The Big Ratchet: How Humanity Thrives in the Face of Natural Crisis (8 page)

Regardless of the precise reasons why brain size grew, there’s no doubt that humans have the brain capacity to be stellar social learners. But many other species, albeit with smaller brains in relation to their body size, are social learners as well. The puzzle is why our species, and not our close ape relatives, is so agile and adaptable, with the capacity for cultures to develop around the world.

In the 1930s, Winthrop and Luella Kellogg set out to answer this question in a way that
few parents would dare. They raised their son, Donald, who was just ten and a half months old when the experiment began, alongside Gua, a female chimpanzee that had been born just
three months after Donald. It turned out that Donald learned more from Gua than the other way around. Donald learned to walk on his knuckles, to scrape his teeth against walls, and to grunt like a chimpanzee. Gua learned to comb her hair. Donald imitated chimp behavior with remarkable accuracy. Gua was far less successful in imitating human behavior. The experiment lasted for nine months.

More recently, researchers presented a puzzle to groups of capuchin
monkeys, chimpanzees, and children. All three species are capable of high degrees of social learning. The researchers tasked them with figuring out how to open a box by turning dials and pushing buttons. Food was the reward. By the end of the experiment, only one of thirty-three chimpanzees, and none of the capuchins, had solved the puzzle, while fifteen of thirty-five children had figured out how to push the right buttons and turn the right dials. The children had imitated each other with great accuracy and cooperated as a group. The chimpanzees and capuchins had not.

The verb “to ape” implies that chimps, gorillas, and orangutans—but not humans—are masters at copying and imitating. But humans are actually the masters at aping, and children are the best imitators of all. Our exceptional abilities to imitate, learn, and cooperate add a crucial strategy to the communication toolkit: cumulative learning.

Cumulative learning allows the inheritance of skills not possible for one person alone to develop. The Japanese macaque Imo displayed great intelligence in figuring out how to wash sand from her potatoes in the stream. But the skill did not go much further. Other macaques learned to wash potatoes, but no other macaque improved on the technique or taught the improved skill to others. In cumulative learning, skills are not just passed from one generation to the next and between peers, but generations can improve or modify the skills and pass on the improved versions. Innovations can build up more quickly in cumulative learning than in hard-wired natural selection; trial-and-error learning, which
restarts with each generation; or social learning that does not spread beyond a few peers. Cultures can adapt to changing conditions that might otherwise spell calamity for a species or, even worse, extinction. The ratchet can turn on a time frame not possible for other species.

We are not the only species that takes advantage of cumulative learning, though the strategy is far less common than genetic inheritance, trial and error, or social learning from peers and parents. Successive generations of songbirds, for example, modify and pass on distinct song dialects for cumulative learning, and chimpanzees in the Congo can improve their technology to unearth termite nests by puncturing the ground with a stout stick before probing the
ground with a slender stick. But humans have taken cumulative learning to a level unprecedented in other species. As a result, our species is agile, adaptable, and capable of the ingenuity it has used to dominate the world.

Cumulative learning gives rise to cultures and their many human-made institutions, including laws, marriage, money, mascots, and icons. The very definition of culture, at least as given by Richerson and Boyd, as “information capable of affecting individual’s behavior that they acquire from other members of their species through teaching, imitation, and other
forms of social transmission,” relies on cumulative learning. Cumulative learning explains why the Inuit can survive in the harsh climate and Franklin’s crew could not, despite being members of the same species.

Culture based on cumulative learning is so strong in humans that it can change the
direction of natural selection itself. The ability of people from different cultures to digest lactose in milk is a clear example. Mammals generally produce the enzyme lactase needed to digest lactose only in their early years when drinking mother’s milk. People who have lost the ability to produce the enzyme know the unpleasant gastrointestinal consequences of drinking milk without it. In cultures where dairy products are not consumed, there’s no need for the enzyme beyond infancy,
and the body generally stops producing it after weaning. The majority of adult humans around the world have lost the ability to produce the enzyme—but some do produce it, about
thirty-five out of every hundred people. Most lactose-tolerant adults descended from northern Europeans and pastoral cultures in Africa and the Middle East.

The ability to produce the enzyme beyond infancy harks back to cultures that domesticated goats, sheep, and cows and made dairy a part of their farming. Only someone with a genetic mutation could produce the crucial dairy-digesting enzyme and reap the nutritional benefit as a weaned child and adult. Those with the mutation were more likely to survive and pass their genes to their offspring than those without it. With more people with the beneficial genes surviving, the culture nudged further toward farming practices with more dairy. In turn, more lactase-producing adults benefited to pass those genes to their offspring. And on it went until a large portion of the population had the gene and dairy farming was firmly established as part of the culture. Cheeses for the French, raw cow’s milk for the East African Masai, and a variety of soured milks for the Scandinavians are today the norm. Culture and evolution intertwined, one spurring the other. Culture alters genes, and genes alter culture. The cumulative learning that gave rise to dairy farming cultures changed the course of evolution for Europeans and for Middle Eastern and African pastoralists.

Cumulative learning is so embedded in our daily lives that we hardly take notice. Common wisdom admonishing people not to “reinvent the wheel,” or encouraging people to “stand on the shoulders of giants,” acknowledges the centrality of cumulative learning for human culture. The path to humanity’s cumulatively learned culture began roughly 8 million years ago, when humans, chimpanzees, bonobos, and gorillas all shared a common
tree-climbing African ancestor. At some point during the next 4 million years, early human ancestors came down from the trees, started to walk upright, and began the journey from an ordinary mammal to a world-dominating species.

Approximate dates for the evolution and extinction of early human species

Tools, Fire, and Words

At least five other species in the human family, all of whom shared the distinguishing characteristic of large brains in proportion to their bodyweight, existed within the past 2.5 million years. They spread far and wide from their origins in Africa, moving into central and eastern Asia, then Europe and Australia, and finally the Americas.
Homo habilis
, the handyman, was the oldest. This species became extinct about 1.5 million years ago after a 1-million-year span of existence.
Homo erectus
, the upright runner, survived the longest, migrating into Eurasia before going extinct roughly 150,000 years ago. The common ancestor of modern humans and Neanderthals,
Homo heidelbergensis
, was the first to migrate to the cold European climate.
Homo floresiensis
, the Asian hobbit, is the most recent to go extinct, an event that took place about 17,000 years ago. Our species,
Homo sapiens
, is the sole survivor in the human lineage. Our existence, beginning about 200,000 years ago, so far spans merely
one-ninth that of
Homo erectus
.

Each of these early human species was capable of progressive accumulation of knowledge and culture. The first known prehuman stone tools coincide in the fossil record with the existence of the ape-like bipedal australopithecine species with smallish brains. The oldest definitive evidence of stone tools, found in the dry valley of the Gona River in current-day Ethiopia, date back 2.6
million years. Similar tools from 2 million years ago have been found in the
Olduvai Gorge in northern Tanzania. The tools were crafted by hitting one stone against another to make a sharp edge, presumably to cut meat and extract marrow from the bones. Although older tools have not yet been found, other evidence suggests that they did exist. Archaeologists working not far from the Gona riverbed site found two bones from animals the size of modern-day goats and cows with scrape marks that might date the use of tools to cut flesh even further back in time, 3.4 million years.

Humans are hardly the only species to use tools, as the famous anthropologist Jane Goodall demonstrated with her study of chimpanzees in Tanzania’s
Gombe National Park in the 1960s. Chimps, she informed the rest of the world, pull leaves off of small sticks to make termite fishing rods, dip them in the ground, and pull up a mouthful of termites. Many other chimpanzee technologies have since been documented—they crack open nuts with stones, dig with sticks for honey in underground beehives, and sharpen
sticks to spear prey. And it is not only our ape relatives that use tools: crows fashion twigs to extract insect larvae from burrows in trees, and some fish crush sea urchins against rocks
to break their shells.

Homo habilis
, with a more human-like brain than its chimp-like predecessor, crafted stone tools and probably also tools made from wood and other materials that left no trace. Our
Homo sapiens
forebears fashioned hand axes, cleavers, picks, scrapers, and knives out of stone. Around 35,000 years ago,
tool use exploded. Spears, fishhooks, and bows and arrows sculpted from bone, ivory, shell, stone, and wood made hunting a promising endeavor. People could stitch clothes with bone
sewing needles to shield themselves from the cold as groups expanded into more northern climes. They made cave paintings and sculptures, hallmarks of burgeoning culture. Our cumulative knowledge was off to a running start, nudging us toward metals, machines, and complex societies relying on tools for their existence.

It was probably a
Homo erectus
who first had the idea of picking up a smoldering branch. Once embers were in our ancestors’ grasp, they could decide where and when to start a fire. Million-year-old burned bones and ashes in a cave in South Africa may be the earliest evidence of
controlled use of fire. But whenever and wherever our ancestors first controlled fire, the effects rippled through many spheres of
Homo erectus
life. Besides providing nutritional benefits, cooking could have set our ancestors on a path toward cumulative learning, thanks to time together around the campfire that promoted social interaction. Tasks needed to be divided up, some for collecting food, some for preparing it, and some for defending the group against animals and others who might try to steal a free meal. Cooperation and communication went hand in hand with the transition from a diet of raw foods to cooked meals.

Control of fire also ushered in new ways of hunting. A burning torch could set fire to the forest to drive out game, or to grassland to promote green growth that would attract prey. As protection against cold, darkness, and predators, fire may have prodded our human ancestors to live in the open rather than retreating to the safety of trees like the other great apes.

Unlike tool use, control of fire is uniquely human. Humans are the only species to have tamed fire. Of the many ways that we have twisted nature for our benefit, control of fire might be the most far-reaching and profound of them all. It sets our species apart from others, and not only because it allows us to cook our food. Control of fire made clearing land for farming possible, for example. It reverberated into the industrial age, as we learned to ignite coal for energy and went on to pursue civilization-changing innovations such as steam power.