The 10,000 Year Explosion Read online

  Changes in domesticated plants can be just as impressive. Corn, or maize, which is derived from a wild grass named teosinte, has changed wildly in only 7,000 years. It's hard to believe that maize and teosinte are closely related.

  Such dramatic responses to selection aren't isolated cases— they've occurred in many domesticated species and continue to occur today. Evolutionary genetics predicts that substantial change in almost any trait is possible in a few tens of generations, and those predictions are confirmed every day. Selection is used routinely in many kinds of agriculture, and it works: It grows more corn, lots more. You can't argue with corn.

  Teosinte and corn

  That doesn't keep some people from trying, though. One argument is that domesticated animals and plants are examples of artificial selection and so not relevant. But the process in which some gene variants are favored and gradually increase in frequency is the essence of evolutionary change for both natural and artificial selection. There is no fundamental distinction in the process, just a difference in scale. Furthermore, we have on record examples of entirely natural adaptive change over a few thousand years—the time since the end of the Ice Age.

  AFTER THE ICE

  The Ice Age ended (or paused, at any rate) some 11,500 years ago. That caused dramatic environmental changes in many parts of the world, especially in the Northern Hemisphere. The American Southwest turned warmer and drier, becoming the desert it is today, and as it did, the creosote bush appeared there.

  Originally from Argentina, its seeds were probably transported north by migratory birds. It thrived in the desert, thanks to its resin-coated leaves; dense lateral roots, which starve out competing plants; and taproot, which can grow up to 15 feet deep into the earth. A number of insects now live in and on creosote bushes: Some have become so specialized that they can eat nothing else. The creosote bush walking-stick looks just like the creosote stems, while a grasshopper, which even has silvery patches that match the shine of the plant's resin, mimics the leaves. All of these creosote-specialized insects in the Southwest have North American ancestors, not South American—and so all their specializations have come into existence in the past 10,000 years.6

  The end of the Ice Age also brought about a global rise in sea level. Mile-thick continental ice sheets melted, and the sea level rose hundreds of feet. As the waters rose, some mountains became islands, isolating small groups of various species. These islands were too small to sustain populations of large predators, and in their absence the payoff for being huge disappeared. Instead, small elephants had an advantage over large ones, probably because they required less food and reproduced more rapidly. Over a mere 5,000 years, elephants shrank dramatically, from an original height of 12 feet to as little as 3 feet. It is worth noting that elephant generations are roughly twenty years long, similar to those of humans.

  But simply getting smaller is hardly a dramatic example of evolution. Indeed, John Tooby and Leda Cosmides (two of the founders of modern evolutionary psychology) have said that "given the long human generation time, and the fact that agriculture represents less than 1 percent of the evolutionary historyof the genus Homo, it is unlikely that we have evolved any complex adaptations to an agricultural (or industrial) way of life."7 A complex adaptation is a characteristic contributing to reproductive fitness that involves the coordinated actions of many genes. This means that humans could not have evolved wings, a third eye, or any new and truly complicated adaptive behavior in that time frame. Tooby and Cosmides have argued elsewhere that, therefore, deep mental differences between human populations cannot exist.8

  We think that this argument concerning the evolution of new complex adaptations is correct, but it underestimates the importance of simple adaptations, those that involve changes in one or a few genes. The conclusion that all humans are effectively the same is unwarranted. We will see not only that we have been evolving at a rapid rate, but that evolution has taken a different course in different populations. Over time, we have become more and more unlike one another as differences among populations have accumulated.

  DOGS

  Let's look at dogs again, as they are well understood examples of rapid evolution. They've been domesticated for roughly as long as humans have farmed, and in that time they have changed a great deal. You can see that dog behaviors are derived from the behavioral adaptations of wolves, their ancestral species. There are breeds like the Irish setter that point, and breeds like the Border collie that live to herd other animals. Both breeds show elaborations of behaviors we see in wolves. When wolves first scent a prey, the leading pack members freezeand point rigidly in the direction of the scent. Border-collie herding instinct must also derive from wolf game-herding patterns, but it is greatly exaggerated.

  Dogs are much more playful than wolves, and this can probably be understood as retention of juvenile behavior (called "neoteny"). Retaining existing juvenile behavior is accomplished far more easily than evolving a behavior from scratch. Many of the ways in which dogs interact with humans can be understood as a new application of behavioral adaptations designed for a pack—the owner takes on the role of the leader of the pack.

  There is no complex behavioral adaptation in dogs without a recognizable precursor in wolves, but that hardly means that all breeds of dogs are the same, or close to it. The testimony of accident statistics is stark: Biting—universal in dogs—is disproportionately distributed among breeds. A survey of U.S. dog attacks from 1982 to 2006 found 1 record of bodily harm attributable to Border collies, but 1,110 records attributable to pit bull terriers.9

  While there has probably not been enough time for dogs to develop wholly new complex adaptations, there has certainly been enough time to lose some, sometimes in all breeds, but other times only in a subset of dog breeds. Wolf bitches dig birthing dens; a few breeds of dogs still do, but most do not. Wolves go into season in a predictable way, at a fixed time of the year; a few dog breeds do, but most do not. Wolves regurgitate food for weaned cubs, but dogs no longer do so. Male wolves help care for their offspring, but male dogs do not. Any adaptation, whether physical or behavioral, that loses its utility in a new environment can be lost rapidly, especially if it has any noticeable cost. Fish in lightless caves lose their sight over a fewthousand years at most—much more rapidly than it took for eyes to evolve in the first place.

  In some sense these are evolutionarily shallow changes, mostly involving loss of function or exaggerations and redirections of function. Although changes of this sort will not produce gills or sonar, they can accomplish amazing things. Dogs are all one species, but as we have noted, they vary more in morphology than any other mammal and have developed many odd abilities, including learning abilities: Dog breeds vary greatly in learning speed and capacity. The number of repetitions required to learn a new command can vary by factors of ten or more from one breed to another. The typical Border collie can learn a new command after 5 repetitions and respond correctly 95 percent of the time, whereas a basset hound takes 80-100 repetitions to achieve a 25 percent accuracy rate.

  DIALS AND KNOBS

  In the same way, we expect that most of the recent changes in humans are evolutionarily shallow, one mutation deep for the most part. Old adaptations could have been lost in some groups but retained in others. We know of at least one example, which we'll discuss in Chapter 4: Some light-skinned populations, in particular northern Europeans, have lost most of their ability to produce melanin.

  Many such changes can be thought of as turning switches or twirling knobs: Biological processes that were once tightly regulated can be turned on all the time, as with lactose tolerance; turned off entirely, as with the caspase 12 gene, which increases the risk of sepsis when intact and which is inactivated in mostpopulations;10 or turned off selectively, as with the Duffy mutation, a malaria defense that keeps a certain receptor molecule from being expressed on red cells while continuing to be expressed everywhere else. Some other changes are more like turning up the volume (someti
mes all the way to eleven), as in some groups that have extra copies of the gene producing amylase, an enzyme present in saliva that aids in digesting starch.11

  In addition, some behaviors may be the manifestations of genetically influenced alternative behavioral strategies, such as those in a hawk-dove game, as we discuss in Chapter 3: Recent natural selection might have eliminated a particular strategy in some populations or might have materially changed the frequency of existing strategies. Such strategies probably exist in many social animals—wolves, for example—and it seems plausible that dogs exhibit a subset of wolf behavioral strategies, the ones that worked well under domestication. If some wolves are genetically inclined to try to become pack leaders, others are probably natural followers, and dogs likely have higher frequencies of such "sidekick" characteristics. We expect that differences between human ethnic groups are qualitatively similar to those between dog breeds—that the differences are evolutionarily shallow, mostly involving loss of function, exaggerations of already-existing adaptations, neoteny, and so on. Although such changes cannot generate truly complex adaptations, changes in all those hundreds or thousands of genetic switches and knobs can still cause the sorts of evolutionary changes we see in dogs and other domesticated species; and these differences—such as those between Great Danes and Chihuahuas, or between teosinte and modern maize—are not so small. In other words, very significant evolutionary changes in response to agriculture were still possible.

  Not only are there strong reasons to believe that significant human evolution over the past 50,000 years is theoretically possible, and in fact likely, but it's completely obvious that it has taken place, since people look different. This is especially true of populations separated by great distances and geographical barriers. These differences are often so great that there is high contrast in appearance between populations: No Finn could be mistaken for a Zulu, no Zulu for a Finn. Differences in appearance have a genetic explanation, so we know that there has been substantial genetic change since modern humans spread out of Africa— change that has not taken the same course in every population.

  It has been said that the differences between human populations are superficial, consisting of surface characteristics such as skin color and hair color rather than changes in liver function or brain development. In a letter to Vince Sarich and Frank Miele in eSkeptic, the e-mail newsletter of the Skeptics Society, Chuck Lemme said that "our insides do not vary like our out- sides" and that differences in appearance are only skin-deep.12 Lemme thinks that these superficial differences are probably driven by sexual selection, which would make them rather like fads.13 Of course, since experts can easily determine race from skeletal features, it appears that those skin-deep differences go all the way to the bone. In fact, recent work has shown that there are population differences in genes affecting brain development, which we'll mention in Chapter 4.

  It was natural for previous generations of physical anthropologists to concentrate on differences in easily observed characteristics, but that never implied that all differences would be easily observable. It was the scientists that were superficial, not the differences.

  Some argue that differences between human populations are small and not very significant. As was eloquently pointed out by Richard Lewontin in 1972, most genetic differences are found within human populations rather than between different groups. Approximately 85 percent of human genetic variation is within-group rather than between groups, while 15 percent is between-group. Lewontin and others have argued that this means that the genetic differences between human populations must be smaller than differences within human population groups.14 But genetic variation is distributed in a similar way in dogs: 70 percent of genetic variation is within-breed, while 30 percent is between-breed. Using the same reasoning that Lewontin applied in his argument about human populations, one would have to conclude that differences between individual Great Danes must be greater than the average difference between Great Danes and Chihuahuas. But this is a conclusion that we are unable to swallow.

  It turns out that although the distribution of genetic variation is as Lewontin said, his interpretation was incorrect. Information about the distribution of genetic variation tells you essentially nothing about the size or significance of trait differences. The actual differences we observe in height, weight, strength, speed, skin color, and so on are real: It is not possible to argue them away. Genetic statistics do not tell you what sort of differences in size, strength, life span, or disposition you can expect to see between populations.

  It turns out that the correlations between these genetic differences matter. If between-group genetic differences tend to push in a particular direction—tend to favor a certain trend— they can add up and have large effects. For example, there areundoubtedly a number of genes that affect growth in dogs, in the sense that some variants of those genes enhance growth and others inhibit it. Even if we find pro- and anti-growth gene variants in both Great Danes and Chihuahuas, the trend must be different. Growth-favoring variants must be more common in Great Danes. Even though a particular Great Dane may have a low-growth version of a particular gene, while a particular Chihuahua has the high-growth version, the sum of the effects of many genes will almost certainly favor greater growth in the Great Dane. We feel safe in saying this, since as far as we know, no adult Chihuahua has ever been as big as any adult Great Dane. In just the same way, on a given day it may rain more in Albuquerque, New Mexico, than in Hilo, Hawaii—but over the course of a year, Hilo is almost certain to be wetter. This has been the case for every year for which we have records.

  More to the point, consider malaria resistance in northern Europeans and central Africans. Someone from Nigeria may have the sickle-cell mutation (a known defense against falci- parum malaria), while hardly anyone from northern Europe does, but even the majority of Nigerians who don't carry sickle cell are far more resistant to malaria than any Swede. They have malaria-defense versions of many genes. That is the typical pattern you get from natural selection—correlated changes in a population, change in the same general direction, all a response to the same selection pressure.

  For that matter, changes in a single gene can occasionally have a large effect: We know that terrifying genetic diseases can be caused by a change to a single gene, and we know that some of the key changes that occur in domestication are caused by mutations in a single gene.

  For example, wild almonds contain amygdalin, a bitter chemical that turns into cyanide when the almonds are eaten. Eating a few wild almonds can be lethal. But in domesticated almond trees, mutations in a single gene block the synthesis of amygdalin, making the almonds edible.15

  Such dramatic consequences of small genetic changes are possible because DNA is a bit like a recipe or a computer program: A change in a single letter can sometimes have a huge effect. In a striking example, the most common kind of dwarfism is caused by a change in a single nucleotide, rather like the meaning of an entire book changing because of one typographical error. In principle, differences in a single gene could cause significant trait differences between human populations.

  The effect of genetic differences on body and mind must depend on the importance of the effects of the genes that vary between populations compared to those that vary within populations. Variants that have large effects will matter more than those that have small effects, right? Lewontin's argument assumed that the average impact of variants in those two classes was the same, which is incorrect. Since all humans have a fairly recent common ancestry («100,000 years), while humans outside of Africa have an even more recent common ancestry («50,000 years), observable differences between populations must have evolved rapidly, which can only have happened if the alleles (gene variants) underlying those differences had strong selective advantages. The alleles that are regional, those underlying the differences between populations, must also have had important effects on fitness. That's what population genetics implies, and genomic information now confirms it.
Most or all of the alleles that are responsible for obvious differences inappearance between populations—such as the gene variants causing light skin color or blue eyes—have undergone strong selection. In these cases, a "big effect" on fitness means anything from a 2 or 3 percent increase on up. Judging from the rate at which new alleles have increased in frequency, this must be the case for genes that determine skin color (SLC24A5), eye color (HERC2), lactose tolerance (LCT), and dry earwax (ABCC11), of all things.16

  In many cases common ancestry is even more recent—for example, Amerindians and northern Asians appear to have diverged only 15,000 years ago or thereabouts. In these populations, selection has had even less time to operate, and observed differences must have had even bigger impacts on fitness.

  Thus, we believe that the obvious differences between racial groups are linked to gene variants that have recently increased in frequency and had major fitness effects. Blue eyes, found only in Europeans and their near neighbors, are the result of a new version of the DNA that controls the expression of OCA2 that has undergone strong selection, at least in Europe. Dry earwax is common in China and Korea, rare in Europe, unknown in Africa: The gene variant underlying dry earwax is the product of strong recent selection. We can confidently predict that many (perhaps most) as yet unexplained racial differences are also the product of recent selection. For example, we argue that the epi- canthic eyelid fold found in the populations of northern Asia is most likely the product of strong and recent selection.

  All this means that just as humans 40,000 years ago were significantly different from their ancestors 100,000 years ago (much more inventive, in particular), humans today are different in many ways from our ancestors of 40,000 BC, and, consideringthe accelerated rate of change, different from our ancestors of early historical times as well. We can empathize with the heroes of the Iliad (well, Odysseus at any rate)—but we're not the same.