While strolling last month through one of the dimly lit backrooms in a wing of the National Galleries of Scotland, my inner eye still tingling with thousands of Impressionistic afterimages, pudgy Rubensian cherubs, and gothic quadrangles, one irreverent painting leapt out at me in a very contemporary sort of way. It was part of an early-16th-century triptych showing what appeared to be a solemn, middle-aged clergyman in gilded ecclesiastical robes commanding three naked adolescent boys before him in a bathtub.

Now, I must say, my first thought on seeing this salacious image was that the Catholic Church has been a hebephilic haven for far longer than anyone realized. But my uneasiness was put to rest once I leaned in to read the caption, which stated that the Dutch artist Gerard David, a prolific religious iconographer based in Bruges, Belgium, was merely painting a scene of starvation cannibalism. Phew! What a relief it was only an innocent case of anthropophagy (the eating of human flesh by humans) and nothing more sinister than that. The boys had been killed by a butcher, you see, and their carcasses were salting in a makeshift vat awaiting ingestion by famished townspeople. Fortunately, that most notorious child-lover himself, St. Nicholas, just happened to be passing through town when he caught wind of the boy-eating scandal and resurrected the lads in the tub.

In any event, my time in Edinburgh offered plenty of food for thought on the subject of human meat. From the art gallery, my partner, Juan, and I galloped over to the Surgeons’ Hall Museum, where we wandered through aisles packed floor-to-ceiling with pickled gangrenous feet, hairy severed arms of Industrial Age elderly women, trephined heads and a sundry of sickly genitals. Also on display was an elegant leather notebook, composed of a substance resembling cowhide but, in fact, made of the skin of the famous corpse supplier-cum-murderer William Burke.

All of this got me thinking about the logistics of cannibalism. The slick commercialization of the food industry has changed things dramatically, but there were, at one time, relatively frequent conditions—crop failures, habitat depletion, famine—in which cannibalism would have had lifesaving adaptive utility for our species. One pair of anthropologists, for example, actually crunched the numbers, concluding that the average human adult provides 66 pounds of edible food, including fat, connective tissue, muscle, organs, blood, and skin. Protein-rich blood clots and marrow are said (by the rare connoisseur) to be special treats. And indeed, at least one prominent evolutionary theorist, Lewis Petrinovich from the University of California, Riverside, has argued that cannibalism is a genuine biological adaptation common to all human beings—including those of you dry-heaving as you’re reading this.

Anthropophagy routinely emerges, says Petrinovich, under predictable starvation conditions, and such examples of human cannibalism are not as rare as many people believe. “The point is that cannibalism is in the human behavioral repertoire,” writes Petrinovich in his 2000 book The Cannibal Within:

… and probably is exhibited for a number of reasons—a common one being severe and chronic nutritional deprivation. A behavior might be exhibited only under extreme circumstances and still be part of our biological inheritance, and the fact that its course follows a systematic pattern argues against the hypothesis that it is psychotic in character.

Petrinovich wends his way through a human history littered with the gnawed-on bones of our cannibalized ancestors, revealing that—contrary to critiques arguing that man-eating is a myth conjured up by Westerners to demonize “primitives”—we really have been gobbling each other up for a very, very long time. We’re just one of 1,300 species for which “intraspecific predation” has been observed. Among primates, cannibalism can usually be accounted for by nutritional and environmental stress, or it appears as a reproductive strategy in which mothers, for example, consume their unhealthy infants to make way for more viable offspring.

Pinpointing the specific factors that cause cannibalism is a rather difficult affair in the laboratory, mainly because of those pesky university ethics review boards. Still, an intrepid Japanese researcher shrugged off these considerations and induced cannibalism among a captive population of squirrel monkeys by feeding the pregnant females a low-protein diet. This led to a high rate of abortion and the mothers’ devouring their aborted fetuses—a much-needed bolus of protein. Now imagine doing this same study with human beings under similar controlled laboratory conditions. Rather horrific, I should say, but that doesn’t mean the findings wouldn’t generalize to our own species. And don’t get me started on the many ways that mammalian mommas feast on placental afterbirth. Some of our own prefer it with a dash of paprika, others as a spaghetti and “meatballs” dish.

But the fact that cannibalism is motivated in primates, including human beings, by starvation is precisely the point that Petrinovich is arguing. Where he differs from other evolutionary theorists, however, is in his assertion that anthropophagy represents a true adaptation in our species, just as cannibalism does for other animals. It is not simply an anomalous behavior found in a handful of depraved individuals. Such people do exist, to be sure—like this man who was so curious to know what his own flesh tasted like (“autocannibalism”) that he … well, I’ll let the clinical psychiatrists who examined him tell you in their own words:

After he cut the first toe, he first showed it to his flatmates before he ate it raw while he walked the streets. He chewed as much of the bone as possible and then spat it out. He recalls eating it ‘for the experience’ and that it was a ‘once in a lifetime opportunity to eat human flesh’. He was excited by the shock value of doing so. The second toe was cooked in an oven before eating. In between cutting his toes he continued to work on renovating houses.

He’s now safely medicated, a successful and happy builder, and presumably wearing special orthopedic shoes. But again, whereas cannibalism can certainly be deviant, in other cases it’s perfectly normal—even healthy. Our close cousins the Neanderthals, essentially carnivorous predators, were driven to cannibalism at the end of the last glacial maximum in the face of dwindling numbers of large game animals. Osteoarchaeological research at a cave in southeast France yielded a bundle of roasted Neanderthal bones from about six individuals, haphazardly discarded bones that had been deliberately defleshed, disarticulated, and the marrow extracted.

In our own species, those bloodthirsty Aztecs and their prehistoric descendants were notorious for their sacrifice and cannibalism rituals. These were largely symbolic religious events, but some scholars have suggested that the greasy surfeit of Aztec sacrifice victims may have also been a high-energy nutritional supplement for the wealthy elite, who had first dibs on this “man corn.” Noncannibalistic people may be the weird ones, cross-culturally speaking. Researchers have documented evidence of ritual anthropophagy throughout non-state societies in Africa (Zandelande, Sierra Leone, the Belgian Congo), South America (Eastern Brazil, Ecuador, Western Columbia, Paraguay), the New Hebrides (Fiji, Papau, New Guinea, Vanuatu, East New Guinea Highlands), and Native America. It’s appeared in modern “civilized” societies, too, including famine-stricken China and Soviet-era Russia.

The bottom line, says Petrinovich, is that when you’re hungry enough, ravenous really, and when all other food sources—including “inedible” things you’d rather not stomach such as shoes, shoelaces, pets, steering wheels, rawhide saddlebags, or frozen donkey brains—have been exhausted and expectations are sufficiently low, even the most recalcitrant moralist among us would shrug off the cannibalism taboo and savor the sweet meat of man … or woman, boy or girl, for that matter. It’s either that or die, and among the two choices, only one is biologically adaptive.

A behavior can be adaptive without being an inherited biological adaptation, of course. But because starvation occurred with such regularity in our ancestral past, and because the starving mind predictably relaxes its cannibalistic proscriptions, and because eating other people restores energy and sustains lives, and because the behavior is universal and proceeds algorithmically (we eat dead strangers first, then dead relatives, then live slaves, then foreigners, and so on down the ladder to kith and kin), there is reason to believe—for Petrinovich, at least—that anthropophagy is an evolved behavior. The taboo against cannibalism is useful in times of health and prosperity; groups wouldn’t survive very long if members were eating one another up. Yet starvation has a way of releasing the cannibal within.

In fact, starvation cannibalism may have been so prevalent in the ancestral past that it literally changed our DNA. Modern human populations appear to contain specific genetic adaptations designed to combat cannibalistic viruses. Typically, when a predator species consumes a prey species, there are substantive differences in immune systems between the two, with different varieties of pathogens. But the more similar are the eater and the eaten, the more vulnerable is the former to debilitating food-borne disease. This is what likely happened with the New Guinea Fore people in the case of kuru, a neurodegenerative disease that devastated that population in the early half of the last century. Epidemiologists traced the disease to mortuary cannibalism; women and children were eating the brains of the recently deceased as part of a funerary ritual. (Pork had fallen into short supply, so human brains infused a healthy dose of protein.) The interesting thing is that kuru is a variant of Creutzfeldt-Jakob Disease and probably resulted, originally, from a single case of cannibalism among the Fore of a CJD-ridden brain, with kuru then evolving on its own course. In a 2003 issue of Current Biology, University of Nottingham geneticist John Brookfield speculated that over the past 500,000 years, human beings developed increasing variation in the gene for the human prion protein. Those who are heterozygous for this gene, points out Brookfield, were protected against CJD through cannibalism. “This sustained heterozygote advantage [was possibly] created by a lifestyle of habitual cannibalism,” suggests Brookfield, “implying a new vision of the lifestyles of our ancestors.”

As we’ve seen, not all cases of cannibalism are due to nutritional needs. Sociopathic individuals such as Jeffrey Dahmer, Armin Meiwes and Issei Sagawa lived in urban environments peppered with fast-food restaurants and overflowing grocery stores, yet still they dined on people. In his book SuperSense, University of Bristol psychologist Bruce Hood argues that such cases reflect essentialism beliefs, the idea that the victims’ hidden “essences” or personality attributes are acquired by physical ingestion. It’s also interesting that many such cases have a sexual component. As the author of To Serve Man: A Cookbook for People wrote teasingly: “There is no form of carnal knowledge so complete as that of knowing how somebody tastes.” I suspect there’s some truth to that uncomfortable joke. Essentialism beliefs may account for our species’ peculiar—and surprisingly recent—history of medical cannibalism as well. The conquistadors and their New World heirs were known to have used human fat from agile natives to grease their arthritic joints. Long before Armin Meiwes was even a twinkling in his mother’s eye, pregnant Ache women of Paraguay were nibbling on boiled penises in the hopes it would bring them sons.

So with all of these scenes swimming in my head, and pragmatist that I am, I’m left wondering why, exactly, it is that the consumption of already dead human bodies is such a taboo, especially for societies in which the soul is commonly seen as flitting off at death like an invisible helium balloon. If you subscribe to such dualistic notions, after all, the body is only some empty shell that the now-liberated spirit no longer needs. All those poor starving children of the world, surrounded by—as some epicures swear—the most succulent meat on the planet. Even resurrectionists should gleefully feed the impoverished with their own flesh, lest they, God forbid, allow such a bounty of edible meat to go to rot. All those wasted commercial goods, burned down to sticky dust in crematories, squirreled away behind ornate vaults, fed extravagantly to bloated subterranean organisms! If you’d rather not eat meat from aged or possibly diseased dead people, and if you’re worried about the dignity of the individual, it would be easy enough to breed and then factory-farm brain-dead or free-ranging anencephalic human beings *, treating them humanely, of course, but enforcing food safety standards to control for outbreaks.

A parting tip for the entrepreneur: Rumor has it there’s already some prime breeding stock—unfortunately a bit marbled with adipose tissue but with all the lucidity of dairy cows—readily available as founder lines in select communities. Want to buy in? E-mail me for details.

Watch: Robert Wright of BloggingHeads.tv and psychologist Paul Bloom of Yale on what motivates cannibals.

Like Slate on Facebook. Follow us on Twitter.

Correction, Dec. 20, 2010: The original incorrectly referred to “hydrocephalic” human beings, who would have a less severe condition characterized by an excess of cerebrospinal fluid. (Return to the corrected sentence.)