Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live (29 page)

Such no-free-lunch findings are in keeping with what we know about adaptation to disease, or for that matter, adaptation to any other threat in nature. It is not a matter of all genes serving their own essential task, so that selection cannot interfere without throwing a monkey wrench into the DNA. Scientists have known for many years that the genome contains much duplication of function, not to mention what is sometimes called “junk DNA,” genetic material that seems to float around in the chromosomes, shockingly unemployed. No modern geneticist would expect to see a system so tightly orchestrated that the slightest deviation would send the whole shebang into disarray.

Indeed, it is precisely because of that multiplicity of function that a new mutation in one area of the genome, rendering resistance to a particular disease, may well have effects on other areas. No one stood over our chromosomes assigning each gene a single task, like a worker on an assembly line. Instead, it is as if the worker making the green widgets also happens to be in charge of cleaning the bathrooms, not to mention staffing the reception desk. Changing the widget color that a person produces from green to purple might vastly increase widget sales, which is terrific for the company’s bottom line, but it might also mean that phone calls go unanswered and toilets overflow.

Similar win-some-lose-some scenarios apply to the genetics of vulnerability to several other modern diseases. Cystic fibrosis, for example, is the most common fatal inherited disorder in populations of European origin. People with the disease have thickened mucus in their respiratory and digestive tracts, leading to a variety of complications. Those with one copy of the cystic fibrosis gene, however, do not show the disease; and recent research suggests that the gene may have persisted in human populations because it also confers some degree of resistance to cholera, another often-fatal bacterial disease, spread via contaminated water.

All of these trade-offs underscore the point that we did not evolve to be in perfect harmony with our environment, whether in the Pleistocene or otherwise. You win some (in the form of increased immunity to AIDS or the ability to withstand dehydration), you lose some (maybe via susceptibility to West Nile virus or in a nasty lung disease). What’s more, ’twas ever thus, and cavemen were not any more likely to escape those balancing acts than we are.

Bred in the bone

One ailment has long been the poster disease for the woeful consequences of adopting agriculture: tuberculosis. Unlike many other infectious diseases, it leaves marks on the skeleton that allows its diagnosis in ancient remains, so it has provided fodder for theories about how health has worsened in recent times. A variety of irregularities, including spinal curvatures and bone destruction followed by new bone formation, are associated with chronic tuberculosis infection, and they can be used to infer the rates of the disease in ancient populations. In mummified bodies, like those from ancient Egypt, the lungs and other organs can also show signs of the disease.

Tuberculosis is caused by a bacterium called
Mycobacterium tuberculosis
, and because a similar type of bacterium,
Mycobacterium bovis
, is found in cattle, it was often assumed that the disease spread from cows to people after the domestication of animals became common. Further support for this idea came from the relative rarity of tuberculosis-infected remains from the period before people began to settle down.

More recent research, however, calls that scenario into question. In addition to simply examining ancient bones, scientists can now detect the bacteria themselves in specimens. They can even determine the precise strain of either
Mycobacterium tuberculosis
or
Mycobacterium bovis
, so that they can tell, for example, whether all members of a family harbor the same strain, suggesting they passed the disease among themselves; or they differ, suggesting that each person acquired his or her case independently. This technology has revealed tuberculosis infections in skeletons that showed no visual signs of disease, and it has suggested that tuberculosis might well have been common far earlier than was previously supposed.

New detailed studies of the differences between
Mycobacterium tuberculosis
and
Mycobacterium bovis
also cast doubt on the idea that the cattle form was the ancestor to the human disease; the DNA in the two types of bacteria is just too dissimilar to easily imagine one form evolving into the other in the time frame that would be required. Furthermore, if the cattle version had passed to early humans, one would expect to see
Mycobacterium bovis
in numerous ancient human populations; instead, however, the cattle form of the disease has been detected in only one small sample of Iron Age pastoralists from southern Siberia. The latest estimates now place the evolution of
Mycobacterium tuberculosis
at least at the time of the early hominins, between 2.6 and 2.8 million years ago, though these dates are still debated by scientists.

Further support for the idea of tuberculosis as an ancient human disease comes from Helen Donoghue of the Centre for Infectious Diseases and International Health at University College London, who suggested that tuberculosis might have coexisted with early humans while they lived in small hunter-gatherer groups.
¹⁷
If infected adults are healthy, the disease may cause relatively little damage, becoming virulent only if the person harboring the bacteria grows old and becomes immunocompromised or has other stressors and diseases at the same time. This idea, that tuberculosis can be a chronic but not bothersome infection unless the immune system is otherwise compromised, is also in keeping with the current concern about outbreaks of tuberculosis in HIV-positive populations, since they, too, have weakened immunity.

The tuberculosis-as-curse-of-modernity idea is not totally dead, however, and the latest twists in the story of this disease showcase the continuous nature of our evolution with, and against, sickness. Donoghue suggests that as humans lived in larger and larger groups, selection on the bacteria would favor strains of tuberculosis that spread quickly and were quite harmful to their hosts.
¹⁸
Just as in the farmed animals—or the city—if a new host lives in the same house, or the same neighborhood, as the current one, it is no longer beneficial to the pathogen to keep its old host alive for as long as possible, since a juicy new victim is literally just around the corner. Hence, a strain of tuberculosis that ruthlessly exploits its hosts will pass on more of its genes in a crowded environment, while a somewhat more restrained form will be more successful in a sparsely populated one, all else being equal. And indeed, the newest strains of
Mycobacterium tuberculosis
progress more quickly to full-blown disease.

Tuberculosis doesn’t get the last laugh, though. A group of scientists from the United Kingdom and Sweden led by Ian Barnes looked for the frequency of a gene variant associated with resistance to tuberculosis in DNA samples from seventeen human populations across the globe that had been living under crowded urban conditions for differing lengths of time.
¹⁹
A population of Anatolian Turks, for example, had been living in settlements since 6000 BC, whereas a Sudanese group had been urbanized only since 1919. Barnes and his colleagues predicted that the longer the population had been living in closely settled conditions, the more common would be the resistance genes. They were right; the more urbanized groups had higher levels of resistance, with a response that evolved in a mere handful of generations.

Why doesn’t everyone, or at least everyone in urban populations, show the same resistance? Once again, there is no disease-free lunch. People with the tuberculosis-resistant genotype appear to be more susceptible to autoimmune diseases, in which the immune system is so good that in effect it turns on itself, causing self-inflicted damage. Nevertheless, in this case—and probably many others—sticking to the genes of our ancestors, or assuming those genes were better adapted to the environment, would have been a bad idea.

Cancer: Old enemy or newfangled foe?

Cancer is a frightening disease, and most of us have either experienced it firsthand or had a close friend or relative diagnosed with the disease. It is the second leading cause of death in the United States, according to the American Cancer Society, which further notes that as of 2011, half of all men and a third of all women in the United States will develop cancer during their lifetimes.
²⁰
At the same time, perhaps because of all the high-tech detection and treatment involved in cancer, it often seems as if the prevalence of the disease is a recent phenomenon, and that back in the day people just didn’t suffer from cancer the way they do now. A 2007 survey, also from the American Cancer Society, found that nearly 70 percent of Americans believed the risk of dying from cancer to be increasing in the United States.
²¹
Along these lines, early anthropologists living with foraging peoples have sometimes remarked on how healthy and apparently cancer-free their subjects’ lives seemed to be.

At least some of the paleo diet and lifestyle enthusiasts place the blame squarely on, you guessed it, the postagriculture environment and its diet of grains and other processed foods. This idea is not new; in 1843, French physician Stanislaw Tanchou gave a presentation to the Paris Medical Society arguing that grain intake was a strong predictor of cancer incidence.
²²
He also predicted that cancer would not occur in hunter-gatherer populations—a notion that, according to the website NewTreatments.org, was born out by “a search among the populations of hunter-gatherers known to missionary doctors and explorers. This search continued until WWII when the last wild humans were ‘civilized’ in the Arctic and Australia. No cases of cancer were ever found within these populations, although after they adopted the diet of civilization, it became common.”
²³

The actual data from Tanchou’s research seem to be unavailable, and it’s hard to know how far to trust a website that includes the statement, “The author and publisher can’t be held responsible for anything.”
²⁴
Nevertheless, the question raised is legitimate: Is cancer one of the manifestations of our deviation from the environment for which we are best suited? Or has it always been with us? Scientists have been divided on the subject, with some medical researchers noting the types of cancers attributable to modern environmental influences such as pollutants, and others claiming that improved diagnosis makes it seem as if the disease has become more common.

Interest in the idea of cancer as a curse of civilization, or at least of modern urban living, was renewed in 2010, when Egyptologists Rosalie David and Michael Zimmerman published a review of literature on cancer in prehistoric peoples, including surveys of mummified remains, as well as ancient writings.
²⁵
They detected remarkably few instances of cancer, leading them to conclude that “cancer was rare in antiquity.” In their paper, David and Zimmerman did not implicate diet in the apparent rise in cancer rates in modern times, but speculated that it “might be related to the prevalence of carcinogens in modern societies.” David was further quoted as saying, “There is nothing in the natural environment that can cause cancer. So it has to be a man-made disease, down to pollution and changes to our diet and lifestyle.”
²⁶

This finding attracted a great deal of attention, prompting headlines like “Mummies Don’t Lie: Cancer Is Modern and Man Made”
²⁷
and “Cancer Caused by Modern Man.”
²⁸
It seemed to confirm our worst fears that we had gone down the wrong path by taking up agriculture and settling down to live in cities. The paleo-diet aficionados took comfort in the anticancer nature of their food, while others fretted over the rise in cancer among not only people but their pets. (Meanwhile, the National Cancer Institute notes a slight decrease in new cancer diagnoses, as well as a drop in cancer-caused deaths, over the period between 2003 and 2007, a time when one would not imagine that the pressures or toxins of the civilized world would be decreasing.)
²⁹

Other scientists, as well as many cancer-related organizations, were not so eager to accept David and Zimmerman’s results, or their conclusions that cancer is caused by modern lifestyles. The first problem lies in whether the discovery of few bone-related cancers in the samples reviewed by David and Zimmerman really means that cancer was rare. Many skeletons are incomplete, and of course cancers do not always spread to bone when they originate in soft tissue; the person could die before such metastasis occurs. What’s more, most cancers appear in older people, so a sample needs to include enough of the over-fifty crowd to have a hope of detecting many cancers at all. The sample of mummies and skeletons that David and Zimmerman studied had a preponderance of remains from much younger individuals, so not that many cancers should have been expected in the first place.

In a fascinating paper published in 1996, Tony Waldron at University College London asked how much cancer should hypothetically be expected in any given sample of human remains.
³⁰
He then applied his reasoning to a robust ancient data set to see whether his predictions were correct. To generate the predictions, Waldron used the medical literature to create a simple formula for cancer incidence based on the proportion of deaths due to cancer of a particular organ or tissue and the proportion of tumors at that site that metastasize to the bone. This calculation gave him an idea of how much evidence of cancer in bone would be expected out of the total number of cancer cases. He then examined the deaths from cancer in a set of records from 1901 to 1905 and compared these to evidence in burials from the crypt of Christ Church, Spitalfields, used between 1729 and 1857. The period between 1901 and 1905 was selected because records were reliable but tobacco use was not yet prevalent enough to have inflated the cancer rate with associated lung cancer.