Science Behind the Fiction: Could aliens pass for cats or humans, like in Captain Marvel?

With Captain Marvel still dominating the box office as well as our hearts and minds, Marvel fans are both rabidly dissecting the movie and posing questions about what's next for Carol Danvers (Brie Larson) and the MCU at large. Here at SYFY WIRE, we also have a bunch of questions about the science of Captain Marvel.

Last week, we explored the reality of Skrull memory retrieval devices and learned that our minds may not be as sacrosanct as we hope. (At least, that's what I remember.) This week we're focusing in on the real star of the show: the adorable, cuddly, and moderately terrifying Goose.

**Spoiler alert: There are spoilers for Captain Marvel below**

There's a moment in the film, played off for comedic effect, wherein Carol's cat Goose is revealed to be anything but a house cat. Talos (Ben Mendelsohn), a Skrull refugee, encounters Goose and is adamant that she is, in fact, a Flerken. The on-screen moment mirrors one from the comics when Carol encounters the Guardians of the Galaxy and Rocket outs her cat (named Chewie) as a Flerken, while simultaneously demanding the feline be killed. Flerkens are dangerous, no matter how well they cuddle.

This fuzzy alien species looks surprisingly like Earth cats, so much so that Carol lives with one for years without ever realizing the truth. And who could blame her? Any strange behavior could easily be dismissed as a cat simply being a cat. If there's anything nearer to a malicious other-worldly entity than a cat, I don't know what it is. But, looks can be deceiving and Flerkens exhibit a number of differences from our familiar feline friends. Specifically, access to pocket dimension carried within their bodies which allow them to store massive objects, including attack tentacles.

Enjoying Marvel movies — or most any movie for that matter — requires a certain suspension of disbelief. But one has to wonder, is it possible for life from another world to closely mirror life found here at home? And if not, what might alien life look like?

Familiar Forms or Untold Variation

There's some debate within the scientific community regarding the answer to this question. On the one hand, much of life on Earth, diverse as it is, follows a similar basic blueprint. On the other tentacle, life on another world would evolve under a different set of environmental pressures, potentially resulting in unimaginably different creatures.

Here at home, there are some pretty easily identifiable similarities throughout the animal kingdom. With the exception of some notable examples, animals tend to layout in roughly the same way: a head on one end and a body with some sort of appendages arrayed symmetrically. For higher animals, those appendages almost always number four (with the possible addition of a tail), with the exception of octopods which took one look at the body layout plan before tossing it out.

It makes sense that life on Earth would have this sort of similarity throughout, considering what we know about evolution through natural selection.

While the fine details of a species, and the individuals within it, change over time, we're all operating from a shared starting point. And those genes which determine body plans, known as Hox genes, remain more or less the same.

The question is, whether an animal evolved independently, on another world, would necessarily be bound by those same rules. After all, life has a way of adapting to bizarre and extreme circumstances, why wouldn't it be capable of finding a way to take hold on worlds we wouldn't recognize as hospitable, in ways we couldn't imagine?

On the surface, it seems obvious that it would. Life in the universe is not limited by the confines of human imagination. But, some scientists think when we do finally encounter complex life on another planet, we might be surprised by its similarities to life we're familiar with.

Charles Cockell, professor of astrobiology at the Unversity of Edinburgh in Scotland suggests there may be a sort of universal biology which confines the way life arises throughout the universe.

By Cockell's estimation, life elsewhere would, by necessity, be bound by the same laws of physics that we are. No argument on that score. Likewise, carbon is the best element for building life (that we know of) and water is the best environment for it to work its magic.

Taking all of this into account, the similar building blocks and similar circumstances (physical pressures) we would expect to see similar results.

To be clear, Cockell is not suggesting that life on another world could pass for a specific Earth animal, but that it may not seem entirely alien to us. Instead, it might at least look like something we might find on Earth, hidden in some as yet undiscovered recesses. Already there are animals being discovered every day that could easily pass for otherworldly visitors.

Cockell's hypothesis, however, is less than universally accepted, and different experts have a whole host of different answers to this extra-terrestrial question.

But, a few key elements seem to be regarded as probably necessary. Intelligent life elsewhere will likely have two eyes, facing forward, to allow for depth perception. This is common in predator species, indicating any intelligent life we encounter will have a predatory past.

It's likely intelligent life would be symmetrical in layout, have a mouth, a hard casing for a brain, and have something like hands with digits capable of fine manipulation. This is necessary for the design and utilization of tools, a requirement for developing technology.

But, that doesn't mean they have to look like humans. There is at least one animal which pretty closely fits the bill right here on Earth.

Having evolved in the sea, octopods, are about as different from humans as higher order animals can be. They tick off most of those boxes while landing on a particularly alien design. They've got two front-facing eyes, a symmetrical body, limbs capable of manipulation, and are incredibly intelligent.

It's not difficult to see how, given time, they might be capable of developing higher-level intelligence that includes tool usage and some form of technology. They do not, however, have a hardened case to hold their brain (or brains, rather, since they have nine of them).

All told, unless our alien counterparts are capable of shape-shifting and changing their external appearance (cue the cephalopods one more time) it is unlikely they could pass for a human, or a cat. And we've not even addressed the internalized pocket universes. We'll have to leave that for another time. But that doesn't mean they might not fit in with the sorts of creatures we're used to.

Maybe one day we'll know for sure.