that is itself changing. It is clear, furthermore, that races are best studied as products of a process; and, finally, that racial differences involve the relative frequency of genes and characteristics rather than absolute and mutually exclusive distinctions.” 10

  Races emerge as part of the process of evolutionary change. At the level of the genome, the driving force of evolution is mutation. Mutation generates novelty in the sequence of DNA units that comprise the hereditary information. The new sequences are then acted on—either eliminated, made more common or ignored—by the evolutionary processes of natural selection, genetic drift and migration.

  The chemical units of which DNA is composed are long lasting but not permanent. Every so often, from spontaneous decay or radiation, a unit will disintegrate. In every living cell, repair enzymes constantly patrol up and down the strands of DNA, proofreading the sequence of chemical units, or bases, as chemists call them. The four bases are known for short as A (adenine), T (thymine), G (guanine) and C (cytosine). The structure of a DNA molecule consists of two strands that spiral around each other in a double helix, with each base on one strand lightly cross-linked to a base on the other strand. The cross-linking system requires that where one strand of the double helix has A, there will be a T at the same site on the opposite strand, with G and C being similarly paired. If the base opposite a T is missing, the repair enzymes know to insert an A. If a C is missing its partner, the enzymes will provide a G. The system, though amazingly efficient, is not perfect. A wrong base is occasionally inserted by the proofreading system, and these “typos” are called mutations. When the mutations happen to occur in a person’s germ cells, whether eggs or sperm, they become evolutionarily significant, because they may then get passed on to the next generation.

  Other kinds of mutation occur through copying errors made by the cell in manipulating DNA. All these types of mutation are the raw material for natural selection, the second evolutionary force. Most mutations affect only the copious regions of DNA that lie between the genes and are of little consequence. It’s the sequence of bases in the genes that codes the information that specifies proteins and other working parts of the cell. This coding DNA, as it is called, occupies less than 2% of the human genome. Mutations that do not meaningfully alter the coding DNA or the nearby promoter regions of DNA, used to activate the coding DNA, generally have no effect on the organism. Natural selection has no reason to bother about them, and for this reason geneticists call them neutral mutations.

  Of the mutations that do change the genetic sequence, most degrade or even destroy the function of the protein specified by the gene. These mutations are detrimental and need to be eliminated. “Purifying selection” is the phrase geneticists use for the action of natural selection in ridding the genome of harmful mutations. The bearer of the mutation fails to live or has few or no offspring.

  It’s just a handful of mutations that have a beneficial effect, and these become more common in the population with each succeeding generation as the lucky owners are better able to survive and breed.

  The individuals with a beneficial mutation possess a new gene, or rather a new allele—a version of the old gene with the new mutation embedded in it. It’s because of mutations and alleles that there exists a third force of evolutionary change, called genetic drift. Each generation is a genetic lottery. Your father and mother each have two copies of every gene. Each parent bequeaths one of their two copies to you. The other is left on the cutting room floor. Suppose that with a particular gene there are just two versions, called alleles A and B, in a population. Suppose too that 60% of a present population carries allele A and 40% allele B. In the next generation, these proportions will change because, by the luck of the draw, allele A will be passed on to children more often than allele B, or the other way round.

  If you follow the fate of allele A down the generations, it does a random walk in terms of its frequency in the population, from 60% in one generation, say, to 67% in the next to 58% to 33% and so on. But the walk cannot continue forever, because sooner or later it will hit one of two numbers, either 0% or 100%. If the frequency falls to 0%, allele A is permanently lost from the population. If it hits 100%, it’s allele B that is lost and allele A that becomes the permanent form of the gene, at least until a new and better mutation crops up. This fluctuation in frequency is a random process known as genetic drift, and when the walk ends in allele A hitting 100%, geneticists say that it has become fixed or has gone to fixation, meaning it’s the only game in town.

  An important part of the genome that has gone to fixation is the DNA of the energy-producing mitochondria, former bacteria that were captured and enslaved long ago by the ancestor of all animal and plant cells. The mitochondria, little organelles within every cell, are inherited through the egg and passed down from a mother to her children. At some early stage in modern human evolution, one woman’s mitochondrial DNA went to fixation by edging out all other versions of mitochondrial DNA.

  The same winner-take-all victory was attained by a particular version of the Y chromosome, which men alone carry because it includes the male-determining gene. At a time when the human population was quite small, a single individual’s Y chromosome increased in frequency until it became the only one left. As described below, the genetic legacies of the mitochondrial Eve and the Y-chromosomal Adam have proved immensely useful for tracing the migration of their descendants around the globe.

  This rise and fall of the alleles depends on the blind chance of which are cast aside and which pass into the next generation when the egg and sperm cells are created. Genetic drift can be a powerful force in shaping populations, particularly small ones, in which the drift toward either loss or fixation can happen within a few generations.

  Another force that shapes the genetic heritage of a species is migration. As long as a population stays together and interbreeds, everyone draws fr
om a common gene pool in which each gene exists in many different versions or alleles. An individual, however, can carry at most two alleles of each gene, one from each parent. So if a group of individuals breaks off from the main population, it will carry away only some of the alleles in the general pool, thus losing part of the available genetic endowment.

  Mutation, drift, migration and natural selection are all unceasing forces that drive the engine of evolution ever onward. Even if a population stays in the same place and its phenotype, or physical form, remains the same, its genotype, or hereditary information, will remain in constant flux, running like the Red Queen to stay in the same place.

  A population can stay more constant if it interbreeds, with everyone drawing from the same pool of alleles. As soon as any barrier to interbreeding occurs, such as a river encountered as the species spreads out, the populations on either side of the river will become subtly different from each other because of genetic drift. They will have taken the first step toward becoming subspecies, or races, and will continue to accumulate minor differences. Eventually one of these minor differences, perhaps a shift in the season of mating or in mate preference, will create a reproductive barrier between the two subspecies. As soon as individuals in the two populations cease to mate freely, the two subspecies are ready to split into distinct species.

  The Peopling of the World

  So consider how this mechanism of differentiation, of a species developing into races, would have applied to humans. The change agents of migration, drift and natural selection bore down on the human population with particular force as soon as people started to disperse from the ancestral homeland. Those leaving Africa seem to have comprised a few hundred people, consisting perhaps of a single hunter-gatherer band. They took with them only a fraction of the alleles in the ancestral human population, making them less genetically diverse. They spread across the world by a process of population budding. When a group grew too big for the local resources, it would split, with one band staying put and the other moving a few miles down the coast or upriver, a process that further reduced the diversity at each population split.

  Because the modern humans of 50,000 years ago were a tropical species, the first people to leave Africa probably crossed the southern end of the Red Sea and kept to roughly the same latitude, hugging the coast until they reached Sahul, the Ice Age continent that then included Australia, New Guinea and Tasmania. The earliest known modern human remains outside Africa, about 46,000 years old, come from Lake Mungo in Australia.

  The modern human exodus from Africa occurred at a time when the Pleistocene Ice Age had another 40,000 years to run. To begin with, hunter-gatherer bands were probably stretched out through a strip of mostly tropical climates from northeast Africa to India to Australia. To judge by the behavior of modern hunter-gatherers, these little groups would have been highly territorial and aggressive toward neighbors. To get away from one another and find new territory, bands started moving north into the cold forests and steppes of Europe and East Asia.

  The evolutionary pressures for change on these small isolated groups would have been intense. Those migrating eastward faced new environments. Living by hunting and gathering, they would have had to relearn how to survive in each new habitat. The groups moving northward from the equatorial zone of the first migration would have encountered particularly harsh pressures. The last ice age did not end until 10,000 years ago. The first modern humans who moved northward had to adapt to conditions very different from those of their tropical homeland and develop new technologies, such as making tightly fitting clothes and storing food for the winter months. The climate was far colder, the seasonal differences were more pronounced and the problems of keeping warm and finding sustenance during the winter months were severe.

  If these obstacles were not daunting enough, the people moving northward encountered armed opposition as well. An earlier wave of humans had left Africa some 500,000 years before and now occupied the Eurasian continent. These humans, called archaic to distinguish them from modern people, included the Neanderthals in Europe and Homo erectus in East Asia. Both disappeared about the time that modern humans entered their territories. In the case of the Neanderthals, the archaeological record makes clear that the area of their settlements steadily shrank as that of the modern humans increased, implying that the moderns drove the Neanderthals to extinction. The record from East Asia is not yet detailed enough for the fate of Homo erectus to be understood, but a strong possibility is that the species met the same fate as the Neanderthals.

  After the occupation of Eurasia, the single gene pool that existed among the small group that left Africa was now fragmented into many different pools. The vast terrain across which humans were now spread, from southern Africa to Europe, Siberia and Australia, prevented any substantial flow of genes between them. Each little population started to accumulate its own set of mutations in addition to those inherited from the common ancestral population. And in each population the forces of natural selection and drift worked independently to process these mutations, making some more common and eliminating others.

  If marriage partners had been exchanged freely throughout the human population as it dispersed around the globe, races would never have developed. But the opposite was the case. People as they spread out across the continents at the same time fragmented into small tribal groups. The mixing of genes between these little populations was probably very limited. Even if geography had not been a formidable barrier, the hunter-gatherer groups were territorial and mostly hostile to strangers. Travel was perilous. Warfare was probably incessant, to judge by the behavior of modern hunter-gatherers. Other evidence of warfare lies in the slow growth of the early human population, which was far below the natural birth rate and could imply a regular hemorrhage of deaths in battle.

  Once the available territory had been occupied, people overwhelmingly lived and died in the region where they were born. The fact that people were pretty much locked within their home territories until modern times is one of the surprises that has come out of the genome. Several lines of evidence point to this conclusion. All men carry copies of the same original Y chromosome, which, as mentioned above, became universal early in modern human evolution. But mutations started to accumulate in the Y, and each mutation forms a branch point on the human family tree between the men who have it and the men who don’t. The root of this branching tree lies in Africa, and its limbs extend around the world in a pattern that follows the path of human migrations. There is not a lot of tangling between the branches, showing that the world filled up in an orderly way and that once it had done so, people then stayed put.

  The same story is told by the mitochondrial DNA in tracking the migration of women. More recently, geneticists have been able to survey populations using devices called gene chips that sample the whole genome and provide a much more detailed picture. The gene chips are arrays of short lengths of DNA chosen to recognize half a million sites along the human genome where the sequence often varies. (The variable sites, known as SNPs, or “snips,” tell where people differ; the sites on the genome where everyone has the same DNA unit are uninformative.) Because two pieces of DNA will link up chemically if their sequence of bases is exactly complementary* to each other, each short piece of DNA on a gene chip in effect interrogates the genome being tested, saying, “Do you have an A at this site or not?” Thus an entire genome can be scanned to test its sequence at sites that are known to vary from one population to another.

  Using a 500,000 snip chip, researchers at Stanford University have found a strong correspondence between the genetics and geographical origins of Europeans. In fact, 90% of people can be located to within 700 kilometers (435 miles) of where they were born, and 50% to within 310 kilometers (193 miles). Europeans are fairly homogeneous at the genetic level, so it is quite surprising that enough genetic differences exist among them to infer a person’s origin so precisely.11

  An
other group of researchers looked at Europeans in isolated regions who weren’t likely to move much. One site was a Scottish island, another a Croatian village and the third an Italian valley. Anyone who didn’t have all four grandparents living in the same region was excluded. Under these conditions, the researchers found they could map individuals to within 8 to 30 kilometers (5 to 19 miles) of their village of origin.

  The finding shows that the world’s human population is very finely structured in each geographic region in terms of its genetics, with human genomes changing recognizably every few miles across the globe. Such a situation exists only because, until the past few decades, most people have taken marriage partners from very close to where they were born. Such a high degree of local marriage “was probably the norm in rural Europe due to lack of transport or economic opportunities,” the researchers conclude.12

  Evolutionary Stresses

  Once the human population had spread out across the globe, it was subject to a variety of strong evolutionary stresses in the form of a radical makeover of human social organization and population movements that swept over the original settlement pattern. These population shifts were caused by climate change, the spread of agriculture and warfare.

  A clue to major population movements after the exodus from Africa is provided by human skin color, which evolved to be dark in equatorial latitudes and pale in northern ones. If one could look at the global population of 25,000 years ago, its differentiation might have been much simpler to trace. Agriculture hadn’t yet been invented, and population growth had not yet seriously upset the social structure of small hunter-gatherer groups. Anyone who could have flown around the globe would have seen dark-skinned people inhabiting its equatorial belt, pale-skinned people in its high northern latitudes and a smooth gradation of skin color between them.