WEBVTT 00:00:27.000 --> 00:00:29.000 The far north of our planet 00:00:30.640 --> 00:00:32.800 is a land of extremes. 00:00:37.880 --> 00:00:39.440 When winter takes hold… 00:00:43.160 --> 00:00:46.160 …arctic winds cause temperatures to plummet. 00:00:49.800 --> 00:00:54.080 Even the hardy Alaskan brown bear must take refuge. 00:01:02.520 --> 00:01:05.160 Unable to endure the freezing conditions, 00:01:07.040 --> 00:01:10.280 they are forced to embark on an extraordinary journey. 00:01:15.040 --> 00:01:18.800 As the North Pole tilts into the darkness and cold of winter… 00:01:24.000 --> 00:01:25.680 the bears begin climbing 00:01:27.000 --> 00:01:28.680 high into the mountains… 00:01:32.960 --> 00:01:35.400 to find shelter in caves or dens 00:01:36.600 --> 00:01:38.480 dug into the steep slopes. 00:01:41.640 --> 00:01:43.160 Hundreds of meters up. 00:02:12.680 --> 00:02:14.880 Bears living at the ends of the Earth 00:02:16.760 --> 00:02:20.880 must contend with the incredible challenges each season brings. 00:02:23.920 --> 00:02:24.800 To do it, 00:02:26.320 --> 00:02:30.240 they rely on a very special connection to the universe, 00:02:31.240 --> 00:02:34.680 because written into a bear's biology 00:02:34.760 --> 00:02:36.840 is every critical moment… 00:02:38.400 --> 00:02:41.960 …of Earth's four-and-a-half-billion-year history. 00:02:45.040 --> 00:02:47.520 Events of unimaginable power… 00:02:48.800 --> 00:02:50.040 …and destruction 00:02:51.480 --> 00:02:56.400 that have created a world like nowhere else in our universe. 00:03:33.320 --> 00:03:36.480 This is Luna, a six-year-old female. 00:03:41.480 --> 00:03:43.880 Having reached the safety of her den… 00:03:46.800 --> 00:03:50.000 this will be her home for the next six months. 00:03:57.960 --> 00:03:59.120 A cozy spot 00:04:00.560 --> 00:04:04.400 to see out the winter in hibernation. 00:04:22.640 --> 00:04:24.520 As the days get shorter… 00:04:27.200 --> 00:04:29.880 her body responds to the rhythm of our planet. 00:04:36.360 --> 00:04:38.480 Her heart rate begins to slow… 00:04:48.280 --> 00:04:50.280 …to just eight beats per minute. 00:05:04.880 --> 00:05:06.520 As she settles down… 00:05:08.600 --> 00:05:10.600 her body temperature drops. 00:05:30.800 --> 00:05:35.880 And she falls into a deep, dreamful sleep. 00:05:52.280 --> 00:05:56.760 The powerful connection between bear and universe… 00:05:59.880 --> 00:06:03.520 compels her to spend the harsh winter months underground. 00:06:07.960 --> 00:06:12.080 Blissfully unaware of the long journey ahead. 00:06:17.160 --> 00:06:19.120 Because outside the den… 00:06:22.480 --> 00:06:24.240 …Earth is on the move. 00:06:34.280 --> 00:06:38.400 Every day, the planet spins once on its axis. 00:06:55.320 --> 00:07:00.080 Each month, the Moon completes one full circuit around the Earth. 00:07:07.040 --> 00:07:08.680 And as the months pass, 00:07:09.720 --> 00:07:13.320 the Earth and the Moon orbit the Sun. 00:07:19.200 --> 00:07:21.320 During the time she'll spend asleep, 00:07:22.320 --> 00:07:26.520 Luna will travel 470 million kilometers. 00:07:32.960 --> 00:07:34.920 An epic cosmic voyage 00:07:36.640 --> 00:07:38.640 halfway around the Sun. 00:07:47.560 --> 00:07:48.680 And this year, 00:07:49.560 --> 00:07:52.200 she has some new traveling companions. 00:07:55.560 --> 00:07:58.200 While in and out of sleep… 00:07:59.520 --> 00:08:01.640 …Luna has given birth… 00:08:04.040 --> 00:08:05.880 to two little bears. 00:08:12.320 --> 00:08:16.480 The cubs will eat and sleep in the den until springtime. 00:08:19.600 --> 00:08:21.400 But two more mouths to feed 00:08:23.320 --> 00:08:26.000 is taking its toll on Luna's body. 00:08:33.120 --> 00:08:35.800 Fat cells beneath her skin begin to shrink 00:08:37.440 --> 00:08:40.800 as her energy stores are used up producing milk. 00:08:48.920 --> 00:08:50.960 After five months underground, 00:08:51.840 --> 00:08:55.400 Luna has already lost more than a quarter of her body weight. 00:09:01.120 --> 00:09:04.440 And with outside temperatures still locked below zero… 00:09:06.560 --> 00:09:09.800 Luna urgently needs spring to arrive 00:09:10.640 --> 00:09:14.000 if she is to continue feeding her newborn cubs. 00:09:22.080 --> 00:09:22.960 Luckily, 00:09:24.880 --> 00:09:26.360 Earth has a happy quirk. 00:09:29.800 --> 00:09:31.480 It's tilted on its side. 00:09:35.920 --> 00:09:38.800 And so, as it moves through space, 00:09:39.720 --> 00:09:42.240 at different times of the year, 00:09:42.320 --> 00:09:45.400 different parts of the planet lean toward the Sun, 00:09:47.280 --> 00:09:49.640 causing the seasons to turn. 00:10:00.360 --> 00:10:02.000 Which means, in Alaska, 00:10:03.200 --> 00:10:05.200 something remarkable is happening. 00:10:12.880 --> 00:10:14.760 With each passing day, 00:10:14.840 --> 00:10:17.000 the Sun gets higher in the sky. 00:10:20.840 --> 00:10:23.680 Its warming rays transforming the landscape. 00:10:34.080 --> 00:10:37.920 Just when Luna and her cubs need it most. 00:10:42.520 --> 00:10:46.080 Nerve endings in her skin detect the warmth of the Sun. 00:10:53.880 --> 00:10:55.560 And as the days get longer… 00:11:01.120 --> 00:11:04.000 …specialized cells in the back of her eye 00:11:05.480 --> 00:11:09.000 sense the tiny increases in light levels. 00:11:14.680 --> 00:11:16.960 These secret messages, 00:11:17.040 --> 00:11:19.200 direct from our nearest star… 00:11:22.400 --> 00:11:26.280 tell Luna it's time to wake from hibernation. 00:11:40.720 --> 00:11:45.160 As they signal the gentle turning of our planet's seasons. 00:11:54.320 --> 00:11:56.000 But out in the solar system… 00:11:58.200 --> 00:12:00.040 seasons like Earth's 00:12:01.880 --> 00:12:03.200 are far from typical. 00:12:09.640 --> 00:12:12.160 Our nearest neighbor, Venus… 00:12:14.920 --> 00:12:16.680 …is much closer to the Sun 00:12:17.640 --> 00:12:20.360 and so is much hotter than Earth. 00:12:25.160 --> 00:12:28.120 And because the planet has no tilt, 00:12:28.960 --> 00:12:31.080 it has virtually no seasons. 00:12:35.480 --> 00:12:39.400 Temperatures remain searingly hot all year round. 00:12:44.280 --> 00:12:45.560 Further away, 00:12:46.080 --> 00:12:47.480 out past Earth… 00:12:49.600 --> 00:12:50.920 …is Mars, 00:12:52.880 --> 00:12:55.680 whose orbit causes its distance from the Sun 00:12:56.200 --> 00:12:58.440 to vary dramatically through the year, 00:13:00.160 --> 00:13:01.960 making seasons on Mars… 00:13:03.600 --> 00:13:06.440 …far more extreme than our own. 00:13:09.200 --> 00:13:13.880 During winter, over a quarter of the planet's atmosphere freezes solid. 00:13:15.880 --> 00:13:17.040 While in summer, 00:13:18.120 --> 00:13:20.440 when Mars passes closer to the Sun, 00:13:21.240 --> 00:13:24.280 the increased heat creates dust storms 00:13:24.360 --> 00:13:29.280 so severe they can engulf the entire planet. 00:13:33.360 --> 00:13:34.800 But beyond Mars, 00:13:36.480 --> 00:13:37.680 past Jupiter… 00:13:41.080 --> 00:13:42.000 …and Saturn… 00:13:48.080 --> 00:13:51.000 …three billion kilometers from the Sun, 00:13:52.600 --> 00:13:57.200 Uranus has the most extreme seasons in the solar system. 00:13:59.840 --> 00:14:03.240 This planet spins on its side. 00:14:04.280 --> 00:14:06.240 So as it orbits, 00:14:06.320 --> 00:14:08.480 the summer side faces the Sun… 00:14:12.040 --> 00:14:17.080 …while the winter side won't see daylight for 42 years. 00:14:29.400 --> 00:14:32.800 Only Earth has seasons 00:14:34.440 --> 00:14:37.440 that, even at the extremes of our planet, 00:14:38.200 --> 00:14:40.440 allow life a fighting chance. 00:14:54.520 --> 00:14:56.120 With the arrival of spring, 00:14:57.280 --> 00:14:59.600 Luna's family emerge from the den. 00:15:06.280 --> 00:15:11.000 The cubs can now enjoy their first moments in the Sun. 00:15:35.880 --> 00:15:40.360 But the turning seasons mean the cubs must now set off on a daunting journey. 00:15:48.280 --> 00:15:49.760 Because to find food, 00:15:50.840 --> 00:15:53.480 Luna must lead them back to the lowlands 00:15:54.760 --> 00:15:56.760 on the other side of the mountain. 00:16:13.720 --> 00:16:16.000 It's a grueling climb for little legs. 00:16:23.840 --> 00:16:27.320 With hundreds of meters of ice and rock to overcome. 00:17:21.040 --> 00:17:25.280 Once they're over the summit, the hardest part of the trek is over. 00:17:36.760 --> 00:17:39.920 But survival is far from guaranteed. 00:17:43.560 --> 00:17:45.920 To make it through the rest of the year, 00:17:47.240 --> 00:17:48.840 the family will rely… 00:17:50.000 --> 00:17:52.000 …on Luna's instinctive understanding 00:17:53.200 --> 00:17:54.640 of the seasons ahead. 00:17:58.920 --> 00:18:00.280 Each one… 00:18:02.640 --> 00:18:05.480 a result of Earth's yearly journey through space. 00:18:12.440 --> 00:18:13.840 A journey that began… 00:18:15.920 --> 00:18:18.960 …before the Earth even existed. 00:18:33.640 --> 00:18:39.600 A time when our solar system was little more than ash. 00:18:43.120 --> 00:18:44.120 No Sun, 00:18:44.880 --> 00:18:45.720 no planets, 00:18:46.440 --> 00:18:47.600 and no moons. 00:18:50.360 --> 00:18:53.920 Just a vast cloud of gas and dust. 00:19:07.320 --> 00:19:10.960 Then, in a distant solar system… 00:19:13.400 --> 00:19:14.920 …a star explodes. 00:19:16.920 --> 00:19:21.800 A supernova that sends a shock wave across the galaxy. 00:19:41.280 --> 00:19:46.280 On impact, the shockwave causes the cloud to fall in on itself. 00:19:51.680 --> 00:19:53.440 And begin to rotate. 00:20:00.440 --> 00:20:02.640 As it spins faster and faster, 00:20:04.040 --> 00:20:08.280 the cloud flattens out into a disc. 00:20:10.200 --> 00:20:13.200 A maelstrom of gas and rock. 00:20:16.600 --> 00:20:19.400 In the center, under intense pressure… 00:20:23.040 --> 00:20:24.480 …the Sun is born. 00:20:28.080 --> 00:20:31.840 While further out, rock collides with rock… 00:20:34.080 --> 00:20:36.200 and our planet begins to grow. 00:20:41.080 --> 00:20:43.480 Until after 20 million years, 00:20:44.880 --> 00:20:46.760 Earth is fully formed. 00:20:49.360 --> 00:20:52.720 Still circling the Sun like the cloud from which it grew. 00:20:58.920 --> 00:21:02.280 Orbiting once every 365 days, 00:21:05.520 --> 00:21:07.440 which is how Earth got its year. 00:21:14.240 --> 00:21:16.600 But at this point in the planet's story, 00:21:17.760 --> 00:21:19.520 it's a year without seasons. 00:21:24.080 --> 00:21:26.720 Because the chaos that created them… 00:21:29.200 --> 00:21:31.080 is still to come. 00:21:41.560 --> 00:21:42.680 In Alaska, 00:21:43.640 --> 00:21:45.360 the vibrancy of spring… 00:21:46.600 --> 00:21:49.280 …has given way to lazy summer days. 00:22:01.480 --> 00:22:03.880 The cubs are now five months old. 00:22:06.600 --> 00:22:08.120 And still learning… 00:22:10.400 --> 00:22:11.560 how to be bears. 00:22:25.000 --> 00:22:27.080 Because they're growing so quickly, 00:22:28.040 --> 00:22:29.960 the cubs are constantly hungry. 00:22:36.520 --> 00:22:38.800 And while it might look idyllic… 00:22:41.040 --> 00:22:44.480 summer in Alaska is a time of hardship. 00:22:54.680 --> 00:22:57.520 Without a decent meal to be found in the meadows, 00:22:58.040 --> 00:23:00.200 the bears must make do with grass. 00:23:11.240 --> 00:23:16.240 Which means the cubs still rely on their mother's milk 00:23:16.320 --> 00:23:17.920 for the bulk of their food. 00:23:35.520 --> 00:23:39.800 But Luna hasn't eaten properly since before she went into hibernation, 00:23:41.560 --> 00:23:44.040 so she's running on empty. 00:23:53.080 --> 00:23:56.040 But nearby, out in space, 00:23:57.520 --> 00:23:59.160 help is on the way. 00:24:05.880 --> 00:24:10.400 Because today, our planet has a constant companion. 00:24:13.000 --> 00:24:14.040 The Moon. 00:24:16.320 --> 00:24:18.280 Locked in orbit around the Earth, 00:24:19.720 --> 00:24:22.320 it's our nearest celestial neighbor. 00:24:23.920 --> 00:24:27.960 The largest and brightest object in the night sky. 00:24:32.120 --> 00:24:35.520 And because the Moon is very big and very close… 00:24:37.760 --> 00:24:40.480 it exerts a powerful gravitational pull. 00:24:47.880 --> 00:24:51.480 Strong enough to influence our world. 00:24:56.080 --> 00:24:59.160 It lifts the oceans away from the planet's surface… 00:25:07.520 --> 00:25:09.680 …creating Earth's tides. 00:25:15.960 --> 00:25:18.680 And on a few days around the full and new Moon… 00:25:20.800 --> 00:25:25.400 the lowest summer tides reveal an entirely new landscape. 00:25:29.760 --> 00:25:32.800 Mudflats extend hundreds of meters from the shore. 00:25:39.120 --> 00:25:42.800 Allowing hungry bears access to buried treasure. 00:25:50.560 --> 00:25:54.480 At just the right time for Luna and her young family. 00:26:02.480 --> 00:26:04.720 But finding what's hidden in the mud 00:26:05.480 --> 00:26:08.200 is a job best left to the grown-ups. 00:26:17.080 --> 00:26:21.680 Luna's sense of smell is 2,000 times better than our own. 00:26:24.480 --> 00:26:26.880 Perfect for detecting tasty morsels. 00:26:31.200 --> 00:26:32.680 Soft-shelled clams. 00:26:53.680 --> 00:26:56.200 She's so skilled at prizing them apart, 00:26:57.400 --> 00:27:00.200 she can get through almost one a minute. 00:27:20.440 --> 00:27:24.000 The clams are a welcome supplement to her meager summer diet. 00:27:31.720 --> 00:27:34.600 Vital energy she can pass along to the cubs. 00:27:39.800 --> 00:27:43.360 These gifts, delivered to her by the Moon, 00:27:43.880 --> 00:27:46.760 will help Luna sustain her family through summer. 00:27:49.880 --> 00:27:52.480 But the bear's relationship with the Moon… 00:27:55.440 --> 00:27:58.440 …runs much, much deeper. 00:28:03.240 --> 00:28:08.280 Because it was during the Moon's formation that Earth got its seasons. 00:28:13.880 --> 00:28:16.040 Not long after it formed, 00:28:16.960 --> 00:28:18.760 our planet gained a companion. 00:28:31.000 --> 00:28:32.800 But it wasn't the Moon. 00:28:34.920 --> 00:28:39.080 It was another planet that formed in Earth's orbit. 00:28:44.520 --> 00:28:46.880 A sibling called Theia. 00:28:48.680 --> 00:28:50.800 It was roughly the size of Mars. 00:28:52.760 --> 00:28:56.680 -A vast ball of rock and lava. 00:28:59.440 --> 00:29:01.200 Throughout their childhood, 00:29:03.040 --> 00:29:05.120 Earth and Theia lived in harmony. 00:29:11.320 --> 00:29:16.280 Until gravitational disturbances in the young solar system… 00:29:19.680 --> 00:29:23.400 set the two planets on a collision course. 00:29:28.880 --> 00:29:30.040 In this moment… 00:29:32.040 --> 00:29:35.040 …Earth's future hangs in the balance. 00:29:54.000 --> 00:29:56.040 Theia is obliterated… 00:29:59.480 --> 00:30:02.400 as the impact melts both worlds. 00:30:29.120 --> 00:30:31.400 For an instant in cosmic time… 00:30:34.560 --> 00:30:36.480 our planet had two moons. 00:30:46.360 --> 00:30:53.160 But then gravity dragged one of them back down to be consumed by Earth. 00:31:06.720 --> 00:31:09.880 Leaving only the moon we know today. 00:31:17.040 --> 00:31:21.160 And the collision transformed our planet forever. 00:31:28.280 --> 00:31:30.600 The impact knocked it over to one side 00:31:31.600 --> 00:31:35.040 so that today it spins with a tilt of 23 degrees. 00:31:40.960 --> 00:31:43.760 So, as it moves around in its orbit, 00:31:45.600 --> 00:31:47.040 first one hemisphere 00:31:47.720 --> 00:31:51.640 then the other is angled toward the Sun… 00:31:55.000 --> 00:31:57.880 giving Earth its seasons. 00:32:19.600 --> 00:32:21.920 As summer draws to a close, 00:32:22.680 --> 00:32:25.200 the longer nights are a sign 00:32:25.280 --> 00:32:28.280 that the harsh Alaskan winter is fast approaching. 00:32:32.560 --> 00:32:35.200 The cubs are now seven months old. 00:32:41.120 --> 00:32:45.640 And Luna knows that if they are to survive their long winter sleep, 00:32:46.360 --> 00:32:49.400 the family must build up their fat reserves. 00:33:06.000 --> 00:33:08.240 But the Moon is a constant reminder… 00:33:12.920 --> 00:33:14.400 …help is on the way. 00:33:19.800 --> 00:33:22.960 A thousand kilometers off the coast of Alaska… 00:33:24.960 --> 00:33:28.480 another creature is tuned in to Earth's seasonal rhythm. 00:33:32.200 --> 00:33:34.200 Sockeye salmon. 00:33:36.760 --> 00:33:39.080 After two years roaming the Pacific Ocean… 00:33:42.760 --> 00:33:44.120 …they've reached maturity. 00:33:46.080 --> 00:33:48.400 And now, the changing day length… 00:33:49.600 --> 00:33:52.680 …triggers hormones to be released into their blood. 00:33:55.720 --> 00:33:59.080 Chemical signals that alter their behavior. 00:34:08.080 --> 00:34:09.440 Calling them home 00:34:10.920 --> 00:34:13.800 to breed in the rivers where they were born. 00:34:34.280 --> 00:34:36.080 Luna's exhausted and hungry, 00:34:37.120 --> 00:34:41.600 but still she leads her cubs to the river's edge. 00:34:54.080 --> 00:34:59.000 Because she senses something magical is about to happen. 00:35:04.080 --> 00:35:08.960 Today, the unique clockwork of the seasons 00:35:09.040 --> 00:35:12.400 will deliver a vital lifeline to the bears. 00:35:33.520 --> 00:35:37.680 The Earth has reached a pivotal position in its journey around the Sun. 00:35:41.720 --> 00:35:44.240 And now the Moon raises the tide. 00:35:50.000 --> 00:35:55.280 Providing the final push that drives the salmon into the rivers. 00:36:40.480 --> 00:36:45.680 At long last, the cubs get their first taste of salmon. 00:36:51.160 --> 00:36:54.960 And although a single fish will not see the bears through winter, 00:36:55.920 --> 00:37:01.400 it's the cubs' first real meal in nearly eight months of life. 00:37:18.040 --> 00:37:21.880 The arrival of the first salmon is a relief to Luna. 00:37:24.720 --> 00:37:26.600 Because experience has told her 00:37:27.720 --> 00:37:31.200 there will soon be much richer pickings upstream. 00:37:50.720 --> 00:37:52.720 As the days grow ever shorter, 00:37:54.400 --> 00:37:58.280 the very same hormones that set the salmon off on their journey 00:37:59.200 --> 00:38:01.400 now dramatically transform them. 00:38:08.960 --> 00:38:10.560 Their heads change shape. 00:38:13.000 --> 00:38:15.680 And their bodies blush bright red. 00:38:19.040 --> 00:38:20.960 As in their millions, 00:38:21.480 --> 00:38:25.920 they surge upriver into their spawning grounds. 00:38:36.120 --> 00:38:40.400 And the final piece of the bears' story falls into place. 00:38:49.120 --> 00:38:52.520 A story as old as the planet itself. 00:39:02.680 --> 00:39:04.280 The formation of the Earth… 00:39:09.760 --> 00:39:12.040 the collision that created the Moon… 00:39:15.000 --> 00:39:18.840 and our planet's unique relationship with its moon and star… 00:39:25.400 --> 00:39:31.520 have combined to create a world like nowhere else in the universe. 00:39:33.920 --> 00:39:35.600 A Goldilocks planet. 00:39:38.600 --> 00:39:42.680 The only place we know that's just right for life. 00:40:04.320 --> 00:40:05.360 And for Luna, 00:40:06.000 --> 00:40:09.000 four and a half billion years of cosmic history 00:40:10.920 --> 00:40:14.000 is now distilled into this moment. 00:40:21.760 --> 00:40:27.200 The annual convergence of salmon and bear. 00:40:38.160 --> 00:40:40.400 And the precious opportunity 00:40:42.160 --> 00:40:43.440 to feed her family. 00:40:51.120 --> 00:40:55.080 Luna and her cubs now have what they need to make it through winter. 00:40:57.400 --> 00:41:01.960 And while ultimately, their future on our changing planet is uncertain, 00:41:03.320 --> 00:41:06.480 for this year, the salmon will sustain them… 00:41:10.400 --> 00:41:13.120 as our planet continues on this relentless voyage… 00:41:15.680 --> 00:41:17.000 …around the Sun. 00:41:26.640 --> 00:41:29.720 Next in the story of our universe, 00:41:31.160 --> 00:41:34.720 the ocean wanderer whose life cycle reveals 00:41:35.400 --> 00:41:39.320 how everything in the universe was made. 00:41:41.120 --> 00:41:43.400 Because in every turtle's egg 00:41:44.280 --> 00:41:46.680 are atoms from The Big Bang… 00:41:49.000 --> 00:41:52.560 …forged as stars die. 00:41:56.160 --> 00:41:59.000 But how are stars and planets created? 00:42:00.840 --> 00:42:07.080 And how do the same ingredients transform into living things?