She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (75 page)

The Industrial Revolution, which came about ten thousand years after the Agricultural Revolution, was an acceleration of this feedback. Instead of using animal-drawn plows, farmers could run tractors powered with new fuels like gasoline. Instead of spreading manure from their own livestock, they could spread fertilizers extracted from mines or produced from petroleum. The cotton gathered by New World slaves no longer had to be woven by people; it was now turned over to coal-powered looms. As railroads cut across continents, cattle could be grazed on lands thousands of miles from the people who would ultimately eat them. The influence of human culture now produced a worldwide ecological inheritance.

By some measures, this cultural feedback loop has been a great success. Before the Agricultural Revolution, a square kilometer of land could typically feed fewer than ten hunter-gatherers. Today, if it's intensively farmed, it can feed thousands. In the early 1800s, more than 90 percent of the world lived in extreme poverty, scraping by on the equivalent of about two dollars a day. Today, less than 10 percent are. A child born in the United States in 1900 had an average life expectancy just short of fifty years. Children born in 2016 will live, on average, to age seventy-nine.

I feel fortunate that my children get to inherit this world created by cumulative culture. But I can also see that they inherit an environment that is suffering in many respects. Since the dawn of agriculture, three-quarters of the terrestrial biosphere has been converted from wilderness. Somewhere between a quarter and a third of the planet's biological productivity—its
ability to convert sunlight into biomass—is now appropriated for human use. If the same cultural practices that have reworked the planet so dramatically over the past ten thousand years are inherited by future generations, we may push many species to extinction and threaten our own well-being.

Our cumulative culture has even altered the atmosphere. We're not the first organisms to change the chemistry of the air—photosynthetic bacteria began pumping oxygen into the sky two billion years ago, and every generation of living things has had to adapt to an oxygen-rich planet ever since. But it's unheard-of for just one species of animal, using tools of its own making, to manage such a feat.

When hunter-gatherers set fire to meadows or forests, they could loft carbon dioxide and other molecules into the air. Because their numbers were so low, early humans barely nudged the atmosphere's makeup. But once farmers started clearing land for planting, the soils released carbon dioxide at a greater rate. By three thousand years ago, mining operations were belching particles of lead and other pollutants into the air. The traces of this pollution are trapped in Bronze Age layers of ice in Greenland.

The same forces that led to the destruction of most wild land on Earth also polluted the air. By the Industrial Revolution, the pollution had become so thick in cities that it cut millions of lives short. As people began burning coal, oil, and gas, they also flooded the atmosphere with so much carbon dioxide that it began trapping extra heat—enough to raise the average temperature of the entire planet. By the early twenty-first century, humans had raised the level of carbon dioxide in the atmosphere to its highest level in millions of years. In response, the planet had warmed about 2 degrees Fahrenheit since 1880.

Much of the pollution that humans put into the atmosphere washes out swiftly. The lead-laced fumes from gasoline, for instance, disappeared soon after it was barred. Carbon dioxide is different.
It hangs in the atmosphere for centuries, still trapping heat and warming the planet. If tomorrow we cut our emissions of carbon dioxide to zero,
the planet would keep warming another degree or more. Future generations would inherit a planet
with endangered coastlines, increased wildfires, and farmland threatened with drought.

Three million years ago, the bipedal apes who were teaching each other how to break apart rocks were a small part of a vast ecosystem. But the open-ended power of cultural heredity, transferring knowledge across generations, gave humans the power to rework the land into its own ecological inheritance, which has now led to a climatic inheritance.

Today, we sense that we are close to being able to alter human heredity,” David Baltimore declared at the international gene-editing meeting in 2015. He was speaking shorthand, one that an audience at a meeting about CRISPR intuitively understood. To them,
human heredity
was the transmission of genes from human parents to human children. And to them, the looming ability to alter it was a new chapter in human history to be entered with awe and fear.

We certainly need to come to a collective decision about using CRISPR on human embryos, to use it only in ways that help people without creating serious dangers of their own. But this shorthand about heredity poses dangers, too. We risk coming to see ourselves as merely the product of the genes we inherited from our parents, and the future as nothing more than carrying those genes forward. The prospect of altering genetic heredity becomes wildly thrilling or terrifying. Soon no one will suffer from a genetic disease again, we're promised. Soon China will breed an army of supergeniuses, we're assured. This shorthand makes it hard to think clearly about genetic heredity. It leads us to overvalue our ambiguous knowledge of how genes work and dismiss the other factors that shape our lives—and could be reshaped to improve the world.

None of this is to say we should dismiss the power of heredity, or shy away from altering it. Instead, we can switch from a shorthand to a longhand. Thinking about heredity more broadly could lead plant scientists to better crops, for example. The first plant breeders manipulated the genetic makeup of crops, picking out good plants to cross to make better varieties.
In recent decades, plant breeders have become more aware of the genes that their crops inherit.
The epigenetic side of plant biology has only started to emerge, and some plant breeders are starting to investigate how they can tinker with it to improve crops even more.

Plants sometimes naturally change their epigenetic profile. The methylation that decorates its DNA may get altered, a methyl group falling away from a gene, for example. That change may awaken the gene and improve a plant's growth. Scientists are searching for such changes and trying to propagate the plants so that the new generations inherit the same epigenetic profile.

Transgenerational epigenetic inheritance is a real phenomenon in plants, but many scientists are skeptical that it matters very much in nature. It's wrong to call it Lamarckian, because Lamarck had a very different vision for the inheritance of acquired characters. He thought that inheritance could produce intricate adaptations. Skeptics like Robert Martienssen see little evidence for such adaptations in wild plants.

Yet that doesn't mean such adaptations are impossible. In fact, Martienssen told me, he thinks we know enough now about epigenetics to try to build one.

Martienssen can imagine a plant that could respond to disease outbreaks by turning on immune defenses, and then pass down RNA molecules to their offspring to keep those defenses turned on. If, over the generations, the disease faded away, the plants could shut the resistance genes down so that they didn't have to use their energy to make proteins they no longer needed.

“We could easily engineer a plant to be epigenetically adaptive—to be Lamarckian,” Martienssen said.

Thinking broadly about heredity might help us outside of laboratories as well. In the United States, it has proven all too tempting over the centuries to blame poverty and inequality on biology. A woman like Emma Wolverton could be institutionalized for life because she was judged a
genetically doomed moron. The relative poverty of African Americans could be written off, even by some psychologists, as the result of their inheriting the wrong genes.

Others have argued that the gulfs in the United States are the product of the environment into which people are born and grow up. But the word
environment
is too bland to help us understand much about this problem. The stubborn inequalities in the United States are not the result of some people living in a physical environment. Their environment is built by social forces, and those forces last for centuries because they are regenerated across the generations.

After blacks were emancipated from slavery, they still had to contend with structural racism as well as the racist attitudes of individuals. This racism did not keep springing out of the void year after year. Children learned it, either implicitly or outright, from parents and other adults, and then passed it on to their own children. The social environment then shaped the physical environment into which later generations of blacks were born. Housing discrimination and segregation created neighborhoods where children ended up in poorly performing schools, had to contend with far greater odds of getting shot, and had fewer opportunities for work.

Cumulative culture allowed our species to make giant leaps in technological progress, but it also made us prone to inequality. Hunter-gatherers tend to keep these differences in check, although in a society like the Nootka of Vancouver Island, some people ended up impoverished slaves serving wealthy masters. Once farmers began building up surpluses of food, the gulf could begin to open. They could grow not just over the course of one farmer's lifetime but across generations, because now there were goods to inherit. At first, children might inherit farms and stores of grain from their parents; later, gold and houses and other goods enriched them. The Industrial Revolution made the entire world richer, but some people became vastly richer than others. Francis Galton's ancestors built an empire on guns and banking, which left him able to hire all the math tutors he cared to.

In 1931, the historian James Truslow Adams contrasted the United
States with countries like Great Britain by what he called “the American dream.” He defined the dream as being “that life should be made richer and fuller for everyone and opportunity remain open to all.” For much of the 1900s, the United States lived up to that dream fairly well. Immigrants fared better there than they had in their home countries. As the United States grew wealthier, much of that wealth flowed to the poorest half of American citizens and allowed them to climb the economic ladder.
Raj Chetty, a Stanford economist, has estimated that Americans born in 1940 had a 90 percent chance of making more money than their parents at age thirty.

But Chetty and his colleagues have found that those odds then steadily dropped. Americans born in 1984 had only a 50 percent chance of making more than their parents. The shift was not the result of the United States suddenly running out of money. It's just that wealthy Americans have been taking much of the extra money the economy has generated in recent decades. Chetty's research suggests that if the recent economic growth in the United States was distributed more broadly, most of the fading he has found would disappear. “The rise in inequality and the decline in absolute mobility are closely linked,” he and his colleagues reported in 2017.

Inheritance has helped push open that gulf. About
two-thirds of parental income differences among Americans persist into the next generation. Economists have found that American children who are born to parents in the
ninetieth percentile of earners will grow up to make three times more than children of the tenth percentile.

This inheritance is not simply what parents leave in their wills but the things that they can buy for their children as they grow up. In the United States, affluent parents can afford a house in a good public school district, or even private school tuition. They can pay for college test prep classes to increase the odds their children will get into good colleges. And if they do get in, their parents can cover more of their college tuitions.

Poor parents have fewer means to prepare their children to get into college. Even if their children do get accepted, they have fewer funds, and they're more vulnerable to layoffs or medical bankruptcy. Their children may graduate saddled with steep college debts or drop out before getting a degree.

The gifts that children inherit can keep coming well into adulthood. Parents may help cover the cost of law school, or write a check to help out with a septic tank that failed just after their children bought their first house. Protected from catastrophes that can wipe out bank accounts, young adults from affluent families can get started sooner on building their own wealth.

Inheritance also goes a long way to explain
the gap in wealth between races in the United States. In 2013, the median white American household had thirteen times the wealth of the median black household, and ten times that of the median Latino household. In 2017, a team of researchers from Brandeis University and the public policy group Demos sifted through a number of hypotheses that might account for the differences. Going to college didn't close the gap. In fact, the researchers found, the median wealth of white people who didn't finish high school was greater than that of blacks who went to college. Black families actually save money at a greater rate than their white counterparts. Nevertheless, the median white single parent has 2.2 times more wealth than the median black household where there are two parents.

The one big difference the researchers did find was inheritance. Whites are five times as likely to receive major gifts from relatives, and when they do, their value is much greater. These gifts can, among other things, allow white college students to graduate with much less debt than blacks or Latinos. And the effects of these inheritances have compounded through the generations as blacks and Latinos were left outside the wealth feedback loop that benefited white families.

Left unmanaged, these cultural inheritances will roll on, and future generations will be born into systems of economic inequality. The same is true for the environmental inheritance we leave. One of the most important things each new generation learns from the previous one is how to get enough energy to survive. That usually means liberating the Earth's supply of organic carbon and putting some of it in the air. Some people learn how to cut down forests to make charcoal. Others pilot cargo ships across the ocean, leaving a diesel plume trailing behind. If we carry on this way, we
may manage to burn through
the remaining 12 billion tons of fossil fuels tucked away inside our planet by 2250.