It's the time of year when seeing flesh-eating zombies on the streets is actually kind of normal. So let's talk about cannibalism. You know you're wondering.
When did all this craziness start?
Paleoanthropologic evidence suggests that Neanderthals were butchering each other as far back as 100,000 years ago. Bones from sites in France, Croatia, and Italy all bear marks from stone tools indicative of defleshing. Analysis of some of the bones in France revealed that the marks are concentrated in places consistent with butchery, and not ritual defleshing.
Evidence also suggests* that humans in Europe, North and South America, India, New Zealand, Australia, the Solomon Islands, New Guinea and Sumatra practiced cannibalism at various times beginning just prior to the Upper Paleolithic period. American anthropologist Marvin Harris has argued that it was common practice for humans living in small groups, but disappeared as societal groups got bigger and states were formed. Eventually, cannibalism became taboo in many cultures, and by the 19th century it persisted only among a few isolated groups in the South Pacific. Today, very few cultures are still believed to engage in the practice, though isolated instances involving individuals or small groups have been confirmed in the last twenty years (several of them involving soldiers engaged in wars in Africa).
Why would you want to eat another person?
In the days of pre-modern medicine, cannibalism was explained by a proposed black humour (the body fluids that Hippocrates believed caused moods, emotions and behaviors) that filled the ventricle and caused hunger for human flesh. Our understanding of cannibalism is a little better today, and we even have a technical term for it: anthropophagy (anthropos, or "human being," plus phagein, meaning "to eat").
Anthropologists divide anthropophagy into two categories, both rather broad: survival cannibalism and learned, or customary, cannibalism.** Survival cannibalism is what's about to happen whenever you see two cartoon characters stuck in a life raft and one of them has a thought balloon above their head depicting the other one with a roast chicken for a body. Outside of cartoons, survival cannibalism may be—given extreme and desperate enough situations—the easiest form of cannibalism to accept, and Western society has historically been relatively forgiving of it. In 18th and 19th century seagoing communities, it was pretty much accepted as something that happened from time to time as a hazard of the occupation and lifestyle. By the 19th century, sailors and fishermen had even worked out some general guidelines should the "custom of the sea" need to be performed. Straws were drawn to decide who would be killed and eaten and who would have to do the killing (usually the second shortest straw made you the killer, and the shortest made you dinner).
Perhaps the most famous example of survival cannibalism is the Donner party, a group of eighty-seven settlers heading to California in 1846. When the party reached the Sierra Nevada Mountains, the weather turned, and they were blocked by snow at a point now called Donner Pass. The party splintered into three groups. One set up camp at a nearby lake, one camped in the nearby Alder Creek Valley, and one group of 15 travelers, later dubbed the Forlorn Hope, made snowshoes and began the 100-mile journey to Sutter's Fort. While individuals in all three groups eventually resorted to cannibalism, it wasn't the feeding frenzy that most people imagine. The people who turned to eating human flesh did so as a last resort (after eating everything from boiled rawhide to leather scraps) for a very brief period of time before their rescue.
Another example, that many people know from the movie Alive, is the 1972 plane crash that left the players and staff of a Uruguayan rugby team and some of their friends and family members stranded in the Andes Mountains. As people died from their crash-related injuries, the survivors resorted to cannibalizing the dead. Some refused to eat human flesh and starved to death. Of the 45 people onboard the plane, only 16 survived the 72 day ordeal.
Learned or customary cannibalism is pretty much what it sounds like: the consumption of human flesh in a socially prescribed, ritualized manner, often passed down through the generations. Learned cannibalism can be divided into two categories: endo- and exocannibalism. Endocannibalism is the consumption of the flesh of a person who is a member of the same group (whether family, tribe, society, culture, etc.—any defined group fits the bill), often practiced as a funeral rite. The Wari' people of the Amazon consumed the flesh of their deceased in order to transform their tribesmen into spirits that could take animal form and provide food for the tribe. Anthropologists also found that the tribe's endocannibalism also helped survivors cope with grief. Endocannibalism among Wari' ended, as it did for most other groups, in the 1960s, when missionaries and governments began to encroach on their societies.
Exocannibalism is the consumption of the flesh of a person outside of one's own social group, often as a way to intimidate an individual or group, steal another's life force, or express domination of an enemy in warfare. Certain tribes in the Fiji islands maintained ritualized acts of cannibalistic "battle rage," where captured enemy warriors were publicly tortured, killed, and consumed.
The accusation of exocannibalism may be even more damaging to enemies than eating them. When Christopher Columbus encountered the Carib Indians, he described them as "sub-human eaters of men," labeling them inferior to Europeans and not much better than animals. They were seen as a dangerous "other," and the murder of their people and theft of their land was easily justifiable because of that. The slur of cannibalism goes both ways, however. When the Spaniards arrived in Mesoamerica, and when explorer David Livingstone encountered certain African cultures, both the Aztecs and the African tribes assumed their white visitors were cannibals.
Do Other Animals Do It?
Cannibalism is a common occurrence in thousands of species, even herbivorous and detritivorous ones, to the point where zoologists refer to it as "ubiquitous" in the natural world. Female black widow spiders and praying mantises famously practice sexual cannibalism, killing and consuming males of their species during, or after, reproduction.
Filial cannibalism, where adults eat the young of their own species, is also common among non-human animals. Groups of adult male chimpanzees have been observed to attack and eat infant chimps. Adult male elephants, dogs, bears, lions, and even some types of fish have all been observed to kill and consume infants when replacing a previous dominant males and taking over a group.
Sharks in the order Lamniformes, which includes great whites and sand tigers among others, have been known to exhibit intrauterine cannibalism, where multiple embryos are created during impregnation and the larger or stronger individuals consume their weaker siblings during development in utero.
Is That It?
Well, no. "Deep down," science writer Carl Zimmer says, "we are all cannibals. Our cells are perpetually devouring themselves, shredding their own complex molecules to pieces and recycling them for new parts." Zimmer's exploration of cellular cannibalism (and the sexual cannibalism I mentioned above) can be found in the New York Times.
* "Suggest" is the key word here, as it is with the Neanderthals. While most anthropologists agree that ritual cannibalism has occurred in certain societies around the world over the course of history, researchers are sometimes reluctant to associate it with a particular group of people without concrete evidence. The conservative view is that there is no definitive proof that cannibalism exists in a group until an anthropologist sees, with their own two eyes, a member of that group take a piece of flesh off a body and eat it. For archaeologists, the best proof that cannibalism took place in a group that no longer exists is the presence of human muscle protein in fossilized human feces.
** Cannibalism in the vein of Jeffrey Dahmer or Hannibal Lecter is known as pathological anthropophagy, the consumption of human flesh because of insanity. It's generally outside the scope of anthropology.