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The Tibetan high plateau lies about 4500 meters above sea level,
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with only 60% of the oxygen found below.
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While visitors and recent settlers struggle with altitude sickness,
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native Tibetans sprint up mountains.
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This ability comes not from training or practice,
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but from changes to a few genes that allow their bodies
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to make the most of limited oxygen.
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These differences are apparent from birth—
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Tibetan babies have, on average, higher birth weights,
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higher oxygen saturation,
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and are much likelier to survive than other babies born in this environment.
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These genetic changes are estimated to have evolved
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over the last 3,000 years or so, and are ongoing.
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That may sound like a long time,
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but would be the fastest an adaptation has ever evolved in a human population.
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It's clear that human evolution isn't over—
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so what are other recent changes?
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And will our technological and scientific innovations impact our evolution?
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In the past few thousand years,
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many populations have evolved genetic adaptations to their local environments.
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People in Siberia and the high arctic are uniquely adapted to survive extreme cold.
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They're slower to develop frostbite,
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and can continue to use their hands in subzero temperatures
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much longer than most people.
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They've undergone selection for a higher metabolic rate
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that increases heat production.
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Further south, the Bajau people of southeast Asia can dive 70 meters
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and stay underwater for almost fifteen minutes.
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Over thousands of years living as nomadic hunters at sea,
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they have genetically-hardwired unusually large spleens that act as oxygen stores,
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enabling them to stay underwater for longer—
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an adaptation similar to that of deep diving seals.
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Though it may seem pedestrian by comparison,
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the ability to drink milk is another such adaptation.
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All mammals can drink their mother's milk as babies.
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After weaning they switch off the gene that allows them to digest milk.
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But communities in sub-Saharan Africa, the middle east and northwest Europe
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that used cows for milk have seen a rapid increase in DNA variants
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that prevent the gene from switching off over the last 7 to 8000 years.
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At least in Europe, milk drinking may have given people a source of calcium
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to aid in vitamin D production, as they moved north and sunlight,
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the usual source of vitamin D, decreased.
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Though not always in obvious ways,
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all of these changes improve people's chance of surviving to reproductive age—
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that's what drives natural selection,
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the force behind all these evolutionary changes.
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Modern medicine removes many of these selective pressures
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by keeping us alive when our genes,
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sometimes combined with infectious diseases,
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would have killed us.
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Antibiotics, vaccines, clean water and good sanitation
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all make differences between our genes less important.
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Similarly, our ability to cure childhood cancers,
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surgically extract inflamed appendixes, and deliver babies
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whose mothers have life-threatening pregnancy-specific conditions,
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all tend to stop selection by allowing more people to survive
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to a reproductive age.
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But even if every person on Earth has access to modern medicine,
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it won't spell the end of human evolution.
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That's because there are other aspects of evolution besides natural selection.
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Modern medicine makes genetic variation
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that would have been subject to natural selection
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subject to what's called genetic drift instead.
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With genetic drift, genetic differences vary randomly within a population.
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On a genetic level, modern medicine might actually increase variety,
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because harmful mutations don't kill people and thus aren't eliminated.
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This variation doesn't necessarily translate to observable, or phenotypic,
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differences among people, however.
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Researchers have also been investigating whether genetic adaptations
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to a specific environment could appear very quickly
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through epigenetic modification: changes not to genes themselves,
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but to whether and when certain genes are expressed.
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These changes can happen during a lifetime,
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and may even be passed to offspring—
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but so far researchers are conflicted over whether epigenetic modifications
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can really persist over many generations
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and lead to lasting changes in populations.
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There may also be other contributors to human evolution.
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Modern medicine and technology are very new,
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even compared to the quickest, most recent changes by natural selection—
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so only time can tell how our present will shape our future.