같은 족속의 새끼를 잡아먹고 있는 원시의 현장 (뉴욕타임즈 6월 24일자 Science섹션)

새, 파리, 사마귀, 침팬지, 사자, 물고기, 그리고 인간. 모두 동족을 잡아 먹거나 같은 족속의 새끼를 먹어치운 경험이 있는 생명체들이다. 6월 27일자 Science에 실릴 미국 하버드-스페인 공동 연구진에 의한 연구 결과가 주목을 끌고 있다. 더러는 전염병으로부터 사멸되어 가는 같은 종족을 보존하기 위해 발병 부위에 해당하는 이웃의 신체 부위를 뜯어다 먹은 적이 있는 인간. 그들은 세월이 흐르면서 매우 유사한 유전자를 지닌 집단으로 판명되었다. 생존의 본능에 의한 행위인가 자연을 거스르는 몰염치인가. 사회, 정신학은 물론 동물학 및 행태학자들에게 큰 관심을 불러일으키고 있는 공동 연구진의 연구 결과에 주목한다 (뉴욕타임즈)

사진 上: 물고기가 자신과 같은 종류의 새끼 물고기를 잡아 먹고 있다.
사진 下: 박테리아(녹색)가 자신의 일부(적색)를 먹어 치우고 있다.

They Eat Their Own; the Question Is Why

By DONALD G. McNEIL Jr. (NY Times, 6/24/2003)

Birds do it. Robber flies that look like bees do it. Even chimpanzees do it.

And now researchers say that a tiny voice from near the bottom of life's evolutionary ladder is chiming in on the chorus: Let's do it. Let's eat our own.

It is time to get over the old notion that only the advanced, highly intelligent beings of this world practice cannibalism.

It has just been discovered by Harvard and Madrid researchers that even bacteria do it.

Higher orders, like humans, may practice cannibalism for complex psychological reasons, like appeasing gods or honoring ancestors. Chimps seem to do it as an act of revenge or in a burst of malice: females have been known to snatch and eat the babies of smaller females. Lions do it for genetic preservation: when a male takes over a pride, he eats the previous king's cubs, eliminating the old royal bloodline and speeding the lionesses back into estrus to continue his.

And for praying mantises, it's just about good sex. As the finale to a successful copulation, the female bites the heads off her mate and devours him. It's not known if the plat du jour sacrifices himself so his offspring will be nourished, or if he entertains naïve hopes of escaping before the big Sadie Hawkins Day chowdown.

Bacillus subtilis bacteria, though, act more like the Donner Party. They use cannibalism only in extremis, and only for survival, according to a study that appeared in Science on Friday.

In fact, what this mild relative of the anthrax bacterium engages in could be described as cannibalism for couch potatoes: it takes the path of least energy.

Its other option for surviving starvation is to turn itself into a hardy spore, as anthrax does. But that means taking 10 hours to build a thick cell wall and eject the water in its interior.

"It becomes like freeze-dried food — almost crystalline, resistant to heat and time and radiation," said Dr. Richard M. Losick, a molecular biology professor at Harvard and one of the study's authors.

But should its dirt environment suddenly become enriched with the organic matter it needs to survive, a spore is at a disadvantage because it takes 36 hours to morph back into a bacterium.

For B. subtilis, the lazy alternative is to pump out an antibiotic that pops open its brethren so it can digest their innards, which is what Dr. Losick and a team from the National Biotechnology Center in Madrid caught it doing.

This seems to be the lowest order so far that engages in cannibalism, Dr. Losick said. There is a slime mold, dictyostelium discoideum, that sometimes engulfs fellow amoebas instead of its usual bacterial prey, but it does so as part of a reproductive process that is more about incorporating everyone nearby into a multicellular "giant zygote" than about simply digesting them.

What B. subtilis gets is a fraternal snack to tide it over in the hopes that dinner will arrive. Who eats whom is "just by chance," Dr. Losick said. "Some start down the road faster than others."

Human cannibalism is not by chance, of course, though social scientists debate how common it was. Around the world, sites have been found with defleshed skulls, bones with shiny "pot polish," femurs cracked for marrow and even one set of petrified human feces with traces of human muscle in it.

In April, a powerful argument that cannibalism was widespread among our ancestors was made by researchers from University College, London, and from Australia and Papua New Guinea.

They found that all ethnic groups harbor genetic signatures that protect against infection by prions, proteins in meat that lay waste to the brain. Prions can come from animals, as mad cow disease showed, but they kill off cannibals far more effectively.

The best-known example of that is the Fore people of Papua New Guinea, who from the 19th century to the late 1950's held "mortuary feasts" on deceased relatives.

From the 1920's to the 60's, an epidemic of kuru, a prion disease, killed more than 200 Fore a year, especially women, who ate brains while men ate muscle. A study of 30 elderly Fore women who survived the era showed that 23 had the prion-protection gene, which was far less common among Fore born later, suggesting that it had protected them.

The discovery of similar genes in ethnic groups around the world, the researchers said in the April paper, is powerful evidence that cannibalism had at some time made such protection important.

It should be noted that forms of human cannibalism still go on. "Muti murders" are well-known to the South African police. Each year they find a few bodies missing the lips, tongue or other parts used in traditional medicine — muti in Zulu. Last year, a murderer was arrested in Krugersdorp trying to sell a head for $1,000, and journalists clandestinely filmed a morgue attendant offering two human hands for $400.

Eating human placentas has its fans, especially in Britain, among some vegetarians who think of it as meat that does not involve killing as well as women who give birth at home and believe it helps the uterus contract. In 1998, the British Broadcasting Standards Commission criticized a program featuring a mother making placenta pâté, and a Web site for older mothers, www.mothers35plus.co.uk, still offers a recipe for roast placenta in tomato sauce.

There have been philosophical defenses of human cannibalism. The best-known, of course, is "A Modest Proposal," the 1729 satire by Jonathan Swift, who suggested solving the Irish overpopulation problem by judicious application of knife and fork.

Studying lower species is, at least, less fraught with political touchiness. Researchers who believe that the Anasazi people of the American Southwest widely practiced cannibalism have been accused of racism by their descendants, the Hopi.

But lower-order cannibalism, not driven by emotion or religion, is in some ways more of a puzzle.

Presumably, it confers some evolutionary advantage, but it seems such a backward-turning gear in the Darwinian drive train. After all, each cannibal has just as good a chance of being eaten as of eating. That ensures survival of the fittest, but ultimately of only one fittest — not an ideal species-preservation goal.

Among robber flies — so called because some with orange stripes sneak up to hives to snatch honeybees for food — the females are so fast and vicious that they sometimes grab the smaller males and eat them without its being clear that they realize what they are eating. Male courting dances, done from a distance, seem to be a way of letting females know that their intentions are sexual, not culinary.

And there are powerful medical arguments against cannibalism. As the Fore showed, dining on kin magnifies disease even faster than marrying kin.

A 1998 study of cannibalistic tiger salamanders led by Dr. David W. Pfennig, a University of North Carolina biologist, tested that notion.

Four groups of cannibalistic tiger salamander larvae were fed, respectively, healthy tiger salamanders, healthy small-mouthed salamanders and salamanders of both types that had been sickened by letting them swim in a bacterial soup. The tigers that ate healthy prey or sickened small-mouths all survived and grew. But nearly half those that ate sick members of their own species died. The tigers, autopsies showed, had eaten bacteria best suited to killing tigers.

But they keep eating one another, despite the risks, because they breed in ephemeral snow-melt ponds. "Their pools dry up really fast, so they need to get out of the larval stage quickly, and the best way to do that is to get lots and lots of food," Dr. Pfennig said.

The biologist, who also studies spadefoot toad tadpoles in desert rainwater ponds, says such time pressures drive species toward cannibalism. "Disease is something you can potentially overcome, but starvation is pretty permanent," he said. Tadpoles, he said, can taste each other, and tend to spit out close kin.

In species that hatch many offspring, some serve as surplus protein, all of it presumably infection-free. Sharks eat their siblings in the womb, tiny copepod crustacean females eat their young, as do burying beetles, who match the number they leave alive to the size of the mouse corpse they bury with them.

But sometimes one's more mature brethren provide nutrients no one else can. Rattlebox moth larvae, which eat poisonous plants for toxic alkaloids that will protect them as moths, may eat alkaloid-laced siblings if they cannot get to the plants themselves.

Down at the bacterial level — where species mutate rapidly and Darwin is the only god — the mystery is why cannibalism persists.

"There's a big incentive to cheat, to evolve a strain resistant to the killing factor," Dr. Losick said. "But we don't see cheaters. And we still don't understand: What's in it for the siblings?"