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00:00:06,480 --> 00:00:08,440
[thunder crashing]

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[rainfall]

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[dog barking]

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[birds calling]

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[varied bird calls]

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[wings fluttering]

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[narrator] The Earth,
home to millions of species.

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[man talking on radio]

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[woman singing]

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00:00:33,520 --> 00:00:36,800
But what might live... beyond?

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00:00:42,320 --> 00:00:45,680
Astronomers have discovered 
thousands of planets

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00:00:45,760 --> 00:00:47,760
outside our solar system.

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00:00:50,640 --> 00:00:53,840
They believe there are trillions more.

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If life exists on only a fraction of them,

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then the universe must be... alive.

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[creatures calling]

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All living things have the same needs.

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To feed...

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reproduce...

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and evolve.

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By applying the laws of life on Earth

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00:01:44,240 --> 00:01:46,520
to the rest of the universe...

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it's possible to imagine
what could live...

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on alien worlds.

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[cicadas chirping]

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Planets beyond our solar system 
are known to astronomers as exoplanets.

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00:02:38,440 --> 00:02:41,960
They are all 
trillions of miles from Earth.

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And yet, it might be possible
to detect a faint signature of life,

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from the light of the stars they orbit.

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Every star is sending to us 
light of all different colors.

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So, if we can catch that light 
in our telescope

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and put it through an instrument
that spreads the light out into a rainbow,

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00:03:09,240 --> 00:03:11,600
and we look in great detail 
at that rainbow,

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we will find the chemical fingerprints

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of the elements in the atmosphere 
of that star.

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This is one of those rainbows captured 
by the telescope of a distant star

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00:03:23,400 --> 00:03:26,400
about 12 light-years away,
the star Tau Ceti.

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We can see these dark lines, like 
from hydrogen in the star's atmosphere.

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Down here, we see these 
three characteristic lines of magnesium.

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So, this is the atmosphere of the star,

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00:03:39,760 --> 00:03:41,960
but what we wanna do is catch light

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that passes through 
the atmosphere of an exoplanet,

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to capture the chemical fingerprint of 
that atmosphere in exactly the same way.

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In 2012, this sort of fingerprint

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was visible in our own solar system.

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When Venus passed in front of the sun,

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it was silhouetted against it.

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For a moment, a tiny halo appeared,
the atmosphere of Venus.

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It's this sliver of light
astronomers are looking for.

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If they can analyze
the atmosphere of distant exoplanets

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00:04:36,240 --> 00:04:38,880
silhouetted against distant stars,

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they might find proof of life.

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[seabirds calling]

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[bird calls slowing]

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[Natalie] We've thought very carefully
about what the signs of life will be.

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So we look around and we look
at what's in the atmosphere

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that could be remotely detectable 
from light-years away.

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And it always leads us back 
to the same gas.

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Oxygen.

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50 to 70 percent of oxygen
in our atmosphere

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is actually coming from the ocean...

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which is the lungs of planet Earth.

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The process that drives the growth 
of the giant kelp in the oceans,

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and, in fact, all the forests 
that we see on the land,

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is photosynthesis.

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What interests me most 
about photosynthesis

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is the waste product 
that gets tossed away,

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and that's the oxygen
that goes into the atmosphere.

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We think that almost all
of the oxygen in our atmosphere

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is produced by photosynthesis,

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by life here on planet Earth.

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If you see it in abundance,

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you know that
the planet is a living world.

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Among trillions of worlds
out there,

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those with more oxygen
are more likely to sustain life.

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Imagine... a planet like Earth

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orbiting not one star...

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but two.

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This is Eden.

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[creatures singing and calling]

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The light from its twin stars
powers photosynthesis...

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pumping oxygen into the atmosphere...

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allowing life to thrive.

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[cooing]

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[sniffing]

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Grazers feed on low-lying fungus...

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[twig snaps]

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...but they're constantly alert to danger.

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The canopy is home to predators...

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[chattering]

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...perfectly evolved
to live among the trees...

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[snuffling]

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[chattering]

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...waiting for their moment to strike.

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[chattering]

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-[rustling]
-[purring]

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[screeching]

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On the ground, the grazer has the edge.

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It's faster in a chase.

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[squawking]

101
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But the predator has evolved 
a special weapon.

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This time, the grazer escapes...

103
00:09:06,120 --> 00:09:09,560
and the predator conserves its energy.

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[birdsong]

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On any planet, energy is precious.

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Starlight is a virtually infinite source.

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But the trick is
to turn that light into life.

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[man] We are in the tropical rainforest.

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There are more types of plants and
animals here than anywhere else on Earth.

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The trees and plants 
are soaking up the sun's energy.

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00:10:11,440 --> 00:10:16,040
By the magic of photosynthesis,
they convert sunlight into glucose energy,

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in other words, sugar.

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You can see it and even taste it.

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You can actually taste 
the energy from the sun.

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Plants act like batteries,

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capturing the sun's energy as glucose.

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Herbivores eat plants.

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Carnivores eat herbivores.

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Every link in the food chain

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depends on the transfer 
of this glucose energy.

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And in this forest,

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one animal can't get enough of the stuff.

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[humming]

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The hummingbird.

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[Alejandro] Hummingbirds can move
their wings over a hundred times a second.

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[chirping]

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[humming]

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You can feel how much energy 
they are spending just by being there.

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This frenzy of activity

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is fueled by glucose-rich nectar.

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[Alejandro] This is a device that
from the front, it looks like a flower,

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so a hummingbird comes
and it feeds from it.

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And there is one.

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It's going back and forth.
Come on, you can do it.

135
00:12:06,400 --> 00:12:08,800
Uh... Okay, so now, he's drinking.

136
00:12:12,120 --> 00:12:15,840
The device measures
the nectar the bird is drinking

137
00:12:16,280 --> 00:12:18,600
and the oxygen it is breathing.

138
00:12:21,120 --> 00:12:23,960
We calculate the energy intake 
from the nectar

139
00:12:24,040 --> 00:12:26,280
and the energy output from the oxygen

140
00:12:26,360 --> 00:12:30,440
and we can calculate very accurately
how much energy the bird is using.

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00:12:33,920 --> 00:12:39,320
A hovering hummingbird
consumes oxygen at an incredible rate.

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00:12:40,520 --> 00:12:43,720
Ten times faster than an Olympic sprinter.

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00:12:47,840 --> 00:12:53,240
The more oxygen it can get into its cells,
the faster it can burn glucose,

144
00:12:54,920 --> 00:12:57,280
which unlocks the energy within.

145
00:13:05,240 --> 00:13:10,200
On Earth, 21 percent
of the atmosphere is oxygen...

146
00:13:12,400 --> 00:13:15,680
fueling the diversity of life around us.

147
00:13:30,640 --> 00:13:35,160
On Eden, there's ten percent more oxygen 
than on Earth...

148
00:13:38,920 --> 00:13:41,880
so life here can be more diverse...

149
00:13:45,080 --> 00:13:46,920
more energetic...

150
00:13:49,720 --> 00:13:51,720
more competitive.

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00:13:56,440 --> 00:13:58,560
[panting]

152
00:14:01,240 --> 00:14:03,240
[animals and birds calling]

153
00:14:05,520 --> 00:14:08,760
Summer is breeding season for grazers.

154
00:14:16,400 --> 00:14:20,920
Always fearful of predators,
they don't spend time finding a mate.

155
00:14:23,040 --> 00:14:26,400
Instead, they produce worm-like spawn.

156
00:14:29,240 --> 00:14:31,400
Each needs to fuse with another

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00:14:32,360 --> 00:14:34,400
to create an embryo.

158
00:14:40,240 --> 00:14:43,600
For protection, they form a cocoon.

159
00:14:56,560 --> 00:15:00,400
Suspended above the ground, 
the embryos can grow,

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00:15:01,720 --> 00:15:03,800
away from predators.

161
00:15:19,680 --> 00:15:24,160
Here on Earth,
the threat of death is a huge factor

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00:15:24,720 --> 00:15:26,840
in the evolution of life.

163
00:15:30,720 --> 00:15:32,200
[David] Predation is everywhere.

164
00:15:32,280 --> 00:15:33,920
There's not an organism out there

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00:15:34,000 --> 00:15:37,120
that doesn't run the risk
of being eaten by another organism.

166
00:15:39,200 --> 00:15:43,680
The aspect that I'm interested in 
is how predation shapes reproduction.

167
00:15:54,320 --> 00:15:56,480
They're gonna be comin' in here.

168
00:15:59,040 --> 00:16:02,760
This river is perfect
for studying guppy fish...

169
00:16:04,120 --> 00:16:06,200
and their patterns of reproduction.

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00:16:07,920 --> 00:16:10,120
-[man] Here. Here's another male.
-[David] Yeah, yeah.

171
00:16:12,520 --> 00:16:14,560
This is an easy neighborhood.

172
00:16:14,640 --> 00:16:17,680
Here, guppies live
with only one other species of fish,

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00:16:17,760 --> 00:16:21,920
and it's a fish that rarely eats guppies 
or harasses guppies.

174
00:16:22,400 --> 00:16:26,680
They can swim wherever they want without
any risk of somebody trying to eat them.

175
00:16:28,720 --> 00:16:31,200
-Oh, right. There you go. All right.
-[man] Here we go.

176
00:16:32,680 --> 00:16:35,840
Here we have some babies that 
we just caught. We have five of them here.

177
00:16:36,720 --> 00:16:40,440
They may look small to you, but that's
really big for a newborn baby guppy.

178
00:16:43,720 --> 00:16:46,440
The guppies here invest a lot of effort

179
00:16:46,520 --> 00:16:48,400
in relatively few offspring.

180
00:16:51,880 --> 00:16:54,640
Downstream, it's a different story.

181
00:16:57,600 --> 00:16:58,520
[David] Okay!

182
00:17:00,160 --> 00:17:01,360
Wow.

183
00:17:06,360 --> 00:17:10,840
This boulder
is a natural barrier, dividing the river.

184
00:17:15,560 --> 00:17:18,080
Below it lurks danger.

185
00:17:21,120 --> 00:17:25,160
A guppy life down here is
pretty much like the life of a fugitive.

186
00:17:25,240 --> 00:17:27,280
You're always on the run from predators.

187
00:17:27,360 --> 00:17:29,320
If a guppy were to swim out here...

188
00:17:31,240 --> 00:17:33,080
it wouldn't last a second.

189
00:17:36,000 --> 00:17:38,160
The only defense
the guppies have

190
00:17:38,240 --> 00:17:40,800
is to produce lots of offspring,

191
00:17:42,120 --> 00:17:45,640
in the hope that some will survive 
to adulthood.

192
00:17:49,560 --> 00:17:54,160
Okay, I've two babies from up there, 
where guppies live without predators,

193
00:17:54,240 --> 00:17:57,640
and four from down here,
where guppies live with predators.

194
00:17:57,720 --> 00:17:59,520
There's a dramatic difference in size.

195
00:17:59,600 --> 00:18:02,160
Those that are from here
are much, much smaller

196
00:18:02,240 --> 00:18:04,560
than the ones that I caught 
above the barrier.

197
00:18:07,520 --> 00:18:11,600
Moms don't have a big prospect of living
to the future, no matter what they do.

198
00:18:12,160 --> 00:18:15,680
So the best strategy down here
is to make many small babies.

199
00:18:16,280 --> 00:18:18,280
-[birds calling]
-[animal barking]

200
00:18:19,560 --> 00:18:23,240
The guppies have evolved
two different ways of breeding.

201
00:18:25,200 --> 00:18:27,240
Having a few large babies...

202
00:18:28,920 --> 00:18:31,000
or lots of small ones.

203
00:18:34,520 --> 00:18:38,640
The deciding factor
is the threat from predators.

204
00:18:45,240 --> 00:18:48,000
[David] If you're one of the prey items, 
you're gonna die young,

205
00:18:48,080 --> 00:18:49,840
so you better live fast.

206
00:18:50,320 --> 00:18:54,600
You better put a lot into having babies,
because if you don't, you'll go extinct.

207
00:19:01,520 --> 00:19:04,800
Such a strategy
would apply on any planet

208
00:19:05,280 --> 00:19:08,800
wherever one species preys on another.

209
00:19:18,040 --> 00:19:21,760
On Eden, the grazers have evolved 
to breed fast.

210
00:19:25,720 --> 00:19:29,880
They produce as many offspring 
as quickly as possible...

211
00:19:32,720 --> 00:19:36,000
to ensure the survival of the species.

212
00:19:42,760 --> 00:19:44,800
[birds squawking]

213
00:19:47,440 --> 00:19:50,920
But in the forest, 
they're never truly safe.

214
00:19:56,360 --> 00:20:00,680
As summer ends,
the fungi grow orange-colored fruit...

215
00:20:02,840 --> 00:20:04,880
which attract grazers.

216
00:20:04,960 --> 00:20:06,240
-[whoosh]
-[squeaking]

217
00:20:10,280 --> 00:20:14,200
Spores within the fruit
spread an infection.

218
00:20:21,760 --> 00:20:23,360
[growling]

219
00:20:27,960 --> 00:20:28,840
[chatters]

220
00:20:28,920 --> 00:20:30,720
Now, when predators attack,

221
00:20:32,120 --> 00:20:34,720
infected grazers don't run.

222
00:20:35,520 --> 00:20:37,680
They've lost their fear instinct.

223
00:20:40,680 --> 00:20:41,920
[grazer squeals]

224
00:20:42,000 --> 00:20:43,640
[predator chattering]

225
00:20:44,800 --> 00:20:45,920
[squeals]

226
00:20:46,000 --> 00:20:47,320
[growling]

227
00:20:52,320 --> 00:20:54,000
It's an easy meal...

228
00:20:55,840 --> 00:20:57,680
but a poisonous one.

229
00:21:00,960 --> 00:21:02,440
[wails]

230
00:21:05,040 --> 00:21:11,160
The fungi have used the grazers 
to infect and kill the predators.

231
00:21:28,640 --> 00:21:34,000
Such complex relationships 
between species exist on Earth...

232
00:21:36,920 --> 00:21:39,120
but not all are so lethal.

233
00:21:44,120 --> 00:21:48,680
The Hadza are hunter-gatherers,
living off the land.

234
00:21:50,840 --> 00:21:53,080
[man, speaking Hadza] My name is Palango.

235
00:21:56,200 --> 00:22:01,160
As a man, my job is to hunt.

236
00:22:03,200 --> 00:22:07,360
I always carry a bow and arrow

237
00:22:08,400 --> 00:22:11,120
for protection...

238
00:22:15,240 --> 00:22:16,640
because the bush

239
00:22:17,480 --> 00:22:21,120
has many dangerous animals.

240
00:22:24,240 --> 00:22:28,440
One high-calorie food is prized 
above all else.

241
00:22:30,040 --> 00:22:31,040
Honey.

242
00:22:33,520 --> 00:22:36,200
But finding fresh honey is difficult.

243
00:22:40,640 --> 00:22:44,360
The bees move their nests 
from season to season.

244
00:22:47,920 --> 00:22:50,360
So the Hadza call on help.

245
00:22:50,880 --> 00:22:52,280
[whistling]

246
00:22:55,760 --> 00:22:57,080
[continues whistling]

247
00:22:57,680 --> 00:23:01,160
[Palango, in Hadza]
Honeyguides are very clever birds.

248
00:23:01,240 --> 00:23:02,240
[whistles]

249
00:23:02,320 --> 00:23:07,040
They know where honey is,
and they will take you there.

250
00:23:11,960 --> 00:23:13,800
[honeyguide chirping]

251
00:23:14,760 --> 00:23:16,400
She makes a special cry...

252
00:23:17,000 --> 00:23:18,800
[whistling]

253
00:23:19,280 --> 00:23:20,800
...and I whistle back.

254
00:23:22,120 --> 00:23:24,120
[honeyguide chirping]

255
00:23:28,080 --> 00:23:29,640
[whistling]

256
00:23:29,720 --> 00:23:34,400
If I follow her, she will lead me
directly to the honey.

257
00:23:39,320 --> 00:23:40,800
[whistling]

258
00:23:44,520 --> 00:23:46,680
-[honeyguide chirping]
-[Palango whistling]

259
00:23:50,840 --> 00:23:53,480
The honeyguide has lived up to its name.

260
00:23:54,480 --> 00:23:58,080
It's led them to a bees' nest 
in a baobab tree.

261
00:24:03,480 --> 00:24:06,880
The hunters need the bird 
to find the bees.

262
00:24:08,640 --> 00:24:12,160
And the bird needs the hunters 
to access the honey.

263
00:24:13,920 --> 00:24:17,400
[Palango] The honeyguides
take us to the bees.

264
00:24:20,560 --> 00:24:23,440
But the bees are aggressive.

265
00:24:24,160 --> 00:24:25,920
So she needs our help.

266
00:24:26,360 --> 00:24:30,280
Climbing up a baobab tree...

267
00:24:33,720 --> 00:24:39,480
is very hard and dangerous.

268
00:24:44,120 --> 00:24:46,840
One day I fell.

269
00:24:52,480 --> 00:24:58,400
We use smoke to get the bees drunk

270
00:24:59,640 --> 00:25:02,840
and chase them out of the nest.

271
00:25:35,320 --> 00:25:37,480
The risk is worth it.

272
00:25:40,040 --> 00:25:42,400
Fresh, rich, honey.

273
00:25:54,080 --> 00:25:56,480
Palango makes sure
the bird gets its share.

274
00:26:00,000 --> 00:26:02,000
It's a win-win situation.

275
00:26:08,520 --> 00:26:11,840
A complex relationship between species

276
00:26:12,440 --> 00:26:14,800
is known as symbiosis.

277
00:26:21,320 --> 00:26:25,680
The richer the ecosystem,
the more complex the relationship.

278
00:26:37,840 --> 00:26:42,800
On Eden, a deadly three-way symbiosis 
has evolved...

279
00:26:44,640 --> 00:26:46,720
centered around the fungi.

280
00:26:50,120 --> 00:26:51,480
-[twig snaps]
-[chattering]

281
00:26:53,440 --> 00:26:55,960
First, they feed the grazers.

282
00:26:57,160 --> 00:26:58,360
-[whoosh]
-[squeaks]

283
00:26:59,400 --> 00:27:01,400
Then, they infect them...

284
00:27:05,240 --> 00:27:07,600
to poison the predators.

285
00:27:15,000 --> 00:27:16,640
[wails]

286
00:27:24,520 --> 00:27:29,400
And the fungi do all this
to grow their next generation...

287
00:27:36,240 --> 00:27:40,400
which feeds on the bodies 
of the dead predators.

288
00:27:41,360 --> 00:27:43,360
[bird calls echoing]

289
00:27:50,080 --> 00:27:54,480
On Earth, fungi play a crucial,
but invisible role

290
00:27:54,560 --> 00:27:56,680
in the life of any forest,

291
00:27:59,400 --> 00:28:01,280
behind the scenes.

292
00:28:14,040 --> 00:28:17,840
[man] So, I think most people think
the first complex life on our planet

293
00:28:17,920 --> 00:28:20,720
was something like insects,
or maybe even the dinosaurs.

294
00:28:21,440 --> 00:28:24,760
But, actually, 
hundreds of millions of years before that,

295
00:28:25,320 --> 00:28:30,600
really the first complex 
multicellular organisms were these things.

296
00:28:31,320 --> 00:28:32,360
Fungi.

297
00:28:36,720 --> 00:28:40,600
We know of at least 140,000 species,

298
00:28:41,440 --> 00:28:45,000
but that's likely to be less
than ten percent of the real total.

299
00:28:49,320 --> 00:28:52,240
There's probably thousands 
of different fungal species

300
00:28:52,840 --> 00:28:54,720
just in this forest alone.

301
00:29:06,680 --> 00:29:10,000
Okay, so we've actually got
three mushrooms here

302
00:29:10,680 --> 00:29:13,920
and you can actually see these dotted 
all throughout the forest.

303
00:29:14,640 --> 00:29:16,800
But what's amazing
is that these mushrooms

304
00:29:16,880 --> 00:29:19,120
are really just the tip of the iceberg.

305
00:29:19,680 --> 00:29:21,280
When we look below the surface,

306
00:29:21,720 --> 00:29:25,800
most of the fungus is actually 
in these little... tiny little threads,

307
00:29:25,880 --> 00:29:28,200
that go all throughout the soil.

308
00:29:28,280 --> 00:29:29,840
And this is called the mycelium,

309
00:29:29,920 --> 00:29:32,720
and it connects
all of the other mushrooms in this area.

310
00:29:35,120 --> 00:29:40,000
There can be tens of kilometers 
of these tiny microscopic fungi

311
00:29:40,080 --> 00:29:42,280
spreading throughout the entire soil.

312
00:29:43,720 --> 00:29:47,400
The tree needs those fibers to survive,

313
00:29:48,000 --> 00:29:51,240
because the tree can capture 
carbon from the atmosphere,

314
00:29:51,320 --> 00:29:53,720
and it provides that carbon to the fungus.

315
00:29:54,200 --> 00:29:58,480
In contrast, the fungi access 
nitrogen and phosphorus from the soil,

316
00:29:58,560 --> 00:30:00,720
which they give
in exchange for that carbon.

317
00:30:01,240 --> 00:30:05,080
So, really, it's a mutualism
that benefits both of the organisms.

318
00:30:11,720 --> 00:30:14,080
Possibly the most extraordinary thing

319
00:30:14,520 --> 00:30:16,520
about this mycelial system

320
00:30:16,920 --> 00:30:18,800
is just how connected it is.

321
00:30:20,520 --> 00:30:23,560
So, the fungi that are attached
to the roots of this tree

322
00:30:23,640 --> 00:30:26,120
will also be attached
to the roots of that tree,

323
00:30:26,640 --> 00:30:28,560
and that tree over there,

324
00:30:28,640 --> 00:30:31,800
and they will also be connected 
to their neighbors

325
00:30:31,880 --> 00:30:34,760
via the same mycelial system

326
00:30:34,840 --> 00:30:38,080
that is really going tree to tree to tree.

327
00:30:44,160 --> 00:30:49,160
The mycelial network
is like a circuit of wires and nodes

328
00:30:49,840 --> 00:30:52,280
through which information can flow.

329
00:30:54,920 --> 00:30:59,280
And with it,
fungi can maintain the forest.

330
00:31:02,720 --> 00:31:05,360
So, if we have one tree over there
that's dying,

331
00:31:05,440 --> 00:31:08,640
it might reallocate more nutrients
towards that tree,

332
00:31:08,720 --> 00:31:10,560
so that the tree can do better

333
00:31:10,640 --> 00:31:13,480
and, as a result,
the fungal system does better.

334
00:31:14,120 --> 00:31:18,080
Similarly, if there's disturbance 
at some part of the fungal network,

335
00:31:18,160 --> 00:31:20,120
it will remove nutrients away

336
00:31:20,200 --> 00:31:23,360
so that it can minimize
the impact of that disturbance.

337
00:31:25,320 --> 00:31:28,480
It's these invisible ecosystem engineers

338
00:31:28,560 --> 00:31:33,280
that are keeping the entire system 
functioning in a really healthy way.

339
00:31:41,320 --> 00:31:44,680
This system has become known 
as the Wood Wide Web,

340
00:31:44,760 --> 00:31:46,560
and it sounds like a bit of a joke,

341
00:31:46,640 --> 00:31:51,720
but, really, it doesn't function 
so differently from the internet,

342
00:31:51,800 --> 00:31:55,240
keeping all of the organisms 
connected in the forest system.

343
00:31:55,320 --> 00:31:57,320
[whirring sound effects]

344
00:32:02,720 --> 00:32:05,960
This process is happening
all across the planet,

345
00:32:06,040 --> 00:32:08,160
in every ecosystem we can imagine.

346
00:32:12,840 --> 00:32:17,800
So, if we're gonna get a lush ecosystem
on some alien planet out there,

347
00:32:17,880 --> 00:32:20,000
I bet you that it's underpinned

348
00:32:20,080 --> 00:32:23,160
by something like
this massive mycelial system.

349
00:32:26,480 --> 00:32:28,480
[birds calling]

350
00:32:28,560 --> 00:32:30,560
[insects chirping]

351
00:32:34,120 --> 00:32:35,440
-[squeaks]
-[whoosh]

352
00:32:36,520 --> 00:32:39,920
Life in the forests of Eden is a trade-off

353
00:32:40,680 --> 00:32:44,600
between fungi, grazers and predators.

354
00:32:44,680 --> 00:32:46,160
[growling]

355
00:32:47,520 --> 00:32:49,160
[wails]

356
00:32:55,520 --> 00:32:59,160
But there are greater
celestial forces at play.

357
00:33:05,760 --> 00:33:09,280
Eden has stronger seasons than on Earth,

358
00:33:12,480 --> 00:33:17,080
greater fluctuations
of light and warmth from its twin stars.

359
00:33:21,480 --> 00:33:23,080
[sniffing]

360
00:33:23,840 --> 00:33:25,480
[chattering]

361
00:33:27,520 --> 00:33:29,680
As winter approaches,

362
00:33:29,760 --> 00:33:32,000
the remaining predators migrate...

363
00:33:34,320 --> 00:33:35,880
chasing the light.

364
00:33:35,960 --> 00:33:37,120
[chattering]

365
00:33:41,440 --> 00:33:43,960
The remaining grazers die.

366
00:33:46,920 --> 00:33:49,040
It's the same every winter.

367
00:33:50,240 --> 00:33:52,960
Nothing grows in the dark forest...

368
00:33:53,040 --> 00:33:54,720
[rustling]

369
00:33:54,800 --> 00:33:56,520
[creaking sounds]

370
00:33:56,600 --> 00:34:00,400
...except the embryos in their cocoons.

371
00:34:01,280 --> 00:34:03,080
[crunching sounds]

372
00:34:03,160 --> 00:34:04,680
[rumbling sounds]

373
00:34:07,240 --> 00:34:09,840
Just as the season starts to change...

374
00:34:13,320 --> 00:34:15,320
the grazers hatch...

375
00:34:18,760 --> 00:34:22,880
taking their cue 
from the return of the twin stars.

376
00:34:27,720 --> 00:34:29,720
[birdsong]

377
00:34:31,720 --> 00:34:35,240
On Earth, life does the same thing,

378
00:34:35,320 --> 00:34:37,480
using the seasons as a clock.

379
00:34:44,880 --> 00:34:46,360
It's midsummer,

380
00:34:47,200 --> 00:34:49,200
the longest day of the year.

381
00:34:59,000 --> 00:35:01,320
[woman] There's a lot of fly fishermen 
out this time of year

382
00:35:01,400 --> 00:35:04,560
because this is the time of year
that the fish are biting,

383
00:35:05,760 --> 00:35:08,760
because there is an abundant food source
in the river,

384
00:35:08,840 --> 00:35:12,400
and that food source
are all these guys, the mayfly nymph.

385
00:35:13,240 --> 00:35:17,520
So, we think of mayflies as flying around
on the land with us,

386
00:35:18,440 --> 00:35:21,680
but they spend the vast majority 
of their life cycle living down

387
00:35:21,760 --> 00:35:24,680
on the bottom of the river,
developing, growing,

388
00:35:25,240 --> 00:35:28,560
and it's all really building up 
to this one time in the year

389
00:35:28,640 --> 00:35:29,960
where they're all going to emerge.

390
00:35:36,920 --> 00:35:38,800
Yellow Breeches River

391
00:35:39,880 --> 00:35:42,120
feeds the Susquehanna.

392
00:35:47,200 --> 00:35:48,640
Beneath these waters,

393
00:35:49,360 --> 00:35:53,840
millions of mayflies 
are primed and ready to go.

394
00:35:59,040 --> 00:36:02,040
And when the sun sets, they emerge...

395
00:36:02,120 --> 00:36:03,720
[crowd cheering and applauding]

396
00:36:06,040 --> 00:36:07,320
en masse.

397
00:36:13,320 --> 00:36:17,120
[Allison] There are just so many insects
that seemingly came out of nowhere.

398
00:36:20,520 --> 00:36:22,840
It really almost feels like
an alien invasion.

399
00:36:30,080 --> 00:36:33,800
Mayflies live, at most, for two days.

400
00:36:35,520 --> 00:36:40,080
In that time, they must find a mate 
to pass on their genes.

401
00:36:44,320 --> 00:36:47,120
[Allison] They have one shot,
they have one chance, and if they miss it,

402
00:36:47,200 --> 00:36:48,960
it's game over for that mayfly.

403
00:36:50,760 --> 00:36:52,760
[cheering and applause]

404
00:37:00,720 --> 00:37:02,160
Once they've mated,

405
00:37:02,600 --> 00:37:06,360
the females return to the river 
to lay their eggs,

406
00:37:06,440 --> 00:37:08,000
and then die.

407
00:37:10,920 --> 00:37:13,080
[Allison] Each of these little white dots
is a mayfly

408
00:37:13,160 --> 00:37:16,320
that successfully made it back 
to the water and has laid eggs.

409
00:37:17,920 --> 00:37:21,880
They are floating down by the thousands,
by the hundreds of thousands.

410
00:37:22,520 --> 00:37:24,480
They have started the next generation

411
00:37:25,480 --> 00:37:27,800
that will continue on 
and do this again next year

412
00:37:27,880 --> 00:37:30,120
in this really precisely timed event.

413
00:37:36,240 --> 00:37:40,080
The rhythm of the seasons 
is always dictated

414
00:37:40,160 --> 00:37:43,480
by the tilt of a planet towards its star.

415
00:37:49,400 --> 00:37:54,080
Earth is tilted at an angle 
of 23-and-a-half degrees.

416
00:38:00,520 --> 00:38:04,120
On Eden, that angle is 40 degrees,

417
00:38:06,840 --> 00:38:10,800
so more light spreads across
more of the planet's surface.

418
00:38:14,640 --> 00:38:18,040
And more light means more life.

419
00:38:24,800 --> 00:38:27,680
It's spring, and there's food to eat...

420
00:38:30,120 --> 00:38:34,400
but the new grazers need to grow 
as fast as possible...

421
00:38:35,400 --> 00:38:37,120
[distant roaring]

422
00:38:39,440 --> 00:38:40,600
[roaring and chattering]

423
00:38:40,680 --> 00:38:42,800
...before the predators return...

424
00:38:49,320 --> 00:38:52,080
and the life cycle repeats.

425
00:39:03,720 --> 00:39:05,720
Wondrous creatures...

426
00:39:08,480 --> 00:39:10,480
fed by glucose...

427
00:39:17,960 --> 00:39:19,960
powered by oxygen...

428
00:39:22,480 --> 00:39:24,640
nourished by starlight.

429
00:39:35,440 --> 00:39:40,680
What is true on Eden
may be true throughout the universe...

430
00:39:43,360 --> 00:39:47,640
that all life depends
on the flow of energy.

431
00:40:00,120 --> 00:40:03,400
How might life adapt 
on a different world,

432
00:40:05,520 --> 00:40:08,240
inhabited by intelligent beings,

433
00:40:10,360 --> 00:40:13,200
intelligent enough to make a new home...

434
00:40:14,480 --> 00:40:16,840
among the stars?