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  • The Tibetan high plateau lies about 4500 meters above sea level,

  • with only 60% of the oxygen found below.

  • While visitors and recent settlers struggle with altitude sickness,

  • native Tibetans sprint up mountains.

  • This ability comes not from training or practice,

  • but from changes to a few genes that allow their bodies

  • to make the most of limited oxygen.

  • These differences are apparent from birth

  • Tibetan babies have, on average, higher birth weights,

  • higher oxygen saturation,

  • and are much likelier to survive than other babies born in this environment.

  • These genetic changes are estimated to have evolved

  • over the last 3,000 years or so, and are ongoing.

  • That may sound like a long time,

  • but would be the fastest an adaptation has ever evolved in a human population.

  • It's clear that human evolution isn't over

  • so what are other recent changes?

  • And will our technological and scientific innovations impact our evolution?

  • In the past few thousand years,

  • many populations have evolved genetic adaptations to their local environments.

  • People in Siberia and the high arctic are uniquely adapted to survive extreme cold.

  • They're slower to develop frostbite,

  • and can continue to use their hands in subzero temperatures

  • much longer than most people.

  • They've undergone selection for a higher metabolic rate

  • that increases heat production.

  • Further south, the Bajau people of southeast Asia can dive 70 meters

  • and stay underwater for almost fifteen minutes.

  • Over thousands of years living as nomadic hunters at sea,

  • they have genetically-hardwired unusually large spleens that act as oxygen stores,

  • enabling them to stay underwater for longer

  • an adaptation similar to that of deep diving seals.

  • Though it may seem pedestrian by comparison,

  • the ability to drink milk is another such adaptation.

  • All mammals can drink their mother's milk as babies.

  • After weaning they switch off the gene that allows them to digest milk.

  • But communities in sub-Saharan Africa, the middle east and northwest Europe

  • that used cows for milk have seen a rapid increase in DNA variants

  • that prevent the gene from switching off over the last 7 to 8000 years.

  • At least in Europe, milk drinking may have given people a source of calcium

  • to aid in vitamin D production, as they moved north and sunlight,

  • the usual source of vitamin D, decreased.

  • Though not always in obvious ways,

  • all of these changes improve people's chance of surviving to reproductive age

  • that's what drives natural selection,

  • the force behind all these evolutionary changes.

  • Modern medicine removes many of these selective pressures

  • by keeping us alive when our genes,

  • sometimes combined with infectious diseases,

  • would have killed us.

  • Antibiotics, vaccines, clean water and good sanitation

  • all make differences between our genes less important.

  • Similarly, our ability to cure childhood cancers,

  • surgically extract inflamed appendixes, and deliver babies

  • whose mothers have life-threatening pregnancy-specific conditions,

  • all tend to stop selection by allowing more people to survive

  • to a reproductive age.

  • But even if every person on Earth has access to modern medicine,

  • it won't spell the end of human evolution.

  • That's because there are other aspects of evolution besides natural selection.

  • Modern medicine makes genetic variation

  • that would have been subject to natural selection

  • subject to what's called genetic drift instead.

  • With genetic drift, genetic differences vary randomly within a population.

  • On a genetic level, modern medicine might actually increase variety,

  • because harmful mutations don't kill people and thus aren't eliminated.

  • This variation doesn't necessarily translate to observable, or phenotypic,

  • differences among people, however.

  • Researchers have also been investigating whether genetic adaptations

  • to a specific environment could appear very quickly

  • through epigenetic modification: changes not to genes themselves,

  • but to whether and when certain genes are expressed.

  • These changes can happen during a lifetime,

  • and may even be passed to offspring

  • but so far researchers are conflicted over whether epigenetic modifications

  • can really persist over many generations

  • and lead to lasting changes in populations.

  • There may also be other contributors to human evolution.

  • Modern medicine and technology are very new,

  • even compared to the quickest, most recent changes by natural selection

  • so only time can tell how our present will shape our future.

The Tibetan high plateau lies about 4500 meters above sea level,

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B1 中級 英國腔

人類演化有多快

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    shuting1215 發佈於 2020 年 10 月 08 日
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