When you picture Earth, what are the colours you first think of?
There’s the blue of our oceans, the greens and browns and oranges of the landmasses,
and the swirls of white that make up our clouds.
But, according to some theories, it wasn’t always this way.
And, in our planet’s ancient past, the predominant shade was actually… purple.
This is Unveiled, and today we’re answering the extraordinary question; was ancient Earth
a different colour?
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On Valentine’s Day in 1990, one of the most famous photographs of all time was taken,
the Pale Blue Dot.
It was snapped by the Voyager 1 Space Probe on its way out of the solar system, from some
3.7 billion miles away.
It isn’t much to look at, and the significance of it could easily be missed… but what this
seemingly blank image actually shows is Earth as a tiny, blueish speck against the vast
expanse of space.
It’s what our planet looks like set against the rest of the cosmos.
And we generally take this pale blue colour for granted.
We know that around seventy-one percent of the surface of our world is covered by ocean,
and we know that from far enough away all of that water merges into a deep blue hue.
We also know that the gasses in our atmosphere serve to shape and refract the light around
our planet, thereby dictating how it appears from a distance.
But, by now, we’re used to seeing visuals from the International Space Station, as well,
as it races around the Earth providing a gods-eye view of what, for all human beings, is home.
So, we’re very aware that so much of Earth when viewed from above also appears to be
green… thanks to our lush forests, fields and woodlands.
That green colour comes from chlorophyll, a pigment in most plants and trees.
It’s vital for photosynthesis and providing plants with the energy they need to grow and
function.
Therefore, it’s a crucial piece of chemistry in how life on Earth works in general, ensuring
that oxygen and carbon dioxide levels remain relatively stable.
Importantly for today’s question, though, chlorophyll also absorbs red and blue light
while reflecting green light - which is why leaves and vegetation are predominantly the
colour that they are; green.
But what happens if we substitute chlorophyll for something else?
Well, according to one theory, that might have been exactly what was going on during
Earth’s earliest years.
The Purple Earth Hypothesis was first put forward by the renowned biochemist Shiladitya
DasSarma in a feature in “American Scientist”, in 2007.
And today, it increasingly shapes what we imagine Earth in ancient times to have looked
like.
DasSarma and his team conducted extensive studies on Haloarchaea, a microscopic, single-celled
archaea found in highly salty environments.
These microbes are what’s known as retinal-based (rather than chlorophyll-based), which means
that they photosynthesise light energy from the sun differently.
Rather than reflecting green and yellow light as most plants on Earth today do, they absorb
it.
Instead, they reflect reds and blues… which gives them a purple colour when we look at
them.
Under the microscope, they’re said to have a purple membrane.
And, in some ways, they could be described as our more commonplace green cells, but as
they might appear in negative.
Haloarchaea aren’t exactly prominent in the world today… if they were, then every
time you walked through the park or cycled through the forest you’d be met with a sea
of purple.
Instead, they’re typically found in extreme environments, only.
But the theory is that there may have been a time when retinal-based cells, in general,
were more widespread.
Particularly when there was less oxygen on Earth, period… so that the entire planet,
by our standards, was extreme.
To put it into context, the retinal pigment is described as a far simpler molecule, compared
to chlorophyll… leading to the suggestion that it dominated Earth in the days before
what scientists refer to as the Great Oxygenation Event.
The Great Oxygenation Event has also been known as the Oxygen Crisis and the Oxygen
Revolution.
It occurred on this planet between roughly 2.3 and 1.9 billion years ago, transforming
Earth’s atmosphere from one where oxygen was scarce… into one where (as today) it’s
abundant.
Currently, little is definitely known about what the Earth was like just after it formed,
nor during its first billion years or so.
But we can chart chloroplasts - i.e., structures within cells that conduct photosynthesis via
chlorophyll - to around 3.5 billion years ago.
The broad idea is that by the time of the Great Oxygenation Event, chlorophyll-based
microbes had become dominant… and Earth turned green.
Before then, though, it was all about retinal-based microbes and, so, the Earth was purple.
These could survive in anaerobic conditions without oxygen and, thanks in part to their
simpler structure, they were incredibly efficient during the first half of Earth’s life.
Up until chlorophyll took over.
The theory is still very much up for debate but, in the meantime, it provides us with
an all-new strategy when looking for signs of life not on Earth.
When looking at alien planets and star systems, we so often assume that they would ideally
have properties similar to our own world’s as it is now.
The conditions for liquid water and the promise of oxygen in the atmosphere are two key points
of interest, for example.
And if we were to ever get a close enough look at a far-off world and it was green all
over, then the scientists and astrobiologists amongst us would certainly get excited…
because of the suggestion of life that that would hold.
But now, we’re keeping our eyes peeled for purple, as well.
Because we know that, if the Purple Earth Hypothesis rings true, then a planet boasting
reddish, blueish shades could perhaps be considered a precursor for life as we know it.
A first stage toward life eventually thriving.
And so, according to some, purple-ness should be considered a biosignature of sorts whenever
we’re searching the skies for an extraterrestrial presence.
One fun coincidence is that the colour purple has infiltrated recent pop culture ideas on
what aliens would look like.
For years during the mid-twentieth century, the “little green man” had been the go-to
image for anyone penning (or filming) science fiction.
Nowadays, that motif isn’t quite so all-encompassing, and aliens in books and films come in a greater
variety of shapes and colours.
They’re no longer always humanoid, and every so often they are purple.
Among the most purple are Oh from the 2015 animated movie, “Home”, Widget from the
early ‘90s cartoon of the same name, and the villainous Ivan Ooze from the 1995 “Power
Rangers” feature film.
So, perhaps the Purple Earth Hypothesis has already permeated our collective consciousness?
What’s clear is that this theory is one which potentially transforms the way in which
we look at space, and at the history of our own planet.
Imagine what the world would look like now if chlorophyll had never taken over from retinal,
in the way that the Purple Earth Hypothesis suggests that it has done.
For one, Earth may never have oxygenated in the same way, meaning life may never have
appeared.
But even if life had miraculously emerged under these alternate conditions, the purple
equivalents of leaves, trees and forests (which all could have been totally different multicellular
structures in all other ways, as well) are a bit of a head trip to think about!
In today’s world, there are various places you can go to get a small sense of what might
have been.
All across the world map there are salt ponds and lakes curiously tinged with purple, thanks
to the modern day, retinal-based microbes that still dwell there.
Indeed, it was locations like this that first provided Shiladitya DasSarma and his team
with grounds to formulate the Purple Earth Hypothesis to begin with.
The somewhat ironic situation is that with photographs of these lakes, it could be argued
that there’s an almost unnatural element to them.
That, because of their purple hint, they look as though they don’t belong here.
Or as though the images themselves have been edited for effect.
When, in reality, the chemical process needed to make stuff purple… is potentially one
of the oldest-known natural features of our planet.
Older even than the grass being green.
A long time ago, everything was purple.
And, in our ongoing search for alien life, we could be far better off if we specifically
looked for purple places of interest.
Or, at least, that’s how this particular theory goes.
And that’s why it’s thought that ancient Earth could truly have been a different colour.
What do you think?
Is there anything we missed?
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