Avatar's Pandora, complete with flying blue beasties and pretty forest waterfall.

If Earth had made different choices early in its life-hosting career, it could've ended up something like Pandora (the setting from the 2009 film Avatar).

From the little green men of Mars to the rubber foreheads of Star Trek, aliens persist as a tried-and-true staple of science fiction. Though the concept of alien life isn’t unrealistic, scifi representations are almost always implausible. We’ve recently deduced that Earth-like planets should number in the billions in our galaxy alone, so life – even intelligent life – should be common. But there’s no reason aliens would look, sound, or act remotely like us.

You don’t need to look beyond Earth for an example of why this makes sense: the two most intelligent animals besides apes are dolphins and the crow family. Minor tweaks to history could give Earth very different representatives on the galactic council: large aquatic creatures possessing an entirely alien sense (echolocation) or tiny flying scavengers descended from dinosaurs. A single planet with a relatively narrow range of environments* and just one biochemical “language” can give rise to incredible diversity.

What’s more, the differences between life forms on Earth can give us hints about how aliens might look and behave. Life has evolved to solve specific problems. Some of these problems – and the solutions evolution comes up with – are weird, random, or arbitrary. But other problems are universal physics quandaries: how to streamline for maximum swim speeds, how to create lift and stay flying, how to focus light in order to see.

To discover aliens from Earth-like worlds, we don’t have to travel trillions of miles to seek them out – let’s just go back in time.* We’re not going to give Hitler an art scholarship or prevent the fall of the Roman Empire; we’ll rewind time to when Earth was a lifeless rock and force biology to do a hard reset. When we re-run Earth’s “Make Life” program from an infinitesimally different random seed, what happens? What aspects of life are arbitrary, and what are set in stone?*

Luckily, we have a dimension ship at our disposal, so we can visit this alternate Earth and gaze in wonder at its Earthenoid inhabitants!* What unfathomable alien vistas await us?

What if there is no life here, only our deaths?

This journey to an alternate Earth assumes that complex life developed to begin with. It’s possible that when we step out of our dimension ship we suffocate in the atmosphere of nitrogen, carbon dioxide, and methane. Whoops. Guess we should have looked out the window and checked for obvious signs of photosynthesis before getting outside. Large complex life probably requires oxygen, (it’s a waaaay better fuel source than anything else on this planet) and an oxygen-rich atmosphere is only going to exist after the Earth evolves the right kind of photosynthetic bacteria or algae.

It’s even possible that no life exists on Earth at all. It’s difficult to gauge how unlikely life is going to be, just as it is difficult to gauge how likely it is for complex life to arise once single-celled life exists. All we know is that both things take a lot of time – time measured in billions of years. Resetting the Earth’s life might be like resetting the lottery after discovering that you have the winning ticket.

Even if complex life exists where we’re headed, and doesn’t immediately try to eat us or dissect us in a lab, we need to be concerned about carbon dioxide levels. Significantly lower or higher levels of CO2 could cause us to hyperventilate and possibly fall unconscious. Probably not lethal, but problematic enough to limit our tour to a few minutes.

Dark Crystal Landstrider
The Dark Crystal (1982 movie) is a great place to find inventive and (mostly) plausible alien life – all of it puppet-based! (A landstrider epic mount is pictured.)

Earthenoid Animals

Alter-Earth’s animals – let’s call them earthenoids – would be the first things to grab your attention* after you step out of your dimension ship. They skitter, they lope, they screech, they reek. They’re none of the right colors or proportions, and they’re too slow or too fast. All in all, they look, sound, move, and smell entirely alien.

That is, except for a few noticeable similarities:

Bilateral Symmetry

Their left sides will look like their right, though it may not be true symmetry. For instance, our heart is on our left side, and our liver is on our right. Simpler organisms resembling Earth’s sea stars and jellyfish may have radial symmetry.

Eyes and Mouths

Everything that wants to navigate the world and feed itself needs eye(s) and mouth(s). And since eyes and mouths have similar design constraints regardless of species, they’ll probably be recognizable even for beings that are otherwise totally alien.


Size is limited by Earth’s gravity and atmosphere. These earthenoid beings might be a bit bigger or a bit smaller than us, but assuming they’ve had a while to evolve, they won’t be bafflingly enormous or microscopic. All bets are off for water-based creatures!* While it’s true that some of our Earth’s dinosaurs were pretty huge, during the “age of the dinosaurs” most creatures were roughly human-sized or smaller; the big stuff just stands out.


Unrelated species can share common traits because they have the same physics problems to solve.* This is why unrelated orcas, tuna, and earthenoid goobleyfish* all have sleek, torpedo-shaped bodies.* Similarly, all flying creatures will be as light as possible with large wingspans.

The “how to move over land” problem has many more solutions than the physics problems associated with flight or swimming. Just like on our Earth, ground-bound earthenoids walk, lope, run, scuttle, slither, ooze, hop, skitter, and… bound. Legs of some kind are common, but they’re not required, and they could come in any number.

Evolution (the 2001 film) had some refreshingly creative alien critters... saddled with laughable scientific justifications.
Evolution (the 2001 film) had some refreshingly creative alien critters… saddled with laughable scientific justifications.

Earthenoid Plants

The backdrop for these bizarre earthenoid animals isn’t quite so strange. The earthenoid plants are odd, to be sure, but the basic shapes of forests, savannas, and (especially) deserts are the same. Plants have simple jobs: soak up sunlight, grow, reproduce, don’t get eaten (much). While plants on the Earth we came from had all sorts of ways to do these jobs, there’s one generic catch-all strategy that remains the same: grow really, really tall. So while the undergrowth might appear a bit confusing and alien, earthenoid forests will be recognizably forest-like, if oddly colored.*

Trees and large glowing plants of Pandora (2009 Avatar movie)
The large, tree-like plants of Pandora (from the 2009 movie Avatar) tend to be less “alien” than the animals or smaller plants. (Don’t ask me about the glowing, though. That doesn’t seem scientifically justifiable; it’s almost as if the glowing was included just to make the movie look cool. Weird!)

Like animals, plants have size limits imposed by gravity and atmosphere. Competition would ensure that tree-like plants grew high enough to actively push against these limits – just like trees do on Earth.*

Under the Microscope

Earthenoid life would still be made of cells organized into tissues organized into organs organized into organisms. True, there are some animals from our Earth that exist as giant undifferentiated blobs (looking at you, sponges and coral). But amorphous simplicity doesn’t work well on land, especially not after a billion years of evolution herding life toward more compartmentalization and specialization of tissues.

It’s fair to expect earthenoid organs to look freakishly weird, but some basics will still be in place for complex life larger than an Earth worm: heart(s), lung(s)/gills, brain, digestive tract, muscles, and frame (bone or exoskeleton). That’s because blood-like fluid needs to get pumped, oxygen needs to get harvested from the air or water, motions and behaviors need to be coordinated, and food needs to be digested. Also possible are stingers, poison sacs, gas bladders, livers, gall-bladders, spleens, and all sorts of other “optional organs” that we see on our own Earth.*

Like the physics problems experienced by flying or swimming life, cells experience identical chemistry problems, forcing them toward the same solutions. Thus the similarities between earthenoid life and the life from our Earth increase as we zoom in to look at cells and molecules. At the most basic level, earthenoid life is still carbon-based, still using water as a solvent, and still (for the larger, hungrier life) using oxygen metabolism. There are alternatives to carbon molecules, water solvent, and oxygen metabolism out there, but these are by far the best options for life on any Earth-like planet; they won’t be changing when we do our reset.*

We find Earthenoid life organized into cells. In order for complex life to survive, these cells must have some basic functions carried out by specific sets of building blocks:

  • Libraries of data (built with DNA & closely related RNA)
  • Molecular machinery and scaffolding (mostly proteins, but also some RNA)
  • Walls, shells, and compartments (lipids, also known as fats)
  • Fuels stores (often sugar or starch, but could be any organic molecule)

While these functions won’t be provided by the exact molecules listed, only a handful of those molecules’ close relatives would be up to the task under Earth-like conditions. We’ll find that earthenoids use a slightly different chemical language or have slightly different “building codes.” Maybe a chemical relative like PNA replaces our DNA, or the proteins use a different “alphabet.”* Their libraries might all be backwards or written right to left instead of left to right. Directionality is just as arbitrary in biochemical languages as it is in written languages. But this alphabet would be, to us English readers, more like Cyrillic than Chinese: many of the letters will be recognizable.

We could take a gamble and eat some earthenoids if we got really hungry, because they’ll probably use the same energy-rich building blocks as life from our Earth. But even if they do, there’s a fifty-fifty chance that all their molecules are mirror-image (chiral) versions of ours. That’s not a matter-on-antimatter mega-explosion scenario; rather, it would just make us sick and not fulfill any of our protein requirements.* This is probably for the best, since it also means that bacteria and viruses that we’ve inadvertently brought with us probably won’t be able to thrive here when they don’t find the right sort of food.

Intelligent Eartheniods

Weird alter-Earths filled with strange plants and animals can make great scenery, but we won’t get as much mileage out of an alien world without, well, aliens. Though there’s no guarantee we’d find life of human-or-better intelligence here, it’s not unlikely. What is unlikely is that it would look anything like us; intelligence can arise anywhere.

Crows and Orca
Some of the runners-up in the “race to global dominance” game. Note that they look nothing like humans. (First Place in the race to global dominance goes to bacteria.)

We can expect intelligent eartheniods to be social creatures. Evolution is actually less about competition between species and more about competition between individuals of the same species. And when a species has an intricate social structure, greater intelligence increases an individual’s ability to navigate it – just like saying and doing all the right things on a date will help a human get laid.

Increasing intelligence allows for more complex social structures, which demand increased levels of intelligence. This upward spiral to sapience can happen for any species, though it helps if the animal had some smarts to begin with.* The process is limited by how cumbersome, energy-consuming, and vulnerable the brain becomes. Evolution works around these limits by making increasingly efficient brains.

This process is less likely when members of a social group share genetic material. For example, ants and bees are highly social, and their societies are complex, yet they face little pressure to gain intelligence. They aren’t in competition with their siblings; their genes do just fine if they always put their group’s welfare above their own. So earthenoid hive minds are unlikely, though if humans are any indication, earthenoid cultures could still create groupthink.

And don’t forget, there could be multiple intelligent species sharing the world – so long as they occupy very different niches. Humans are land-based species able to exploit almost any niche. We also actively killed off all our sapient rivals because we’re jerks.* But imagine if crows and dolphins simultaneously beat us to the intelligence threshold. They’d be much more amenable to sharing the planet, since neither would bother with the others’ territory.

There’s not much else we need to see here – unless the intelligent earthenoids are so far beyond humans* that they’ve explored space and contacted true aliens. But even that would be superfluous; we’ve already seen the truly alien. Hopefully the experience has been strange enough that we vow to keep humanoid “rubber forehead aliens” out of our science-fiction stories from now on. The realistic options are less relatable but infinitely more varied and interesting.

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