December 16, 2010Link
Let us take a moment to celebrate the fiftieth anniversary of the greatest take-down of human hubris. In November 1960, a 29-year-old British woman named Jane Goodall was wrapping up a long field season among the chimpanzees of Tanzania. She had won their trust, or at least their indifference, and so Goodall could observe the chimpanzees up close, discovering things about their behavior that no one had seen before. One day, walking alone through a valley, she passed by a termite mound with a tree stump nearby. It occurred to Goodall that there was no stump there. She dropped to the ground, realizing that a chimpanzee was crouched over the mound, fifty yards from her. He was eating termites.
"Then," Goodall later wrote, "very deliberately he pulled a thick grass stalk towards him & broke off a piece about 18 inches long."
The chimpanzee, whom she later named Greybeard, turned his back on Goodall, making it impossible for her to see what he was doing with the stalk. He then spotted his human observer and ran off.
Two days later, Goodall returned to the termite mound. Greybeard was back, another stalk in his hand, and now she could see what he was doing. He was dipping the stalk into the termite mound. The enraged termites clamped onto the grass, which Greybeard then drew out.
"It was held in the left hand," she wrote, "poked into the ground, and then removed coated with termites. The straw was then raised to the mouth & the insects picked off with the lips, along the length of the straw, starting at the middle."
At the time, this was a shattering thing to see. Greybeard was using a tool: he was picking out an object from nature and using it to get something he could not get with his own body. But tools were supposed to have been unique to humans. In fact, making tools was supposed to have been crucial for the evolution of humans, full stop. Tools had allowed early humans to kill more animals and dig up more tubers, to farm and build cities. In 1960, while Goodall was wandering the mountains of western Tanzania, computer engineers were unveiling the PDP-1, the first computer that didn't need to be in an air-conditioned room. AT&T was releasing Dataphone, the first commercial modem. The Industrial Age was starting to give way to the Information Age, thanks to humanity's unique gift of tool-making.
Goodall sent a cable to her patron, Louis Leakey, to let him know that our unique gift with tools might not be so unique after all.
Leakey cabled back his astonishment: "Now we must redefine 'tool,' redefine 'man,' or accept chimpanzees as humans."
Fifty years later, the PDP-1 has given way to cloud computing; the Dataphone to WiFi. And over the past five decades, our understanding of animals using tools has changed dramatically, too. As University of St Andrews researchers Amanda Seed and Richard Byrne write in the new issue of Current Biology, Greybeard was not a freakish outlier. In fact, chimpanzees are far from the only animals to make and use tools. Leakey's cable resonates more than ever.
Scientists have observed chimpanzees making a great many tools in the wild. They will gather up stones and carry them to forest kitchens, where they use them to crack open nuts. They sharpen sticks to spear bush babies. They can fashion sandals from leaves to walk over thorny ground cover. Some chimpanzees carry tool kits: a stout stick to break into termite mound, and a slender one to fish out the termites.
Since chimpanzees are our closest living relatives, their newly discovered technology might mean that the origin of tool use should be pushed back, from the oldest hominin stone tools 2.5 million years ago to the common ancestor of humans and chimpanzees some seven million years ago. But reality turns out to be more complicated than that. Gorillas and orangutans use tools as well. Gorillas will poke a stick into water to test its depth. Orangutans use wooden picks to tease out the irritating hairs in fruits before they eat them.
All right then--can we just push tool use back to the common ancestor of great apes, 20 million years ago? Not quite. Capuchin monkeys in Brazil can break nuts with rocks as well. Dolphins can stick sponges on their snouts to protect their delicate skin from the rough sea bottom as they look for food. And on the remote Pacific islands of New Caledonia, even crows are tool makers.
In 1992, Gavin Hunt of the University of Auckland discovered that New Caledonian crows regularly broke off sticks and twisted their tips to create hooks. They then used these hooks to fish for insect larvae that lived in the crevices of candlenut trees. If they can't find the right stick, the leaf of pandanus plants will do. They peck away at the edge of the jagged-edge leaf, so that they can dip it into crevices. When the crows draw the leaves back out, their jagged edges snag insects.
Dolphins, crows, New World monkeys, and great apes: these four groups of animals must have evolved their abilities to make and use tools on their own. Seed and Byrne consider their different technologies to see what these animals have in common. To begin with, their brains need to rewire themselves. When we pick up a knife, our brains use a representation of our outstretched arm to guide it to the right destination. But once we start using the knife to cut an onion, the knife becomes part of that representation, as an extension of our bodies. Studies on monkeys reveal that they reorganize their brains to encompass tools as well. As I write in Brain Cuttings
, some philosophers have argued that we should think of all of our technology today as an extended mind. Google is not out to make us stupid in other words; it's actually more like a vast extension of our memory.
But tool use requires more than just the necessary brain power to handle a tool. An animal has to know why it's using the thing in the first place. New Caledonian crows, for example, don't simply start prying at sticks at the sight of food. They size up the situation first. Alex Taylor, a colleague of Hunt's at the University of Auckland, set up a remarkable experiment to test this ability in crows. He put a piece of meat at the far end of a narrow transparent box. The crows could see the food but not reach it. Near the food box was a second box with a long stick inside. But Taylor put bars on the box so that the crow couldn't reach it with its beak. Finally, Taylor tied a short stick to a piece of string, so that it dangled nearby.
The crows inspected the boxes and the sticks and came up with a solution. They hopped to the string, and pulled it up. They pulled the short stick free and then used it to fish out the long stick. And then they used the long stick to reach the food.
Tool-making also demands a capacity for innovations--for coming up with new solutions that other animals haven't discovered before. New Caledonian crows can bend pieces of wires to make larva-fishing hooks. Innovations require not just the ability to picture a goal, but an understanding of physics--how different objects will function in response to gravity, friction, and other forces.
The capacity to use tools has only evolved a few times, which raises the question, why hasn't it evolved more often? Seed and Byrne observe that tools demands time and effort. While one animal is busy struggling with its equipment, other animals may be able to get food with nothing but their mouths. It's possible that the evolution of tool use has been favored in places where the benefits outweigh the costs--where easy-to-eat food is scarce or the food supply is unpredictable. This hypothesis has some evidence in its favor. Capuchin monkeys in dry forests will use stones to dig for tubers. But in moister forests where fruits are abundant, capuchins have never been seen digging tubers. On the other hand, orangutans use sticks to probe for insects most frequently in places where the insects are most abundant, even when they have other choices for food.
It's also possible that tool use only evolves in lineages that have already acquired skills such as goal-following, planning, and understanding rules. New Caledonian crows are closely related to other species of crows, as well as jays, forming a group called corvids. Other corvids cannot match New Caledonian crows in using tools, but they are capable of other kinds of complex cognition. Scrub jays, for example, can hide seeds in thousands of caches over many square miles, and they can accurately recall the location of every one. Once a lineage evolves this level of cognition, tool-making may then become a possibility.
Social intelligence may also be a prerequisite . At first, social intelligence and tool use might not seem like a very good match. When we think of social intelligence, we might think of organizing a food drive or a start-up company. And when we think of tool-use, we think of sitting alone in front of a computer. But a great deal of evidence suggests that the two kinds of cognition are intertwined. Primates that live in complex societies tend to have bigger brains than those in smaller groups, for example. It's possible that the demands of a social life foster bigger, more powerful brains--which can then be turned to new challenges like tools.
Corvids are also very social birds, and they also have large brains. But they don't share the correlation between group size and brain size found in primates. In fact, New Caledonian crows, the most cognitively impressive of the corvids, actually live in small families, rarely interacting with other families. What they lack in quantity, they make make up for in quality. Unlike many bird species, in which the young leave their parents as soon as possible, juvenile New Caledonian crows stay with their parents for over a year. The juvenile crows don't have to invent tools on their own. Instead, they can watch their parents make tools and use them. This time for learning is probably essential for the crows, because it takes over a year for New Caledonian crows to figure out how to use hooks to get food. In an intriguing parallel, adult chimpanzees also allow juveniles to watch them crack nuts and use other kinds of tools.
The ability that chimpanzees and New Caledonian crows have to learn and to come up with new solutions to old problems has also endowed them with something else: culture. The tools used in one population of crows are different in certain ways from the tools made by other populations. Some populations shape panadus leaves into wide probes, while others make them slender. Still others cut a stepped series of notches in the leaves. Chimpanzee technologies also vary from one site to the next. Not all chimpanzees use rocks to smash nuts, for example. Many populations of apes have unique combinations of tools and techniques that neighboring populations lack.
After our ancestors branched off from other apes, they evolved into the best tool-makers on Earth. But it took a long time. The oldest hominin fossils and the oldest stone tools are separated by four million years. Even then, the tools were just chipped rocks. But then, at an accelerating pace, our ancestors got better at making tools. Our social life may have been helping to push that acceleration--humans are far more aware of each other, and what they're thinking, than other apes. Children were not just able to learn from adult tool-users; the adults could start to teach the children, too. As our brains expanded--thanks in part to our increasingly complex social life--they may have become better able to carry out the thinking required for using tools. Seed and Byrne suggest that one key skill we evolved was keeping many different representations of reality in our heads all at once.
Still, we shouldn't look at animal tool-makers as nothing but living fossils, showing us how simple our skills once were. Animal technology is constantly developing, just like ours. On New Caledonia, for example, it's startling to consider the fact that the candlenut trees where crows use their tools are not native. They were brought from Indonesia by early settlers perhaps a few thousand years ago. Faced with a new source of food, the crows of New Caledonia invented a new kind of technology.
Fifty years after Goodall spotted a fishing chimpanzee, our redefinition continues.
Copyright 2010 Carl Zimmer