Before the Dawn: Recovering the Lost History of Our Ancestors
Page 33
Evolution in the Recent Human Past
At its most basic, the process of evolution is simply a change in gene frequencies between generations; one version of a gene, in other words, becomes more common in a population and other versions less common.336 It may take many generations, however, for the shift in frequencies to become significant or for one version of the gene to supplant the alternative versions altogether. The belief that human evolution is essentially complete rests on the conjecture that it moves too slowly for significant changes to have occurred recently, for instance within the last 50,000 years.
But the human genome, just like those of all other species, can adapt quite quickly to changes in the environment. Without this capacity, humans would long ago have lapsed into extinction. At least four types of recent evolutionary change have already become evident and many more will doubtless come to light. The recent genetic changes already discovered include defenses against disease; increases in fertility; responses to cultural changes that affect the human environment; and changes in cognitive behavior.
Disease and other parasites are among the most serious threats to the welfare of large animals, and few diseases have presented a greater threat to human existence than malaria. Though the malaria parasite is very ancient, malaria is thought to have become a common disease among people only within the last 10,000 years, and perhaps within the last 5,000 years or so when slash-and-burn agriculture was introduced into West Africa. The sun-lit pools in the clearings would have provided an ideal breeding place for the mosquitoes that carry the parasite.
Confronted with a severe and sudden threat like the Plasmodium falci parum form of malaria, natural selection will favor any helpful mutation that crops up. Several blood diseases, such as sickle cell anemia, have arisen because of changes in hemoglobin that protect against malaria. Another natural defense against the parasite is the impairment of an enzyme known as G6PD (for glucose-6-phosphate dehydrogenase) which kicks off the train of reactions leading to the metabolism of glucose. Genetic variations that sharply reduce the efficiency of the enzyme work wonders against the malaria parasite, although they also cause a serious blood disorder.
There are two principal variants of the G6PD gene, which seem to have arisen independently of each other. One is found in African populations, the other in peoples of the Mediterranean. Sarah Tishkoff of the University of Maryland has dated the age of the two G6PD variants. The statistical method she used gives a broad range of possible ages, but all are consistent with the idea that the human genome has evolved its resistance to malaria only recently. The variation of the G6PD gene that is common in Africa arose sometime between 4,000 and 12,000 years ago, according to Tishkoff’s calculations. The Mediterranean variant started to spread between 2,000 and 7,000 years ago.337
An even more recent protection against disease evolved some 1,300 years ago among people of northern Europe. The protection consists of a modification to a protein, known as the CCR5 receptor, that is embedded in the surface of white blood cells. The variant gene, known as CCR5-delta-32 because it has lost 32 units from its DNA, occurs in some 14% of Swedes, diminishes to 5% in Mediterranean populations and is rare to nonexistent in non-European populations.338 It presumably achieved its sudden prominence through the natural selection caused by some serious epidemic that raged through Europe at that time.
The variant gene was discovered because it is also protective against AIDS. But AIDS is far too recent a disease to have driven the frequency of the CCR5-delta-32 variant to such a high level in the European population. That driving force was at first thought to have been the Black Death, which killed some 25 to 40% of Europeans in the years 1346-1352, and a further 15 to 20% in 1665-1666. But it now seems that smallpox, which in the long term killed a larger number of people though in less dramatic fashion, was the probable agent of selection.339 Presumably the loss of 32 units from the gene alters the structure of the surface protein used by both the AIDS virus and the smallpox virus to gain access to a cell.
Diseases, especially those that kill people before the age of reproduction, are potent selective forces. So too are genes that affect fertility. A genetic change that favors fertility has been observed at high frequency in European populations. It occurs rarely in Africans but bears the marks of being under strong selection in Europe, as if promoted by something in the European environment. The change consists of a large segment of chromosome 17, some 900,000 DNA units in length, which has become flipped, or inverted. The inversion carries several genes but it is not clear which of them is responsible for conferring greater fertility.340 The inversion evidently rose to high frequency among Europeans after the exodus from Africa and perhaps in the last 10,000 years.
From a historical point of view, the most interesting class of evolutionary changes are those that have occurred in response to human culture. When people first started to abandon their way of life as hunters and gatherers some 15,000 years ago, they had much less need for two kinds of gene, the olfactory genes that mediate the sense of smell, and the genes that are used by the liver to detoxify the natural poisons with which wild plants defend themselves. When a gene is vital for an organism’s survival, any mutation in the gene will be lethal and the mutated version will be lost from the population. But when mutations crop up in genes that don’t matter anymore, the gene may survive, even though it has lost its function.
This has been the fate of many human olfactory genes. Mammals possess a standard suite of about 1,000 of these genes. The proteins that each makes are embedded in the surface of the cells that line the nose and serve to detect specific odors. Once people settled down and grew their food, they no longer depended on their noses to detect which fruits were ripe or which wild plants were relatively safe to eat. On evolution’s use-it-or-lose-it principle, more than 60% of olfactory genes in people are now inactive. Yoav Gilad and colleagues at the Max Planck Institute for Evolutionary Anthropology in Leipzig find that humans are losing their olfactory receptor genes four times faster than other higher primates. “This process is probably still ongoing in humans,” they conclude.341 Distressing news as this may be to gourmets and oenophiles, the price of civilization is that the faculty of smell is inexorably being degraded.
In parallel with the loss of olfactory genes, people are also losing genes that detoxify natural plant poisons. The enzymes made by these genes are no longer needed for their original purpose but have assumed an unexpected role in modern societies—that of metabolizing medicinal drugs. This unnatural stimulus does not occur often enough, however, and many of the genes are being lost through disuse. (This process explains much of the variability in the response to drugs, including why some people have severe side effects or require different doses. People who have lost the gene that breaks down a certain drug will maintain a high dose of it in their bloodstream, whereas those who still retain the gene will clear the drug rapidly.)
After settlement and agriculture came the rearing of livestock. Lactose tolerance, as discussed earlier, is the genetic response to the availability of animal milk. The genetic change evolved some 6,000 years ago among cattle herders of northern Europe and later among peoples of Africa and the Near East who took up pastoralism.
Of two recent evolutionary cognitive changes, which are also responses to culture, one is the postulated development of genes for enhanced intelligence among the Ashkenazi Jews of medieval Europe. As noted in the previous chapter, the hypothesis holds that because of the intellectually demanding occupations to which Jews were confined for some 900 years, any mutation that released constraints on the growth of the brain was favored, and that these mutations are the ones familiar as causing a variety of genetic diseases among people of Ashkenazic descent.
The other cognitive change is one that can be inferred from the striking recent rise to prominence of versions of two brain genes. As described in chapter 5, the genes first came to light because mutated versions cause microcephaly, a condition in which people are born with an unusually small
head and brain. The new version of the microcephalin gene appeared around 37,000 years ago, rapidly became more common under intense selective pressure, and is now carried by most people in Europe and East Asia.342 The other gene, a new version of ASPM, emerged 6,000 years ago and is now carried by 44% of Caucasians.343 Both genes are thought to be involved in determining the number of neurons formed in the cerebral cortex in the early embryo. The rapid spread of these two alleles indicates that the human brain has been subject to intense evolutionary pressures in the recent past and may still be evolving.
These instances of recent evolutionary change are probably just the first of many that remain to be discovered. Since all occurred after the dispersal from Africa, the alleles that cause them are present to a different extent in different populations. Human diversity therefore cannot be a purely cultural phenomenon, as many social scientists sometimes seem to believe. It has a genetic component too. The component remains to be defined and quantified, but it could prove to be substantial.
Evolution in History
Given these new examples of evolutionary change, it seems clear that human evolution has continued at the very least until the recent past, nor is there any reason to think it will ever cease. An obvious but far-reaching conclusion follows. Evolution and history are not two distinct processes, with one following another like the change between royal dynasties. Rather, evolution and history overlap, with the historical period being overlaid on a still continuing process of evolutionary change.
The implications are clearly of possible interest to historians and social scientists. Historians are concerned with motivation, but seldom consider sexual selection as a driver of national politics. Students of the Mongol empire have proposed many sophisticated reasons for the expansion of the Mongol empire, such as Genghis’s supposed desire to prevent any future group of steppe dwellers rising to power in the way the Mongols had done. The discovery that 8% of Asian men in the lands ruled by Genghis Khan carry the Y chromosome of the Mongol royal house offers a quite different motive, but one of unusual specificity.
Sexual selection, in this case the effort by one male to propagate his genes at the expense of others, has been a powerful force throughout primate history, from chimpanzee societies to those of the Yanomamo. It has operated with little change in more complex societies, especially during times when access to women was one of the accepted rewards of power. Even in many contemporary societies, where at least a pretense of monogamy is expected of rulers, the old instincts have not disappeared. True, procreation played no evident role in the drive to power of dictators like Hitler or Stalin. But Mao Tse-tung, as revealed in the memoir by his personal physician Li Zhisui, lived like an emperor, with villas and swimming pools and a stream of girls procured by the Cultural Work Troupe of the Central Garrison Corps. “He was happiest and most satisfied when he had several young women simultaneously sharing his bed,” writes his unadmiring Boswell.344
Presidents John F. Kennedy and Bill Clinton conducted affairs while serving in the White House. “I don’t know a single head of state who hasn’t yielded to some kind of carnal temptation, small or large. That in itself is reason to govern,” said François Mitterrand, the French president whose funeral in 1996 was attended by both his wife and his mistress.345 Yet the drive for reproductive success is not a motive cited in many histories. Perhaps the possibility that a brute desire to procreate might drive the affairs of state is a concept that historians find too gross to contemplate.
Given that physical characteristics, such as the ability to digest lactose, have evolved in recent history, so too may have many other traits, including changes in social behavior. At least two conditions are necessary for the human genome to be significantly modified: there must be a selective pressure applied steadily for several generations, and those who adapt to the pressure must have more descendants than others. Such conditions may have occurred quite often in the human past, although it is hard at present to identify them.
Even evolutionary changes need not be permanent. The aggressiveness of the Yanomamo could have a lot to do with the marginal nature of the environment in which some of them live. Under conditions in which aggressive men have more children, genes that favor aggression would become more common. If the Yanomamo should suddenly become peaceful traders for many generations, then a new set of genes might be favored. The fierce Vikings of the tenth century became the peaceful Scandinavians of today. A cultural explanation is usually taken for granted, on the assumption that genes cannot change so quickly. But maybe the speed with which natural selection can act in human populations has been underestimated. Biologists are only just beginning to understand the genes that affect social behavior, some 30 of which have so far been detected, mostly in various species of laboratory animal. One of the most interesting findings is of a genetic mechanism for bringing about quick evolutionary change in a gene for behavior.i346
A possible subject of future inquiry is whether longstanding traits of certain societies may have an evolutionary basis, perhaps because over many generations they allowed people with a certain kind of personality to enjoy greater reproductive success than others. Some scholars have remarked on long term cultural differences between societies of East and West. Richard E. Nisbett, a social psychologist at the University of Michigan, believes there are “dramatic differences in the nature of Asian and European thought processes,” principally that Westerners view the behavior of physical objects and organisms as being governed by precise rules, whereas East Asians seek to understand events in terms of the complex web of interrelationships in which they are embedded. The social structures of Europe and China are built to match, in Nisbett’s view, with Asian societies being interdependent and Western societies individualistic.347
Another scholar, the military historian Victor Davis Hanson, attributes the continuous prowess of Western militaries since the era of classical Greece some 2,500 years ago to democratic institutions and willingness of the free yeomanry to accept effective military discipline while retaining their independence and initiative. “Western ideas of freedom, originating from the early Hellenic concept of politics as consensual government and from an open economy . . . were to play a role at nearly every engagement in which Western soldiers fought,” Hanson writes.348
To the extent that such long term cultural traits indeed exist, what might be their origin? Nisbett cites the fact that Chinese civilization was founded on rice farming, which required irrigation and central control; hence ordinary Chinese found themselves living in a world of complex social constraints, whereas the ecology of ancient Greece favored activities like hunting, herding, fishing and trade, which could be pursued without an elaborate social organization.
Did rice farming encourage the conformity for which eastern societies are known and small-scale farming the rugged individualism of the west? Given the propensity of the human genome to adjust to its environment, including the social environment, it is not impossible that many societies have left their imprint in the genetics of their members, and that the character of different societies reflects the personality traits of those who were the most reproductively successful in them. This is perhaps what Darwin had in mind in allowing that people might take some credit for their evolutionary progress.
The extent to which such a process may have happened in history cannot yet be determined. The novel issue of recent human evolutionary change is of particular interest, however, because it bears on the question of which future directions human evolution is likely to take. As Darwin noted, the fact that man has evolved to his present state “may give him hope for a still higher destiny in the distant future.”
Future Directions of Human Evolution
The most improbable feature of science fiction movies is not the faster-than-light travel or the transporter beams but a feature that audiences accept without a second thought: the people. The inhabitants of the far future are always portrayed as looking and behaving exactly like people today.
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All that is certain about future evolution is that people will not remain the same as they are today. They will differ because the environments of the future will be different, and people will either have adapted to them or perished.
Many futures are possible. Perhaps the gracilization of the human form will continue as societies favoring trade and cooperation extend their advantage over those more inclined to aggression. Perhaps our distant descendants will be far more intelligent, having evolved in response to the ever increasing intellectual demands of a more complex society. Perhaps they will be stockier, with shorter arms and legs, having followed the standard biological rules for adapting to cold climates as the Earth plunges back again into the inevitable next ice age. Perhaps the human lineage will resume its speciation, dividing into two or more castes inhabiting different social niches. And if self-sustaining populations are ever established on Mars or Europa, they will certainly follow independent evolutionary paths, adapting in form and social behavior to the ecological rules of their new planet.
Theorists have not yet reached agreement on the future evolution of the human species. Two of the founders of population genetics, Ronald Fisher and Sewall Wright, disagreed strongly on the conditions that favor evolutionary change. Wright believed that evolution worked best when a population was divided into small independent groups with limited gene flow between them; a genetic innovation that emerged in one group could then be allowed to spread to the others. Fisher, on the other hand, thought beneficial innovations were more likely to arise in large populations with a high degree of mixing. “Which of them is right? No one really knows,” says Alan Rogers, a population geneticist at the University of Utah.
As it happens, the conditions favored by Wright apply well to most of recent human history before 10,000 years ago, when the population was divided into small, distant groups spread across the globe. But the world at present, with increasing travel and migrations, seems much closer to Fisher’s ideal conditions for evolutionary change. “You used to marry a lass from your local village, now it’s anyone you can track down on the internet,” says Mark Pagel, an evolutionary biologist at the University of Reading in England.349