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The Earth, home to millions of species.

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But what might live... beyond?

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Astronomers have
discovered thousands of planets

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outside our solar system.

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

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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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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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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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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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silhouetted against distant stars,

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they might find proof of life.

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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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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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Grazers feed on low-lying fungus...

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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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perfectly evolved to
live among the trees...

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waiting for their moment to strike.

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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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But the predator has
evolved a special weapon.

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This time, the grazer escapes...

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and the predator conserves its energy.

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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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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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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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The hummingbird.

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Hummingbirds can move their
wings over a hundred times a second.

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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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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.

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Uh... Okay, so now, he's drinking.

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The device measures the
nectar the bird is drinking

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and the oxygen it is breathing.

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We calculate the energy
intake from the nectar

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and the energy output from the oxygen

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and we can calculate very accurately
how much energy the bird is using.

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A hovering hummingbird consumes
oxygen at an incredible rate.

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Ten times faster than
an Olympic sprinter.

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The more oxygen it can get into its
cells, the faster it can burn glucose,

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which unlocks the energy within.

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On Earth, 21 percent of
the atmosphere is oxygen...

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fueling the diversity of life around us.

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On Eden, there's ten percent
more oxygen than on Earth...

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so life here can be more diverse...

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more energetic...

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more competitive.

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Summer is breeding season for grazers.

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Always fearful of predators, they
don't spend time finding a mate.

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Instead, they produce worm-like spawn.

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Each needs to fuse with another

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to create an embryo.

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For protection, they form a cocoon.

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Suspended above the
ground, the embryos can grow,

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away from predators.

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Here on Earth, the threat
of death is a huge factor

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in the evolution of life.

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Predation is everywhere.

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There's not an organism out there

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that doesn't run the risk of
being eaten by another organism.

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The aspect that I'm interested in is
how predation shapes reproduction.

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They're gonna be comin' in here.

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This river is perfect
for studying guppy fish...

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and their patterns of reproduction.

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- Here. Here's another male.
- Yeah, yeah.

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This is an easy neighborhood.

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Here, guppies live with
only one other species of fish,

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and it's a fish that rarely eats
guppies or harasses guppies.

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They can swim wherever they want without
any risk of somebody trying to eat them.

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- Oh, right. There you go. All right.
- Here we go.

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Here we have some babies that we
just caught. We have five of them here.

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They may look small to you, but that's
really big for a newborn baby guppy.

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The guppies here invest a lot of effort

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in relatively few offspring.

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Downstream, it's a different story.

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Okay!

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Wow.

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This boulder is a natural
barrier, dividing the river.

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Below it lurks danger.

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A guppy life down here is pretty
much like the life of a fugitive.

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You're always on the run from predators.

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If a guppy were to swim out here...

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it wouldn't last a second.

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The only defense the guppies have

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is to produce lots of offspring,

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in the hope that some
will survive to adulthood.

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Okay, I've two babies from up there,
where guppies live without predators,

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and four from down here,
where guppies live with predators.

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There's a dramatic difference in size.

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Those that are from here
are much, much smaller

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than the ones that I
caught above the barrier.

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Moms don't have a big prospect of living
to the future, no matter what they do.

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So the best strategy down here
is to make many small babies.

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The guppies have evolved
two different ways of breeding.

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Having a few large babies...

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or lots of small ones.

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The deciding factor is
the threat from predators.

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If you're one of the prey
items, you're gonna die young,

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so you better live fast.

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You better put a lot into having babies,
because if you don't, you'll go extinct.

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Such a strategy would
apply on any planet

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wherever one species preys on another.

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On Eden, the grazers
have evolved to breed fast.

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They produce as many
offspring as quickly as possible...

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to ensure the survival of the species.

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But in the forest,
they're never truly safe.

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As summer ends, the fungi
grow orange-colored fruit...

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which attract grazers.

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Spores within the fruit
spread an infection.

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Now, when predators attack,

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infected grazers don't run.

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They've lost their fear instinct.

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It's an easy meal...

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but a poisonous one.

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The fungi have used the grazers
to infect and kill the predators.

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Such complex relationships
between species exist on Earth...

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but not all are so lethal.

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The Hadza are hunter-gatherers,
living off the land.

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My name is Palango.

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As a man, my job is to hunt.

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I always carry a bow and arrow

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for protection...

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because the bush

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has many dangerous animals.

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One high-calorie food
is prized above all else.

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Honey.

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But finding fresh honey is difficult.

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The bees move their nests
from season to season.

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So the Hadza call on help.

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00:22:57,680 --> 00:23:01,160
Honeyguides are very clever birds.

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They know where honey is,
and they will take you there.

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She makes a special cry...

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and I whistle back.

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If I follow her, she will lead
me directly to the honey.

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The honeyguide has lived up to its name.

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It's led them to a bees'
nest in a baobab tree.

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The hunters need the
bird to find the bees.

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And the bird needs the
hunters to access the honey.

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The honeyguides take us to the bees.

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But the bees are aggressive.

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So she needs our help.

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Climbing up a baobab tree...

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is very hard and dangerous.

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One day I fell.

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We use smoke to get the bees drunk

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and chase them out of the nest.

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The risk is worth it.

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Fresh, rich, honey.

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Palango makes sure
the bird gets its share.

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It's a win-win situation.

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A complex relationship between species

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is known as symbiosis.

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The richer the ecosystem, the
more complex the relationship.

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On Eden, a deadly three-way
symbiosis has evolved...

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centered around the fungi.

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First, they feed the grazers.

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Then, they infect them...

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to poison the predators.

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And the fungi do all this to
grow their next generation...

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which feeds on the bodies
of the dead predators.

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On Earth, fungi play a
crucial, but invisible role

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in the life of any forest,

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behind the scenes.

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So, I think most people think
the first complex life on our planet

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was something like insects,
or maybe even the dinosaurs.

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But, actually, hundreds of
millions of years before that,

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really the first complex multicellular
organisms were these things.

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Fungi.

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

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

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There's probably thousands
of different fungal species

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just in this forest alone.

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Okay, so we've actually
got three mushrooms here

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and you can actually see these
dotted all throughout the forest.

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But what's amazing is
that these mushrooms

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are really just the tip of the iceberg.

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

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most of the fungus is actually
in these little... tiny little threads,

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that go all throughout the soil.

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And this is called the mycelium,

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and it connects all of the
other mushrooms in this area.

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There can be tens of kilometers
of these tiny microscopic fungi

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spreading throughout the entire soil.

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The tree needs those fibers to survive,

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because the tree can capture
carbon from the atmosphere,

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and it provides that
carbon to the fungus.

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In contrast, the fungi access
nitrogen and phosphorus from the soil,

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which they give in
exchange for that carbon.

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So, really, it's a mutualism that
benefits both of the organisms.

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Possibly the most extraordinary thing

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about this mycelial system

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is just how connected it is.

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So, the fungi that are
attached to the roots of this tree

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will also be attached
to the roots of that tree,

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and that tree over there,

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and they will also be
connected to their neighbors

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via the same mycelial system

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that is really going
tree to tree to tree.

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The mycelial network is like
a circuit of wires and nodes

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through which information can flow.

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And with it, fungi can
maintain the forest.

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So, if we have one tree
over there that's dying,

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it might reallocate more
nutrients towards that tree,

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so that the tree can do better

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and, as a result, the
fungal system does better.

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Similarly, if there's disturbance
at some part of the fungal network,

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it will remove nutrients away

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so that it can minimize the
impact of that disturbance.

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It's these invisible ecosystem engineers

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that are keeping the entire system
functioning in a really healthy way.

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

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and it sounds like a bit of a joke,

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but, really, it doesn't function
so differently from the internet,

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keeping all of the organisms
connected in the forest system.

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This process is happening
all across the planet,

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in every ecosystem we can imagine.

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So, if we're gonna get a lush
ecosystem on some alien planet out there,

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I bet you that it's underpinned

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by something like this
massive mycelial system.

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Life in the forests
of Eden is a trade-off

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between fungi, grazers and predators.

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But there are greater
celestial forces at play.

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Eden has stronger seasons than on Earth,

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greater fluctuations of light
and warmth from its twin stars.

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As winter approaches,

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

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chasing the light.

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

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

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Nothing grows in the dark forest...

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except the embryos in their cocoons.

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

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the grazers hatch...

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

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On Earth, life does the same thing,

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using the seasons as a clock.

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It's midsummer,

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the longest day of the year.

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

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

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

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

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

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

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

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

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

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Yellow Breeches River

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

342
00:37:22,520 --> 00:37:24,480
They have started the next generation

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

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

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

346
00:37:40,160 --> 00:37:43,480
by the tilt of a planet
towards its star.

347
00:37:49,400 --> 00:37:54,080
Earth is tilted at an angle
of 23-and-a-half degrees.

348
00:38:00,520 --> 00:38:04,120
On Eden, that angle is 40 degrees,

349
00:38:06,840 --> 00:38:10,800
so more light spreads across
more of the planet's surface.

350
00:38:14,640 --> 00:38:18,040
And more light means more life.

351
00:38:24,800 --> 00:38:27,680
It's spring, and there's food to eat...

352
00:38:30,120 --> 00:38:34,400
but the new grazers need
to grow as fast as possible...

353
00:38:40,680 --> 00:38:42,800
before the predators return...

354
00:38:49,320 --> 00:38:52,080
and the life cycle repeats.

355
00:39:03,720 --> 00:39:05,720
Wondrous creatures...

356
00:39:08,480 --> 00:39:10,480
fed by glucose...

357
00:39:17,960 --> 00:39:19,960
powered by oxygen...

358
00:39:22,480 --> 00:39:24,640
nourished by starlight.

359
00:39:35,440 --> 00:39:40,680
What is true on Eden may be
true throughout the universe...

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

361
00:40:00,120 --> 00:40:03,400
How might life adapt
on a different world,

362
00:40:05,520 --> 00:40:08,240
inhabited by intelligent beings,

363
00:40:10,360 --> 00:40:13,200
intelligent enough to make a new home...

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