0:00:06.957,0:00:13.088
The Tibetan high plateau lies [br]about 4500 meters above sea level,

0:00:13.088,0:00:17.058
with only 60% of the oxygen found below.

0:00:17.058,0:00:21.390
While visitors and recent settlers [br]struggle with altitude sickness,

0:00:21.390,0:00:24.749
native Tibetans sprint up mountains.

0:00:24.749,0:00:27.860
This ability comes not from training [br]or practice,

0:00:27.860,0:00:31.590
but from changes to a few genes [br]that allow their bodies

0:00:31.590,0:00:34.720
to make the most of limited oxygen.

0:00:34.720,0:00:37.660
These differences are apparent from birth—

0:00:37.660,0:00:41.510
Tibetan babies have, on average, [br]higher birth weights,

0:00:41.510,0:00:43.690
higher oxygen saturation,

0:00:43.690,0:00:49.400
and are much likelier to survive than [br]other babies born in this environment.

0:00:49.400,0:00:52.840
These genetic changes are estimated [br]to have evolved

0:00:52.840,0:00:57.167
over the last 3,000 years or so, [br]and are ongoing.

0:00:57.167,0:00:59.364
That may sound like a long time,

0:00:59.364,0:01:05.737
but would be the fastest an adaptation [br]has ever evolved in a human population.

0:01:05.737,0:01:08.857
It’s clear that human evolution[br]isn’t over—

0:01:08.857,0:01:10.937
so what are other recent changes?

0:01:10.937,0:01:16.562
And will our technological and scientific[br]innovations impact our evolution?

0:01:16.562,0:01:18.462
In the past few thousand years,

0:01:18.462,0:01:23.345
many populations have evolved genetic [br]adaptations to their local environments.

0:01:23.345,0:01:29.943
People in Siberia and the high arctic are [br]uniquely adapted to survive extreme cold.

0:01:29.943,0:01:32.013
They’re slower to develop frostbite,

0:01:32.013,0:01:36.097
and can continue to use their hands [br]in subzero temperatures

0:01:36.097,0:01:38.517
much longer than most people.

0:01:38.517,0:01:41.817
They’ve undergone selection [br]for a higher metabolic rate

0:01:41.817,0:01:44.737
that increases heat production.

0:01:44.737,0:01:50.418
Further south, the Bajau people [br]of southeast Asia can dive 70 meters

0:01:50.418,0:01:54.418
and stay underwater [br]for almost fifteen minutes.

0:01:54.418,0:01:58.301
Over thousands of years living [br]as nomadic hunters at sea,

0:01:58.301,0:02:05.282
they have genetically-hardwired unusually [br]large spleens that act as oxygen stores,

0:02:05.282,0:02:08.522
enabling them to stay underwater [br]for longer—

0:02:08.522,0:02:13.482
an adaptation similar [br]to that of deep diving seals.

0:02:13.482,0:02:16.232
Though it may seem pedestrian [br]by comparison,

0:02:16.232,0:02:20.009
the ability to drink milk [br]is another such adaptation.

0:02:20.009,0:02:23.159
All mammals can drink [br]their mother’s milk as babies.

0:02:23.159,0:02:28.128
After weaning they switch off the gene [br]that allows them to digest milk.

0:02:28.128,0:02:33.374
But communities in sub-Saharan Africa, [br]the middle east and northwest Europe

0:02:33.374,0:02:38.324
that used cows for milk have seen [br]a rapid increase in DNA variants

0:02:38.324,0:02:44.206
that prevent the gene from switching off [br]over the last 7 to 8000 years.

0:02:44.206,0:02:48.876
At least in Europe, milk drinking may [br]have given people a source of calcium

0:02:48.876,0:02:53.016
to aid in vitamin D production, [br]as they moved north and sunlight,

0:02:53.016,0:02:56.769
the usual source of vitamin D, [br]decreased.

0:02:56.769,0:02:59.207
Though not always in obvious ways,

0:02:59.207,0:03:04.223
all of these changes improve people’s [br]chance of surviving to reproductive age—

0:03:04.223,0:03:06.463
that’s what drives natural selection,

0:03:06.463,0:03:10.007
the force behind all these [br]evolutionary changes.

0:03:10.007,0:03:13.507
Modern medicine removes [br]many of these selective pressures

0:03:13.507,0:03:16.207
by keeping us alive when our genes,

0:03:16.207,0:03:19.097
sometimes combined [br]with infectious diseases,

0:03:19.097,0:03:20.667
would have killed us.

0:03:20.667,0:03:25.057
Antibiotics, vaccines, clean water [br]and good sanitation

0:03:25.057,0:03:28.893
all make differences between our genes [br]less important.

0:03:28.893,0:03:32.424
Similarly, our ability to cure [br]childhood cancers,

0:03:32.424,0:03:36.690
surgically extract inflamed appendixes, [br]and deliver babies

0:03:36.690,0:03:41.223
whose mothers have life-threatening [br]pregnancy-specific conditions,

0:03:41.223,0:03:46.014
all tend to stop selection by allowing [br]more people to survive

0:03:46.014,0:03:48.144
to a reproductive age.

0:03:48.144,0:03:52.272
But even if every person on Earth [br]has access to modern medicine,

0:03:52.272,0:03:55.246
it won’t spell the end of human evolution.

0:03:55.246,0:04:00.192
That’s because there are other aspects [br]of evolution besides natural selection.

0:04:00.192,0:04:02.452
Modern medicine makes genetic variation

0:04:02.452,0:04:05.692
that would have been subject [br]to natural selection

0:04:05.692,0:04:09.756
subject to what’s called [br]genetic drift instead.

0:04:09.756,0:04:14.902
With genetic drift, genetic differences [br]vary randomly within a population.

0:04:14.902,0:04:19.970
On a genetic level, modern medicine [br]might actually increase variety,

0:04:19.970,0:04:24.270
because harmful mutations don’t kill [br]people and thus aren’t eliminated.

0:04:24.270,0:04:29.315
This variation doesn’t necessarily [br]translate to observable, or phenotypic,

0:04:29.315,0:04:32.605
differences among people, however.

0:04:32.605,0:04:36.338
Researchers have also been investigating [br]whether genetic adaptations

0:04:36.338,0:04:40.018
to a specific environment [br]could appear very quickly

0:04:40.018,0:04:44.408
through epigenetic modification: [br]changes not to genes themselves,

0:04:44.408,0:04:48.685
but to whether and when certain genes [br]are expressed.

0:04:48.685,0:04:51.409
These changes can happen [br]during a lifetime,

0:04:51.409,0:04:54.030
and may even be passed to offspring—

0:04:54.030,0:04:58.780
but so far researchers are conflicted [br]over whether epigenetic modifications

0:04:58.780,0:05:02.296
can really persist over many generations

0:05:02.296,0:05:06.550
and lead to lasting changes [br]in populations.

0:05:06.550,0:05:10.302
There may also be other contributors [br]to human evolution.

0:05:10.302,0:05:12.990
Modern medicine and technology [br]are very new,

0:05:12.990,0:05:17.420
even compared to the quickest, [br]most recent changes by natural selection—

0:05:17.420,0:05:22.307
so only time can tell how our present [br]will shape our future.