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  • Picture the view of Earth from space.

  • Beautiful, right?

  • What colors do you see?

  • Besides the blue of the oceans, the dominant color of our planet, as we know it, is green.

  • But imagine a time when the Earth looked a littlepurple.

  • All that green we see today is because of chlorophyll, the pigment that plants use to

  • harness the sun's energy through photosynthesis.

  • Chlorophyll looks green because it absorbs the blue and red parts of the visible spectrum

  • to use in photosynthesis, while reflecting green light.

  • But the fact is, the sun actually emits more photons in the green wavelengths than any

  • other part of the visible spectrum.

  • And this has biologists puzzled.

  • Why didn't living things evolve to take advantage of all that literal green energy,

  • by absorbing green light, instead of reflecting it?

  • Well, scientists think that green light is so plentiful that absorbing all of its energy

  • would actually be harmful, causing damage to the pigment -- similar to a sunburn.

  • But another, more interesting explanation has been proposed too.

  • It's called the Purple Earth Hypothesis.

  • So the idea goes, maybe the very first photosynthesizers on Earth didn't use chlorophyll at all.

  • Instead, maybe they used another, simpler light-sensitive molecule -- one that did absorb

  • all of that abundant green energy, and reflected purple light.

  • The idea is at least plausible, because such tiny, lavender forms of life exist on Earth

  • today: They're called halobacteria.

  • Halobacteria aren't actually bacteria -- they're Archaea, single-celled organisms that mostly

  • thrive in extreme environments, where almost nothing else can live.

  • For example, they're known for living happily in concentrated salt solutions.

  • Their membranes contain a light-sensitive pigment called retinal, which absorbs green

  • light, making it appear purple as a result.

  • Retinal is a simpler molecule than chlorophyll, and it's easier to produce, while making

  • the most of that abundant green light.

  • So the Purple Earth Hypothesis suggests that, back in the Archaean Eon, before chlorophyll

  • was a thing, Earth's oceans may have been dominated by microbes that were a lot like

  • halobacteria -- ones that used retinal, or some other purple pigment, to harness the

  • sun's energy.

  • This idea was first proposed in the mid 2000s by microbial geneticist Shil DasSarma at the

  • University of Maryland.

  • He says his hypothesis might help explain why today's photosynthesizers don't absorb

  • the green light from the sun -- because they adapted to a world where an abundance of other

  • organisms was already monopolizing it.

  • According to the Purple Earth Hypothesis, chlorophyll eventually evolved in a different,

  • competing lineage of microbes, to take advantage of the wavelengths of light that purple Archaea

  • weren't using.

  • Picture a mat of green, chlorophyll-based microbes underneath a raft of purple, retinal-based

  • microbes, soaking up the leftover dregs of light.

  • In this scenario, halobacteria -- or something like them -- could have been among the earliest

  • forms of life on our planet.

  • And their world would've been very different from the one we know today -- hot, bombarded

  • by UV rays, and rich in sulfur and methane.

  • Unlike today's chlorophyll-using photosynthesizers, these organisms wouldn't have produced oxygen

  • -- in fact, they would have thrived in the oxygen-deprived environment of early Earth.

  • That is, of course, until those lowly chlorophyll-producers started taking over.

  • Chlorophyll is a more complex molecule than retinal, and it doesn't soak up those abundant

  • green wavelengths of light.

  • But it is more efficient -- making better use of the light that it absorbs, and converting

  • more of it into usable energy.

  • And in the long run, that's what may have mattered most.

  • Of course, this is all speculation -- the fossil record is pretty limited when it comes

  • to microbes from billions of years ago, so the purple Earth hypothesis can't be proven

  • either way.

  • But, it does fit with what we know about the atmosphere of early Earth.

  • For instance, we know that, around three billion years ago, there was barely a trace of oxygen

  • in our planet's atmosphere -- so there couldn't have been a lot of chlorophyll-based photosynthesis

  • going on.

  • But we also know that, about two billion years ago, some microbes that did use chlorophyll

  • -- like cyanobacteria -- came on the scene, and began to flourish, releasing tons of oxygen.

  • The flood of oxygen would have killed off many of the simpler -- possibly purple! -- microbes

  • that came before them.

  • This microbial carnage came to be known as the Great Oxygenation Event, which we've

  • talked about before.

  • But if DasSarma's hypothesis is right, then why are there still halobacteria and other

  • Archaea around today?

  • Wouldn't they have been wiped out?

  • Well, his work suggests that some crafty halobacteria may have managed to snatch a few genes from

  • the DNA of other microbes that allowed them to survive in the presence of oxygen, which

  • has helped them persist into the present.

  • Again, it's all speculation.

  • But a purple Earth is fun to think about.

  • And if this idea is true, it could also have big implications for the search for life on

  • other planets.

  • One way we seek out other living worlds is by looking for planets that reflect less red

  • light than we'd expect.

  • This could mean they have organisms with chlorophyll that are absorbing those red wavelengths.

  • But, if life can evolve with more than one way of making energy from light, maybe we

  • shouldn't just be searching for planets that are green like ours.

  • Maybe ... we need to be looking for purple worlds as well.

  • What do you want to know about the story of life on Earth?

  • Let us know in the comments.

  • And don't forget to go to youtube.com/eons and subscribe!

  • Now, don't stop exploring!

  • Check out some of our sister channels from PBS Digital Studios, and find out what you'll

  • discover next!

Picture the view of Earth from space.

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B2 US chlorophyll purple green earth retinal green light

When The Earth Was Purple

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    joey joey posted on 2021/05/02
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