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← Is human evolution speeding up or slowing down? - Laurence Hurst

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Showing Revision 4 created 09/22/2020 by lauren mcalpine .

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