War: What is it good for?

by Ian Morris

  Some ten thousand species of ants are known, and far more wait to be classified. Some of these species are very peaceful, while others fight constantly. Just as some cells in an animal’s body turn into blood while others become teeth, some female ants in each colony turn into reproductive queens while others become sterile workers, and in warlike species some also grow up to be soldiers. Without ever really thinking about what they are doing, they wage savage wars driven by smell.

  Since there are so many kinds of ants, there are many different patterns, but one of the commonest is that soldier ants “smell” the workers in their colony by tapping them with their antennae (which function rather like our noses). If foragers go out in the morning but do not come back, the absence of their smells triggers a response that sends the soldiers rushing out to confront whatever is detaining the foragers. After about one-fifth of the soldiers have marched out, the remaining four-fifths react to the new chemical balance in the air by staying put as a reserve in case some other ant colony should exploit their absence to seize the unoccupied nest.

  If the expeditionary force finds that enemy ants are killing the missing foragers, it does not just rush to attack the foe. Instead, the soldier ants do more tapping and smelling, and if this tells them that they outnumber the enemy, they charge, clamping their jaws around the hostile ants’ abdomens and breaking them in two (Figure 6.6). If the odds seem balanced, they will have a standoff, waving their feelers, and if they sense that they are outnumbered, they run home. When the imbalance in numbers is extreme, the stronger force might storm the weaker’s nest, massacring its queen and soldiers and carrying off its babies to raise as slaves.

  Figure 6.6. Six-legged soldiers: Plectroctena ants battling to the death in Tanzania

  Biologists draw three broad conclusions from this. First, since some species of unintelligent ants and some species of highly intelligent apes wage lethal gang warfare while other species do not, powerful brains are neither necessary nor sufficient for this kind of behavior. Second, we can conclude that sociability is necessary for lethal gang warfare, because only sociable animals can form gangs, cooperating to attack enemies at such unfair odds that they can safely fight to the death. The third conclusion, however, is that sociability by itself is not a sufficient cause for lethal violence, because some species of sociable apes and ants do not form murderous mobs.

  For animals to make killing part of their evolutionarily stable strategy, some other factor must be driving up the payoff from lethal aggression, and the natural histories of ants and apes suggest that this secret ingredient is territory. When animals have valuable territories to compete over, the payoff from killing enemies rises. Each time the chimpanzees of Kasekela raided into Kahama during the Gombe War, the Kahama chimps retaliated by raiding back into Kasekela. If the Kasekelans had scared Godi but let him get away on January 7, 1974, they could have been confident he would have joined the next raid against them. But if they killed him, they could be certain he would not. And if they killed all the Kahaman males, they could take over their land and surviving females.

  Here we confront one of the biggest paradoxes of war. Territoriality drove up the payoff from killing for those ants and apes sociable enough to be able to do it safely, but when, at the end of the Ice Age, population growth and farming began caging human societies in the lucky latitudes, this extreme version of territoriality pushed our ancestors into productive war, which raised the payoffs from not killing defeated enemies. Instead, societies that recognized signals of submission and absorbed losers became safer and richer and outcompeted their rivals until—eventually—one of them turned into a globocop.

  I will return to this odd outcome toward the end of the chapter. For now, though, I want to focus on the facts that—for all their differences—chimpanzees, bonobos, and humans are all sociable and territorial and all descend from a shared ancestor (usually called proto-Pan, from the Greek words for ancestral ape). Clearly, this ancestral species had the potential to develop wildly different evolutionarily stable strategies. Something happened 7 or 8 million years ago to set chimpanzees and humans on the road toward violence; then, around 1.3 million years ago, something else happened to push bonobos away from using violence against their own kind (although they do hunt monkeys for meat, and in one disturbing case, several adult bonobos were seen to cannibalize a dead bonobo baby, with the baby’s mother leading the way). Finally, in the last 10,000 years, yet another development made us humans react to caging by becoming less violent. But what?

  Planet of the Apes

  I want to look first at the chimpanzee/bonobo split, which, study of the two apes’ DNA tells us, began around 1.3 million years ago. This makes it much more recent than the human/proto-Pan split (dating back some 7.5 million years). Unfortunately, though, we know less about it, because fossils do not survive well in the tropical rain forest where it happened. This forces us to work with indirect lines of evidence.

  DNA analysis suggests that as recently (on an evolutionary scale) as 2 million years ago, the now-extinct proto-Pans roamed over a central African rain forest the size of the continental United States. But nothing lasts forever, and as the climate fluctuated over the following half-million years, a great inland lake in East Africa burst its banks. The water flowed north and west toward the Atlantic, turning into what is now the mighty, mile-wide Congo River (Figure 6.7). Impassable to apes, this split proto-Pan’s kingdom in two. By 1.3 million years ago, apes north of the Congo were evolving into chimpanzees and those south of the river into bonobos.

  Figure 6.7. Planet of the apes: the ranges of modern chimpanzees, bonobos, and gorillas, and major sites with protohuman fossils between one million and six million years old

  The forests on either side of the river were not wildly different, and apes in both places ate mostly fruit, seeds, and (when they could catch them) monkeys. South of the Congo, however, the apes that eventually evolved into bonobos expanded their diet by eating young leaves and shoots. Their bodies adapted to this diet, growing teeth with long shearing edges to tear up their greens. Bonobos do not find leaves and shoots as tasty as fruits, seeds, and monkey meat, but leaves and shoots are more plentiful, and they keep bonobos full between real meals. Leaves and shoots, says the biological anthropologist Richard Wrangham, are bonobo “snack food.”

  Just why bonobos fill up on these snacks and chimpanzees do not remains controversial, but in their book Demonic Males, Wrangham and his co-author Dale Peterson suggest that it is because gorillas—who also eat shoots and leaves—went extinct south of the Congo but hung on north of it. This left the southern branch of proto-Pan without competitors for shoots and leaves, and so any random genetic mutation that made it easier for an ape to eat this extra food flourished. The mutations spread through the gene pool, and proto-Pan began evolving into bonobos. North of the river, however, proto-Pan still lived alongside gorillas, and since no hundred-pound proto-Pan that challenged the proverbial four-hundred-pound gorilla for a leaf would last long enough to pass on its genes, chimpanzees did not evolve to eat these foods.

  Other primatologists suggest different explanations, such as small differences between the two sides of the Congo in climate or the concentration of good foods, which might have made growing new kinds of teeth and adapting to new foods worthwhile for bonobos but not for chimpanzees. Eventually, as techniques improve and data pile up, scientists will surely answer this question. For our purposes, though, what really matters is not the cause of the divergence in diets but its consequences, because—unlikely as it may sound—snacks sent bonobos down the path toward peace and love while chimps took off along the hard road of violence.

  Because they can fill up on leaves and shoots when they cannot find fruits and their other favorite foods, bonobos can travel in large, stable groups (typically about sixteen animals). Chimpanzees, however, regularly have to split up into very small groups of two to eight animals, because they cannot find enough fruit to feed larger parties. Godi’s
disastrous decision to strike off on his own in 1974 was entirely typical for a chimpanzee but would have been very eccentric for a bonobo. The result, of course, is that bonobos almost never find themselves outnumbered eight to one.

  But that is not all. Chimpanzee groups also tend to split up in very specific ways when they go foraging. Males can travel faster than females (especially females burdened with babies), and so males often head off in single-sex groups. Female chimps, however, often resort to foraging individually, because they move too slowly to cover enough ground in a day to find enough food to support a bigger group. All this is in striking contrast to the snack-rich bonobos. As well as being large and stable, their foraging parties normally have roughly equal numbers of males and females.

  At this point, the absence of snacks in chimpanzee-land turns ugly. Groups of half a dozen males are regularly running into isolated females. The males do not always rape the females, but it happens with alarming regularity. At these odds, females have no real chance of fighting off attackers; what fighting does take place instead tends to be among the males, over who gets access to the female.

  Over the last million-plus years, male chimpanzees have evolved two very specific features because of their inability to subsist on snacks: hawkishness and huge testicles. Because rape is always an option, males who will fight are more likely to pass on their genes than males who will not, and because females often end up having sex with multiple males in a single day, males who have large testes (to pump out the biggest possible load of sperm, increasing the odds of being the lucky fellow who fertilizes the egg) have a reproductive advantage over those who do not.

  So important is this quirk of ape evolution that biologists have created an entire subfield called sperm competition theory. On average, chimpanzee testicles weigh a whopping quarter pound, while gorillas, despite having bodies four times as big, have testicles weighing just an ounce. This is because each alpha-male gorilla monopolizes a harem of females and faces little competition from other gorillas’ sperm.

  Bonobos have huge testicles too, because male bonobos, like male chimpanzees, are locked in competition to impregnate females who have multiple sexual partners. Unlike what goes on among chimps, however, bonobo sperm competitions are almost entirely nonviolent. Males rarely outnumber females, and if a male courts his intended too aggressively, other females are likely to gang up on him, chasing him away with hoots and threats. (Female chimpanzees do sometimes cooperate against rapists, but nowhere near so effectively.)

  Male bonobos win the sperm competition not by fighting each other but by making themselves agreeable to females. One of the best methods, it seems, is to be a good son; bonobo mothers use their friendships among the females to make sure that their own sons find girlfriends. In the land of the bonobos, mama’s boys finish first.

  Across a million or so years, the payoffs of dovishness soared among bonobos. The meek inherited the rain forest, and bonobos of both sexes evolved to be smaller, more delicate, and just plain nicer than chimpanzees. “In all my experience,” Robert Yerkes (the founding father of primatology) said of Prince Chim, the first bonobo in captivity, “I have never met an animal the equal of Prince Chim in approach to physical perfection, alertness, adaptability, and agreeableness of disposition.” Whether Prince Chim felt the same way about Yerkes, who locked him up in Cambridge, Massachusetts, and trained him to eat with a fork at a miniature table, we will never know.

  The Naked Ape

  The evolutionary paths that led to Chim and Yerkes branched about 7.5 million years ago. Around then, apes living at the edges of the great central African rain forest began evolving away from proto-Pan and toward us—the only animals with the capacity to cage their own Beast.

  Once again, food seems to have been at the center of things. Because fruit trees thin out in these dry borderlands, giving way first to mixed woodland and then to open savannas, apes had to find new things to eat if they were to live there. Since adversity is the mother of evolutionary invention, all kinds of genetic mutations flourished as the apes adapted. Anthropologists have given these creatures wonderful, exotic names—Sahelanthropus north of the rain forest, Ardipithecus east of it, and different kinds of Australopithecus all around it—but I will call them collectively protohumans.

  To the nonexpert eye, protohuman bones look much like any other ape’s, but great changes were under way. Over a few million years, molar teeth grew bigger and flatter, thickly coated with enamel. This made them ideal for crunching up hard, dry foods, and chemical analysis shows that the foods in question were tubers and the roots of grasses. These are good sources of carbohydrates and are available even in dry spells, when the aboveground parts of plants shrivel up—if apes can dig them up and chew them. Any mutation that made paws nimbler would therefore make protohumans fatter, stronger, and probably better at fighting too, and altogether more likely to spread their genes through the population.

  The anatomy of ankles and finds of actual footprints, left by protohumans taking strolls through soft ash and mud that then hardened into stone, show that the shift was under way by four million years ago. Protohumans had begun walking on their hind legs, freeing up their front legs to turn into arms. These creatures were certainly still very different from us, however. They were only four feet tall, probably covered in hair, and still spent a lot of time in trees. They rarely—if ever—made stone tools and certainly could not talk, and it is a fair bet that the males still had testicles on the chimpanzee/bonobo scale.

  But however apelike they were, they more than made up for it by mutating toward bigger and bigger brains. Four million years ago, the average Australopithecus sported twenty-two cubic inches of gray matter (less than modern chimpanzees, which typically have twenty-five cubic inches). By three million years ago, this had increased to twenty-eight cubic inches, and another million years after that, to thirty-eight. (Today we average eighty-six cubic inches.)

  It might seem self-evident that big brains are better than small ones, but the logic of evolution is more complicated. Brains are expensive to run. Our own typically make up 2 percent of our body weight but use up 20 percent of the energy we consume. Mutations producing bigger brains only spread if the brain tissue that is added pays for itself by bringing in the extra food it needs. In the middle of the rain forest, this was rarely the case, because apes did not need to be Einsteins to find leaves and fruit. In the dry woodlands and savanna, however, brainpower and food supply rose together in a virtuous spiral. Smart woodland apes dug up roots and tubers, which paid for bigger brains; the even smarter apes this produced figured out better ways to hunt, and meat paid for even more of the expensive gray cells.

  Armed with all this brainpower, protohumans went straight to work on inventing weapons. Modern chimpanzees and bonobos have been known to use sticks and stones to catch food and hit each other, but by 2.4 million years ago protohumans had already realized that they could bash pebbles together to make sharp cutting edges. Telltale marks show that they used these choppers (as archaeologists call them) to slice meat off animal bones, although so far we have found no signs that they used them to slice each other.

  Biologists conventionally treat the combination of brains over thirty-eight cubic inches and the ability to make tools as the threshold at which apes became Homo (“mankind” in Latin), the genus to which we, Homo sapiens (“wise man”), belong, and over the next half-million years Homo began looking and acting much more like us. Around 1.8 million years ago, in the space of a few thousand generations—the blinking of an evolutionary eye—average adult height shot up above five feet. Bones became lighter, with jaws that protruded less and noses that protruded more. Sexual dimorphism—the size difference between males and females—declined toward the range we find among modern people, and protohumans shifted permanently from tree to ground living.

  The label that biologists use for these new creatures is Homo ergaster, “working man,” chosen to reflect their skill at making tools and weapo
ns. Some of these can be quite beautiful, made from carefully selected stones and finished with delicate touches from wood and bone “hammers”—all of which required careful coordination, forward planning, and, of course, bigger brains still (fifty-three cubic inches by 1.7 million years ago).

  Homo ergaster paid for its huge head with a peculiar trade-off: its guts got smaller. Earlier protohumans had rib cages that flared out at the bottom, like those of modern apes, to accommodate enormous intestines, but Homo ergaster’s ribs were more like ours (Figure 6.8). This left less room for yards of digestive tubing, which poses a difficult question for anthropologists. Apes have huge bowels so they can digest the fibrous raw plants they live on. Smaller guts would mean that Homo ergaster was extracting less energy from its food—but its bigger brain called for more energy. So what was going on?

  Figure 6.8. La bella figura: on the left, the best-preserved Homo ergaster skeleton so far found (known as the Turkana Boy), belonging to a boy roughly ten years old who died 1.5 million years ago; on the right, the famous Lucy, an adult female Australopithecus afarensis who lived 3.2 million years ago

  The answer, we can be fairly certain, is that Homo ergaster was the first protohuman that could make fire at will and used this new skill to cook. Cooking makes food easier to digest, which made enormous intestines, along with the huge flat teeth and powerful jaws earlier protohumans had needed to chew up raw tubers, roots, and grass, redundant. All now disappeared.