Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (8 page)

Furthermore, appealing to the EEA doesn’t help. First of all, let’s look at those creaky old genes. What does it mean to say that our genes are old, but our environment new? Our genes come from our ancestors, who got them from their ancestors, and so on ad infinitum, or at least “ad Precambrian-um.” Some of our genes are identical to those of worms, chickens, and even bacteria, while others arose much more recently. A gene crucial to sperm production, called
BOULE
, is found in virtually all sexually reproducing animals and is 600 million years old, far preceding, of course, the time when humans were on the African savanna, or were even mammals.

Genes change when mutation provides the raw material and then natural selection or other forces, such as individuals moving to a new place, or sheer random chance, act on that material. But they change piecemeal, in fits and starts, and the rest of the genome is dragged along higgledy-piggledy. Organisms don’t get to shed their whole set of genes in one fell swoop, like a pair of ill-fitting trousers, even during major transitions like the shift from water to land—or from ape to human.

New molecular techniques are allowing scientists to pinpoint which genes are evolutionarily conserved—that is, essentially unchanged as different groups split off from each other in history—and which are more recent. While it is true that more recently separated groups, like humans and apes, share more genes than do distant relatives, like humans and carnations, that relationship does not mean that those shared genes arose at any particular point, in our hunter-gatherer past or elsewhere, and now cannot catch up. As anthropologists Beverly Strassmann and Robin Dunbar point out, “From a genetic standpoint, the Stone Age may have no greater significance than any other period of our evolutionary past.”
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Which genes change is also important. Much has been made of the proverbial 98 percent genetic similarity between humans and chimpanzees (the actual percentage changes slightly depending on which expert you consult or what metric is used, with biologist Roy Britten recently suggesting that 95 percent is a more accurate figure
³²
). But the add-’em-all-up approach is not likely to yield any insight into what genetic differences, whether small or large, really mean. Anthropologist Jonathan Marks points out that we share perhaps a third of our genes with daffodils. It all depends what scale of measurement you use. “So from the standpoint of a daffodil, humans and chimpanzees aren’t even 99.4% identical, they’re 100% identical. The only difference between them is that the chimpanzee would probably be the one eating the daffodil.”
³³
Without going so far as to argue for the rights of flowering bulbs, as people have done for chimps and other great apes, Marks notes that it is difficult to know what to make of the similarity free of context. But Loren Cordain says, “DNA evidence shows that genetically, humans have hardly changed at all (to be specific the human genome has changed less than 0.02 percent) in 40,000 years.”
³⁴
This purported lack of genetic progress is used to support Cordain’s prescription of a hunter-gatherer diet, before agriculture came along with its newfangled ideas about growing grain and living in houses.

Setting aside whether we are in fact 2 percent, 1 percent, or 5 percent different from chimpanzees, or whether our genes really are less than 1 percent different from those of our Pleistocene ancestors, what that 5 percent or 0.08 percent contains is crucial. The vaunted statistics are often obtained by counting up the differences in the components of DNA between two populations, or two species. Because these components occur in a pattern of chemicals called bases, we often speak of DNA sequences. But simply comparing sequences tells little about the function of the DNA. Rebecca Cann, a human geneticist and anthropologist at the University of Hawaii, is skeptical about extrapolating from DNA to meaningful difference. She points out that while “it is true that it is difficult to find coding sequence differences between two modern humans, it is not true that therefore the ones that do exist are unimportant. And we won’t be able to tell this just looking at the ‘parts list.’ ”
³⁵

In other words, if all you had was an alphabet, you could easily end up concluding that
Hamlet
and the script for an episode of
The Sopranos
were the same thing, since they use exactly the same letters. Perhaps that idea is a little far-fetched, but I trust the analogy is clear. And when it comes to genes, the “parts list” is woefully inadequate. The big question is not how many genes differ between ape and human, or between today’s human and our ancestors of 50,000 years ago, but
which
genes differ. Changes in the fine biochemical structure of DNA happen over time, simply by chance. Other changes occur because of selection on human characteristics such as language ability. But as eminent evolutionary biologist Sean B. Carroll says, “How can we identify the ‘smoking guns’ of human genetic evolution from neutral ticks of the molecular evolutionary clock?”
³⁶
Using the alphabet analogy, he means that we need tools to help us distinguish Shakespeare from soap opera in a way that shows the difference in content, not just a difference in the number of times the letter “a” or “b” is used.

Carroll and other geneticists are now focusing their attention on regulatory and developmental genes, the ones that direct the rest of the show and determine when in the early growth of an organism its genes are switched on or deactivated. Much of the genome contains noncoding DNA, or genetic material that does not produce proteins. These sections can direct other genes, or simply clutter up the chromosome like jars of rusty bolts in a garage. Their functions, and the rate at which they seem to have changed in comparison with other genes, are a hot area of research in evolutionary biology. What they do not tell us, however, is that our genes are so similar to those of other organisms, or to those of our ancestors, as to render us stuck in the past, or that the number of changes per se is a valuable yardstick. Carroll puts it this way: “The rate of trait evolution tells us nothing about the number of genes involved.”
³⁷
But the converse is also true: knowing how many genes have changed doesn’t tell us about how fast a trait has become altered.

What’s more, whether old or new, human genes are also far from uniform, even after all this time. Although we are more similar to each other than are the members of a group of chimpanzees, human beings are still remarkably genetically diverse. Some genes, such as those involved in lactose tolerance, are far more likely to be found in people whose ancestry comes from some parts of the world than in people originating from other parts. Even within groups, the most casual scrutiny shows genetic variation in traits ranging from ear shape to the ability to taste bitter compounds. Such genetic variation among individuals is the fodder for evolution because it provides a menu of options for natural selection. If the environment changes, one or another of those menu items might be suited for the new conditions. This means that we still have an ample supply of genes that can evolve, and we are not simply dragging around a set of genes that were best suited for the Pleistocene.

What about the other argument supporting the need to use our EEA—that we spent far longer as hunter-gatherers in small bands than as cubicle workers in an urban sea? It does stand to reason that longer periods of time give evolution more scope to work, and by that standard, 10,000 years doesn’t provide as much of an opportunity as 100,000, or a million. But sheer time simply isn’t the only relevant variable. My students often complain that if they had just had more time for an exam, or to write a term paper, they would have done better. More time means more opportunity to work for them too, or so the student frantically clutching a test paper after the bell goes off would have me believe. The sad truth, however, is that some of those students wouldn’t get the correct answer, or write an A essay, if you gave them from now until doomsday, or the next geological epoch. Time matters, and of course if I allowed only fifteen minutes for students to write a five-page paper, I would get shoddy work from everyone. But time isn’t the only thing that matters.

The same goes for evolution and our ability to adapt to a new environment, whether that is agriculture or life on land instead of water. Large changes take a long time. Olives don’t become petunias in a few generations. But how long does it take for them to become bigger olives? We no longer have to satisfy ourselves with generalities like “the time since agriculture is too short.” We can look for the answers. The length of time required for a change in genes to become common in a population is a question we can now at least partly answer with data, as I’ll detail in the chapters that follow. In the meantime, while it is true that, as Tooby and Cosmides point out in the title of a 1990 paper, “The Past Explains the Present,” the present has not stayed still.
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Genes, peaks, and mismatch

As an alternative to the EEA, prominent anthropologist Bill Irons suggested a modification: the Adaptively Relevant Environment.
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The Adaptively Relevant Environment is a set of environmental features, such as the amount of rainfall or the abundance of snakes, that is important to a trait, such as having a fear of reptiles. In an environment brimming with cobras, those who shun snakes are at an advantage. If the environment becomes reptile-free, and people instead run screaming from garden hoses, telephone cables, and other wiggly cylindrical objects, the trait is no longer adaptive.

Irons’ notion does not rest on life in a foraging society, and hence avoids what he calls “Pleistocentrism in which all human psychological adaptations are tightly tied to the conditions of Pleistocene foraging societies.”
⁴⁰
He sketches several human behaviors, including incest avoidance and striving for high status, and then analyzes them using the concept of an Adaptively Relevant Environment, arguing that the former evolved as a mechanism that avoids the deleterious effects of inbreeding when close relatives have children. The evolution of incest avoidance thus required “a social environment in which close kin, siblings, parent, and children are in intimate contact during the critical period of the first two or three years of a child’s life, and in which intimate contact is rare between nonkin or distant kin when one or both parties are in the critical age range of newborn to three years.”
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This somewhat pedantic mouthful boils down to having an aversion to sex with those one is raised with from birth or thereabouts—a situation likely to have been common both in foraging societies and more recent ones. We therefore do not need to invoke a particular way of life as a reason for the behavior.

Irons also notes the difficulty of defining the precise environment in which any adaptation, whether dietary, psychological, or otherwise, occurred, since humans, and the other hominins before us, did so many different things in so many different places during the hundreds of thousands of years before agriculture. He also points out that many environmental changes occurred more recently than the end of the Pleistocene and do not seem to have hinged on the transition to agriculture.

I do not find any particular fault with Irons’ concept, but I am not sure we need a new framework for understanding the evolution of human behavior in addition to the usual principles of evolutionary biology. Traits in organisms, human or not, evolve in a particular environment, and although I agree with the evolutionary psychologists, and Irons, that understanding that environment helps us understand the adaptation, we may not need a brand-new dedicated term for it.

Perhaps it would be just as helpful to invoke a concept that has been in use by biologists in various forms since the 1930s, when the distinguished evolutionary biologist Sewall Wright imagined that populations and their genes could be viewed as if they were in a three-dimensional landscape, with hills and valleys.
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The vertical axis, or height of the peaks, indicates the success or fitness of a group of genes. If a population on a mountaintop changes the composition of its genes, it is likely to move to a less successful point, and hence any small changes probably will be selected against. Conversely, a population in a valley is likely to improve with small changes. The entire fitness landscape may well contain peaks that are even higher than the one that a given population, even one on a mountain, is already on, but those peaks might have valleys between them. Hence, a population on a peak cannot move very easily to a higher one, whereas a population in a valley probably will get better no matter what direction it takes.

From the perspective of the EEA, the point is that we already have a way to think about inertia in evolution. Populations get “stuck,” and it may be difficult for their gene frequencies to change without having their overall level of fitness—the degree to which they are suited to their environment—get worse before it gets better. But that is a different, and more nuanced, claim than the declaration that we arrived at the Pleistocene, or at a way of life with small hunter-gatherer bands, and will be unable to escape until millions of years pass.

This is not to argue that our modern lives are not sometimes, perhaps frequently, mismatched with our ancestral environment, or that we cannot use our past to inform our present. The evolutionary psychologists, among others, have reminded us that not all human behaviors are currently adaptive. It is extremely plausible, for example, that we crave sugar and not fiber because we evolved in an environment where ripe fruit was both nutritious and in short supply. Seeking it out meant gaining calories that in turn made it more likely the seeker would have enough nutrition to survive and reproduce, passing on his or her cravings. Nowadays, in a world full of processed sugar in everything from ketchup to Mars bars, this eagerness to consume sweets backfires, resulting in high rates of diabetes, obesity, and other woes.

Fiber is also good for us, yet we seem to lack that same enthusiasm for filling our diets with bran. Why wouldn’t natural selection have instilled a drive to seek out high-fiber foods similar to the drive it instilled for sweet foods? The answer is simple: fiber was abundant in our ancestral environments, and no one had to do anything special to acquire it. People eating a diet similar to that eaten by hunter-gatherers can consume up to 100 grams of fiber per day, in contrast to the standard American intake of less than 20 grams, just because their food is all unprocessed. No one who craved the prehistoric equivalent of broccoli or bran muffins in the Pleistocene would have been at a particular advantage over those who did not.