Imagine for a moment the immense history of life on earth.
While this planet hasn’t been here since the formation of the milky galaxy itself,
it has been here for a significant portion of it.
Given that life appeared on earth, rather conspicuously, the very first moment it could,
we live on a world where the vast majority of its existence has been that of a habitable
living world.
At least for simple life.
This gives Earth an unusually long window for having been detected by someone else in
the galaxy as a planet well worth investigation, especially if occurences of even microbial
life are rare in the universe.
We ponder this now, thinking of missions to ice shell moons and life finder missions to
Venus and Mars to try to establish if life in its microbial form exists in those places,
or once existed, or the even more interesting question of will life someday exist in places
that might develop conditions conducive for it.
One can envision a future brighter sun or transitional sun swelling into a red giant
that makes these ice shell moons, or at least one or two of them, open ocean worlds for
a time where life might arise long after the first genesis of life in this solar system
has gone utterly extinct.
That aside, it may also be that Earth is rare and that few exoplanets that have periods
of habitability, or urability, last long enough to produce civilizations.
And that in itself assumes that abiogenesis is easy, there may be aspects of it that we
don’t yet understand that say no, it isn’t, and that any life in the universe is rare.
That may motivate alien civilizations to push further to explore alien life wherever they
find it, or think it may exist.
And if they have a more complete picture of planetary science, they may be able to pick
these interesting worlds out with much greater capability than we can now.
As a result, we can’t rule out that this star system and our planet hasn’t at some
point in its immense history been visited by an alien civilization.
And there are several methods for it to have happened, ranging from depositing a probe
in systems of interest whenever you happen to pass by them in stellar encounters to sending
out self-replicating, self-repairing probes to monitor worlds long term.
And while we often think of aliens coming here to interact with us science fiction style
and establish contact, there is also a scenario where aliens may have visited a very different,
very early earth that bears little resemblance to the earth we know.
Through exploring past periods in earth’s history long before we were a glimmer in the
planet’s eye we find periods in earth’s history that showcase and give us an idea
of what inhabited alien worlds really might look like above and beyond the guesswork often
presented in sci fi.
There are many such periods, you could imagine the age of the dinosaurs for example with
an earth covered in the myriad species of the tyrant lizards, as they were once known,
though the reality of the dinosaurs was much more complex than that.
Hot blooded reptiles pursuing prey with stereo vision eyes such as in the case of the tyrannosaurus
rex; perhaps one once ate an unfortunate visiting alien, or the skies filled with flying reptiles,
one branch of which still wages their age-old aerial battles in the form of the birds.
Often when coming up for ideas for videos on this channel, I quietly sit on my porch
and look up.
Sometimes I see the stars, but in daylight I see smaller birds attacking large hawks,
owls at night hunting their prey scanning the landscape around them, spring nesting
sometimes in the eaves of my house, and many other things that I wonder must hearken back
to the dinosaurs themselves.
But even the age of the dinosaurs was not the most alien landscapes this world has ever
presented.
There are other periods so foreign to our modern earth, that had aliens ever visited,
and studied, it might have been so alien to what they know that they found this place
a treasure trove in the study of astrobiology and just what life can do.
The first of these hypothetical earthly alien landscapes involves an unconfirmed hypothesis
in astrobiology.
The Purple Earth Hypothesis.
This is somewhat of a mystery, photosynthetic life on earth actually uses the more complex
and difficult to work with chemical Chlorophyll for photosynthesis.
But there is an alternative chemical that would on its face seem a simpler way to get
to the same end.
Known as retinal, this chemical actually still is used by some forms of life on earth in
the form of simple photosynthesizers known as Haloarchaea, so we know it is used by life,
but there may have been a time where it was much more extensively used meaning that instead
of a green earth, it would have been a purple earth.
Indeed the Haloarchaea are indeed purple.
This is an interesting state of affairs in photosynthesis, since the retinol type actually
likes the green through yellow area of the visible light spectrum, where the sun emits
very strongly as opposed to the blue and red light chlorophyll based plants prefer, which
avoid green light and reflect it, which is why plants are generally green.
Some evidence shows that purple photosynthesis was the first type, as opposed to green.
There is also evidence of co-evolution and almost symbiotic coexistence between purple
and green photosynthetic species.
So with that state of affairs, it may be that the type of photosynthesis an exoplanet with
life experiences may be dependent on evolution and the type of star that life has to work
with.
There could be purple worlds with active biospheres that could be searched for within exoplanet
SETI.
That would be a good biosignature indeed, since it’s hard to envision a planet turned
purple by retinol without a biosphere being present to produce it.
Indeed, if there are inhabited worlds with intelligent life out there that only know
the retinal type of photosynthesis, and chlorophyll never developed, they might not know to even
look at modern earth for its signature and miss our biosphere completely.
They might hypothesize such biochemistry could exist, but dismiss it as unlikely based on
its complexity and the fact that they’ve never seen it in nature.
In short, there may be aliens out there that don’t believe earth could exist and never
look for it.
Or they might know all too well, having stationed a probe here as their star system passed by
long ago during Earth’s microbial only phase.
If they had watched long enough, they might literally have been witness to earth turning
from purple to green.
To imagine, if they have words and terms as we understand them, that may be their name
for this world.
The formerly purple now green water world, Glorkon the Brilliant’s Photosynthizer …. or
simply the changeling world.
Generally how you would study this in SETI is to start with looking for how worlds reflect
the light from their star.
That tells you a lot about whether you’re dealing with a signature of chlorophyll or
retinal.
While we need much larger telescopes to look for this, we are just at the cusp with the
James Webb Space Telescope to begin the search for this in situational cases.
That said, let’s look at a much later period in the alien earth’s history.
If an alien species had visited earth in the Ordovician or Devonian periods, roughly 470
to 360 million years ago, and decided to explore earth’s land mass in those days, they would
have seen a very bizarre landscape very much unlike anything we see today.
If they were in the right place, they’d see a kind of forest with giant organisms,
something like trees, but were actually radically different.
They’d have found a forest of giant fungi.
Known as Prototaxites, this group of extinct fungi are weird.
While it would be inaccurate to call it a mushroom, imagining how mushrooms and the
other fungi are is useful to imagine this organism, which at the time was the largest
land dwelling organism on earth that we are aware of.
Imagine a mushroom that had no cap, but rather one giant trunk up to a meter wide sticking
up in the air as much as 8 meters with no branches.
You might also picture the form of a giant cactus with no arms, but with very different
physiology with a trunk made up of interwoven tiny tubes.
As is so common in paleontology, the story of the discovery of this species is a rough
one.
Fossils were first found in 1843 by John William Dawson.
Dawson’s interpretation is that they must have been fossilized conifer trees that preserved
the remnants of the fungi that digested them after they died.
In 1872 however, comes along William Carruthers who absolutely destroyed the idea to the point
that Dawson after fighting against the idea eventually was forced to accept it as new
findings came to light and then promptly claimed he never actually thought it was a tree.
That said, his original name stuck to this day it actually references a tree rather than
a fungus.
Oddly, just why these giant fungi went extinct is still a mystery.
There is some evidence that it was really more like a lichen and hosted a coating of
symbiotic algae.
Or the algae might have been eating them, indeed there is evidence of arthropods and
borers in the fossils raising the specter that they might have not been able to survive
the onslaught of newly evolved species.
That said, aliens visiting this strange world have beheld a strange landscape indeed of
towering fungi, and a forest floor of sorts of different fungi and early land plants very
much unlike what we see on our landscape today.
Incidentally, this period saw something really unusual.
The End Devonian extinction event.
The cause of this mass extinction isn’t clear, and it seems to only have affected
marine life.
There are a variety of candidates including a known impact crater structure, but the effect
of this extinction event created a major defining watershed for our civilization millions of
years later.
Essentially, the ocean went anoxic, meaning below a certain level the water became depleted
of oxygen.
This had the effect of stopping decomposition deep in the ocean.
That along with the geology in certain areas, created the conditions for the formation of
a huge amount of the world’s oil fields, particularly in North America.
So while the idea of aliens visiting earth in the distant past can’t ever be ruled
out, it remains an interesting idea.
There are many other periods where earth was very different than it is today, but it’s
fun to project past conditions here and try to imagine alien exoplanets with biospheres
radically different than anything we have here on earth today.
Probably even biospheres that resemble earth at no period in its history.
But I leave you with this, imagine if those aliens came back to check on how things panned
out here and they find us and fill us in on the blank spots we currently have in the story
of the natural history of earth.
Perhaps that’s the point of a Von Nuemann probe.
After it has finished its work and transmitted home, or if it’s still actively doing so,
the moment you make first contact with an active or dead probe, you find a full natural
history of your world with the implication that the aliens were simply saying glad you
found our probe, here is a gift of your full natural history.
Be well, and maybe someday, we’ll meet.
Admittedly, I’d like that.
Thanks for listening!
I am futurist and science fiction author John Michael Godier currently wondering if it goes
further than that.
What if these probes are in all systems, and someone long ago decided the best thing they
could do for the good of the galaxy is to collect data and make it freely available
to all that run across it.
Thus the entire history of the habitable period of the Milky Way could be out there.
That’s a deep thought, even for me and be sure to check out my books at your favorite
online book retailer and subscribe to my channels for regular, in-depth explorations into the
interesting, weird and unknown aspects of this amazing universe in which we live.