When The Earth Was Purple

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
little … purple. 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!


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