James' World 2

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The Indigenous peoples of the Bolivian highlands are survivors. For thousands of years, they have lived at altitudes of more than two miles, where oxygen is about 35 percent lower than at sea level. This type of setting is among the harshest environments humans have ever inhabited. Scientists have recognized for some time that these residents of the Andes Mountains have evolved genetic adaptations to the thin air of their lofty home. Now, researchers are learning that they have also evolved another remarkable genetic adaptation since their ancestors first settled the highlands of South America around 10,000 years ago.

In the volcanic bedrock of the Andes, arsenic is naturally abundant and leaches into the drinking water. The dangers it poses are well known: inorganic arsenic is associated with cancers, skin lesions, heart disease, diabetes, and infant mortality in other populations. But the biochemistry of Andeans has evolved to efficiently metabolize this notoriously toxic substance. Populations in Bolivia—along with groups in Argentina and Chile—have evolved variants around the gene AS3MT, which makes enzymes that break down arsenic in the liver. It is a prime example of natural selection, the evolutionary process by which organisms adapt to their environments to survive longer and produce more offspring. Apparently, natural selection among the Uru, Aymara, and Quechua peoples of the Bolivian Altiplano took DNA sequences that are present but rare in other populations around the world and increased their frequency to the point where the normally uncommon sequences are predominant in these groups. The case is one of many discoveries of relatively recent biological adaptation that could upend a long-standing idea about the evolution of our species.

For most of the 21st century, many evolutionary biologists have assumed that humans evolved at a leisurely pace in recent millennia, in contrast to the dramatic transformations that occurred earlier in our prehistory. The oldest known members of the human family evolved in Africa around six million to seven million years ago and looked apelike in many ways. Our own species, Homo sapiens, arose in Africa a few hundred thousand years ago and began venturing into other parts of the world in significant numbers around 60,000 years ago. By that point, our physical appearance seems to have settled into an evolutionary plateau, with only minor differences among human populations around the globe. After natural selection had worked its wonders for millions of years, transforming small-brained quadrupeds into large-brained bipeds, it appeared that biological evolution had slowed to a crawl in our lineage as H. sapiens developed agriculture, founded civilizations, and transformed the planet.

Early studies of the DNA of modern people turned up few fixed differences—genetic variants possessed exclusively by one population, which seemed to confirm this apparent stasis. Consequently, many scholars believed that the latest chapter of the human saga revolved around cultural changes rather than biological ones—figuring out more reliable means of obtaining food instead of changing our digestive or metabolic systems, for instance.

But advances in the sequencing of ancient and modern DNA have allowed scientists to look more closely at how our genetic code has evolved over time, and the results are startling. Genetic studies suggest that H. sapiens experienced many major episodes of natural selection in the past few thousand years as our ancestors fanned across the globe and entered new environments containing foods, diseases, and toxic substances they had never before encountered. “It shows the plasticity of the human genome,” says Karin Broberg of the Karolinska Institute in Sweden, who studies the genetics of susceptibility to environmental toxic substances. “We’ve spread throughout the world, and we live in very extreme environments, and we’re able to make them our homes. We are like rats or cockroaches—extremely adaptable.” This research offers fresh insights into how our species conquered every corner of the planet. We didn’t manage this feat through cultural adaptation alone, as some scientists previously supposed. Rather, humans continued to evolve biologically to keep pace with the radical changes they were making in their ways of life as they pushed into terra incognita.

To appreciate how these evolutionary changes came about, it helps to know the basics of how DNA is structured and how it can vary among individuals and populations. The human genome contains about three billion nucleotide base pairs, the matched sets of two complementary nucleic acids that form the basic unit of our genetic code. The DNA sequences of people today are extremely similar; we differ on only about one tenth of a percent of the genome, or about one out of 1,000 positions. A difference between two people at any position on the genome is called a single nucleotide polymorphism, or SNP (pronounced “snip”). A variant of genetic code, which may be a single position or thousands, that differs between individuals is called an allele. In general, human populations share most of the same genetic variation and evolutionary history.

New research raises the possibility that recent human history involved far more dynamic evolution than previously thought.

In Darwinian biology, the classic conception of natural selection is a “hard sweep,” in which a beneficial mutation allows some individuals to survive longer or produce more offspring, such that eventually that variant becomes fixed in the population. In the early 2000s, when researchers were starting to look for signs of hard sweeps in the genomes of contemporary peoples, the clearest examples came from populations that had adapted to unique circumstances. For instance, around 42,000 years ago, a selective sweep changed a protein on the surface of red blood cells in Africans to boost their resistance to malaria. People in the Tibetan Highlands underwent selective sweeps for genes that helped them tolerate low oxygen (intriguingly, populations of the Himalayas, Andes, and Ethiopian highlands adapted to high altitude with different assortments of genes, taking different evolutionary paths to solve similar problems).

Some of the best-known selective sweeps happened in western Eurasia and involved alleles associated with diet, skin pigmentation, and immunity. Many of these sweeps are linked to the profound shifts wrought by the transition to agriculture. Around 8,500 years ago, early farmers spread an allele that helped them synthesize long-chain polyunsaturated fatty acids from plant-based foods. These fatty acids are essential for cell membranes, particularly in the brain, and hunter-gatherers obtained them easily from meat and seafood. The new genetic variant allowed agricultural populations to synthesize them from short-chain fatty acids found in plants. This variant was rare at first, but now it is present in about 60 percent of Europeans.

Likewise, as dairy farming rose, so, too, did a gene variant that helped people consume milk products into adulthood. When Stonehenge was built around 5,000 years ago, virtually no Europeans possessed the genes people need to digest milk as adults. In most mammals—and most human populations—the body ceases producing the milk-digesting enzyme lactase after weaning. Yet around 4,500 years ago, a gene that kept the lactase turned on in adulthood began to spread through Europe and South Asia. Another series of sweeps beginning around 8,000 years ago gave Eurasians their distinctive pale complexion. These changes reduced their production of the dark skin pigment known as melanin, which is believed to have allowed more sunlight to penetrate their skin and help them synthesize vitamin D, a nutrient in short supply among early agriculturalists.

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Chris Gash

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