One might imagine that some form of birth control could also effectively limit population, but of course that only works if everyone adopts it. Even smallish groups that do not limit their fertility will rapidly displace (in a few centuries at most) those that do, which brings us back to where we started—a trap where population growth and limitations on population growth keep pace, causing us to remain near the Malthusian limit without achieving lasting improvements in standard of living. In the future, under a disciplined world state, the imposition of birth control could conceivably invalidate the principle, as it could lower the rate of population growth and enable a higher standard of living to take root, but birth control has certainly never worked that way in the past.
War (defined broadly to include all kinds of interpersonal violence) might limit population before starvation occurred if it increased strongly as human density increased. If humans had been unable to form large, well-organized societies, war might have saved us from penury: In fact, war probably has been an important limiting factor in many species other than our own and was probably important for early humans. But humans can cooperate, particularly if there is something worth stealing. In apopulation with a storable surplus, state formation eventually limited local violence—and peace led to the poorhouse.
Infectious disease is the most serious rival to famine as a population limiter. Certainly the two can work together and often have, since malnutrition can lead to reduced disease resistance, while infectious agents can reduce work output—and thus food production. Furthermore, an infectious disease made worse by population density, or one that killed even well-fed people, could, in principle, be the key population-limiting factor in a society. In such a situation, humans would generally have plenty of food, but other trade-offs would be present. There would, for example, be weaker selection for metabolic efficiency than in a classic famine-driven Malthusian trap. Depending on how most of the people made a living, women might become self-supporting and have a reduced need for paternal investment. There would be strong selection for resistance to the organism or organisms responsible for that strong disease pressure.
Each of those three horsemen—war, pestilence, and famine— has dominated in different populations and time periods.
Primitive warfare was apparently the dominant mechanism limiting population among most foragers before the development of agriculture in the Neolithic period. Infectious disease must have mattered in hunting-gathering societies, but its impact was mitigated by foragers' low population density. Strong climatic swings, such as major droughts or cold waves, must have sometimes rapidly reduced the land's carrying capacity and caused famine—particularly during the climatically unstable glacial periods. But, judging from the abundant evidence of homicide and cannibalism in the archaeological record, our guess is that local violence had a stronger effect. In this sort ofsystem, people were egalitarian, and it shows in the genes: The fraction of men fathering the next generation seems to have been markedly higher than in agricultural societies. Infectious disease, in particular falciparum malaria, may have been the limiting factor in tropical Africa. From what we know, it seems that until very recently population densities stayed lower in Africa than in Europe, the Middle East, or East Asia. The female-dominated farming system seen in much of Bantu Africa, in which women were largely self-supporting, indicates that producing food was fundamentally easier there than in most of Eurasia. In much of Eurasia, hard work from two parents barely allowed break-even reproduction. Disease may have limited the complexity of African state systems—but of course there were many other factors, ranging from Africa's relative isolation from rest of the Old World to elephants attacking the fields of pioneers.13
In many parts of the Old World, particularly among farmers living under strong states, famine and malnutrition were the main factors limiting population. With internal peace, population rapidly bumped up against carrying capacity. In those societies, people living on the bottom rungs of society were regularly short on food, so much so that they often couldn't raise enough children to take their place. However, elites must have had above-replacement fertility, and their less successful offspring would have replaced the missing farmers. Gregory Clark, in A Farewell to Alms, shows that in medieval England the richest members of society had approximately twice the number of surviving offspring as the poorest.14 The bottom of society did not reproduce itself, with the result that, after a millennium or so, nearly everyone was descended from the wealthy classes. Thereis reason to think that this happened in many places (eastern Asia and much of western Europe, for example), but wealth was not acquired in the same way everywhere, so selection favored different traits in different societies.
UNDER THE YOKE
As Rousseau wrote, "Man was born free, and he is everywhere in chains."15
In the days before agriculture, governments didn't really exist. Most of the hunter-gatherers were egalitarian anarchists: They didn't have chiefs or bosses, and they didn't have much use for anyone who tried to be boss. Bushmen today still laugh at wannabe "big men." Perhaps we could learn from them.
But farmers do have chiefs: It goes with the territory. Grain farmers store food, and so they have something valuable to steal, which wasn't the case among hunter-gatherers. Elites, defined as those who live off the productive work of others, came into existence in farming societies because they could. Interestingly, some peoples seem to have curbed the growth of elites just by growing root crops such as yams that rot quickly unless left in the ground, and thus are hard to steal.16 Another point is that the strongest early states often had natural barriers that made it difficult for "citizens" to escape the tax collectors. Egypt, with a strip of very fertile land embedded in uninhabitable desert, is a prime example.17
Of course, once your neighbors form states, there's pressure on your group to do the same, both for self-defense and for the benefit of those locals who will form the new elite. Today, practically everyone lives under some kind of government.
Once elites became possible, elite reproductive advantage kicked in. This is the most basic kind of class struggle—the struggle for existence—but it has seldom been noticed by historians, or for that matter by the participants. It could take various forms. In some cases, tremendous advantage accrued to a single male lineage—it's good to be the king! Researchers have found a surprisingly common form of Y chromosome in 8 percent of Ireland's male population. That Y chromosome is also fairly common in parts of Scotland that are known to have had close ties with Ireland, and among the Irish diaspora. Worldwide, 2 million to 3 million men carry this chromosome, and it appears to be the marker of direct male descent from Niall of the Nine Hostages, a high king of Ireland around AD 400. For some 1,200 years (until 1609), his descendants held power in northern Ireland.18
The most spectacular example is Genghis Khan, otherwise known as the Scourge of God, the Master of Thrones and Crowns, the Perfect Warrior, and Lord of All Men. About 800 years ago, Genghis and his descendants conquered everything from Peking to Damascus. Genghis knew how to have a good time. Here's his definition of supreme joy: "to cut my enemies to pieces, drive them before me, seize their possessions, witness the tears of those dear to them, and embrace their wives and daughters!"19 It appears that the last part of that list especially appealed to him. He and his sons and his son's sons—the Golden Family—ruled over much of Asia for several hundred years, tending to the harem throughout. In so doing, they made the greatest of all genetic impacts. Today some 16 million men in central Asia are his direct male descendants, as shown by their possession of a distinctive Y chromosome. It just shows that one man can make a difference.
The elite's reproductive advantage was usually less concentrated. For example, we often see cases in which a relatively small band of conquerors takes over a society and becomes the new elite. If that ruling elite has a strong reproductive advantage and doesn't intermarry much with the original population, the average inhabitant may eventually be largely descended f
rom that elite, even without any obvious or deliberate genocide. This may have happened when the Anglo-Saxons conquered England; although they were greatly outnumbered by the existing Celtic speakers, they account for a large fraction of the modern English gene pool. There is evidence of an apartheid-like social structure in early Anglo-Saxon England that would have furthered this trend.20
If it was possible for individuals to move into and out of the elite, which was often the case, traits that increased the probability of entry and continuing membership would have been favored by natural selection. This could happen in any class that had above-replacement fertility, not just in a ruling class. As long as there was significant gene flow, traits favored in that class would tend to increase in the population as a whole, not just in the high-fertility groups.
But if a high-fertility subpopulation was reproductively isolated (or nearly so) for long enough, selective pressures specific to that social niche might cause them to evolve in an unusual direction and become significantly different from the surrounding population. We think this happened among the Ashkenazi Jews, as we discuss at length in Chapter 7. Suffice it to say, for now, that the kind of natural selection that occurred among the Ashkenazim was possible because of the persistence over centuries of strong prohibitions against intermarriage and an oddsocial niche in which certain traits conferred high fertility. It's a very unusual case, since few populations appear to have experienced the long-lasting reproductive isolation and unusual job mix required to get those results. There are all sorts of ways in which that process could have been interrupted; it's being interrupted now, for example, through high rates of intermarriage between Jews and non-Jews and by changes in fertility patterns.
We've said that the top dogs usually had higher-than-average fertility, which is true, but there have been important exceptions. Remember that rulers, then as now, made mistakes, had bad luck, and in fact often had no idea what they were doing. Sometimes ruling elites lost wars and were replaced by outsiders, as in the Norman Conquest. Sometimes they got a little too enthusiastic about slaughtering each other, as in the Wars of the Roses. And often ruling elites just made bad choices—bad in terms of reproductive fitness, that is. The most common mistake must have been living in cities, which have almost always been population sinks, mostly because of infectious disease. By "population sink," we mean that city dwellers couldn't manage to raise enough children to break even: Cities in the past, before modern medicine and civil engineering, could only maintain their population with a continuing flow of immigrants from the surrounding countryside.
Wealth could make up for the risks that cities presented, if the disease risk wasn't too bad—immunity to famine is an automatic perk of the ruling class, and it's worth quite a lot. But if disease risks were severe, even complete immunity from famine might not be enough, and the ruling elite would gradually disappear—it may not have been obvious, but it sometimes happened.
The disease risks of cities must have gotten worse with time as new pathogens adapted to humans and as civilizations separated by geographic barriers made contact and exchanged pathogens (as happened to the Hittites). We know, for example, that falciparum malaria did not always exist in Italy, but arrived and spread gradually up the peninsula during classical times.21 Smallpox was also a latecomer to Italy, and it's possible that the addition of those two mighty diseases turned Rome into a population sink for the empire's elites.
Sometimes evolutionarily bad choices on the part of a ruling class are obvious. In classical times, there was a plant called silphium that grew in a narrow coastal strip of Cyrenaica, modern-day Libya. Its resin was used as a contraceptive and abortifacient. The resin appears to have been very effective, preventing pregnancy with a once-a-month pea-sized dose. Sil- phium eventually became too popular for its own good. Never domesticated, it was overharvested as demand grew. As it became scarcer, the price rose until it was worth its weight in silver, which drove further overharvesting and eventually led to one of the first human-caused extinctions in recorded history. However, during the centuries in which it was routinely used by the Greco-Roman upper classes, it must have noticeably depressed fertility, unless they were throwing money out the window.
Eventually, in some populations, elites turned into governments with a local monopoly of force. You would think that the resulting law and order would have been good for the peasants. They were safer, since they were no longer allowed to raid and be raided by their neighbors. This was a major change, since pre-state warfare often killed a larger fraction of the population than major modern wars do. Peasants still experienced war
Man with the Hoe The J. Paul Getty Museum, Los Angeles
Jean-Francois Millet About 1860-1862 Black chalk and white chalk heightening on buff paper 28.1 x 34.9 cm (1l'/u x 133/4in.)
with external foes, but the percentage killed by violence seems to have decreased. However, since births and deaths still balanced, every decrease in death by violence was counterbalanced by an increase in deaths caused by infectious disease (which hit everybody, including elites) and starvation (peasants only). Government, especially good government, eventually led to decreased standards of living, at least in terms of calories.
WE FOUGHT THE LAW (AND THE LAW WON)
If your ancestors were farmers for a long time, you're descended from people who decided it was better to live on their knees than to die on their feet.
Farming led to elites, and there was no avoiding their power. Foragers could walk away from trouble, but farms were too valuable (too important to the farmers' fitness) to abandon. So farmers had to submit to authority: The old-style, independent-minded personalities that had worked well among egalitarian hunter-gatherers ("A Man's a Man for a'That") were obsolete.22 Even when some group had a chance to refound society on a more egalitarian basis, as in the case of the medieval Iceland republic, elites tended to reappear.23
Aggressive, combative people may also have experienced lowered fitness once ruling elites began to appear. With strong states, the personal payoff for aggression may have become smaller, while law and order made combativeness for self-defense less necessary. Sheer crowding must also have disfavored some personality traits that had worked in the past. Intuitively, it seems that a high level of aggressiveness would be less favored when encounters with strangers were frequent. Fight too often and you're sure to lose. Moreover, although the winner of a deadly struggle between two peasants might conceivably gain something, his owners, the elites who taxed both of those peasants, would not, any more than a farmer benefits when one bull kills another.
Farmers don't benefit from competition between their domesticated animals or plants. In fact, reduced competition between individual members of domesticated species is the secret of some big gains in farm productivity, such as the dwarf strains of wheat and rice that made up the "Green Revolution." Since the elites were in a very real sense raising peasants, just as peasants raised cows, there must have been a tendency for them to cull individuals who were more aggressive than average, which over time would have changed the frequencies of those allelesthat induced such aggressiveness. This would have been particularly likely in strong, long-lived states, because situations in which rebels often won might well have favored aggressive personalities. This meant some people were taming others, but with reasonable amounts of gene flow between classes, populations as a whole should have become tamer.
We know of a gene that may play a part in this story: the 7R (for 7-repeat) allele of the DRD4 (dopamine receptor D4) gene. It is associated with Attention-Deficit/Hyperactivity Disorder (ADHD), a behavioral syndrome best characterized by actions that annoy elementary school teachers: restless-impulsive behavior, inattention, distractibility, and the like.
The polymorphism is found at varying but significant levels in many parts of the world, but is almost totally absent from East Asia. Interestingly, alleles derived from the 7R allele are fairly common in China, even though the 7R alleles themselves are extremely rare
there. It is possible that individuals bearing these alleles were selected against because of cultural patterns in China. The Japanese say that the nail that sticks out is hammered down, but in China it may have been pulled out and thrown away.
Selection for submission to authority sounds unnervingly like domestication. In fact, there are parallels between the process of domestication in animals and the changes that have occurred in humans during the Holocene period. In both humans and domesticated animals, we see a reduction in brain size, broader skulls, changes in hair color or coat color, and smaller teeth. As Dmitri Belyaev's experiment with foxes shows, some of the changes that are characteristic of domesticated animals may be side effects of selection for tameness. As for humans, we know of a number of recent changes in genes involving serotonin metabolism in Europeans that may well influence personality, but we don't know what effect those changes have had—since we don't yet know whether they increase or decrease serotonin levels. Floppy ears are not seen in any human population (as far as we know), but then, changes in the external ear might interfere with recognition of speech sounds. Since speech is of great importance to fitness in humans, it may be that the negative effects of floppy ears have kept them from arising.
Some of these favored changes could be viewed as examples of neoteny—retention of childlike characteristics. Children routinely submit to their parents—at least in comparison to teenagers—and it's possible that natural selection modified mechanisms active in children in ways that resulted in tamer human adults, just as the behaviors of adult dogs often seem relatively juvenile in comparison with adult wolf behavior.
The 10,000 Year Explosion Page 10