* * *
Throughout history we’ve built walls around our cities for precisely the same reason the termites do. In military-speak, walls act like a “force multiplier,” permitting a small number of defenders to hold out against a much larger invading army. The moment people settled into stable populations they became vulnerable. Agrarian societies could not wander the way earlier peoples did, since their crops were rooted in place. Surpluses of food could be stored for later distribution during harder times, enabling populations to grow. But stored foods could also be stolen. Like termites, people in early civilizations found themselves in a tight spot. They needed to find ways to protect their food and their brood from nomadic raiders. The earliest remnants of settled towns, scattered along the banks of rivers such as the Tigris, Euphrates, and Nile, all show evidence of walled fortifications, some dating to more than 5500 BCE.8
The first defenses appear to have been ditches fronted by wooden palisades but, by 3500 BCE, cities were surrounding themselves with walls built from mud brick and stone.9 Askut, Semna, Uruk, and Jericho all had populations numbering in the tens of thousands. Each was protected by tall brick or stone walls and was entered only through heavily fortified gates.10 By 1500 BCE, cities were experimenting with double walls, providing two successive lines of defense. Walls were topped with walkways and protected by battlements, alternating plates and slits, which gave cover to archers shooting down onto attackers. Towers bulged out from the walls at regular intervals, and from them defenders could fire at the flanks, or even the backs, of invading troops attempting to scale or breach the wall. Battlement walkways jutted several feet out from the face of the wall, looming over the space below like a balcony. Holes in the floors of these protruding battlements let defenders drop boulders onto the heads of attackers, pour boiling oil, or even empty chamber pots and garbage onto the faces below.11
The Judean city Lachish exemplified this style of ancient fortification. Perched atop a rock butte, the city sat two hundred feet above the plain below, replete with houses for its eight thousand residents, markets, synagogue, and an eight-hundred-foot-deep stone-lined well.12 A narrow road led up the side of the butte, slipping in between two enormous stone towers—the gatehouse. Each gatehouse tower loomed fifty feet high. Surrounded by jutting battlements with archer slits and floor holes, gatehouses promised intense flanking fire from all sides and above, a deadly crossfire to anyone attempting to force his way through the gate. Inside this gate was another, and then another. All told, the Lachish gatehouses comprised six successive “kill zones”—exposed chambers through which intruders needed to pass and in which they would have been exposed to arrows, oil, and boulders from all sides.13
Wrapped all the way around the outcrop were two tall walls. The outer wall, forty feet high and ten feet thick, circled the city roughly halfway up the escarpment, making climbing the hill virtually impossible or at least dangerously impractical. At the very top of the outcrop rose another city wall, taller and thicker than the first, and both walls were capped with protruding balcony-like battlements.14 Although by no means unusual for the time, the fortifications of Lachish were impressive. Combined with the elevated and inaccessible location of the city atop the rock hill, they made a forbidding barrier to any invading army.
But the Assyrians weren’t just any army—they were unmatched in their day—and they were ready for this challenge. Like siafu, Assyrian society revolved around warfare, and they had huge, well-trained, professional standing armies at their disposal. No one dared face Assyrian archers and chariots in open-field battle, which meant they’d had lots of practice overtaking fortified towns. When the Assyrians advanced on Lachish in 701 BCE, and settled into camp on the adjacent hill, they came prepared to breach its walls.
Assyrian siege strategy applied overwhelming force to many parts of the fortress at the same time. By spreading the defending troops thinly, forcing them to cover multiple attack points simultaneously, the Assyrians dramatically increased the odds that one of their assaults would succeed. But assaulting fortress walls required planning and preparation. The tools needed for successful siegecraft were so big they had to be constructed on-site, and this could require many months of labor. In addition to foot soldiers and charioteers, Assyrian armies brought with them thousands of engineers, trained builders skilled at erecting temporary fortifications around their encampments and at building the towers and tunnels needed to overthrow heavily defended fortresses.15
Assyrian siege towers stood three stories tall, permitting attackers to fight from the same height as the defenders. Wooden battlements lined the top floors, giving archers cover as they fired. Each tower was fronted with a drawbridge that could be dropped into place when the tower got close enough to a wall. Middle levels in the tower carried additional troops prepared to rush up ladders as soon as the drawbridge was lowered. The bottom floor housed a battering ram. A giant iron-tipped log hung suspended from the frame so that it could be swung back and forth into the fortress walls. Like a massive crowbar, a wedged ram could also be rocked side to side, prying stones from the wall once cracks started to form.16
The problem with siege towers was that they had to be pushed into place, and defenders positioned ditches, moats, and outer curtain walls to impede such progress. This meant that engineers often had to first drain and then fill in moats with rocks and soil, and then construct roadways leading up to the walls. In the case of Lachish, the problem wasn’t a moat; it was cliffs, and the forty-foot wall that loomed halfway up it. The embankments beneath the wall were way too steep for siege towers, so the engineers built a ramp.
Stone by stone, the Assyrians assembled a massive causeway two hundred feet wide at its base, tapering as it rose to fifty feet wide when it finally abutted the city wall. When it was finished, the ramp climbed high above the plain below, reaching half way up the cliffs of the escarpment—high enough that the tops of the siege towers lined up evenly with the battlements of the outer city wall. The Assyrians used captured prisoners from nearby towns to pile the stones beneath a barrage of hostile fire, forcing the Lachish defenders to shoot their own countrymen to slow the incessant advance of the ramp.17
At the same time as the ramp was being constructed, engineers built five siege towers, each mounted on giant wooden wheels. They also manufactured dozens of tall ladders with hooks that could be thrown against the walls in haste, as soldiers rushed the perimeter. When all of the pieces were finally in place, the Assyrians launched their coordinated attack. Soldiers inside the bottom floors of the siege towers pushed their contraptions side by side up the ramp toward the city walls. The wooden-sided towers shielded soldiers from arrows, and water-soaked hides covered all exposed surfaces to protect against flame. Rows of archers marched alongside the climbing towers showering the defensive battlements with arrows, and each archer was accompanied by a shield bearer to keep his hands free for firing.18
When the wheeled siege towers reached the Lachish wall, other ladder-bearing soldiers launched additional attacks. Flanked by throngs of rapid-firing archers, foot soldiers with shields, pikes, and swords swiftly scaled the walls on all sides of the city, drawing defenders away from the main gates and the wall at the top of the ramp where the battering rams were smashing. The siege towers crashed through the defenses, and Assyrian soldiers poured into the city proper. The result was a savage and total destruction: city walls torn asunder, buildings demolished, and the city leaders skinned alive. Attackers impaled defending troops on stakes or blinded them with swords, and slaughtered thousands of the city’s inhabitants. Those not killed on the spot were deported, marched by the Assyrians to far-off lands where they lived the rest of their lives as slaves.19 Like the termite colony plundered by siafu, Lachish was gone.
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The similarities between these two battles are striking: both involved a sedentary population defending itself against attack from a larger, invading army. Both cities were surrounded by rigid walls, which restricted
entry to a small number of heavily fortified gates. And, in both cases, the fortifications ordinarily sufficed, protecting inhabitants against the majority of likely threats.
Most armies of Lachish’s day lacked the engineers, supplies, and time necessary for waging a prolonged siege. To starve out a defended city, or breach its walls, required tens of thousands of soldiers camped for many months—sometimes years—in hostile territory a long way from home. The invading army had to protect itself during this process, so they often built a fortress around their camp, which required huge quantities of food and timber. Also, since they would be far from home for an extended period, invaders needed to leave the bulk of their armies behind. That is, their armies had to be so vast, and so well organized, that they could move ten thousand troops to a foreign land without leaving their home cities vulnerable. All told, the logistics involved with successful siege warfare vastly exceeded the capabilities of all but the richest of armies.20 Most of the time, the only option available to an invading force was to storm the walls and rush the gatehouse. And in this situation, gatehouses were spectacularly effective.
Walling a city essentially converts it into a tunnel. By forcing intruders to enter through a narrow gate, a city removed the numerical advantage of invading troops. Regardless of how many soldiers waited outside a city’s walls, only a few could enter at a time and, in the narrow confines of gatehouses, soldiers defended from protected positions—their flanks were shielded by slabs of stone; those of the attackers were not.21
Narrow gates work the same way for termites. The strength of siafu armies lies in their numbers. Masses of ants pour over prey, biting and slicing all at the same time. Unlucky grasshoppers or spiders are overwhelmed because they are attacked by thousands of soldiers at once. Lanchester’s square law helps make sense of this outcome. Just like soldiers concentrating their fire on an opposing army, the simultaneous attack of thousands of ants can overwhelm even the best of fighters. But termite mounds eradicate this numerical advantage, since only a few ants can fit into each entrance at a time. Narrow gateways convert the battlefield from one of mass attack to one of individual confrontations, akin to dung beetles defending tunnels.22 In this type of fight, the better-armed soldier prevails.
Army ant soldiers have big heads and crazy jaws, but the weapons of termite soldiers are even bigger. That’s because army ants have to balance multiple tasks, while termite soldiers do not. Ant soldiers march long distances from their nest during an attack, racing to overtake prey. For them, selection for mobility balances selection for bigger heads and jaws, resulting in a compromise. Termite soldiers, on the other hand, hardly move at all. The one thing they do is block the gatehouse and bite everything that approaches. Termite soldiers are bigger and stronger than siafu, and, inside their rigid tunnels, they win. Termites hold their ground, and the masses of ants move on in search of easier plunder.
Both types of city are safe as long as their walls hold. But when these barriers get broken, bad things happen. For Lachish, it took the coordinated efforts of the world’s most powerful army to crash through the walls. For the termites, it took an aardvark.23 Weighing in at 140 pounds—ten million times heavier than a termite soldier—aardvarks are walking bulldozers, lumbering beasts with strong legs and long claws specialized for digging into termite mounds. Aardvarks tear through the side of a mound, slurping up termites with their long, sticky tongues. They amble on once they’ve had their fill, but the holes they dig take time to repair. If an exposed mound is discovered by siafu, the result can be disastrous. As soon as the walls are breached, tactical advantage returns to attackers, since they’ve got the greater numbers. Invading armies swarm in, and cities perish.
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Throughout this book I’ve compared animal weapons to human weapons.24 I’ve drawn parallels between the historical processes surrounding their evolution, including the environments in which they function, the forces of selection shaping their performance, and the ways in which they change through time. In particular, I’ve suggested that the circumstances conducive to extreme weapons—the ingredients triggering an arms race—and the sequence of stages through which weapon evolution unfolds, are the same regardless. But how similar are these processes, really?
Teeth and horns are a part of the body of an animal. Elk antlers, for example, are manufactured as the animal develops, and the instructions relevant to their construction are written into the elk’s DNA. When a bull makes sperm, the sperm carry copies of his DNA. If he succeeds in fertilizing the eggs of a cow, this DNA provides the template for building the antlers of his sons. Information encoded in the DNA is transmitted from parents to offspring and, as a result, the antlers of progeny resemble the antlers of their dads. The weapon is replicated as it is passed from father to son.25
Cultural traditions—instructions for everything from how to dress, how to act or communicate, and how to build shelter or weapons—also get passed from parents to offspring. And, mapped over expanses of time and place, these cultural traditions clearly change, diversifying from population to population just as body parts of animals do. But cultural information is not encoded in DNA and, for the longest time, biologists drew a line in the sand with “biological” evolution on one side, and “cultural” evolution on the other.26 Now that line is disappearing.
Although I believe it is both illuminating and exciting to equate these processes, it’s worth taking a minute to consider the differences between them. We manufacture our weapons using materials from the environment, and these structures exist as entities separate from ourselves—we can throw them away or modify them, if we want to, whereas animals are stuck with the weapons their bodies grow.
But animals manufacture structures, too.27 Termite fortresses are perfect examples. Beaver dams, bird nests, spiderwebs, and mouse burrows are all manufactured structures. They’re not parts of the bodies of the animals that make them, but they’re replicated just the same. Information relevant to their construction is transmitted from one individual to the next and, as a result, the same structures are erected by generation after generation. Often this information is inherited genetically, but occasionally the details of manufacture are learned through apprenticeship and careful practice—precisely the same processes that occur in humans. All of these structures evolve.
Termite fortresses
A second difference between cultural and biological evolution is that cultural information can be transmitted more widely and rapidly than DNA. Cultural information is usually transferred from parents to progeny, but it doesn’t have to be. Instructions for manufacturing an effective rifle or castle can be taught to an apprentice, carried by garrisons to foreign lands, or even stolen by spies. Since cultural information is learned, rather than inherited, it can be transferred from person to person more liberally than DNA. At least, that’s what we used to think.
It turns out that transmission through DNA is far less strict than biologists initially realized. The sequencing of genomes of more and more species has revealed that fragments of DNA get swapped all the time.28 Bacteria gobble up bits of DNA from other species—even distantly related species, such as viruses, plants, and animals—just like spies steal secrets from foreign governments or corporations. A fifth of the bacterial genome may be borrowed chunks of foreign DNA. It’s easy to ignore bacteria when we consider the world around us. After all, they’re really tiny and hard to tell apart. But the reality is that there may be as many as ten million bacteria species worldwide, including roughly forty thousand species thriving inside your body at this very moment.29 The majority of living things on our planet are bacteria in one form or another, so any concept of biological evolution must acknowledge that bacteria swap information around even more readily than cultures do.
The fact is that DNA is not the only medium of information transfer, and this means that other things, too, can evolve. Some viruses house their genetic code in molecules of ribonucleic acid, or RNA, rather than DNA. Viruses most defini
tely evolve (they also swap parts of their genomes all over the place, as when pieces of avian flu mixed with human flu to form the deadly influenza strain of 1918, or when pieces of pig flu, bird flu, and human flu mixed to form the “swine” flu of 2009).30 Scripts of programming code—self-replicating units in a silica world—evolve in ways stunningly like those of natural populations, despite having no RNA or DNA.31 Consequently, although a means of information transfer and replication is essential for evolution, any number of mechanisms will suffice.
A few things do truly distinguish biological and cultural evolution, however. In biological systems the ultimate source of new variation is mutation, copy errors incorporated into the genetic code when DNA replicates itself during cell division. Mutations don’t happen very often, and when they do, they strike at random.32 Novelty in the design of manufactured weapons can arise through random events, too, such as mistakes that crop up during production. But manufactured weapons are usually modified on purpose, as engineers and designers seek to improve them. Brilliant minds like those of Archimedes, Leonardo da Vinci, and J. Robert Oppenheimer, all strived to develop new and better weapons, and the varieties they experimented with were deliberate. This means that new variation can arise faster in cultural traits than in biological ones, and this variation is more likely to be constructive. Once in place, however, variation is variation, and selection drives evolution just the same.
The most important difference is that success of a cultural trait is not linked to the reproductive success of people who use it. When elk antlers evolve, they do so because some individuals produce more offspring than others. Antlers are copied through reproduction, and winners in the battle for breeding make more copies of their antler type than losers do. Antlers evolve as the elk evolve—the processes are inextricably linked, since the mechanism of replication of the weapons is the same as the mechanism of replication of the elk. When we refer to a “population” of antlers, we also mean the population of elk that grow and wield them.
Animal Weapons Page 17