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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 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, then the universe must be alive.

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All living things have the same needs to feed, reproduce.

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And evolve.

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By applying the laws of life on Earth to the rest of the universe,

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it's possible to imagine what could live on alien worlds.

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The Earth, home to millions of species.

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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 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, we will find the chemical fingerprints 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 about 12 light years away, the star Tau Setti.

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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, but what we want to do is catch light 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 was visible in our own solar system.

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When Venus passed in front of the Sun, 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, 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 that could be remotely detectable from light years away.

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And it always leads us back to the same gas, oxygen.

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50 to 70% of oxygen in our atmosphere 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 and in fact all the forests that we see on the land is photosynthesis.

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What interests me most about photosynthesis 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 is produced by photosynthesis, by life here on planet Earth.

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And if you see it in abundance, you know that the planet is a living world.

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Among trillions of worlds out there, those with more oxygen are more likely to sustain life.

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Imagine a planet like Earth orbiting not one star, but two.

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This is Eden.

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The light from its twin stars powers photosynthesis, pumping oxygen into the atmosphere, allowing life to thrive.

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Grazers feed on low-lying fungus, but they're constantly alert to danger.

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The canopy is home to predators, perfectly evolved to live among the trees, waiting for their moment to strike.

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On the ground, the grazer has the edge. 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, 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, 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 in Earth.

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The trees and plants are soaking up the sun's energy. By the magic of photosynthesis, they convert sunlight into glucose energy, 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, capturing the sun's energy as glucose.

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Herbivores eat plants. Carnivores eat herbivores.

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Every link in the food chain depends on the transfer of this glucose energy.

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And in this forest, one animal can't get enough of the stuff.

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The hummingbird.

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Hummbirds 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 is fueled by glucose-rich nectar.

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This is a device that from the front looks like a flower.

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A hummingbird comes and it feeds from it.

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There is one. It's going back and forth.

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You can do it.

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Okay, so now it's drinking.

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This measures the nectar the bird is drinking and the oxygen it is breathing.

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We calculate the energy intake from the nectar 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% of the atmosphere is oxygen, fueling the diversity of life around us.

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On Eden, there's 10% more oxygen than on Earth.

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So life here can be more diverse, more energetic, 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 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 away from predators.

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Here on Earth, the threat of death is a huge factor in the evolution of life.

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Predation is everywhere. There's not an organism out there that doesn't run a 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 going to be coming in here.

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This river is perfect for studying guppyfish and their patterns of reproduction.

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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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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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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 in relatively few offspring.

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Downstream, it's a different story.

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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, it wouldn't last a second.

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The only defense that guppies have is to produce lots of offspring in the hope that some will survive to adulthood.

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I have 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 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 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 going to die young 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, 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 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, which attract grazers.

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Spores within the fruit spread an infection.

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Now, when predators attack, infected grazers don't run. They've lost their fear instinct.

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It's an easy meal, 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. But not all are so lethal.

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

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One high-calorie food is prized above all else. 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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The honey guide has lived up to its name.

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It's led them to a bee's nest in the forest.

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The honey guide has lived up to its name. It's led them to a bee's nest in a baobab tree.

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The hunters need the bird to find the bees. And the bird needs the hunters to access the honey.

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That's dangerous해ä.

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All was very sad.

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I know.

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The risk is worth it.

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Fresh, rich honey.

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The lango 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 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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They are 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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They season the predators.

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The fungi do all this to grow their next generation, which feeds on the bodies of the dead predators.

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On Earth, fungi play a crucial but invisible role in the life of any forest.

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Behind the scenes.

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Most people think the first complex life on our planet was something like insects or even dinosaurs.

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Hundreds of millions of years before that, the first complex multicellular organisms were these things.

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Fungi.

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We know of at least 140,000 species, but that's likely to be less than 10% of the real total.

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There's probably thousands of different fungal species just in this forest alone.

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Okay, so we've actually got three mushrooms here, and you can actually see these dotted all throughout the forest.

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But what's amazing is that these mushrooms are really just the tip of the iceberg.

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When we look below the surface, most of the fungus is actually in these tiny little threads that go all throughout the soil.

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This is called the mycelium, 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 spreading throughout the entire soil.

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The tree needs those fibers to survive, because the tree can capture carbon from the atmosphere, and it provides that carbon to the fungus.

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In contrast, the fungi access nitrogen and phosphorus from the soil, 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 about this mycelial system is just how connected it is.

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So the fungi that are attached to the roots of this tree will also be attached to the roots of that tree, and that tree over there.

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And they will also be connected to their neighbors via the same mycelial system that is really going tree to tree to tree.

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The mycelial network is like a circuit of wires and nodes 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, it might reallocate more nutrients towards that tree so that the tree can do better, 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, it will remove nutrients away so that it can minimize the impact of that disturbance.

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It's these invisible ecosystem engineers that are keeping the entire system functioning in a really healthy way.

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This system has become known as the Wood Wide Web, and it sounds like a bit of a joke, but really it doesn't function so differently from the internet, keeping all of the organisms connected in the forest system.

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This process is happening all across the planet in every ecosystem we can imagine.

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So if we're going to get a lush ecosystem on some alien planet out there, I bet you that it's underpinned by something like this massive mycelial system.

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Life in the forests of Eden is a trade-off 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. Greater fluctuations of light and warmth from its twin stars.

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As winter approaches, the remaining predators migrate, chasing the light.

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The remaining grazers die.

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It's the same every winter. Nothing grows in the dark forest, except the embryos in their cocoons.

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Just as the season starts to change, the grazers hatch, taking their cue from the return of the twin stars.

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On Earth, life does the same thing, using the seasons as a clock.

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It's mid-summer, the longest day of the year.

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There's a lot of fly fishermen out this time of year because this is the time of year that the fish are biting.

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Because there's an abundant food source in the river, and that food source are all these guys, the mayfly nymph.

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So we think of mayflies as flying around on the land with us, but they spend the vast majority of their life cycle living down on the bottom of the river, developing, growing.

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And it's all really building up to this one time in the year where they're all going to emerge.

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Yellow Breeches River feeds the Sasquahana.

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Beneath these waters, millions of mayflies are primed and ready to go.

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And when the sun sets, they emerge on mass.

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There's just so many insects that seemingly came out of nowhere.

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It really almost feels like an alien invasion.

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Mayflies live at most for two days.

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In that time, they must find a mate to pass on their genes.

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They have one shot, they have one chance, and if they miss it, it's game over for that mayfly.

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Once they've mated, the females return to the river to lay their eggs and then die.

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Each of these little white dots is a mayfly that successfully made it back to the water and has laid eggs.

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They are floating down by the thousands, by the hundreds of thousands.

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They have started the next generation that will continue on and do this again next year in this really precisely timed event.

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The rhythm of the seasons is always dictated by the tilt of a planet towards its star.

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Earth is tilted at an angle of 23.5 degrees.

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On Eden, that angle is 40 degrees.

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So more light spreads across more of the planet's surface.

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And more light means more life.

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It's spring and there's food to eat.

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But the new grazers need to grow as fast as possible.

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Before the predators return.

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And the life cycle repeats.

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Wanderous creatures.

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Fed by glucose.

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Powered by oxygen.

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Washed by starlight.

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What is true on Eden may be true throughout the universe.

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That all life depends on the flow of energy.

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How might life adapt on a different world?

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Inhabited by intelligent beings.

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Intelligent enough to make a new home among the stars.