Unnatural Selection

Evolution made the cultural revolution possible 50,000 years ago, but human culture became so powerful that it turned the tables: now culture could influence the course of biological evolution. The fitness of genes determines the course of natural selection, but human inventions can change their fitness. Scientists can even see the imprint of our cultures in our DNA.

The coevolution of culture and genes has made it possible, for instance, for some people to drink milk. Among mammals, this is a bizarre gift. One of the main ingredients in milk is a sugar called lactose, and in order to absorb it, mammal intestines produce an enzyme called lactase that cuts it into digestible pieces. As a rule, mammals only make lactase as nursing infants; as they grow, they stop producing it. For a typical adult mammal, making lactase is a pointless exercise, since it has no need to digest milk.

Like other mammals, the majority of humans stop making lactase by the time they grow up. For them fresh milk is disagreeable–instead of being broken down and digested, lactose builds up and becomes food for a thriving colony of bacteria. The wastes that the bacteria excrete cause gas and diarrhea. (Cheese and yogurt have less lactose, so they’re easier to digest.)

Yet some people (including half of all Americans) have mutations in their DNA that have disabled the lactase switch. They go on making the enzyme as adults, and they have no trouble drinking milk. This ability became widespread only after cattle were domesticated 10,000 years ago. Among groups of people who came to depend on cattle herding for their survival–such as tribes in northern Europe or the southern edge of the Sahara–being able to drink milk beyond infancy represented an evolutionary advantage. Mutations that let adults continue making lactase spread through the tribes. But among people who never relied on cattle, such as Australian Aborigines and Native Americans, genes that favored drinking milk conferred no advantage and never became common.

Over the course of history people have not only taken on new diets but have had to face different kinds of diseases. People who have settled in tropical regions had to struggle against malaria, because the mosquitoes that carry the disease thrive there. In malaria‑prone regions, blood diseases that are rare or nonexistent elsewhere become common. People of African and Mediterranean descent, for example, suffer from sickle‑cell anemia. Their red blood cells contain a defective form of hemoglobin, the molecule that the cell uses to carry oxygen from the lungs. When these cells are deprived of oxygen, the hemoglobin collapses and the cell shrinks from a plump bag to a scrawny tube. In this form red blood cells can get stuck in small blood vessels, forming dangerous clots or even tearing the vessels apart. When they pass through the spleen, white blood cells there can sense that they are defective and destroy them. The clots and the loss of red blood cells can make bones rot and retinas detach; ultimately sickle‑cell anemia can kill.

About 150,000 babies in sub‑Saharan Africa are born with the disease annually, and very few survive to childhood. To get sickle‑cell anemia a person has to inherit two copies of a mutant hemoglobin gene, one from each parent. Far more people carry a single copy of the sickle‑cell gene; with a single copy, a person’s hemoglobin is only slightly defective. But given the harm that sickle‑cell anemia causes, it should be far rarer than it is today. The death of people with two copies of the gene should have drained it almost completely out of the human gene pool.

The sickle‑cell gene survives because it can give life as well as take it away. The parasite that causes the nastiest form of malaria, Plasmodium falciparum, invades red blood cells and eats their hemoglobin. It causes intense fevers, and the blood cells it invades can clump up, quickly forming lethal clots. As the parasite devours the hemoglobin in a cell, the cell loses its oxygen. If an infected cell has a defective hemoglobin gene, the loss of oxygen will make it collapse into a sickle. Now it can no longer clump together with other cells, sparing a person the dangerous blood clots. And sickle cells are so clearly deformed that they are quickly destroyed in the spleen, along with the parasites they carry.

People with a single copy of the sickle‑cell gene thus can survive a bout of malaria that might kill someone who lacked it. Natural selection destroys copies of the gene in people who carry two copies, but it spreads them by allowing people with single copies to have children.

The sickle‑cell gene marks the spread of agriculture. Before people farmed, malaria was probably not as important a scourge as it is today. Five thousand years ago, for example, much of sub‑Saharan Africa was covered in forests. The floors of African forests are relatively mosquito‑free, with most species sucking the blood of birds, monkeys, snakes, and other residents of the canopy. But then waves of farmers spread through sub‑Saharan Africa and began turning the forests into fields. Puddles formed in the eroded, exposed earth, providing the perfect breeding spots for Anopheles gambiae, a formerly rare species of mosquito with a taste for humans. Farmers working in fields and sleeping in villages were easy targets for these mosquitoes, and they carried Plasmodium from one person to another. As malaria became more common, defenses against it began to evolve. Sickle‑cell anemia, in other words, is one of the prices the world pays for farming.

While culture can sometimes drive natural selection in new directions, on the whole it may be slowing human evolution down to a crawl. Genes that might once have lowered our reproductive fitness no longer pose such a danger. One in 10,000 babies in the United States, for example, is born with a genetic disorder known as phenylketonuria, which prevents them from breaking down an amino acid known as phenylalinine. As these children eat food, levels of phenylalinine in their bloodstream build up, until it damages their developing brain and causes mental retardation. Once the mutations that cause phenylketonuria would have lowered a person’s fitness. But now parents of children with phenylketonuria have the medical knowledge to save their children. A diet low in phenylalinine will allow their children to grow up with healthy brains. Thanks to medicine and other inventions, we have blunted the stark differences between successful and failing genes, making it harder for natural selection to create much change.

In the future, cultural evolution may slow biological evolution down even more. Natural selection works fastest when there’s a big difference in the reproductive success of individuals–some individuals have no offspring at all, while others have big families. Today people around the world are enjoying better levels of food, health, and income, and as a result, they are raising smaller families. As the differences in human reproductive success dwindle, natural selection becomes weaker.

Another blow against future evolution is the human genome itself. All humans on Earth descend from a small group of people who lived in Africa somewhere between 60,000 and 170,000 years ago. That small founding population had relatively little genetic diversity to speak of, and little evolutionary time has passed for new diversity to emerge. There is more genetic diversity in the chimpanzees that live in the Tai forest of the Ivory Coast than in the world’s entire human population. A few mutations have cropped up in different populations of humans from time to time, and natural selection has been able to nurture these new genes in some places–places where drinking milk or fighting off malaria is important, for example. But genes also have a way of spreading and mixing, as people come into contact with one another. Human history is, above all, a story of mingling.

In Italy, Luigi Cavalli‑Sforza of Stanford University can see the imprint of genes brought to the country by Greek colonists who settled in the boot and in eastern Sicily, of Phoenicians and Carthaginians in western Sicily, of Celts in the north. There are traces of the mysterious Ligurians, subjugated by the Romans, near Genoa; near Tuscany the genes of Etruscans still survive 2,500 years after their civilization vanished. Around Ancona a cluster of genes survives from a civilization that existed 3,000 years ago, called the Osco‑Umbro‑Sabellian civilization. These genes were able to mingle together through Italy during an age when humans could travel only by masted ships and horseback. As transportation has improved, the mingling has only accelerated. Europeans came to the New World, bringing with them African slaves, and the genes of three continents began to mix together. Today airborne immigrants are accelerating the flow of genes around the planet even more. In such a swirling sea of DNA, there is not enough isolation anymore for any new species of human to emerge. For the foreseeable future, the great hominid tree of 15 species or more will remain pruned down to our own. And even within our own species, it may be very hard for natural selection to produce much change.