[Script Info]
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[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:06.96,0:00:13.09,Default,,0000,0000,0000,,The Tibetan high plateau lies \Nabout 4500 meters above sea level,
Dialogue: 0,0:00:13.09,0:00:17.06,Default,,0000,0000,0000,,with only 60% of the oxygen found below.
Dialogue: 0,0:00:17.06,0:00:21.39,Default,,0000,0000,0000,,While visitors and recent settlers \Nstruggle with altitude sickness,
Dialogue: 0,0:00:21.39,0:00:24.75,Default,,0000,0000,0000,,native Tibetans sprint up mountains.
Dialogue: 0,0:00:24.75,0:00:27.86,Default,,0000,0000,0000,,This ability comes not from training \Nor practice,
Dialogue: 0,0:00:27.86,0:00:31.59,Default,,0000,0000,0000,,but from changes to a few genes \Nthat allow their bodies
Dialogue: 0,0:00:31.59,0:00:34.72,Default,,0000,0000,0000,,to make the most of limited oxygen.
Dialogue: 0,0:00:34.72,0:00:37.66,Default,,0000,0000,0000,,These differences are apparent from birth—
Dialogue: 0,0:00:37.66,0:00:41.51,Default,,0000,0000,0000,,Tibetan babies have, on average, \Nhigher birth weights,
Dialogue: 0,0:00:41.51,0:00:43.69,Default,,0000,0000,0000,,higher oxygen saturation,
Dialogue: 0,0:00:43.69,0:00:49.40,Default,,0000,0000,0000,,and are much likelier to survive than \Nother babies born in this environment.
Dialogue: 0,0:00:49.40,0:00:52.84,Default,,0000,0000,0000,,These genetic changes are estimated \Nto have evolved
Dialogue: 0,0:00:52.84,0:00:57.17,Default,,0000,0000,0000,,over the last 3,000 years or so, \Nand are ongoing.
Dialogue: 0,0:00:57.17,0:00:59.36,Default,,0000,0000,0000,,That may sound like a long time,
Dialogue: 0,0:00:59.36,0:01:05.74,Default,,0000,0000,0000,,but would be the fastest an adaptation \Nhas ever evolved in a human population.
Dialogue: 0,0:01:05.74,0:01:08.86,Default,,0000,0000,0000,,It’s clear that human evolution\Nisn’t over—
Dialogue: 0,0:01:08.86,0:01:10.94,Default,,0000,0000,0000,,so what are other recent changes?
Dialogue: 0,0:01:10.94,0:01:16.56,Default,,0000,0000,0000,,And will our technological and scientific\Ninnovations impact our evolution?
Dialogue: 0,0:01:16.56,0:01:18.46,Default,,0000,0000,0000,,In the past few thousand years,
Dialogue: 0,0:01:18.46,0:01:23.34,Default,,0000,0000,0000,,many populations have evolved genetic \Nadaptations to their local environments.
Dialogue: 0,0:01:23.34,0:01:29.94,Default,,0000,0000,0000,,People in Siberia and the high arctic are \Nuniquely adapted to survive extreme cold.
Dialogue: 0,0:01:29.94,0:01:32.01,Default,,0000,0000,0000,,They’re slower to develop frostbite,
Dialogue: 0,0:01:32.01,0:01:36.10,Default,,0000,0000,0000,,and can continue to use their hands \Nin subzero temperatures
Dialogue: 0,0:01:36.10,0:01:38.52,Default,,0000,0000,0000,,much longer than most people.
Dialogue: 0,0:01:38.52,0:01:41.82,Default,,0000,0000,0000,,They’ve undergone selection \Nfor a higher metabolic rate
Dialogue: 0,0:01:41.82,0:01:44.74,Default,,0000,0000,0000,,that increases heat production.
Dialogue: 0,0:01:44.74,0:01:50.42,Default,,0000,0000,0000,,Further south, the Bajau people \Nof southeast Asia can dive 70 meters
Dialogue: 0,0:01:50.42,0:01:54.42,Default,,0000,0000,0000,,and stay underwater \Nfor almost fifteen minutes.
Dialogue: 0,0:01:54.42,0:01:58.30,Default,,0000,0000,0000,,Over thousands of years living \Nas nomadic hunters at sea,
Dialogue: 0,0:01:58.30,0:02:05.28,Default,,0000,0000,0000,,they have genetically-hardwired unusually \Nlarge spleens that act as oxygen stores,
Dialogue: 0,0:02:05.28,0:02:08.52,Default,,0000,0000,0000,,enabling them to stay underwater \Nfor longer—
Dialogue: 0,0:02:08.52,0:02:13.48,Default,,0000,0000,0000,,an adaptation similar \Nto that of deep diving seals.
Dialogue: 0,0:02:13.48,0:02:16.23,Default,,0000,0000,0000,,Though it may seem pedestrian \Nby comparison,
Dialogue: 0,0:02:16.23,0:02:20.01,Default,,0000,0000,0000,,the ability to drink milk \Nis another such adaptation.
Dialogue: 0,0:02:20.01,0:02:23.16,Default,,0000,0000,0000,,All mammals can drink \Ntheir mother’s milk as babies.
Dialogue: 0,0:02:23.16,0:02:28.13,Default,,0000,0000,0000,,After weaning they switch off the gene \Nthat allows them to digest milk.
Dialogue: 0,0:02:28.13,0:02:33.37,Default,,0000,0000,0000,,But communities in sub-Saharan Africa, \Nthe middle east and northwest Europe
Dialogue: 0,0:02:33.37,0:02:38.32,Default,,0000,0000,0000,,that used cows for milk have seen \Na rapid increase in DNA variants
Dialogue: 0,0:02:38.32,0:02:44.21,Default,,0000,0000,0000,,that prevent the gene from switching off \Nover the last 7 to 8000 years.
Dialogue: 0,0:02:44.21,0:02:48.88,Default,,0000,0000,0000,,At least in Europe, milk drinking may \Nhave given people a source of calcium
Dialogue: 0,0:02:48.88,0:02:53.02,Default,,0000,0000,0000,,to aid in vitamin D production, \Nas they moved north and sunlight,
Dialogue: 0,0:02:53.02,0:02:56.77,Default,,0000,0000,0000,,the usual source of vitamin D, \Ndecreased.
Dialogue: 0,0:02:56.77,0:02:59.21,Default,,0000,0000,0000,,Though not always in obvious ways,
Dialogue: 0,0:02:59.21,0:03:04.22,Default,,0000,0000,0000,,all of these changes improve people’s \Nchance of surviving to reproductive age—
Dialogue: 0,0:03:04.22,0:03:06.46,Default,,0000,0000,0000,,that’s what drives natural selection,
Dialogue: 0,0:03:06.46,0:03:10.01,Default,,0000,0000,0000,,the force behind all these \Nevolutionary changes.
Dialogue: 0,0:03:10.01,0:03:13.51,Default,,0000,0000,0000,,Modern medicine removes \Nmany of these selective pressures
Dialogue: 0,0:03:13.51,0:03:16.21,Default,,0000,0000,0000,,by keeping us alive when our genes,
Dialogue: 0,0:03:16.21,0:03:19.10,Default,,0000,0000,0000,,sometimes combined \Nwith infectious diseases,
Dialogue: 0,0:03:19.10,0:03:20.67,Default,,0000,0000,0000,,would have killed us.
Dialogue: 0,0:03:20.67,0:03:25.06,Default,,0000,0000,0000,,Antibiotics, vaccines, clean water \Nand good sanitation
Dialogue: 0,0:03:25.06,0:03:28.89,Default,,0000,0000,0000,,all make differences between our genes \Nless important.
Dialogue: 0,0:03:28.89,0:03:32.42,Default,,0000,0000,0000,,Similarly, our ability to cure \Nchildhood cancers,
Dialogue: 0,0:03:32.42,0:03:36.69,Default,,0000,0000,0000,,surgically extract inflamed appendixes, \Nand deliver babies
Dialogue: 0,0:03:36.69,0:03:41.22,Default,,0000,0000,0000,,whose mothers have life-threatening \Npregnancy-specific conditions,
Dialogue: 0,0:03:41.22,0:03:46.01,Default,,0000,0000,0000,,all tend to stop selection by allowing \Nmore people to survive
Dialogue: 0,0:03:46.01,0:03:48.14,Default,,0000,0000,0000,,to a reproductive age.
Dialogue: 0,0:03:48.14,0:03:52.27,Default,,0000,0000,0000,,But even if every person on Earth \Nhas access to modern medicine,
Dialogue: 0,0:03:52.27,0:03:55.25,Default,,0000,0000,0000,,it won’t spell the end of human evolution.
Dialogue: 0,0:03:55.25,0:04:00.19,Default,,0000,0000,0000,,That’s because there are other aspects \Nof evolution besides natural selection.
Dialogue: 0,0:04:00.19,0:04:02.45,Default,,0000,0000,0000,,Modern medicine makes genetic variation
Dialogue: 0,0:04:02.45,0:04:05.69,Default,,0000,0000,0000,,that would have been subject \Nto natural selection
Dialogue: 0,0:04:05.69,0:04:09.76,Default,,0000,0000,0000,,subject to what’s called \Ngenetic drift instead.
Dialogue: 0,0:04:09.76,0:04:14.90,Default,,0000,0000,0000,,With genetic drift, genetic differences \Nvary randomly within a population.
Dialogue: 0,0:04:14.90,0:04:19.97,Default,,0000,0000,0000,,On a genetic level, modern medicine \Nmight actually increase variety,
Dialogue: 0,0:04:19.97,0:04:24.27,Default,,0000,0000,0000,,because harmful mutations don’t kill \Npeople and thus aren’t eliminated.
Dialogue: 0,0:04:24.27,0:04:29.32,Default,,0000,0000,0000,,This variation doesn’t necessarily \Ntranslate to observable, or phenotypic,
Dialogue: 0,0:04:29.32,0:04:32.60,Default,,0000,0000,0000,,differences among people, however.
Dialogue: 0,0:04:32.60,0:04:36.34,Default,,0000,0000,0000,,Researchers have also been investigating \Nwhether genetic adaptations
Dialogue: 0,0:04:36.34,0:04:40.02,Default,,0000,0000,0000,,to a specific environment \Ncould appear very quickly
Dialogue: 0,0:04:40.02,0:04:44.41,Default,,0000,0000,0000,,through epigenetic modification: \Nchanges not to genes themselves,
Dialogue: 0,0:04:44.41,0:04:48.68,Default,,0000,0000,0000,,but to whether and when certain genes \Nare expressed.
Dialogue: 0,0:04:48.68,0:04:51.41,Default,,0000,0000,0000,,These changes can happen \Nduring a lifetime,
Dialogue: 0,0:04:51.41,0:04:54.03,Default,,0000,0000,0000,,and may even be passed to offspring—
Dialogue: 0,0:04:54.03,0:04:58.78,Default,,0000,0000,0000,,but so far researchers are conflicted \Nover whether epigenetic modifications
Dialogue: 0,0:04:58.78,0:05:02.30,Default,,0000,0000,0000,,can really persist over many generations
Dialogue: 0,0:05:02.30,0:05:06.55,Default,,0000,0000,0000,,and lead to lasting changes \Nin populations.
Dialogue: 0,0:05:06.55,0:05:10.30,Default,,0000,0000,0000,,There may also be other contributors \Nto human evolution.
Dialogue: 0,0:05:10.30,0:05:12.99,Default,,0000,0000,0000,,Modern medicine and technology \Nare very new,
Dialogue: 0,0:05:12.99,0:05:17.42,Default,,0000,0000,0000,,even compared to the quickest, \Nmost recent changes by natural selection—
Dialogue: 0,0:05:17.42,0:05:22.31,Default,,0000,0000,0000,,so only time can tell how our present \Nwill shape our future.