In the fifth episode of the original series of Star Trek, the show tries to pass off what
is clearly someone’s dog with a horn taped to its head as an ‘alien lifeform.’
It’s pretty hilarious to revisit — not just to see the actors make a heroic attempt
to stay in-character while carrying this thing around — but also because this unicorn dog
doesn’t seem distinct enough from life on earth to make a believable alien.
But this raises an interesting question: if there is life elsewhere in the universe, just
how ‘alien’ will aliens look?
It’s a question many scientists and sci-fi authors have speculated on, and is a fascinating
rabbit hole to go down.
So, for this entry into the archive, we’ll explore relevant phenomena in evolution and
physics to determine if extraterrestrials are more likely to look like Earth animals,
or something far stranger...
For our first example, let’s start by imagining an Earth-like planet.
It’s got a comfortable amount of liquid water, a protective atmosphere, and plenty
of carbon to form the building-blocks of organic compounds.
If life emerges on such a world, what kinds of ecosystems will form?
One of the most important phenomena to consider when imagining extraterrestrial life is something
called convergent evolution — or the independent evolution of similar features across different
species despite being part of different evolutionary branches.
It’s why echidnas and hedgehogs look almost identical, despite one being a marsupial and
the other a placental mammal.
Or why hummingbirds and hummingbird hawk-moths are often mistaken for each other, despite
their last common ancestor predating the first land animal.
Or why crustaceans simply refuse to stop evolving into crabs — a trend we’ll discuss more
in just a bit.
With all these examples, it feels like nature is copying its own homework, and not changing
things enough to get away with it.
But evolution isn’t a conscious force, though we often think of it that way: it’s the
product of random genetic mutations allowing certain members of a species to better survive
and pass on their genes, resulting in change over time.
And a trait that benefits one species might also benefit another in a different time or
place if they face similar obstacles.
One of the best examples of convergent evolution is how sharks — a type of fish, dolphins
— a type of marine mammal, and ichthyosaurs — a type of extinct marine reptile, all
evolved the same general body shape despite coming from vastly different lineages.
They all have streamlined bodies, tail flukes, and dorsal fins that are in roughly the same
place.
This is particularly remarkable for dolphins and ichthyosaurs, as neither of them started
off with any fins at all.
As is often the case, the main reason for this seemingly miraculous convergence has
to do with physics.
Simulations have shown these body shapes are the optimal way of moving through the dense
medium of water.
So, natural selection encouraged parallel attributes in each lineage, with the three
animals essentially finding similar solutions to the same problem.
So, on a habitable alien planet with liquid oceans, perhaps we’d find lifeforms with
the same general body plan swimming beneath the waves…
A more extreme example of convergent evolution is how crustaceans just won’t stop evolving
into crabs.
I told you we’d get to it.
The number of times different lineages have independently converged on a crab-like body
plan is so high that there’s even a word for it: carcinization, meaning to become crab-like.
Today, there is a staggering array of these crab mimics across the globe, from the tiny
strawberry hermit crab that feeds on seaweed, to the coconut crab that looks like it feeds
on your nightmares.
I’ve made jokes about carcinization before on my channel, because there is something
oddly hilarious about the phenomenon.
Are crabs just the pinnacle of evolution?
What is it about the crab body-shape that makes so many unrelated crustaceans seemingly
jealous enough to copy their style?
Well, once again physics is our culprit.
The long tails of crustaceans like shrimp or lobsters can restrict the species’ mobility.
By contrast, the compact crab-shape is efficient and versatile — with few exposed parts that
predators can get to without getting pinched.
So, there’s nothing mystical about why the crab body plan is so trendy — though it’s
fun to think so -- it’s simply a good answer to the question of survival, and likely would
be on other Earth-like planets as well...
But these are all examples of convergence in species that already have a lot in common
— like eyes, legs, and a skeletal structure.
If microbial life were to emerge on an Earth-like planet, would it really end up evolving these
structures?
In his book ‘A Zoologist’s Guide to the Galaxy,’ Cambridge zoologist Arik Kershenbaum
reflects on how certain features like eyes and limbs might be more common across extraterrestrial
life than we’d think.
The eye might seem like a unique feature aliens wouldn’t necessarily converge upon.
But on our planet, eyes are thought to have evolved multiple times across different lineages.
Eyes likely emerge from rudimentary photoreceptive organs, which some lifeforms on Earth still
possess, and then through natural selection become more complex over millions of years.
Yet across these variations, eyes always serve the same essential navigational function.
And on another planet, a creature’s chance of survival will also likely be higher if
they can see where they’re going.
So, if the first aliens we encounter end up having eyes, it won’t be that surprising.
The same can be said for legs.
On earth, arthropods and vertebrates evolved legs independently from one another after
emerging from the water.
While the shape and number of legs might fluctuate, the mechanical advantages legs offer are likely
to be universal across Earth-like planets.
Even more fundamentally, practically all animal life on Earth exhibits Cephalization, or the
concentration of the mouth, sense organs, and nerve ganglia on the front end of the
organism, producing a head region.
Cephalization is likely the simple result of the advantage that having the part of the
animal that goes forward be the one fitted out with sense organs.
And on most planets, it’ll likely always be more important to know where you are going
than where you have been.
Even plants convergently evolve.
Trees — that is to say, large photosynthetic lifeforms protected by a bark-like substance
— have emerged many times from different plant lineages.
The evolution of fruits like apples happened independently from fruits like tomatoes, with
both of these shiny red fruits emerging to get the attention of animals that disperse
their seeds after chowing down.
Likewise, seed dispersal by ants has evolved more than one-hundred times, and is present
across more than eleven thousand plant species — making it one of the most dramatic examples
of convergent evolution in all biology.
Yet convergent evolution doesn’t always produce organisms that look the same.
Take, for example, the woodpecker — a kind of bird, and the aye-aye — a type of lemur.
Both of these animals occupy the same basic role as predators who feed on insects that
hide within trees.
Woodpeckers get to these insects using their beaks, and aye-ayes get to these insects using
their elongated, somewhat unsettling finger.
In terms of their lifestyle, these animals are quite similar, but you’d still never
mistake one for the other.
So, even if an alien occupied a similar niche to an earth animal, there’s not always a
guarantee they’d look alike.
So, on our Earth-like alien planet, we might find mobile animal-like organisms with eyes
and limbs moving around stationary plant-like lifeform, occupying niches that are familiar
to us.
But they might still appear pretty distinct from life on Earth, and certainly aren’t
likely to look like the Star Trek dog.
Still a majestic creature, though.
But what about the possibility of aliens with intelligence comparable to ours?
Would they evolve a convergent form on an Earth-like planet, or would they look entirely
different?
The question of what ‘intelligence’ even means could probably be its own video, but
for now, let’s keep things simple.
Returning to Star Trek, Spock is certainly a… suspiciously humanoid extraterrestrial
— with an appearance identical to humans with the exception of elf ears and funky eyebrows.
This was more the result of budget constraints than anything else, but it’s a trend you
see across a lot of science fiction.
And while it’s easy to assume that an intelligent alien would look like we do, that’s not
necessarily the case.
Human hands might seem like a tool perfectly designed for crafting technology, but our
hands didn’t originally evolve for manipulating objects.
The digits we use to work our smart phones began as a means to better grip on to the
trees we used to swing through.
Aliens would probably need complex manipulatory appendages to create technology, sure, but
that could take any number of divergent forms.
Likewise, our bipedal stature might seem ideal, but ask any orthopedic doctor what they think
of our spines, and you’ll realize the human body plan has some definite drawbacks extraterrestrials
might want to sidestep.
Of course, some things seem likely to be universal across advanced life in the galaxy.
They’d probably need a large brain, right?
Well, that might not even be the case, as crows have relatively tiny brains but are
able to solve logical puzzles and use simple tools.
Their dense brain structure means they actually have a similar number of total neurons to
that of primates, and indicates that alien intellects really might not need to resemble
the human form after all--unless they also happened to evolve from a primate-like species
that began walking upright.
With all the planets in the universe, intelligent humanoid aliens aren’t impossible, just
not all that likely.
But it’s time we addressed the fact that not all planets are the same.
An earth-like level of gravity, for example, isn’t a constant throughout the universe.
Our planet’s gravitational pull is an invisible yet crucial influence across evolution on
Earth, affecting the parameters of animals’ body plans, maximum size, and behavior.
So, on a planet where gravity is, say, significantly weaker, evolution might take a very different
path.
One of the best depictions of low-gravity alien life comes from Sam Vilasboas’s speculative
work of Planet Anu, which I’ve covered before on the channel.
This evolutionary thought experiment depicts extraterrestrials that take advantage of their
planet’s weak gravitational pull in a variety of ways — with many having tall, spindly
bodies that wouldn’t be able to carry their weight on our planet.
Vilasboas does imagine some convergence with Earth life would take place on such a world
— but there are also a variety of lifeforms our planet could never support.
On the other side of the spectrum, we have the possibility of alien life evolving on
a world with a stronger gravitational pull than our own.
Christian Cline’s speculative field guide to alien life, the Teeming Universe, which
I’ve also covered before on the channel, gives us a look at a high gravity world occupied
by heavy, slow-moving lifeforms.
On the fictional exoplanet Borold, Cline imagines extraterrestrials that creep slowly across
the ground like giant terrestrial starfish, their bodies built to withstand over three
times the gravitational forces we experience here on Earth.
And dissimilar gravity isn’t the only variable that might make life on exoplanets different
from what we’re familiar with.
Atmosphere is another thing to consider.
Remember how I mentioned alien life might have eyes?
Well, on a planet with an opaque atmosphere where energy comes not from a sun but from,
say, geothermal vents, you might see a world of sightless creatures.
On Earth, many deep-sea creatures have either undeveloped eyes or no eyes at all.
So, it makes sense that on an alien world with very little light, there wouldn’t be
much point in having light-receptive organs — and organisms might navigate through their
sense of smell or through advanced echolocation.
And even if a planet’s surface is inhospitable, maybe some organisms could thrive in a planet’s
atmosphere.
Returning to the Teeming Universe, there’s a section of the book that outlines the possibility
of life emerging in the stratosphere of a planet somewhere between a terrestrial world
and a gas giant like Jupiter.
The chapter depicts extraterrestrials that float on sacs of internal hydrogen, steering
themselves through the air on specialized fins.
The possibility of life within the atmosphere of a gas giant has been entertained by numerous
scientists over the years, including Carl Sagan.
When considering evolution on planets different from our own, extraterrestrial life becomes
less and less familiar...
And things could get weirder.
Different planetary parameters aren’t the only factor when considering how ‘alien’
extraterrestrial life might be.
So far, we’ve been assuming alien organisms would have similar fundamental chemistry to
our own — but that might not be the case.
Crystalline life based on the silicon atom — instead of carbon like life on earth — has
been a topic of discussion among scientists since before the 20th century.
Since silicon-based bonds are most stable at high temperatures, such life might theoretically
thrive on planets with high geothermal activity.
Even if we visited such planets, we might not recognize them as full of life at a glance,
since such ecosystems might seem barren to our eyes.
Now, more recent studies have shown there are some potential chemical pitfalls to silicon-based
life.
But it’s difficult to disprove such a theoretical concept with any certainty.
Who knows, maybe somewhere out there in the universe a silicon-based scientist is working
on an article about how carbon-based life just isn’t possible.
In fact, some Earth animals have been observed using silicon in their structures.
Diatoms are tiny phytoplankton that encase themselves in a cell wall made of silicate.
Now, these organisms aren’t silicon based, but they do show the material’s potential
to be used in biological life.
And why stop at silicon?
Scientists at Cornell University have modeled a theoretical oxygen-free cell based on methane
that could be the basis for some really unusual lifeforms.
Protected by a cell membrane called an ‘azotosome’ able to withstand extreme temperatures, these
hypothetical cells could potentially survive on exoplanets with weak atmospheres.
And going to the most extreme scenario, it’s possible there are lifeforms that our limited
view of the universe hasn’t prepared us for.
When it comes to extraterrestrials, we don’t know what we don’t know, and maybe our definition
of life will prove too limited in the infinite variations of the cosmos.
So, where does all this leave us?
Well, the truth is, we can’t definitely say what aliens do or do not look like without
ever having met any.
We can make inferences based on evolution on Earth, but with a sample size of one, it’s
difficult to say how much we really know about how life tends to emerge.
With so many planets, it’s not a leap to imagine that some alien ecosystems would be
somewhat similar to Earth life, while others would be completely different.
So, if you’re a worldbuilder or writer who’s worried their fictional creatures are too
similar to Earth life, remember that nature itself isn’t always completely original.
And if you’re worried your fictional life is too out there, remember that we are only
just beginning to discover what’s possible.
Maybe one day we’ll find an ecosystem much like our own, or an ecosystem that stretches
the definition of life.
Or maybe we’ll just find a planet full of crabs.
As always, thanks for watching.
If you enjoyed this entry, please lend your support by liking, subscribing, and hitting
the notification icon to stay up to date on all things Curious.
See you in the next video.