She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (62 page)

That hard-earned skill has led
Alex Mesoudi, a paleoanthropologist at the University of Exeter, and his colleagues to the conclusion that
Homo erectus
used extremely careful imitation to make their hand axes. A careless hand-ax maker might have been blinded by a flying shard or maimed by a badly aimed chop. Observing skilled toolmakers would give novices
the knowledge necessary to avoid such injuries. Careful imitation could also explain why hand axes managed to stay so similar to each other for over a million years. If
Homo erectus
simply looked over old hand axes to guess how to make them, they would have accidentally introduced little variations to their craft. Over a few thousand years, those mismatches would have caused the hand ax to drift far away from its original shape.

Scientists cannot find clear evidence of
full-blown cumulative culture until our own species emerged about 300,000 years ago in Africa. As
Homo sapiens
spread out across the continent, our ancestors developed new shapes and styles for their stone tools. The tools in Morocco became distinctive from those of Kenya and South Africa. Early humans then started using these stone tools to work other materials, like antlers and eggshells. They combined their creations into new inventions, like spear throwers or nets to hunt for game. Artwork—from geometric decorations on eggshells to human figurines—also emerged.

Neanderthals and Denisovans seem to have never crossed this threshold. Language might be one reason for the difference. Our own ancestors may have gained the power of full-blown speech, making it much easier to cooperate on a hunt for game or a search for tubers. It would have made teaching more effective, too, bringing a precision and depth to lessons.

The benefits of language could account for why our ancestors reached a higher population density than Neanderthals and Denisovans. (You can estimate this by measuring the genetic diversity in samples of ancient DNA.) And
our sheer numbers may have then helped bring about cumulative culture. With more people around, our ancestors had more opportunities to meet people and encounter new ideas they could adopt.

To get a rough estimate of what this early social network would have looked like, Kim Hill, an anthropologist at Arizona State University, and his colleagues interviewed hundreds of hunter-gatherers from two tribes: the Ache of Paraguay and the Hadza of Tanzania. Both tribes are made up of small bands; Hill and his colleagues drew up a list of people in a number of bands and asked the Ache and Hadza men if they had ever met anyone on the list. They concluded that each man had a social universe of about a
thousand people. That's far bigger than the social universe of any other primate; male chimpanzees interact with only twenty other males in their entire lifetime.

Once all these pieces fell into place, cumulative culture promptly exploded. Humans could inherit complicated cultural practices, tinker with them, and pass them accurately onward to future generations. It was
the dawn of a new form of heredity, with some striking parallels to the dawn of the first heredity systems on the early Earth. Life's early genomes were so prone to errors as they replicated that they couldn't get large. Once life gained a faithful form of inheritance, it could leap to complex cells. Humans, likewise, may have crossed a threshold and shot into a new universe of complex culture.

An Aboriginal Australian girl born fifty thousand years ago received a tremendous inheritance from her ancestors. It was made up of genes from her parents, possibly along with some epigenetic marks. She inherited some of her microbiome from her mother, along with the mitochondria that were once independent bacteria billions of years ago. As an infant, she began to hear the language of her group and to learn it. She inherited customs for interacting with people, including rules that applied within her family, and others for strangers. Her mother and sisters probably taught her the vast compendium of knowledge previous generations had amassed about how to prepare meals, how to deliver babies, how to use plants to heal the sick. She inherited an Aboriginal cosmology, one that placed her and everyone she knew in a meaningful place in the world.

She also inherited something else:
a human-altered environment.

The environment shapes every species that inhabits it. As species struggle to survive, they evolve adaptations to their surroundings, whether they're fish gaining antifreeze in the Arctic Ocean, or hummingbirds evolving oxygen-hungry blood for flying over the Andes. But some species can reverse the equation. Even as the environment shapes them, they shape their environment. Elephants, for example, tear down tree branches and split their trunks down the center. Lizards, insects, and other animals can
then invade these trees, which were previously off-limits to them. The rampages of the elephants open light into dense forests, allowing small plants to sprout up, providing food to animals like gorillas and bush pigs. Elephants can convert open woodlands into savannas and keep them cleared and fertilized with their dung. The elephants thus live in a habitat of their own making.

At first, our ancestors had little effect on their environment. They were just apes that could walk on two legs in search of fruit, seeds, and tubers, along with the occasional carcass they scavenged along with hyenas and vultures. But then they started to alter their surroundings. Their first environment-changing tool was probably fire.

The oldest known evidence for the use of fire lies in a cave in South Africa. There, scientists have found bits of burned bone and plant matter dating back a million years. For hundreds of thousands of years, hominins probably made fires only in hearths, probably to cook food. But
Homo sapiens
discovered additional uses for it. By 164,000 years ago, people in South Africa were lighting fires in order
to bake soil, turning it into a rocklike material that could be sculpted into tools. And by 75,000 years ago, there's evidence in South Africa that people were setting grasslands on fire, possibly to clear them for earlier hunting. The flames wiped out the aboveground plants, but also stimulated the underground tubers to grow back at a much greater density.

By the time people reached Australia, they were
using fire to reshape entire landscapes. Aboriginals
walked through grasslands with fire sticks perpetually lit. The continent itself still records the first fire sticks, with a thin new layer of charcoal buried several yards in the earth. Fire was so much a part of Aboriginal life that it enveloped their cosmology. In one creation story, people and animals started out looking very different. A sacred spirit used a fire stick to set the entire world ablaze, and only after they were scorched did living things take on the appearance they have today. When the first Europeans finally set eyes on Australia, it was still burning. “We saw either smoke by day, or fires by night, wherever we came,” Captain James Cook wrote in his journal in 1770.

Aboriginals also used fire as a hunting weapon, torching grasslands to flush out kangaroos, lizards, snakes, and other game. These burns could
last for days. Elsewhere they burned forests to foster the growth of plants they wanted to harvest and animals they wanted to eat. These fires were set carefully, so as not to destroy sacred trees. Aboriginals inherited the rules for burning along with the rest of their culture, and they also inherited the landscape that the fires of their ancestors produced.

Fire was just one of many tools that humans used to hunt. They also invented spears, snares, nets, and fishhooks. They taught their children how to make these new tools, and also taught them how to use them. Culture helped turn humans into umatched hunters, driving species like mammoths and ground sloths to extinction. These large mammals were themselves ecosystem engineers, and so their disappearance had profound impacts on their environments. Some trees grew giant fruits, to ensure their seeds would be spread in dung of the giant creature. Now their seeds fell to the ground close by them. Some ecologists have argued that once Siberia's grasslands lost their mammoths and other giant mammals, moss took over and established today's tundras.

But humans also
influenced plants simply through the cultural practices they developed to eat them. People who lived in rain forests gathered fruits from wild trees and brought them to their camps to prepare for meals. After they moved on, the seeds stayed behind, growing into wild orchards that they could revisit in later years. In Iran, foragers brought wild beans from the hillsides to grow alongside rivers where they'd be easier to harvest. When the foragers collected seeds for the next growing season, they unconsciously favored the plants with the variants that let them grow faster in the new environment. The evolution of the plants was now guided by humanity.

Starting roughly ten thousand years ago, with the end of the last Ice Age, some of these tended plants evolved under human care into domesticated crops. In the Fertile Crescent, wheat, millet, beans, and other plants were transformed. In China it was rice; in Africa, sorghum. In Mexico a weed called teosinte became corn. And in some of these same places, wild animals were domesticated into livestock such as cows, goats, and sheep.

The same capacity for cumulative culture that had already spread humans to all the continents save Antarctica now allowed them to convert the
wild lands around them into farm fields and grazing pastures. Children in these agricultural communities inherited traditions for farming, and they also inherited lands that had been converted from wilderness long before they were born. The Agricultural Revolution lofted our species to a far bigger population than before, leaving some farmers desperate for land. They moved into open territories still inhabited by hunter-gatherers, bringing with them the entire package of agriculture: not just the seeds for crops but also their livestock, their saddles, their hoes, and their inherited wisdom about how to use all of it to harvest food, brew beer, sew leather shoes, and all the rest of their cultural practices. And these farmers continued to accumulate new steps to their traditions. As they learned how to work metals, they could make sickle blades or horseshoes. The environment in which much of humanity was now born had become a domesticated landscape, covered by farms as well as by houses, roads, villages, and cities.

It was cultural heredity that led to an agricultural revolution, and it was that revolution that fostered the practice that gave heredity its name. Heirs began to inherit great wealth from their ancestors.

There was nothing new about parents bequeathing valuable things to their offspring. You could argue that our reptilian ancestors were doing it 300 million years ago. The females stocked their eggs with protein-rich yolk, sacrificing some of their own physical resources to pass down to their young. Feeding on those provisions inside their eggs, our ancestors hatched in a stronger condition, more likely to survive to adulthood. When our ancestors evolved into mammals some 200 million years ago, mothers could bequeath milk as well. And when our ancestors evolved into lemur-like primates, their young grew dependent on even more gifts from their parents, in the form of food and protection. Our ancestors grew even more reliant on their parents over the last few million years, because they were growing increasingly big brains.

Brains, ounce for ounce, demand twenty times more energy than muscle. A human infant needs to channel almost half of the calories it gets
every day to fueling its neurons. The human brain doesn't reach full size until about age ten, but even then it's not done developing. The teenage brain furiously prunes connections between neurons while building long-range links between distant regions. The unique anatomy of the human brain is essential for our unique capacity for cumulative culture. But in the time it takes for the brain to develop, human children need help from their parents to get the necessary fuel.

Children fifty thousand years ago couldn't just raid a refrigerator whenever they got hungry. Someone had to kill an animal for them, or harvest some plants and cook them over a fire. The few societies that survive today on hunting and gathering can offer a few hints about what things were like when all humans lived that way. At an early age, hunter-gatherer children start helping to find food and to prepare it. But they still eat more calories than they bring in. This deficit shrinks as they get older and can work harder. But it only turns to a surplus
when they reach their late teens. Until then, families have to work together to make up for the deficit of their children—not just the parents, but grandparents as well.

Some families fare better at this work than others. They may make better arrows, which allow them to take down more game, for example. Hunter-gatherer societies keep this inequality in check with a system of moral judgments. A successful hunter who doesn't share some of his meat with other families will suffer a blow to his reputation. But
such rules only rein in inequality; they don't eliminate it. In hunter-gatherer societies, children from successful families still end up getting more food and enjoy better health than children from other families. Their families ally themselves to a bigger social network, one that can provide more help during a drought or some other disaster. Under the right conditions, this inequality in hunter-gatherer societies can grow from one generation to the next.

Anthropologists have documented one particularly striking case of such inequality on Vancouver Island. The Nootka people have lived there for at least four thousand years, catching salmon that swim up the island's rivers to spawn. They could smoke enough fish to feed themselves and still have plenty left over to trade with inland tribes. It was a prosperous way of
life, giving the Nootka enough resources to build massive wooden houses and totem poles to honor their ancestors. But it did not leave the Nootka in an egalitarian utopia.

The Nootka chiefs held sway over extended families, controlling the best fishing sites on the rivers. Heredity justified and extended their power. Each chief inherited his authority from a distant spirit ancestor. He marked the inheritance of that power by his children with a series of extravagant feasts. Powerful Nootka families grew more powerful over the generations, while others fell into debt. Some Nootka became so destitute that they had to surrender themselves into slavery, moving their families into the houses of their masters.