Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (30 page)

Waldron’s formula turned out to predict the number of remains that should show signs of cancer with surprising accuracy. Furthermore, Andreas Nerlich and Beatrice Bachmeier from Munich used Waldron’s technique on two more ancient samples (an Egyptian population from 1500–500 BC, and a European population from 1400–1800) and found, just as the formula predicted, that only 5 out of the 905 Egyptian skeletons and 13 out of the 2,547 European remains showed signs of cancer.
³¹
These numbers may sound small, but because only a fraction of the total cancer cases leave evidence in the skeleton, they are exactly in keeping with modern cancer incidence. In other words, cancer is not a curse of modernity, and the reason we may see little evidence of it in ancient remains, or in modern foragers, is simple statistics.

David’s remark about no “natural” causes for cancer also sparked criticism from cancer organizations, since solar radiation, radon from rocks, and numerous viruses are known to cause cancer of various sorts. One of the hottest areas in cancer research today is the search for links between cancer and infectious agents. Although it is true that some aspects of modern life—most notably smoking, which causes about a quarter of all cancer cases in the world—are associated with cancer, as Andy Coghlan in the
New Scientist
put it, “Most of them are down to poor lifestyle choices that people can do something about, not, as implied, because they are drowning in a sea of carcinogens from which there is no escape.”
³²

Death, perfection, and the evolution of disease

If cancer is old, does that mean we inherited a predisposition to it from our ancestors, along with the ability to breathe air, stand upright, or see colors? And if so, how far back does our vulnerability go? Finally, can information about cancer in other kinds of organisms help us understand not only why species vary in the degree to which they get cancer, but—a much larger question—what determines how long different species live?

Caleb Finch, an eminent biologist who has studied the evolution of aging and senescence in animals ranging from fruit flies to people, suggests that the cancer propensity and longevity might be related.
³³
While we have some idea about the life spans of various species, it should come as no surprise that obtaining accurate estimates of cancer rates in other species is at least as hard as getting them from skeletal and other human remains. Animals do get cancer, though the frequency varies (and the notion that sharks do not get cancer is a myth, although the group of fish to which sharks belong does seem to have some unique immune system properties that may help in resisting abnormal cell development). Intriguingly, evidence suggests that chimpanzees, at least, suffer from cancer at a much lower rate than humans, even when they live a relatively long time. Nevertheless, humans, even without modern medical care or hygiene, live far longer than any of our primate relatives.

Finch points out that chimpanzees and the other great apes for which we have information seem to suffer little from the neurodegeneration that accompanies Alzheimer’s disease in humans, again even after taking their shorter life spans into account. In other words, a forty-year-old chimp, the equivalent of a human twice that age, is unlikely to suffer from the same age-related diseases that are common in elderly people. Finch further suggests that selection for our long life spans has come at a price. Our immune systems can keep us going for many decades by fending off viruses, bacteria, and other onslaughts, but they also make us prone to inflammation, heart and neurological disease, and cancer. This double-edged sword arises in part because immune system cells regulate many different cell processes, including growth and inflammation, as they distinguish self from nonself and eliminate the latter. But that fine-tuned ability can malfunction, and when it does the result can lead to the unregulated growth that is cancer, as well as a host of other maladies.

This rather literal cost of living, or at least cost of living a long time, has its roots in one of evolutionary biology’s most basic questions: Why does any organism grow old and die? Why don’t we just keep living, and reproducing, for ever-longer time periods? Remember, natural selection works by favoring those who leave the most copies of their genes, so answers like “we evolved to let others replace ourselves” won’t cut it.

The solution to this problem is another variant of the no-free-lunch idea I discussed earlier in the chapter, and in this broader case it is called “negative pleiotropy.” Pleiotropy means that a gene will often have multiple effects, acting at different times during the life span and on different organ systems. As a hypothetical example, the same gene that speeds growth might also strengthen bones or weaken arteries. Any genes that enhance reproduction early in life but are detrimental at later ages, once reproduction is over, will still be favored by natural selection. The pleiotropy is negative because of this “higher-now, lower-later” correlation. Such early acting genes are favored because they will be passed along and the individuals bearing them will outreproduce their competitors before the negative effects are felt. By the time the gene that made someone more fertile begins to make its bearer suffer from Alzheimer’s (again to give a hypothetical example), the genie is out of the bottle: that person has already left more copies of his or her genes than has someone less prone to brain degeneration but with a smaller reproductive output.

Finch and other biologists believe that selection favored our long life relative to other primates perhaps because it enables us to rear our long-dependent children, but it may also have saddled us with cancer and other age-related ills. Mel Greaves from the Institute of Cancer Research in London refers to our evolutionary heritage as dealing us our position in “the cancer lottery . . . a consequence of the ‘design’ limitations, compromises and trade-offs that characterize evolutionary processes.”
³⁴
In keeping with this perspective, evolutionary biologist Bernie Crespi speculates that any genes aiding survival and growth in childhood will be strongly favored by natural selection, even if they increase diseases later.
³⁵
To those who think that this early survival is not really such a big deal, Crespi reminds us that infections and malnutrition used to kill large numbers of children, and they still can in parts of the developing world. In keeping with his idea, a gene that is associated with higher birth weight, a characteristic that can protect infants from such early threats, also confers susceptibility to a number of autoimmune and other disorders. As Crespi puts it, “The evolution of human disease risk becomes inextricably linked with the evolution of modern humans.”
³⁶

The truth is that diseases have indeed always been with us, modern only in the sense that some of them accompanied our evolution into human beings, not our recent adoption of agriculture or urban living. We all wish we could be healthier, and it is easy to fantasize that before Big Macs, or roads, or houses, we were. But evolution doesn’t work that way, with the accomplishment of perfect health or perfect adaptation after some arbitrary period of time. Instead, diseases perfectly demonstrate that life is an endless series of checks and balances, with no guarantees of a happy ending. “What kind of life is that destined to be sick by our own genes?” The answer seems to be, the only kind of life we have.

10

Are We Still Evolving?

A TALE OF GENES, ALTITUDE, AND EARWAX

I
n 2009, Charles Darwin would have been 200 years old, and his landmark book
On the Origin of Species
reached the 150th anniversary of its publication. To commemorate these events, conferences were organized, panels were convened, and volumes were published. The round numbers of the occasions seemed to generate reflective commentary along the lines of those where-are-they-now pieces about movie stars whose careers peaked decades ago, checking to see whether Darwin’s ideas still hold, much the way we periodically try to determine whether Robert De Niro is still a relevant force in film.

Much of the discussion, at least in the popular media, centered on humans, and the topic most frequently addressed—and the question that biologist Jerry Coyne, author of
Why Evolution Is True
, says he is invariably asked when he speaks to the public—was whether humans are still evolving. Some people have strong feelings about the matter; on science writer Carl Zimmer’s blog
The Loom
, a commenter thought further human evolution was inevitable but rued the fact: “Why cant [
sic
] our genes know that we dont [
sic
] want to change anymore. We’re very content with what they’ve achieved till now . . . Why does it [
sic
] evolution have to spoil this temporary bliss for such a wonderful intelligent species?”
¹
On Cavemanforum.com, at least one reader was less optimistic: “I think people are rapidly changing, but let’s face it. This is not a good thing.”
²
Another pettishly remarked, “So we may go to all the trouble of evolving and still not even be able to utilize grains in their raw state, that is unfortunate.”
³
On a more yearning note, a commenter on the blog
Sports Abode
suggests, “I think we are done evolving but I think the next big change will be wings. We could use some wings.”
⁴

Running through these remarks is the feeling that evolution is something extra that has been done to us humans, either spoiling or improving on a product that would have existed in some form regardless of its actions. This is erroneous, of course; people are a product of evolution, and so are all other organisms. Yet no one seems to wonder whether sloths decry their inability to walk on the ground, or if alligators could someday live vegetarian lives.

Part of the curiosity about continued human evolution probably stems from our continuing need to see ourselves as different from other animals—a form of hominin exceptionalism, as it were. It isn’t necessarily related to one’s stance on the origin of humanity; even those who accept that the human species has evolved may believe we are now past all that, having finished up with evolution some time ago, as if checking it off on a grandiose “to do” list. The world of iPads and organ transplants seems far removed from the “nature, red in tooth and claw”
⁵
that set the stage for Darwin’s ideas.

It turns out that although renewed interest in whether modern-day humans are evolving may have been triggered by the
Origin of Species
bicentennial, the question itself arose almost immediately after the book was published. Lawson Tait, a gynecologist and surgeon living in Birmingham, England, was a contemporary of Charles Darwin and enthusiastic promoter of Darwin’s ideas, having read
On the Origin of Species
as a student soon after its 1859 publication. Just a decade later, Tait published a paper in the
Dublin Quarterly Journal of Medical Science
with the worried title “Has the Law of Natural Selection by Survival of the Fittest Failed in the Case of Man?”

Tait was commenting on another article, by William Greg, that had appeared in
Fraser’s Magazine
with a similarly fretful tone: “On the Failure of ‘Natural Selection’ in the Case of Man.” Both authors feared that the increasing ability of the medical profession to prevent deaths from disease would compromise the culling action of selection, so that, in Greg’s words, “those whom [medical science] saves from dying prematurely, it preserves to propagate dismal and imperfect lives.” Thus, “if we eradicated disease, the chances of life and death being equal, beings of an equal calibre, in every respect, would be produced.”
⁶
Tait went on to wonder whether medical practitioners should simply give up, allow nature to take its course, and permit only the most vigorous members of society to survive. But he consoled himself with the thought that “no sooner has science overcome one epidemic than another, far more formidable, makes its appearance.”
⁷
Selection therefore continues, but with different criteria as each epidemic arises. Tait concluded that those in his profession should persist, but with the recognition that their labors would often be Sisyphean.

I have to confess that I found Tait’s even limited optimism about the ability of late-nineteenth-century physicians to eliminate disease a bit ill founded, given the inadequacy of medicine at the time; most drugs were ineffective and sometimes downright dangerous, and bloodletting was still being performed. Be that as it may, his point about modern technology and its ability to buffer us from the forces of nature that promote selection in other species was echoed by Darwin himself, who cited Tait’s and Greg’s work in his 1871 book
The Descent of Man and Selection in Relation to Sex
. “There is reason to believe,” Darwin mused, “that vaccination has preserved thousands, who from a weak constitution would formerly have succumbed to small-pox. Thus the weak members of civilised societies propagate their kind.”
⁸

Along with Tait, Darwin nevertheless believed that humans could evolve, and the two men went on to engage in an extended communication, following up a mutual interest in insectivorous plants such as sundews. Tait also became one of Darwin’s most ardent fans and supporters. Although the two scientists did eventually meet, the way Tait’s biographer J. A. Shepherd describes his subject’s behavior, it bordered on stalker-like, or what passed for such in the 1870s, with increasingly frequent letters and invitations that Darwin often answered only briefly or declined outright.
⁹
Along with Thomas Huxley, however, Tait helped Darwin’s ideas become established among the intelligentsia of Victorian England, so his role in the early history of evolutionary theory remains significant.

Violating the law of the jungle?

In a more modern take on the idea of evolution stopping, geneticist Steve Jones from University College London famously suggested several times over the last decade that it has indeed come to a halt, at least in the Western world, and that we should “look around—this is it. Things have simply stopped getting better, or worse, for our species.”
¹⁰
The argument is similar to Tait’s: most children are surviving, even if they have traits such as myopia or susceptibility to measles that would have been disadvantageous just a few hundred years ago. Contraception allows people to determine the number of children they have, further interfering with the different ability to perpetuate one’s genes that is inherent in evolution by natural selection. In addition, Jones notes that the genetic mutations that provide fodder for changes in the gene pool are more common in older fathers, because errors in DNA accumulate over time, but younger men are having more children now than they were in the past. Finally, he argues that the large, mobile populations of today mean that everyone’s genes are mixing, in effect homogenizing the people of the world into one large group. This means that small groups of individuals can’t respond as quickly to localized forces of natural selection. Furthermore, like Tait and his contemporaries, Jones feels that our genes are now freed from the tyranny of environmental pressures such as predators and devastating diseases, simply because we have removed them through technology.

Although it is initially intuitively appealing, many evolutionary biologists—I among them—disagree with this conclusion. The most obvious objection is that while most of the scientists engaged in the discussion live in developed countries, many of the world’s people do not. They may not be as much at risk of being eaten by a tiger as their, and our, ancestors were, but children in many places still die of malnutrition and a host of diseases, exposing them to natural selection for the ability to survive such assaults. In addition, people have migrated to new and challenging environments within the very recent past, with concomitant evolution; later in the chapter I discuss one such case: Tibetans adapting to life at high altitude. There is no reason to think that such movements might not result in similar adaptations in the future.

Even in more industrialized societies, however, disease remains a very real threat. The extraordinary medical advances of the past century notwithstanding, epidemics continue to take their toll, with SARS, H1N1 influenza, West Nile virus, and of course HIV as recent examples. As we saw in Chapter 9, the latter already may favor bearers of the
CCR5
-D gene variant to become more common in the future. Tait was correct: because pathogens themselves continually evolve new ways to get around the defenses that we, their hosts, put up, the battle with infectious diseases will never be over. The paleo-lifestyle proponents often focus on diet as a central part of human evolution, but disease has arguably left a much larger footprint on our genes.

Medicine and agriculture are both part of the larger human attribute we call culture, along with our ability to create houses, tools, and clothes. These skills and objects allowed humans to occupy parts of the globe that are hostile to most other forms of life, such as the Arctic, and they have changed the playing field for the game of evolution. Jay Stock of the Leverhulme Centre for Human Evolutionary Studies at the University of Cambridge says, “If our culture effectively removes us from environmental stress, then natural selection will no longer occur.”
¹¹

So, does culture insulate us from the environment? Partly, but by no means completely—and it may even make some of those environmental stresses, like crowd-dependent diseases, more apparent. If technology allows us to live in larger societies, the chance of spreading diseases such as tuberculosis or measles, which require a large pool of susceptible victims, increases. And those diseases are very effective agents of selection. In addition, culture itself shouldn’t be discounted as somehow different from the rest of our surroundings. As Meredith Small points out, “Culture may not seem a ‘natural’ force, but because it is part of our environment it is just as natural as disease, weather or food resources . . . humans haven’t really changed the rules of natural selection.”
¹²
What’s more, culture can itself act as an agent of selection; evolution by natural selection requires organisms to have different numbers of surviving offspring, and few forces are as powerful in influencing our family size as the society in which those people live.

John Hawks, the University of Wisconsin anthropologist mentioned in earlier chapters, took on Jones’s contention that younger fathers lack the mutations that will drive evolution. He notes that it is true that, as Jones argues, older men dominated the reproductive pool in the past, partly because older men were better able to afford to marry and reproduce.
¹³
Older men are more likely to bear alterations in their DNA that can be passed on to their children, and such alterations are the source of new genetic variation. This disparity in age of who reproduces is not so common today. But a much more salient change is the enormous growth in the human population. More people means more opportunity for mutations—a point also elaborated on by Gregory Cochran and Henry Harpending in
The 10,000 Year Explosion
. As Hawks puts it, “There are a smaller proportion of older fathers now than in 1700, but the absolute number of older fathers is much, much greater.”
¹⁴
This larger population means a richer source of those novel gene variants, and hence greater scope for evolution.

Drifting along

To fully answer the question of whether we are still evolving, it is important to distinguish between two concepts that are sometimes—incorrectly—used interchangeably: evolution and natural selection. At its core, evolution simply means a change in the frequency of a particular gene or genes in a population, so that, say, in a group of ten hamsters, a gene that makes its bearers wiggle their whiskers starts out in just three individuals, but after several years it is present in six members of the group. The population has evolved, because the proportion of individuals with the whisker-wiggling gene has gone from 30 percent to 60 percent.

The question is,
why
did whisker-wiggling become more prevalent? Here is where the distinction between natural selection and evolution comes in. Biologists commonly recognize four mechanisms by which evolution can occur: genetic drift, gene flow, mutation, and natural selection. Genetic drift is the alteration of gene frequencies through chance events. Imagine that the initial population of hamsters goes for a walk underneath a cliff. Tragedy strikes when a large boulder falls from above on five of the hamsters, killing them instantly. Simply by chance, none of the five unlucky victims carried the whisker-wiggling gene. Once the remaining hamsters reproduce, the population now has a preponderance of whisker wigglers.

When I use a similar analogy in teaching, my students occasionally answer an exam question asking them to define genetic drift with “when a rock falls on some members of a population,” but hopefully the point is clear here: being able to wiggle one’s whiskers had absolutely nothing to do with the likelihood of being killed. Chance events alone caused the trait to increase in frequency nonetheless. Among humans, genetic drift is thought to have caused some of the changes in face shape over the last few million years, from that heavy-browed look of, well, cavemen, to the more delicate skulls of today. Drift is particularly likely to act in small populations, because random events (like that rock falling) can influence a larger proportion of the group and hence more drastically change the gene frequencies.