She Has Her Mother's Laugh: The Powers, Perversions, and Potential of Heredity (31 page)

The farmers expanded from the Near East into Turkey and then westward along the
southern edges of Europe. As the farmers cleared land, planted crops, and grazed livestock, some of the hunter-gatherers retreated to less fertile lands, while others interbred, mixing their DNA with that of the newcomers. Over centuries, the isolated populations of hunter-gatherers winked out, while the farmers—now carrying a mix of Near Eastern and European hunter-gatherer genes—settled the entire continent.

One more major wave arrived in Europe 4,500 years ago. The DNA of skeletons from that age have many variants in common with a vanished people from the Russian steppes. Known as the Yamnaya, they tended vast herds of sheep grazing on the grasslands, following them with horses and
wagons. This nomadic way of life was hugely successful for them. They grew so wealthy, they could build enormous funeral mounds for their dead, which they filled with jewelry, weapons, and even entire chariots.

The DNA of Bronze Age Europeans living 4,500 years ago reveals that the Yamnaya or another closely related people moved into Europe from the Russian steppes. They arrived first in Poland and Germany, building walled cities where they carried on their distinct culture. Within a few centuries, the genetic signature of the steppe people had jumped the English Channel into Britain. At first the steppe people stayed genetically distinct from the farmers and hunter-gatherers around them. But by the end of the Bronze Age, the barriers around the steppe people and the rest of Europe collapsed, as they had so often before. Skeletons younger than 4,500 years contain melanges of ancestry: steppe people, Near Eastern farmers, Near Eastern hunter-gatherers, Gravettians, and Aurignacians. After this last major merger, Europe remained a continent of many cultures. But now its people had taken on a genetic profile from which I and other people of European descent draw their ancestry.

Ancient DNA demonstrated that white people do not share some deep, pure genetic bond reaching back to the earliest days of the human occupation of Europe. The earliest
Homo sapiens
to arrive in Europe have no direct connection to living Europeans at all. Living people of European descent can trace their ancestry to the people who came to the continent in a series of waves, separated by thousands of years. These groups were no more closely related to one another than Laplanders are to Indonesians. But in Europe they encountered each other and mixed their genes. Today's Europeans are fairly uniform, genetically speaking. But that uniformity came out of a biological blender.

Ancient DNA doesn't simply debunk the notion of white purity. It debunks the very name
white.

From the beginning,
skin colors were crucial to Western racial categories. The black skin of Africans was merely the outward mark of an inward curse. Along with whites in Europe and blacks in Africa, there were yellow
Chinese and red Indians. People whose skin was too light or too dark for their race aroused suspicion about their ancestry.

Yet skin color is not a timeless hallmark of human races. It has changed in different places and at different times, probably thanks to natural selection in some cases and thanks to the movement of people in others. Skin-color alleles have traveled the world, working their way into different populations. And the range of light skin tones we call white is only a recent development in this history.

People get their particular color from pigment-producing cells in their skin, called melanocytes. Each melanocyte is stuffed with pouches of pigment called melanosomes. One type of pigment is a yellowish red, and the other is a blackish brown. The amount and the balance of these pigments together set a person's skin color. Since there are many ways to make these adjustments—add more pigment to each melanosome, for example, or increase the number of melanosomes—mutations to a number of genes can produce similar colors.

Today, humans range in color from freckly pale to jet-black. The geography of skin color is complicated. Dark skin is hardly unique to Africa; people in Australia, New Guinea, and parts of southern India are just as dark. Nor is Africa itself uniform: While the Dinka of East Africa are among the darkest people on Earth, the San hunter-gatherers of southern Africa are tan.

Since hominin skin doesn't fossilize, we can't say for sure what skin color our ancestors had four million years ago. But if our closest living primate relatives—gorillas and chimpanzees—are any guide, they likely had light skin. At some point, perhaps two million years ago, our ancestors began adapting to life on the African savanna and lost much of their body hair. Once their skin was directly exposed to sunlight, their skin probably started to change color. That's because the ultraviolet rays in sunlight could now more easily strike skin cells. The damage they caused could lead to skin cancer, and could also destroy an essential molecule in the skin called folate. Mutations that added more pigment to the skin could shield our distant ancestors from this harm.

In 2017,
Sarah Tishkoff, a geneticist at the University of Pennsylvania,
led a study to uncover some clues about the early evolution of human skin. She and her colleagues measured the skin reflectance of 1,570 people in Ethiopia, Tanzania, and Botswana. They then scanned the DNA of their volunteers for variants that were common in people with lighter or darker skin. They found eight variants that were strongly associated with skin color.

The researchers searched worldwide DNA databases and discovered that these variants were also present in some populations scattered across the world. By comparing the DNA surrounding these variants, the scientists could estimate how long ago they arose in common ancestors. To their surprise, the researchers found that all eight variants were hundreds of thousands of years old—older than our entire species, in other words.

On its own, this result can't let us know what color the first members of
Homo sapiens
were. Some of the ancient variants darken skin, but others lighten it. It's possible that they were all present together in early humans, giving them a medium color. Or perhaps there were early humans in Africa who were very dark and others who were light-skinned.

Within Africa, these variants experienced strong natural selection in different places. Close to the equator, wearing few clothes, the Dinka and related people evolved dark skin. In southern Africa, the sunlight was less intense. For the San, dark skin might have actually been a liability. While too much ultraviolet radiation is harmful, too little can cause trouble as well. When the rays strike the skin, they supply the energy our cells need to make vitamin D. Dark skin may have interfered with its production among the San, leading them to evolve their tan skin.

Somewhere between fifty thousand and eighty thousand years ago, a small group of humans expanded out of Africa. Tishkoff and her colleagues discovered that the dark-skinned people of southern India, Australia, and New Guinea all carry the same dark genetic variants she and her colleagues found in Africa. It's possible that one wave of migrants carried the genes for dark skin on a journey across the southern edge of Asia and into the Pacific.

Some of the ancient variants for light skin made their way into light-
skinned populations in Asia and Europe. Along with these African inheritances, Asians and Europeans also gained new mutations that altered their skin color more. One mutation, to a gene called SLC24A5, drastically reduced the pigment that melanocytes make. All living Europeans have it, and so do a substantial fraction of Asian populations.

The discovery of ancient DNA in early humans is allowing researchers to get a better sense of how these newer variants arose. In 2014, researchers studied
a hunter-gatherer who lived in Spain seven thousand years ago. They found that he carried mutations for blue eyes, but he lacked the mutations to genes such as SLC24A5 that are known to make the skin of living Europeans light. Scientists therefore suspect that this seven-thousand-year-old hunter from Spain was a dark-skinned, blue-eyed man.

Of course, this was just one man. But when scientists looked at the DNA from other skeletons across Europe, they found that it fit a broader pattern. Across western Europe—in Spain, France, Germany, and Croatia—Europe's hunter-gatherers lacked the skin-lightening mutations found in Europeans today. Farther east, in places like Sweden and the Baltics, scientists have found light-skin mutations in skeletons of other hunter-gatherer groups dating back almost eight thousand years. Meanwhile, to the south, the farmers who expanded into Europe from Turkey had only one light variant, perhaps meaning that they had olive skin. Only about four thousand years ago, as the different populations in Europe started merging, did their skin start to become uniform.

This long lag is puzzling. If all that mattered to skin color was ultraviolet rays, then the early humans who came to Europe should have swiftly evolved to become light-skinned and then stayed that way. Nevertheless, the fact remains that more than forty thousand years passed before people in Europe began to perceive themselves as joined by the paleness of their skin.

The patterns of inheritance that ancient DNA revealed in Europe—of thousands of years of stability disturbed by sudden
mergings of deeply different peoples—have also come to light in other parts of the world. In
India today, for example, just about everyone has inherited a mix of DNA from two distinct groups of ancestors. One of those groups was most closely related to Europeans, Central Asians, and people from the Near East. The other group, a more enigmatic one, was closely related to the people who live on a tiny archipelago in the Indian Ocean, called the Andaman Islands. The two groups came together to produce the Indian population (as we know it today) within the last four thousand years.

Africa today is home to more than 1.2 billion people, and it is also where most of the fossil record of our species and our more distant hominin ancestors can be found. But that does not mean that living Africans are ancient relics of humanity's past. The genetic profile of many Africans today is profoundly different from that of the people who lived in the same places just a few thousand years ago. While the history of humans in Africa may be far longer than in other parts of the world, Africans are the product of their own turbulent migrations and mixings.

Much of the evidence for this turbulence comes from ancient DNA. The fact that scientists managed to get
ancient DNA from African skeletons came as a huge surprise to scientists themselves. They assumed that the hot climate of Africa would wipe out the DNA in skeletons. But it turns out that the mountains of countries like Malawi, Kenya, and Ethiopia are cool enough to preserve genetic material. In 2017, David Reich and his colleagues published details on ancient DNA from sixteen different Africans who lived as long ago as eight thousand years.

Reich's team found that the branches of living sub-Saharan Africans sprouted early in human history. They first began splitting somewhere between 200,000 and 300,000 years ago, not long after the origin of our species. Over hundreds of generations, the hunter-gatherers in the south, east, and west of the continent gained distinctive genetic profiles. But despite those differences, they did not become entirely cut off from each other. Some genes managed to flow for thousands of miles through a network of small bands.

The ancient DNA Reich and his colleagues studied suggests that at some point the eastern hunter-gatherer population expanded both to the west
and east. To the west, Western Africans inherited a substantial amount of their DNA. To the east, these Africans moved out of Africa altogether; their descendants settled across Europe, Asia, and beyond.

But the flow of genes also traveled back into Africa. When Reich and his colleagues studied a three-thousand-year-old girl from a tribe of Tanzanian cattle herders, they found that a third of her ancestry belonged not in Africa but in the Near East, among the first farmers. Younger fossils in Africa revealed that this Near Eastern DNA flowed all the way into South Africa, where it can be found in many living South Africans. These immigrants also appear to have brought the variant in the SLC24A5 gene that makes European skin light—altering the skin of Africans who inherited it.

It is possible that these migrants brought something else: crops and livestock. Meanwhile, elsewhere on the continent, other Africans were also domesticating native plants like yams and bananas. About four thousand years ago a group of farmers and herders, known as the Bantu, began expanding from the present-day border of Cameroon and Nigeria.

Over the next two millennia they expanded east and south, bringing with them iron tools and a distinctive language. The ancient DNA that Reich and his colleagues extracted from fossils shows that in places like Malawi, the Bantu entirely replaced the earlier hunter-gatherers. In East Africa, they remained distinct for generations before merging with the hunter-gatherers there. Only a handful of tiny tribes in East Africa have a strong genetic link to the people who lived there three thousand years ago without any Bantu ancestry. And in southern Africa, only a few small groups of hunter-gatherers still carry a genetic legacy that once stretched throughout the region.

The people of Madagascar, lying off the east coast of Africa, inherited an even more far-flung combination of genes. Half their genetic ancestry comes from East Africans, and the other half from Southeast Asia. It's likely that a small group of people sailing in the Indian Ocean got swept all the way across. A 2016 study was able to trace the Asian ancestors of Madagascar all the way back to
a single village in Borneo.

As scientists sequence more DNA from living people and find more in
ancient skeletons, they will probably find even more movements and mixings. The further back in time we look, the harder it is to make out the outlines of history, but scientists have already discovered the vestiges of more drastic interbreeding: ancient encounters that introduced DNA into our gene pool from Neanderthals and other extinct humans. To find out about my own inheritance, I took my genome to the place where the study of human heredity had gotten off to such a dubious start: Cold Spring Harbor.