1
00:00:06,172 --> 00:00:07,757
[thunder rumbles]

2
00:00:10,885 --> 00:00:12,887
[poignant music playing]

3
00:00:16,599 --> 00:00:18,560
[Morgan Freeman] For billions of years,

4
00:00:19,352 --> 00:00:23,148
much of our world
was fiercely inhospitable…

5
00:00:23,231 --> 00:00:25,358
[crackling thunder]

6
00:00:25,442 --> 00:00:29,654
…ravaged by the elements
and the forces of nature.

7
00:00:31,740 --> 00:00:33,241
[howling wind]

8
00:00:34,701 --> 00:00:40,248
But this is the story
of what was happening beneath the waves.

9
00:00:44,294 --> 00:00:49,758
Here, shielded from the storms,
life had taken hold…

10
00:00:49,841 --> 00:00:51,885
[rumbling thunder]

11
00:00:52,510 --> 00:00:57,223
…and was about
to change our planet forever.

12
00:00:57,307 --> 00:00:59,559
[sinister music playing]

13
00:01:10,737 --> 00:01:13,198
[sinister music intensifies]

14
00:01:14,240 --> 00:01:16,242
[music ends]

15
00:01:17,368 --> 00:01:19,370
[rousing music playing]

16
00:01:32,967 --> 00:01:34,677
[low growling]

17
00:01:41,768 --> 00:01:44,187
[snarling]

18
00:01:44,270 --> 00:01:46,356
[trilling]

19
00:01:48,191 --> 00:01:49,442
[screeching]

20
00:01:49,526 --> 00:01:52,529
[rousing music continues]

21
00:02:00,537 --> 00:02:02,539
[growling]

22
00:02:02,622 --> 00:02:04,541
[wind howling]

23
00:02:06,626 --> 00:02:08,628
[rumbling]

24
00:02:10,588 --> 00:02:12,423
[rousing music intensifies]

25
00:02:15,343 --> 00:02:17,345
[music fades]

26
00:02:23,351 --> 00:02:25,353
[ominous music playing]

27
00:02:38,074 --> 00:02:40,368
Far back in the depths of time,

28
00:02:41,035 --> 00:02:44,831
Earth looked nothing
like the world we know today.

29
00:02:50,044 --> 00:02:53,548
Nearly 90% of its surface was water,

30
00:02:54,299 --> 00:02:57,093
yet this was no blue planet.

31
00:03:00,096 --> 00:03:05,852
Instead, vast clouds of methane
had turned the atmosphere yellow.

32
00:03:05,935 --> 00:03:07,937
[thunder rumbles]

33
00:03:09,981 --> 00:03:12,192
[explosive blast]

34
00:03:12,275 --> 00:03:15,153
On land, conditions were even worse.

35
00:03:15,236 --> 00:03:16,696
[repeated blasts]

36
00:03:25,330 --> 00:03:30,376
Volcanoes had been erupting
for millions of years.

37
00:03:30,460 --> 00:03:32,462
[rumbling, creaking]

38
00:03:34,923 --> 00:03:36,591
Their explosive power,

39
00:03:36,674 --> 00:03:41,012
one of the few sounds
in this otherwise silent realm.

40
00:03:41,095 --> 00:03:43,097
[ominous music continues]

41
00:03:51,356 --> 00:03:56,694
With the world made up of nothing
but toxic air and rivers of molten rock…

42
00:03:59,072 --> 00:04:03,034
life as we know it was impossible.

43
00:04:05,161 --> 00:04:07,163
[crackling, hissing]

44
00:04:10,667 --> 00:04:12,669
[stillness]

45
00:04:13,294 --> 00:04:15,672
But out in the prehistoric seas,

46
00:04:16,506 --> 00:04:19,008
something is stirring.

47
00:04:19,092 --> 00:04:21,094
[intriguing music playing]

48
00:04:25,598 --> 00:04:30,520
Beneath the surface
hangs a great swathe of plankton.

49
00:04:37,026 --> 00:04:42,323
Microscopic cells like these
have just evolved something new,

50
00:04:43,950 --> 00:04:45,201
photosynthesis.

51
00:04:50,415 --> 00:04:53,793
This means they can harness
the sun's energy to grow,

52
00:04:55,044 --> 00:04:56,754
a breakthrough so important

53
00:04:58,006 --> 00:05:03,094
that plankton will one day give rise
to every single plant on Earth.

54
00:05:03,177 --> 00:05:05,179
[intriguing music continues]

55
00:05:08,474 --> 00:05:09,976
But that's not all.

56
00:05:11,352 --> 00:05:15,982
One byproduct of photosynthesis is a gas,

57
00:05:17,442 --> 00:05:21,863
the missing ingredient
for animal life to exist…

58
00:05:22,405 --> 00:05:24,490
[rousing music playing]

59
00:05:24,574 --> 00:05:25,575
…oxygen.

60
00:05:30,371 --> 00:05:32,707
For nearly two billion years,

61
00:05:32,790 --> 00:05:38,338
plankton produced so much oxygen
that it transformed our planet.

62
00:05:40,131 --> 00:05:42,133
[stirring music playing]

63
00:05:44,886 --> 00:05:49,349
No longer hostile,
Earth became a new world,

64
00:05:51,225 --> 00:05:54,771
where life could finally run wild.

65
00:05:56,481 --> 00:05:58,566
[music fades]

66
00:05:58,649 --> 00:06:00,735
[wind blowing]

67
00:06:03,446 --> 00:06:06,199
Today, millions of years later,

68
00:06:06,282 --> 00:06:12,413
it's still possible to get a glimpse
of what that next wave of life was like.

69
00:06:18,836 --> 00:06:21,714
Because in some parts of our shallow seas…

70
00:06:24,425 --> 00:06:27,887
life still resembles
that more primitive time.

71
00:06:27,970 --> 00:06:30,973
[whimsical music playing]

72
00:06:37,146 --> 00:06:38,481
They may not look like it…

73
00:06:40,858 --> 00:06:43,027
but these are animals…

74
00:06:47,115 --> 00:06:50,576
very similar
to the first complex life forms.

75
00:06:55,248 --> 00:06:59,710
Most are soft-bodied
and rooted to the seafloor.

76
00:07:03,714 --> 00:07:05,174
All have senses.

77
00:07:06,092 --> 00:07:09,929
And some can even see their surroundings.

78
00:07:10,012 --> 00:07:12,014
[majestic music playing]

79
00:07:14,934 --> 00:07:18,229
Many get their food by filter-feeding,

80
00:07:19,814 --> 00:07:21,232
picking out plankton,

81
00:07:21,899 --> 00:07:25,611
sometimes in the most curious of ways.

82
00:07:27,530 --> 00:07:28,781
[slurps]

83
00:07:30,700 --> 00:07:32,618
[slurping]

84
00:07:33,870 --> 00:07:37,415
But not all animals here
are stuck in one place.

85
00:07:42,503 --> 00:07:43,754
Some can move,

86
00:07:44,964 --> 00:07:47,049
exploring the seafloor

87
00:07:47,133 --> 00:07:50,887
like their pioneering relatives
millions of years ago.

88
00:07:58,603 --> 00:08:03,024
It may seem like these shallows
are a kind of Eden,

89
00:08:04,484 --> 00:08:06,194
a safe haven for life.

90
00:08:07,904 --> 00:08:10,615
But this is no paradise.

91
00:08:11,199 --> 00:08:13,618
[ominous music playing]

92
00:08:17,747 --> 00:08:20,541
Living off modern-day California…

93
00:08:24,170 --> 00:08:27,006
this is a rainbow nudibranch.

94
00:08:31,135 --> 00:08:34,639
And its sluggish nature
hides a sinister side…

95
00:08:39,310 --> 00:08:42,063
because, like its ancestors,

96
00:08:42,647 --> 00:08:44,565
it's here to hunt.

97
00:08:50,655 --> 00:08:53,407
It's adapted to a life of predation.

98
00:08:54,659 --> 00:08:56,285
And its prey of choice,

99
00:08:57,745 --> 00:08:59,205
anemones.

100
00:09:05,336 --> 00:09:08,089
But like the very first predators,

101
00:09:08,172 --> 00:09:11,676
the nudibranch has room for improvement…

102
00:09:12,552 --> 00:09:14,554
[tense music playing]

103
00:09:18,182 --> 00:09:21,143
…because here, timing…

104
00:09:25,064 --> 00:09:26,440
is everything.

105
00:09:30,736 --> 00:09:32,697
Luckily for the hunter,

106
00:09:32,780 --> 00:09:35,199
its prey is anchored to the seafloor

107
00:09:36,033 --> 00:09:38,035
and can't hide forever.

108
00:09:38,119 --> 00:09:40,121
[sinister music playing]

109
00:09:52,633 --> 00:09:54,385
The anemone's fate?

110
00:09:55,011 --> 00:09:57,972
To be eaten from the inside out.

111
00:09:58,055 --> 00:10:00,057
[chomping]

112
00:10:13,154 --> 00:10:18,200
The arrival of predation
was a seismic shift in the ancient seas.

113
00:10:21,871 --> 00:10:23,414
The hunted had to adapt

114
00:10:24,290 --> 00:10:25,833
or risk extinction.

115
00:10:30,254 --> 00:10:34,383
And one group adapted
in a way never seen before.

116
00:10:38,888 --> 00:10:41,891
Despite being single animals,

117
00:10:42,725 --> 00:10:46,812
they were able to separate
into layers of individuals,

118
00:10:47,730 --> 00:10:51,025
that then broke free.

119
00:10:53,819 --> 00:10:55,029
Jellyfish.

120
00:10:55,696 --> 00:10:58,616
530 million years ago,

121
00:10:58,699 --> 00:11:04,830
they were the very first animals
to escape the seafloor and swim.

122
00:11:04,914 --> 00:11:06,707
[graceful music playing]

123
00:11:10,294 --> 00:11:12,296
[tense music playing]

124
00:11:12,380 --> 00:11:17,134
But predators were already everywhere.

125
00:11:22,390 --> 00:11:24,392
[dramatic music playing]

126
00:11:34,026 --> 00:11:36,070
Armed with venomous harpoons,

127
00:11:36,570 --> 00:11:39,865
these anemones make quick work
of baby jellyfish.

128
00:11:53,546 --> 00:11:55,047
[tense music playing]

129
00:11:56,132 --> 00:11:57,675
And they are so abundant

130
00:11:58,926 --> 00:12:01,554
escape seems impossible.

131
00:12:04,265 --> 00:12:06,267
[tense music continues]

132
00:12:21,115 --> 00:12:22,116
Caught.

133
00:12:25,035 --> 00:12:26,454
But not for long.

134
00:12:33,169 --> 00:12:34,920
[tranquil music playing]

135
00:12:41,135 --> 00:12:43,220
Free at last.

136
00:12:51,771 --> 00:12:53,773
Thanks to their ability to swim,

137
00:12:54,899 --> 00:12:58,277
jellyfish were the first
to venture out of the shallows

138
00:12:59,820 --> 00:13:01,655
and into the big blue.

139
00:13:09,497 --> 00:13:11,499
[music swells]

140
00:13:11,582 --> 00:13:13,584
[stirring music playing]

141
00:13:23,344 --> 00:13:26,388
Here, with fish yet to evolve,

142
00:13:27,515 --> 00:13:32,144
jellyfish were entirely free of predation.

143
00:13:36,273 --> 00:13:39,193
In time, others would follow.

144
00:13:39,860 --> 00:13:42,488
But more than half a billion years ago,

145
00:13:43,072 --> 00:13:46,158
the open ocean belonged to them.

146
00:13:46,242 --> 00:13:47,868
[music swells]

147
00:13:49,912 --> 00:13:51,914
[music fades]

148
00:13:58,254 --> 00:14:00,256
[water laps gently]

149
00:14:01,966 --> 00:14:04,218
For those still stuck on the seabed,

150
00:14:04,885 --> 00:14:08,848
escaping predation
required a different approach.

151
00:14:21,318 --> 00:14:22,903
This curious creature

152
00:14:24,196 --> 00:14:25,406
is a trilobite.

153
00:14:27,992 --> 00:14:31,745
And its breakthrough adaptation, armor.

154
00:14:34,248 --> 00:14:38,627
Trilobites are one of the first
in a brand-new dynasty,

155
00:14:40,254 --> 00:14:41,881
the arthropods.

156
00:14:47,803 --> 00:14:51,098
They've evolved an external skeleton,

157
00:14:51,640 --> 00:14:56,186
a shield-like shell
that protects their soft insides.

158
00:15:00,941 --> 00:15:04,278
And in these waters, they need it.

159
00:15:05,112 --> 00:15:07,114
[sinister music playing]

160
00:15:11,160 --> 00:15:13,662
This is Anomalocaris.

161
00:15:15,039 --> 00:15:16,999
A bizarre-looking animal,

162
00:15:17,082 --> 00:15:21,337
its name translates
as "the abnormal shrimp."

163
00:15:25,382 --> 00:15:29,762
And in the tropical shallows
508 million years ago,

164
00:15:31,764 --> 00:15:34,975
it's the world's first apex predator.

165
00:15:37,519 --> 00:15:39,521
[tense music playing]

166
00:15:47,488 --> 00:15:52,201
Despite the trilobite's armor,
it makes sense to keep out of sight.

167
00:16:00,334 --> 00:16:04,129
But away from the canyons,
there's nowhere to hide…

168
00:16:06,507 --> 00:16:09,093
because what makes this hunter so special…

169
00:16:11,679 --> 00:16:13,764
…is its speed.

170
00:16:17,601 --> 00:16:19,144
[dramatic music playing]

171
00:16:19,728 --> 00:16:25,943
Unlike jellyfish, Anomalocaris can swim
with both direction and pace.

172
00:16:28,320 --> 00:16:30,280
But that isn't always enough…

173
00:16:35,494 --> 00:16:41,375
because when cornered,
the trilobite's armor comes into its own.

174
00:16:47,131 --> 00:16:52,511
Rolling into a ball,
it becomes completely impenetrable.

175
00:16:55,723 --> 00:16:58,517
Even for the abnormal shrimp.

176
00:17:07,067 --> 00:17:12,656
With the coast clear, the trilobite
continues on its merry way once more.

177
00:17:17,244 --> 00:17:21,999
It's searching for one
of the ancient ocean's greatest sights…

178
00:17:22,875 --> 00:17:24,877
[stirring music playing]

179
00:17:29,840 --> 00:17:32,134
…the trilobite mating grounds,

180
00:17:32,718 --> 00:17:36,013
where thousands come together to breed.

181
00:17:38,724 --> 00:17:43,145
In the story of life,
armor has been such a success

182
00:17:43,228 --> 00:17:46,482
that today, nearly 80% of all animals,

183
00:17:46,565 --> 00:17:50,694
including insects, spiders, and crabs,

184
00:17:50,778 --> 00:17:53,739
are related to this ancient arthropod.

185
00:17:57,493 --> 00:18:01,288
But armor couldn't protect them
from everything.

186
00:18:07,211 --> 00:18:09,338
[ominous music playing]

187
00:18:18,222 --> 00:18:23,102
Forty million years later,
and a new era of life has begun,

188
00:18:24,686 --> 00:18:27,815
with more diversity than ever before.

189
00:18:32,361 --> 00:18:35,531
Here, trilobites are still in their prime,

190
00:18:36,156 --> 00:18:38,867
but their armor is even more robust…

191
00:18:42,830 --> 00:18:48,418
because on this reef,
danger comes from a new type of predator.

192
00:19:00,931 --> 00:19:04,143
Ancestors of today's octopus and squid,

193
00:19:05,769 --> 00:19:11,483
these tentacled giants are part
of a dynasty known as the cephalopods.

194
00:19:15,237 --> 00:19:17,239
[sinister music playing]

195
00:19:21,160 --> 00:19:25,372
The largest,
with a shell eight meters long,

196
00:19:26,498 --> 00:19:28,125
are Cameroceras.

197
00:19:36,133 --> 00:19:39,553
The trilobite is dwarfed
by this towering hunter.

198
00:19:42,431 --> 00:19:45,392
But it's not their size
that makes them such a threat.

199
00:19:48,228 --> 00:19:49,521
[trilobite squeaks]

200
00:19:50,814 --> 00:19:55,360
Cameroceras have evolved
a new way to catch their prey…

201
00:19:57,821 --> 00:19:59,615
even when they can't see it.

202
00:20:01,408 --> 00:20:03,410
[ominous music playing]

203
00:20:08,957 --> 00:20:13,837
Because in these waters,
they hunt using not just sight,

204
00:20:15,464 --> 00:20:16,590
but touch.

205
00:20:28,018 --> 00:20:32,898
All the trilobite can do
is try to stay out of reach.

206
00:20:39,613 --> 00:20:41,615
[ominous music fades]

207
00:20:44,493 --> 00:20:46,495
[dramatic music playing]

208
00:20:52,417 --> 00:20:53,877
With a scissor-like beak…

209
00:20:53,961 --> 00:20:55,087
[crunching]

210
00:20:55,170 --> 00:20:57,589
…able to slice through the toughest armor,

211
00:20:59,341 --> 00:21:02,511
Cameroceras dominate these seas.

212
00:21:05,138 --> 00:21:08,308
Although, there is
another important new arrival.

213
00:21:14,106 --> 00:21:16,817
Graced with a look of permanent surprise,

214
00:21:18,610 --> 00:21:20,988
this is Arandaspis…

215
00:21:23,782 --> 00:21:25,742
an early kind of fish.

216
00:21:26,576 --> 00:21:28,578
[playful music playing]

217
00:21:30,706 --> 00:21:33,000
It hasn't yet evolved a jaw,

218
00:21:33,917 --> 00:21:36,003
so can only suck up the scraps.

219
00:21:40,632 --> 00:21:42,467
But what it has evolved

220
00:21:43,343 --> 00:21:45,804
will change the course of history.

221
00:21:52,561 --> 00:21:58,150
It has a new internal skeleton
that makes it both fast and agile.

222
00:22:00,110 --> 00:22:01,486
A backbone.

223
00:22:03,196 --> 00:22:05,574
And while it may not look like much,

224
00:22:06,158 --> 00:22:10,829
fish like Arandaspis will one day
give rise to all other vertebrates.

225
00:22:11,788 --> 00:22:16,418
Amphibians, reptiles, mammals, and birds.

226
00:22:20,714 --> 00:22:25,469
But for now,
vertebrates are only at the beginning

227
00:22:26,219 --> 00:22:29,514
of a very, very long journey.

228
00:22:29,598 --> 00:22:31,391
[music ends]

229
00:22:41,151 --> 00:22:43,195
[tense ambient music playing]

230
00:22:47,616 --> 00:22:52,704
Twenty million years later,
life faced one of its greatest challenges…

231
00:22:52,788 --> 00:22:54,790
[rumbling]

232
00:22:56,124 --> 00:22:58,126
[howling wind]

233
00:22:59,252 --> 00:23:02,881
…as the climate suddenly began to cool…

234
00:23:04,299 --> 00:23:06,259
[thunder rumbles]

235
00:23:08,512 --> 00:23:12,307
…a development that was to have
dire consequences.

236
00:23:14,434 --> 00:23:16,436
[suspenseful music playing]

237
00:23:32,035 --> 00:23:36,706
Little by little,
the sea started to freeze.

238
00:23:39,835 --> 00:23:41,378
[suspenseful music continues]

239
00:23:52,722 --> 00:23:55,934
For the inhabitants
of Earth's once tropical waters,

240
00:23:57,436 --> 00:24:00,105
this was a catastrophe.

241
00:24:04,860 --> 00:24:06,862
[music fades]

242
00:24:09,114 --> 00:24:10,866
[mournful vocalizing]

243
00:24:19,040 --> 00:24:22,794
The shallow seas
became a frozen graveyard.

244
00:24:26,673 --> 00:24:29,885
The world's first mass extinction.

245
00:24:29,968 --> 00:24:31,970
[mournful vocalizing continues]

246
00:24:41,188 --> 00:24:46,693
The fatal freeze was caused
by a 60% drop in carbon dioxide.

247
00:24:49,946 --> 00:24:52,574
Without this greenhouse gas to warm it,

248
00:24:52,657 --> 00:24:55,410
the planet was plunged into an ice age.

249
00:25:01,917 --> 00:25:05,754
One that would last for 200,000 years.

250
00:25:06,713 --> 00:25:08,715
[sorrowful music playing]

251
00:25:15,847 --> 00:25:20,101
At its peak, almost half the world
was covered in ice,

252
00:25:21,019 --> 00:25:26,608
causing the demise of 85% of all life,

253
00:25:27,651 --> 00:25:31,488
lost from our family tree forever.

254
00:25:38,662 --> 00:25:40,664
With the tropical shallows gone,

255
00:25:40,747 --> 00:25:44,501
the few survivors were those
who could head to deeper waters.

256
00:25:52,133 --> 00:25:54,261
Animals like the nautilus,

257
00:25:54,344 --> 00:25:57,514
a smaller relative
of the giant Cameroceras.

258
00:26:04,688 --> 00:26:07,941
It first evolved
hundreds of millions of years ago,

259
00:26:08,692 --> 00:26:10,819
but still exists today.

260
00:26:14,322 --> 00:26:19,452
It owes its success
to the deep ocean's unchanging nature.

261
00:26:21,371 --> 00:26:26,251
Below 600 meters,
the temperature rarely fluctuates,

262
00:26:26,334 --> 00:26:29,212
making it a sanctuary of stability.

263
00:26:32,007 --> 00:26:33,925
When the Ice Age struck,

264
00:26:35,010 --> 00:26:38,221
all kinds of animals
headed into the abyss.

265
00:26:40,807 --> 00:26:44,477
But it's the cephalopods who have thrived.

266
00:26:46,479 --> 00:26:48,607
[majestic music playing]

267
00:26:52,193 --> 00:26:55,655
Today, many still haunt the deep sea…

268
00:26:58,783 --> 00:27:01,953
including octopus and squid.

269
00:27:08,251 --> 00:27:13,131
Their gelatinous bodies are unaffected
by the deep's immense pressure.

270
00:27:17,260 --> 00:27:22,682
And super senses
let them find their way in the dark.

271
00:27:27,145 --> 00:27:29,439
No longer constrained by shells,

272
00:27:29,939 --> 00:27:34,486
they've evolved
into almost every size and shape.

273
00:27:42,535 --> 00:27:46,122
But this pitch-black world
isn't theirs alone.

274
00:27:51,211 --> 00:27:55,048
Arthropods, like this spider crab,
live here too…

275
00:27:58,093 --> 00:28:00,720
feeling their way across the ocean floor.

276
00:28:04,474 --> 00:28:06,935
Alongside them, the vertebrates.

277
00:28:09,854 --> 00:28:14,192
With little food on offer,
down here, they're rare.

278
00:28:18,113 --> 00:28:22,742
Only around 10% of all fish today
live in the deep.

279
00:28:26,788 --> 00:28:29,165
This is not a place they excel…

280
00:28:31,626 --> 00:28:37,590
unlike the cephalopods, who have become
the deep's near-perfect predators.

281
00:28:45,515 --> 00:28:48,810
Something this shrimp may soon discover.

282
00:28:48,893 --> 00:28:50,895
[quiet, suspenseful music playing]

283
00:29:01,823 --> 00:29:06,745
All around it, the water twinkles
with bioluminescent plankton.

284
00:29:09,330 --> 00:29:14,419
Any disturbance, and for the briefest
of moments, they light up.

285
00:29:20,800 --> 00:29:22,427
But the shrimp has company.

286
00:29:27,515 --> 00:29:29,434
A half-concealed squid

287
00:29:30,435 --> 00:29:32,103
is watching the show.

288
00:29:34,939 --> 00:29:39,778
Its large eyes allow it to see
exceptionally well in the dark.

289
00:29:39,861 --> 00:29:41,821
[tense music playing]

290
00:29:49,871 --> 00:29:51,831
Picking out the dots of light,

291
00:29:52,540 --> 00:29:55,335
it can follow every move the shrimp makes.

292
00:29:57,837 --> 00:30:01,257
All it needs is the right moment.

293
00:30:03,885 --> 00:30:05,887
[tense music continues]

294
00:30:22,320 --> 00:30:24,614
The shrimp never saw it coming.

295
00:30:34,499 --> 00:30:36,751
With such extraordinary senses,

296
00:30:37,377 --> 00:30:43,258
perhaps it's no surprise that squid
have mastered this cold, dark world.

297
00:30:45,969 --> 00:30:48,972
But for those who lived
back in the Ice Age,

298
00:30:49,514 --> 00:30:51,891
things were beginning to heat up.

299
00:30:52,642 --> 00:30:54,644
[rumbling]

300
00:30:54,727 --> 00:30:56,729
[solemn music playing]

301
00:30:59,607 --> 00:31:01,609
[ice creaking]

302
00:31:05,154 --> 00:31:08,408
200,000 years after the big freeze,

303
00:31:10,451 --> 00:31:13,872
there appeared in the ice a vein of blue.

304
00:31:13,955 --> 00:31:16,124
[majestic music playing]

305
00:31:19,127 --> 00:31:20,295
Water.

306
00:31:22,589 --> 00:31:26,009
Carbon dioxide levels had bounced back,

307
00:31:27,719 --> 00:31:30,889
causing global temperatures to rise.

308
00:31:31,764 --> 00:31:34,309
The great melt had begun.

309
00:31:36,519 --> 00:31:40,398
Slow at first,
the thaw rapidly gathered pace.

310
00:31:50,658 --> 00:31:55,538
Before long,
glaciers hundreds of meters tall

311
00:31:55,622 --> 00:31:58,249
cascaded into the warming seas.

312
00:32:03,296 --> 00:32:05,381
[rousing vocalizing]

313
00:32:07,884 --> 00:32:09,344
Within a few millennia,

314
00:32:10,011 --> 00:32:14,307
almost half the planet's ice had melted.

315
00:32:15,600 --> 00:32:19,771
Within a few more, it had entirely gone.

316
00:32:26,194 --> 00:32:29,072
For those who had sought refuge
in the deep,

317
00:32:29,155 --> 00:32:31,699
now was a chance to head back up.

318
00:32:34,243 --> 00:32:36,245
[mysterious music playing]

319
00:32:39,624 --> 00:32:42,251
Off the coast of modern-day Australia,

320
00:32:43,586 --> 00:32:47,006
this is a young giant cuttlefish.

321
00:32:48,758 --> 00:32:51,386
Like his Ice Age ancestors,

322
00:32:51,469 --> 00:32:54,555
he's come to the shallows
in search of a mate.

323
00:33:00,395 --> 00:33:02,689
But others have got here first…

324
00:33:02,772 --> 00:33:05,191
[sinister music playing]

325
00:33:07,402 --> 00:33:10,029
…including an alpha male…

326
00:33:13,199 --> 00:33:15,910
jealously protecting his chosen partner.

327
00:33:20,081 --> 00:33:23,584
And he does not intend to share.

328
00:33:30,508 --> 00:33:33,803
But the smaller male isn't deterred.

329
00:33:35,888 --> 00:33:38,933
Instead, he bides his time

330
00:33:40,393 --> 00:33:41,519
and watches

331
00:33:43,479 --> 00:33:46,607
as a new male enters the arena…

332
00:33:48,776 --> 00:33:50,862
and signals his intention…

333
00:33:53,197 --> 00:33:54,907
to challenge the alpha.

334
00:33:56,159 --> 00:33:58,536
[tense music playing]

335
00:34:02,165 --> 00:34:06,753
With neither backing down,
a fight is inevitable…

336
00:34:06,836 --> 00:34:09,255
[dramatic music playing]

337
00:34:14,510 --> 00:34:18,473
…allowing the young male to make his move.

338
00:34:18,556 --> 00:34:20,099
[whimsical music playing]

339
00:34:20,183 --> 00:34:22,351
[dramatic music playing]

340
00:34:26,397 --> 00:34:30,026
As he approaches,
he tucks in his tentacles

341
00:34:30,985 --> 00:34:32,695
and changes color…

342
00:34:34,906 --> 00:34:36,657
to perfectly mimic

343
00:34:38,034 --> 00:34:39,368
a female.

344
00:34:42,622 --> 00:34:44,999
Using brain, not brawn,

345
00:34:45,541 --> 00:34:47,919
he sneaks straight in.

346
00:34:49,837 --> 00:34:51,839
[whimsical music playing]

347
00:34:52,548 --> 00:34:54,008
[music ends]

348
00:34:56,886 --> 00:35:01,474
The alpha now thinks
he's guarding not one female…

349
00:35:04,477 --> 00:35:05,645
but two.

350
00:35:10,316 --> 00:35:13,611
He's unaware
that this sly young interloper

351
00:35:14,487 --> 00:35:18,032
is mating with his chosen partner.

352
00:35:19,283 --> 00:35:21,285
[tranquil music playing]

353
00:35:32,630 --> 00:35:36,759
Their exceptional intelligence
and remarkable senses

354
00:35:36,843 --> 00:35:41,430
have helped cephalopods
colonize every ocean on Earth.

355
00:35:50,982 --> 00:35:52,692
Yet, in the shallows,

356
00:35:52,775 --> 00:35:57,238
they are rarely the top predators
their towering ancestors once were…

357
00:36:01,159 --> 00:36:03,619
because in the ancient seas,

358
00:36:04,912 --> 00:36:08,249
there was to be a changing of the guard.

359
00:36:08,332 --> 00:36:10,376
[suspenseful music playing]

360
00:36:22,430 --> 00:36:25,391
Seventy million years
after the great melt,

361
00:36:25,474 --> 00:36:28,561
in a period known as the Devonian,

362
00:36:28,644 --> 00:36:31,314
the vertebrates made their move.

363
00:36:31,397 --> 00:36:33,399
[sinister music playing]

364
00:36:37,737 --> 00:36:40,489
This is Dunkleosteus…

365
00:36:43,284 --> 00:36:46,787
a descendant of the tiny Arandaspis.

366
00:36:49,582 --> 00:36:54,503
But at nine meters long,
this fish is no underdog.

367
00:36:59,592 --> 00:37:04,764
Even its young, barely half its size,
are fearsome predators.

368
00:37:09,435 --> 00:37:13,898
And this juvenile
has spotted something out in the blue.

369
00:37:17,360 --> 00:37:18,569
Ammonoids,

370
00:37:19,820 --> 00:37:23,991
close relatives
of today's deep-sea nautilus.

371
00:37:25,159 --> 00:37:27,161
[tense music playing]

372
00:37:33,042 --> 00:37:34,585
Being a vertebrate,

373
00:37:34,669 --> 00:37:38,631
Dunkleosteus has the pace
to catch up with them.

374
00:37:40,800 --> 00:37:42,802
[sinister music playing]

375
00:37:45,763 --> 00:37:49,392
But the ammonoids
boast a trick of their own…

376
00:37:51,936 --> 00:37:53,729
…jet propulsion.

377
00:37:55,106 --> 00:37:58,276
Just what it needs
to outmaneuver the young fish.

378
00:37:59,068 --> 00:38:01,070
[dramatic music playing]

379
00:38:06,242 --> 00:38:07,910
As it starts to circle…

380
00:38:10,288 --> 00:38:12,290
the ammonoid does the same…

381
00:38:16,002 --> 00:38:18,004
shielding its body with its shell.

382
00:38:22,174 --> 00:38:24,176
[growling]

383
00:38:25,678 --> 00:38:29,181
Once upon a time,
this defense would have worked.

384
00:38:29,890 --> 00:38:34,020
But Dunkleosteus has a weapon
its tiny ancestors did not.

385
00:38:38,107 --> 00:38:39,358
Jaws.

386
00:38:40,568 --> 00:38:44,113
It can bite down and crush its prey.

387
00:38:49,952 --> 00:38:52,455
As long as it can find the right angle.

388
00:38:52,538 --> 00:38:54,332
[suspenseful music playing]

389
00:39:10,222 --> 00:39:12,099
Luckily for the ammonoid,

390
00:39:12,183 --> 00:39:15,978
the juvenile's jaws are not yet
strong enough to break its shell.

391
00:39:16,771 --> 00:39:18,773
[teeth clacking on shell]

392
00:39:19,482 --> 00:39:22,276
But the adult's are.

393
00:39:28,324 --> 00:39:30,326
[sinister music playing]

394
00:39:35,456 --> 00:39:37,458
[music fades]

395
00:39:37,541 --> 00:39:42,922
The pairing of backbones with jaws
was to prove a winning combination.

396
00:39:44,090 --> 00:39:47,468
So perfect that, from this moment on,

397
00:39:48,135 --> 00:39:52,139
the vertebrates would never look back.

398
00:39:53,307 --> 00:39:55,309
[rousing music playing]

399
00:40:02,525 --> 00:40:05,986
Today, there are
more than 30,000 species of fish.

400
00:40:11,033 --> 00:40:14,578
Forty times as many as the cephalopods.

401
00:40:22,545 --> 00:40:24,880
While their variety is remarkable…

402
00:40:27,258 --> 00:40:30,136
each still has that perfect pairing.

403
00:40:33,055 --> 00:40:35,391
Backbones give them speed and power…

404
00:40:38,060 --> 00:40:39,645
no matter their size.

405
00:40:43,315 --> 00:40:44,650
While jaws

406
00:40:45,484 --> 00:40:46,986
give them bite.

407
00:40:50,698 --> 00:40:52,950
But there is more to jaws than that.

408
00:40:55,411 --> 00:40:57,997
Some fish use them to protect their eggs.

409
00:40:58,914 --> 00:41:00,916
[serene music playing]

410
00:41:02,751 --> 00:41:05,087
Others to attract a mate.

411
00:41:12,720 --> 00:41:15,681
Then there's the sarcastic fringehead.

412
00:41:16,390 --> 00:41:18,142
[playful music playing]

413
00:41:21,437 --> 00:41:24,148
He doesn't just use his to catch crabs.

414
00:41:33,157 --> 00:41:38,370
He also uses them for one of the most
intimidating displays in nature…

415
00:41:41,916 --> 00:41:44,793
to rid his patch of rivals.

416
00:41:53,260 --> 00:41:57,306
For these fish, size is everything.

417
00:42:07,775 --> 00:42:09,860
[music intensifies]

418
00:42:19,328 --> 00:42:20,871
[music fades]

419
00:42:21,539 --> 00:42:25,501
Big jaws have let this fringehead
rule his strip of seafloor,

420
00:42:26,752 --> 00:42:33,217
but they've also allowed a more infamous
group of fish to rule the waves.

421
00:42:33,300 --> 00:42:35,261
[ominous music playing]

422
00:42:40,641 --> 00:42:44,770
They first appeared
more than 400 million years ago

423
00:42:45,729 --> 00:42:48,607
and have been feared ever since.

424
00:42:48,691 --> 00:42:50,693
[stabbing string music playing]

425
00:42:52,820 --> 00:42:54,822
[music intensifies]

426
00:42:54,905 --> 00:42:55,823
They are

427
00:42:57,449 --> 00:42:58,617
the sharks.

428
00:43:03,038 --> 00:43:06,166
To the winning combination
of backbone and jaws,

429
00:43:07,501 --> 00:43:10,087
they've added senses so sharp

430
00:43:10,170 --> 00:43:13,632
they can detect prey
from hundreds of meters away.

431
00:43:16,760 --> 00:43:20,347
With many also boasting size and strength,

432
00:43:21,390 --> 00:43:23,517
they are as close to perfection

433
00:43:24,518 --> 00:43:27,438
as an ocean predator can get.

434
00:43:37,531 --> 00:43:39,533
[tense music playing]

435
00:43:41,160 --> 00:43:43,621
In the sea off Western Australia,

436
00:43:44,204 --> 00:43:47,249
these fish are balled together
for protection.

437
00:43:50,336 --> 00:43:52,379
But the sharks hold back…

438
00:43:54,757 --> 00:43:58,344
because they have
a special plan of attack.

439
00:44:05,726 --> 00:44:07,645
Appearing to work as a team,

440
00:44:08,270 --> 00:44:13,233
some of the sharks rise from below
to trap the fish against the surface.

441
00:44:16,820 --> 00:44:18,614
Others come in from the side,

442
00:44:20,324 --> 00:44:23,160
corralling the shoal
away from the open ocean

443
00:44:23,827 --> 00:44:25,537
and into the shallows.

444
00:44:33,629 --> 00:44:35,506
Now numbering in their hundreds,

445
00:44:36,423 --> 00:44:38,175
the sharks play for time…

446
00:44:42,012 --> 00:44:45,057
as their prey get ever more tired.

447
00:44:49,853 --> 00:44:52,022
[sinister music playing]

448
00:44:58,278 --> 00:45:02,366
It's a hunting technique that has been
honed over millions of years.

449
00:45:04,952 --> 00:45:08,539
But the carnage starts
in the blink of an eye.

450
00:45:09,456 --> 00:45:11,709
[dramatic music playing]

451
00:45:29,852 --> 00:45:32,062
[music continues]

452
00:45:44,575 --> 00:45:46,243
[music fades]

453
00:45:51,123 --> 00:45:54,084
Virtually unchanged since the great melt,

454
00:45:55,335 --> 00:45:59,423
sharks are one of
the most successful dynasties in history.

455
00:46:03,927 --> 00:46:08,640
But they owe their rise
to the fall of their greatest rivals.

456
00:46:20,235 --> 00:46:21,904
[mysterious music playing]

457
00:46:26,742 --> 00:46:29,036
Back in the ancient seas,

458
00:46:29,119 --> 00:46:33,373
and Dunkleosteus had reigned
for 20 million years.

459
00:46:34,374 --> 00:46:36,418
[sinister music playing]

460
00:46:37,586 --> 00:46:41,673
So vicious a predator
that not even early sharks could compete.

461
00:46:47,554 --> 00:46:51,517
But Dunkleosteus was to suffer
from a devastating quirk of fate…

462
00:46:54,812 --> 00:46:56,814
[mysterious music playing]

463
00:46:57,648 --> 00:46:59,608
…caused by plankton.

464
00:47:02,820 --> 00:47:04,822
Billions of years earlier,

465
00:47:04,905 --> 00:47:07,699
plankton had been the catalyst
for animal life.

466
00:47:11,078 --> 00:47:14,790
But now,
they would almost bring it to an end.

467
00:47:20,003 --> 00:47:22,673
A sudden surge in ocean nutrients

468
00:47:23,257 --> 00:47:27,052
caused the population of plankton
to increase…

469
00:47:30,472 --> 00:47:32,474
on an unprecedented scale.

470
00:47:32,558 --> 00:47:34,560
[ominous music playing]

471
00:47:40,732 --> 00:47:46,697
As it spread through the sunlit shallows,
this thick soup began to rot…

472
00:47:49,575 --> 00:47:52,494
causing the ocean's oxygen to plummet.

473
00:48:02,421 --> 00:48:08,135
Without this critical ingredient for life,
animals started to suffocate.

474
00:48:08,218 --> 00:48:10,220
[mournful music playing]

475
00:48:18,312 --> 00:48:23,775
For those unable to escape the sludge,
there was only one possible outcome.

476
00:48:26,612 --> 00:48:27,946
Extinction.

477
00:48:31,033 --> 00:48:35,370
Dunkleosteus was one of many
to be lost from our planet.

478
00:48:43,045 --> 00:48:45,047
[mournful music continues]

479
00:48:47,090 --> 00:48:51,094
Across the world,
coastlines became littered with the dead.

480
00:49:03,774 --> 00:49:08,028
Only the lucky few,
including early sharks,

481
00:49:08,111 --> 00:49:09,655
were able to survive.

482
00:49:21,917 --> 00:49:27,005
In all, 80% of marine life
was wiped out in this,

483
00:49:27,089 --> 00:49:30,300
the world's second mass extinction.

484
00:49:32,469 --> 00:49:34,471
[music fades]

485
00:49:35,097 --> 00:49:38,976
And yet this is only half the story.

486
00:49:43,480 --> 00:49:46,525
The nutrients that caused
the plankton explosion

487
00:49:48,068 --> 00:49:50,278
didn't start out in the sea.

488
00:49:50,362 --> 00:49:52,364
[uplifting music playing]

489
00:49:53,991 --> 00:49:56,785
They were carried there from the land…

490
00:50:01,623 --> 00:50:07,754
…released by plants during one
of the most amazing events of all time…

491
00:50:15,554 --> 00:50:18,598
…the greening of planet Earth.

492
00:50:19,683 --> 00:50:21,685
[majestic music playing]

493
00:50:24,396 --> 00:50:26,982
What was once a volcanic wasteland

494
00:50:28,316 --> 00:50:32,070
was now overflowing with life.

495
00:50:32,779 --> 00:50:35,282
-[music fades]
-[creatures chirping]

496
00:50:36,450 --> 00:50:40,120
And this new world offered opportunity.

497
00:50:42,622 --> 00:50:44,916
For where plants had paved the way,

498
00:50:45,876 --> 00:50:48,128
animals would follow.

499
00:50:48,211 --> 00:50:50,213
[chirping continues]

500
00:50:51,965 --> 00:50:58,055
And before long,
the race to dominate the land would begin.

501
00:50:59,014 --> 00:51:03,268
-[dramatic music playing]
-[low growling]

502
00:51:06,730 --> 00:51:08,732
[growling, roaring]

503
00:51:08,815 --> 00:51:10,442
[dramatic music intensifies]

504
00:51:10,525 --> 00:51:12,069
[music ends]

505
00:51:13,570 --> 00:51:15,363
[rousing music playing]