Aliens Are Implausibly Hominid

According to the theories most widely believed by UFOlogists, the aliens who have been visiting Earth are anatomically hominids. And that's a problem.

Consider what other taxa could have been Earth's first technological species if we primates hadn't won the race. We'll let the UFOlogists set aside the intelligent aquatic taxa like cetaceans and cephalopods, on the argument that you can't become technological without combustion or electricity. (Neither work very well underwater!) But the other runners-up are still a diverse lot:

• Corvids (e.g. crows) are currently using their fingers for flying, but birds can do intricate work with their beaks.
• Proboscids already have trunks that are arguably better than hands. If elephants had won, they would probably dismiss the idea of trunk-less primates ever developing technology using just their front feet.
• Raccoons have evolutionary potential that is obvious to anyone who's tried to foil these backyard bandits.
• Meerkats are another carnivore that are easy to image replacing humans if we go extinct.
• Many rodent taxa like squirrels and rats have the dexterity and cleverness to be contenders.
• Marsupials have demonstrated convergent evolution into many niches occupied by carnivores and rodents, making them potential contenders too.

But the marsupial case also raises the only* question that could rescue the Greys from being just an obvious and unimaginative creation of our hominid minds. That question is: could the Greys be an example of convergent evolution with our body plan -- the only body plan that we know has created technology?

Some of the Grey body plan seems non-negotiable. You want binocular vision for depth perception (but it might be nice to have extra eyes for rear-view coverage).  Binaural hearing is another minimal requirement. (Note that having only two ears means you can locate a sound source only on the surface of a cone until you turn our head. Ideally you'd have at least one more ear to get instant triangulation, but that's a hard ask with bilateral symmetry.) You want at least one olfactory orifice, but again having more than one would let you detect gradients.

You'll want all these sensors on a movable platform, that can be aimed without repositioning your body. And you'll probably want your brain inside that platform, to minimize sensor lag due to signal speeds that are chemical instead of electromagnetic. So your eyes and ears and nostril(s) will be on a neck-mounted head.

You'll want at least two legs for locomotion, but four is a lot better. (Again, three is a hard ask.) You'll need at least one strong joint per leg. (Having a second joint as weak as the human calf/ankle is pretty sad. We've given up a lot of our leg capabilities just to make it easier to balance on our two hind legs. Standing would be trivial if we were hexapods instead of tetrapods.) Whether that primary joint ("knee") bends forward or backward is arbitrary, as long as it bends opposite of any secondary joint ("ankle"). The legs can attach in various ways.

You'll need at least one grasping appendage, but two are better. You'll need at least two opposable grasping digits, but four would be better. Note that five digits is more than necessary. Our pinky fingers and toes are an embarrassing vestige from our original tetrapod ancestor. Many animals have stopped using at least one of their five digits. And any software engineer can tell you how much better it would be for humans to use base-8 instead of base-10.

There's no hard requirement for the mouth and jaws to be located on the precious head. Jaws can do double-duty as graspers, but any octopus or elephant or crab would tell you you're crazy to use your face to grab your food. Even if you do put your mouth on your head, there's no requirement to mount it on the bottom of a flat snout-less face.

You'll want a skeleton in order to operate outside of water, and your choices are internal or external. Exoskeletons have been very successful on Earth, and are great for defense and harsh environments. However, they don't scale well to large sizes, they limit your respiratory capacity, and they are expensive to upgrade as you grow into adulthood. Endoskeletons seem better (to us!), but it's not a slam dunk.

The alien species most widely attested by UFOlogy theories is the Greys.  Let's list all the similarities between Greys and hominids that are hard to explain as convergent evolution:

• Pentadactyly. It's very unlikely that Grey evolution would repeat our mistake of having too many digits, and never deprecate at least one of them (as done by so many of Earth's tetrapods).
• Hominid-style hips, knees, ankles, shoulders, elbows, and wrists. The hominid joint plan is not required for intelligence, and other joint plans would work equally well.
• Bipedality. This is natural for us tetrapods, but having four limbs seems inferior to having six or eight. Primitive chordates were segmented, and having exactly four limbs was not the only option.
• Mouth placement. The Greys' flat snout-less face is not just suspiciously primate or hominid. It's downright human. Across the scores of primate species, face flatness does not correlate well with intelligence, so it's hard to argue that Greys lack a snout for the same reasons that Homo lost theirs.

These unjustifiable similarities make one suspect that Greys' anatomy (sorry) is not due to evolution but rather is a just-so story. You start with the current H. sapiens body plan.  You finish losing the hair. You finish losing the hominid snout. You keep growing the brain, because aliens are nerds. And you shrink and de-muscle the body, again because nerds. Instead of worrying about alien skulls birthing through alien hips, you de-sex the aliens. (Nerds!) OK, UFOlogists presumably claim that Grey reproduction involves eggs or labs, but you see how the Grey design is pretty on-the-nose for H. sapiens version one jillion.

*There is one other question that could justify Grey similarity to hominids: are they time-traveling future humans? That would explain the amateurish choices of whoever designed Greys. And it would even kinda sorta explain the sillier theories about breeding with current humans. But it raises even thornier questions: Why have future humans changed so much from our current design? It presumably would take many thousands (millions?) of years for humans to find the Grey form preferable. Does it really take us that long to master time travel? Would we really end up looking like that, as opposed to some kind of upload/android/cyborg scenario? And if Greys are time travelers, why are they so haphazard, hesitant, and incompetent in how they interact with us? Why do they think they need to intervene in human history at all? Are they Marty McFly, trying to keep their mom's picture from fading? Just how silly can this story get?

Are there any other options on the table besides convergent evolution and time travel? You could argue for artificial convergence: the aliens have been visiting Earth for at least a million years, directing primate evolution toward our current state. But it's not plausible that the aliens would do it so slowly, in a way that looks exactly like natural selection.

As always, the problems with alien theories are: they are full of unwieldy epicycles that create more problems then they solve; they seem amateurishly fabricated; and they fall apart under scientifically-informed scrutiny.

Update 2021-03-17: When a skeptic considered this question for "Scientific" American in 2009, he completely ignored convergent evolution.