Zoobiquity Read online

  I understand that these narratives allow patients to feel a modicum of control in the face of a terrifying diagnosis. Because that in itself can be healing, I usually just listen quietly as I measure their blood pressure, check their pulses, and place my stethoscope over their heart. But some seem to be seeking medical absolution, so I gently remind them of something they’ve surely heard before: cancer has many causes. Within the DNA we inherited from our parents, from our great-great-great-grandparents, and from ancient animal ancestors lie the blueprints and machinery that instruct cells to create and maintain our body parts. But when this machinery contains errors and then malfunctions, the out-of-control growth we call cancer can develop.

  Here’s what I mean. Living, growing organisms must constantly replace old and dying cells with fresh, new ones. Making a new cell requires copying every single one of the almost three billion building blocks (called nucleotides) in the cell’s DNA. This provides the daughter cell with the exact same information as its parent. When all goes well (and, astonishingly, it usually does), the DNA is copied exactly. But occasionally, about once every ten thousand nucleotides, a mistake is made. Chemical codes can be left out, duplicated, or put in the wrong place.

  Much of the time, these slipups—called mutations—are caught by the cell’s chemical “proofreaders” and fixed before they wind up in a new cell. Often, a “typo” sneaks through but it’s not significant and the cell can continue along normally, even with the misprint. Sometimes these mistakes occur in critical regions of the DNA and actually enhance cell function. These minor changes, over time, can produce new traits, new behaviors, and even new species. For example, alterations or mutations are responsible for size differences in dog breeds. Slight variations in the genes that direct skeletal growth create the most obvious difference between a Chihuahua and a Great Dane.

  However, some mutations harm the cell’s function. For example, normal cells carry “suicide codes” in their DNA. When a cell gets old or is damaged beyond repair, these codes spring into action and cause the cell to self-destruct in a process called apoptosis. But cells can develop mutations in the very genes that direct the destruction. When the destruction instructions go awry or malfunction, the damaged cells will stay alive. They then can replicate—mistakes and all. When that happens, the new defective cell, like its parent, lacks normal cell death instructions. Now there are two cells, each with DNA mistakes and missing the appropriate controls. When these faulty cells replicate, they become four, then eight, then sixteen. Soon an entire population of immortal cells has grown without restraint. This is cancer: initially normal cells, grown out of control, now with different DNA instructions.

  When the out-of-control, mutation-containing cells cluster together, they form a tumor. Sometimes the mutated cells find their way into the bloodstream or lymph system, which are essentially superhighways with mass access to the rest of the body. When the cells travel far from their origins and then replicate in the new location, that’s metastasis. Some cancers, like melanoma, metastasize readily. Others, like chordomas found on the base of the skull, are less ambitious and grow primarily in one region. (By the way, this is the most basic difference between the cancers we call “benign” and those we say are “malignant.” Any mass of abnormal cells is a tumor, but benign growths tend to remain in the same location and refrain from invading nearby tissues.)

  But whether a cancer is sluggish or fleet, a homebody or an adventurer, tumor-forming or what we call “liquid,” what underlies its enormous burden of suffering and death is nothing but errors in the genetic code. Many behaviors and factors in the environment promote these errors and lead to cancer. Smoking, sun exposure, excess alcohol consumption, and obesity have all been linked to DNA damage and to various cancers.

  There’s also a catalog of known, toxic substances that, given adequate exposure, can almost certainly trigger cancer: naturally occurring radon (and other radioactive substances), asbestos, chromium-6, formaldehyde, benzene, and others. The National Institutes of Health (NIH) flags fifty-four documented carcinogens implicated in human cancers. More research will surely add to this list.

  With so many toxins in our environment and so many cancer diagnoses in our communities, it’s easy to point to our polluted surroundings and connect the cancer-causing dots to our neighbors’ suffering. Cancer, many people believe, is unnatural—a disease of our own making. In fact, cancer prevention has become a marketing tool. The simple act of choosing milk or deodorant or tuna fish can feel like a high-stakes exercise in cancer avoidance. Sorting out what’s Madison Avenue from what’s medically accurate has become a challenge for patients and a responsibility for their doctors.

  But cancer can also develop in people who didn’t smoke, drink, or tan and who avoided microwaving food in plastic and cooking on Teflon. It strikes yoga practitioners, breast-feeders, and organic gardeners; infants, five-year-olds, fifteen-year-olds, fifty-five-year-olds, and eighty-five-year-olds. And, pointedly, it’s not uncommon to see elderly patients who have done everything “wrong” … but show no trace of the disease.

  The impulse to blame ourselves or our cultures for our diseases is not unique to modern society or to cancer. As the medical historian Charles Rosenberg has pointed out, “The desire to explain sickness and death in terms of volition—of acts done or left undone—is ancient and powerful.”

  What insights can a species-spanning approach bring? Even the briefest survey of cancer in other animals sheds light on a critical but overlooked truth: where cells divide, where DNA replicates, and where growth occurs, there will be cancer. Cancer is as natural a part of the animal kingdom as birth, reproduction, and death. And, as we’ll see, it’s as old as the dinosaurs. Literally.

  Tessa was just one of the million or so dogs who get diagnosed with cancer each year. Intriguingly, many canine cancers behave very similarly to human cancers. Lethal prostate cancer runs a similar clinical course in men and male dogs. Breast cancer may seek bone tissue in female dogs, just as it can preferentially metastasize to the skeleton in women. Osteosarcoma, which tends to hit human teenagers during their growth spurts, strikes with similar ferocity in many large and giant-breed dogs.

  Sadly, many outcomes are similar, too. As in people, many cancers in dogs become resistant to therapy. And in both species they can recur, even after a patient has been given the all clear.

  Dogs aren’t the only animals in our lives who get cancer. When a cat presents with fever and jaundice, the vet must consider leukemia or lymphoma, leading feline killers in the United States. And when a cat’s owner discovers a lump in her pet’s breast, it may turn out to be a highly aggressive form of breast cancer also diagnosed in many women. For some cats with breast cancer, lumpectomy may suffice. For others, radical mastectomy of the entire chain of all eight mammary glands must be performed.

  Rabbit hysterectomies are commonly recommended due to the high risk of uterine cancer as these pets age. Parakeets are prone to developing tumors on their kidneys, ovaries, or testes. And cancer patients can also be reptiles. Zoo veterinarians have reported on leukemia in pythons and boa constrictors, lymphoma in death adders and hognose snakes, and mesothelioma in rattlesnakes.

  Pediatricians of fair-skinned children aren’t the only doctors who worry about skin cancer in their patients. Equine sunburn is thought to cause skin cancer in light-colored horses, although this “gray horse melanoma” may connect more to a genetic issue in the breed than to too many hours spent basking in the sun. Still, because as many as 80 percent of gray horses will get skin cancer of some kind, their concerned owners, along with those of horses with white “socks” on their legs or blazes on their noses, sometimes apply zinc oxide sunscreen to exposed skin areas. Others, like the parents of towheaded toddlers, insist that their horses wear a hood when out of the stable.

  If your dermatologist reminds you to remove your nail polish before coming in for your yearly mole scan, that’s because she wants to check not only for melanoma
but also for squamous cell carcinoma, a common form of skin cancer and the same kind Tessa had. Tessa’s was in her mouth, but it can also start under a toenail. That’s similar to what happened to a zoo rhinoceros I once examined. Her cancer grew under her horn—which is made from keratin, exactly the same protein that makes up our finger- and toenails. Cattle also develop squamous cell carcinomas in the pale skin encircling their eyes. Some Herefords have been intentionally bred for darker pigmentation around the eyes, which gives them a little more protection from the sun and seems to reduce the incidence of cancer.

  Strike-branding livestock with sheet-metal strips heated to 300° to 600°F can cause tumors to grow around these permanent markings. Like branded cattle, humans who modify their bodies with branding are at increased risk of cancer at the sites of these injuries. Even tattooing may be associated with a rare form of skin cancer.

  Cancer strikes across ecosystems and throughout the animal kingdom. Osteosarcoma, the cancer that forced Ted Kennedy’s son, Ted Junior, to undergo an amputation in the early 1970s, attacks the bones of wolves, grizzly bears, camels, and polar bears. Paul Allen, the cofounder of Microsoft, successfully battled Hodgkin’s lymphoma. Sadly, a killer whale from Iceland succumbed to this cancer of the immune system after months of fever, vomiting, and weight loss. And the neuroendocrine cancer that claimed the life of Apple cofounder Steve Jobs, while rare in humans, is a fairly common tumor of the domestic ferret and has been diagnosed in German shepherds, Cocker spaniels, Irish setters, and other dog breeds.

  Wild sea turtles around the world are dying in large numbers from cancerous tumors possibly triggered by a herpes virus. Genital cancers have become rampant in marine mammals, from North American sea lions to South American dolphins to open-ocean sperm whales. Many of these cancers are brought on by rampaging strains of the papilloma virus, which in humans can cause cervical cancers and genital warts.

  So severely is the disease assailing some animal groups that three wild species are facing extinction because of cancer. Tasmanian devils, found only on their namesake island off mainland Australia, are in the midst of an epidemic of devil facial tumor disease, a cancer that spreads when they fight. Deaths from cancer are hindering conservation of endangered Attwater’s prairie chickens, which used to thrive across Texas, and Western barred bandicoots, an Australian marsupial.

  Cancer can grow in insects, including fruit flies and cockroaches. The disease can even be destructive in the plant world, although plant tumors, sometimes called “galls,” cannot metastasize and so, for plants, cancer is a chronic condition, not a leading killer. Although cancer rarely kills the plant, it does decrease its vigor.

  One thing is clear: cancer is not unique to humans. And neither is it a product of our modern times. More than 3,500 years ago, before soup cans were lined with bisphenol A–laced plastic, before hormones were pumped into meat, and before methylparabens were added to shampoos, Egyptian physicians described human breasts with “bulging tumors.” Ancient Greek doctors, including Hippocrates, explicated cancer in their medical texts (and coined the term karcinos, which means “crab”). The disease appears in ancient Indian Ayurvedic and Persian medical books and in Chinese folklore. Galen, the renowned second-century Greek physician who practiced in Rome, said breast cancer was the most common of the many cancers he saw. In fact, as James S. Olson writes in Bathsheba’s Breast, “Among ancients, breast cancer was cancer,” primarily because it was the one they could easily see.

  In the last few decades, paleopathologists have used X-rays and other methods to survey Egyptian mummies. They’ve examined Bronze Age skeletons from Britain and preserved corpses from Papua New Guinea and the Andes. While their data is admittedly limited—no soft tissue, DNA degradation—the researchers widely agree that cancer did indeed exist in human antiquity. But it’s even older than that.

  In 1997, amateur fossil hunters happened upon the fossilized remains of a female meat eater known as Gorgosaurus, a lanky cousin of T. rex. The paleontologists from the Black Hills Institute of Geological Research who examined her became intrigued by a puzzling finding. In spite of her fearsome, five-inch-long serrated dagger teeth and impressive twenty-five-foot height, this Gorgosaurus was riddled with injuries: a lower-leg fracture, fused vertebrae in her tail, a shattered shoulder, broken ribs, and a raging, pus-filled jaw infection. Examining the fossils with electron microscopes and plain radiographs revealed a possible explanation for these multiple injuries. The scans showed evidence of a mass in the dinosaur’s skull. While paleontologists have argued over the nature of this mass, some experts believe it to be the fossilized remains of a brain tumor.

  A tumor positioned in the ancient animal’s skull would have pressed on her cerebellum and brainstem. These areas are critical regulators of motor activity, balance, memory, and autonomic functions like heart rate. What this meant for the dinosaur is written into her injured skeleton. Researchers suggest that the burgeoning tumor likely affected her daily life.

  “As the tumor grew, the dinosaur—a female perhaps three years old—would have forgotten where she left her last kill, and then she would have forgotten to go to the bathroom,” said one. A tumor in that position meant she wouldn’t have been able to move quickly or make rapid predatory decisions. Like many humans with brain tumors, this ancient creature might have had pain—excruciating headaches upon waking and when bearing down for a bowel movement or any time she bent her head lower than her heart, perhaps to drink or feed or mate.

  Other paleo-oncologists have found tumors in hadrosaurs, the duckbilled prey favored by T. Rex. At the University of Pittsburgh, medical students learn about cancer by studying a 150-million-year-old diseased dinosaur bone on loan from the Carnegie Museum of Natural History. And evidence of probable metastatic cancer has been found in the bone of a Jurassic dinosaur that lived some 200 million years ago.

  Because dinosaur DNA would have been subject to transcription errors similar to those humans face, it’s not surprising that tumors formed in prehistoric creatures. On the other hand, environmental factors may have played a role as well. For most of us, “carcinogens” is synonymous with “man-made toxins.” In fact, however, many mutation triggers are as natural as flowers, plants, and sunshine.

  At times, even the most pristine, “natural” corners of our planet can become as polluted as a Superfund site. A couple of million years ago, for example, you wouldn’t have wanted to be living in what is now Yellowstone National Park’s unspoiled Hayden Valley. That’s when the region’s supervolcano spewed ash over an area that would now cover sixteen states. About sixty-five million years ago, in an area of west-central India called the Deccan Traps, a monster volcano belched more than a quarter of a million cubic miles of lava over the landscape and filled the air with toxic gases like sulfur dioxide. Ionizing radiation, toxic volcanic spew, or even Mesozoic food sources may have wreaked havoc with the DNA of the living creatures inhabiting the Earth in these areas and during these periods. In fact, cycads and conifers, the oldest living seed-bearing plants, and staples of dinosaurs’ diets, contain potent carcinogens. This means that we are not the first (or only) species on Earth whose diet or environment has been infiltrated with carcinogenic substances.

  “Jurassic cancer” demonstrates that while we humans may have coined the term “cancer,” we certainly didn’t create the condition. In fact, the sheer ubiquity of cancer makes it an intrinsic part of life. Yes, toxic exposures created by humans have amplified the risk, in some cases greatly. Several examples of cancer in animals I named earlier have been linked to environmental poisons (more on that in a moment). But the potential to get cancer is simply part of being a living creature on Earth, an organism with cells containing replicating DNA.

  The vulnerability of DNA to mutation means that cancer “becomes a statistical inevitability in nature—a matter of chance and necessity,” as Mel Greaves wrote in Cancer: The Evolutionary Legacy.

  While nothing is likely to dull the devastation a
patient feels upon hearing the dreaded words “You have cancer,” perhaps there’s a small measure of solace to be found in the knowledge that the disease is at least as old as the dinosaurs and as universal among today’s animals as hearts and blood and bones. But a zoobiquitous approach to cancer research could promise more than psychological balm. It could lead to breakthroughs in treatments, therapies, and our understanding of the risks. In fact, it’s already starting to do just that.

  Imagine two animals: a tiny bumblebee bat (weight: .07 of an ounce, the size of a penny) and an enormous blue whale (weight: 420,000 pounds, the size of twenty-five elephants). The huge whale has vastly more cells in its body than the tiny bat and trillions more cell divisions over its longer life. Which animal would you predict would be more likely to get cancer? Because we know that cancer stems from a single cell’s faulty replication, you might think that animals with more cells, more replications, and more mutations would have more cancer.

  Genomics researchers at the University of Pennsylvania tested this hypothesis by calculating the number of cells in the human colon and comparing it to the number of cells in the colon of a giant blue whale. They concluded that if cell division and “proofreading” were identical across species, all whales ought to have colorectal cancer by the time they hit their eightieth birthday.

  But as far as we know, they don’t. In fact, larger species, overall, seem to get cancer less often than smaller species. This fascinating observation is called Peto’s paradox, after the British cancer epidemiologist Sir Richard Peto, who recognized and first described this biologically surprising reality.