Convergent Evolution in the Universe

I think, and I'm a minority in this typically, and also a layman, so I am not trying to say I am more intelligent than anybody else when I say this; it is just my personal speculation, that bipedalism with 4 limbs might be more common than some think, and that the humanoid form might itself be pretty common.

This will be long-winded....

Note: I am fully aware I am basing this on one example, Earth, and that this lends itself to inherent bias from viewing one successful example.

Point One: Bisectional Symmetry

Bisectional symmetry seems to be extremely efficient at achieving the necessary actions for development. If we were based on one singular form, one blob, (one leg, one eye, one ear, etc. etc.) this makes many functions difficult.

One eye does not compute depth perception and environmental dimension very well at all. Even if you could spend optimal biological resources on making one super-powerful eyeball, it would likely require more computation in the brain, thus more brain matter and/or energy usage, to calculate the necessary three-dimensional space at a distance. This is because the dataset it receives (what you see) would lack dimensional context and would have to be internally calculated within the brain using lower-quality "data."

It would also limit field of vision without making the eye take up an extreme amount of space to gain that field, which would leave that one very important organ open to horrible injury, and also reduce its capacity for fine focus, as it would be built for wide view over pinpoint detail. It would be constructed to take in and compute a wider scope of information rather than calculate finer points, which would naturally have its disadvantages.

Threading a needle for such a creature, for example, would be a gargantuan undertaking.

One free appendage (arm) should be pretty obvious when it comes to disadvantage. Carrying while still manipulating other things, or getting one thing to interact with another, etc. etc. You get the idea.

A trisected species? At first thought, more limbs, more stuff, the better, right?

Sure. Three eyes in varying positions. Extremely wide field of view that could overlap for at least some depth perception and calculating 3D space.

Three legs. Extremely stable locomotion, able to move omnidirectionally with extreme ease.

(Though bisectional animals, like on Earth, achieve this easily by simply adding a new pair of limbs when needed, without a third portion of the brain needing to be added to support it, by the way.)

Three arms? Well, seems obvious, right? Three limbs free for manipulation. Pfft, seems a straightforward plus on the face of it.

But here's an issue with having three of everything, or being trisected as a creature. This typically means dedicated computational locations in the computational matter. As in, your brain gets carved into thirds as well.

The fine motions of all three body sections require independent computation to handle their entirely different angles, especially when used in unison. All the muscle groups are in threes, all the organs are made to support three sections, and the brain is working the greater workload of perpetually operating three entire sets of this organism's musculature, external features, and extremities.

To operate EACH of these sections at performance, and to build them at performance comparable to a bisectional species, requires either an entire third more resources to remain on par, in conjunction with a brain—which is an extremely hungry organ throughout life—doing comparative calculations through three independent sections to work them in conjunction.

That makes for an extremely hungry bit of grey matter.

Or, to remain resource-efficient, each of those three sections has to be that much more below par: less strong, less dexterous, comparatively smaller, less complex, or whatever combination solves the issue of finite resource management and investment.

Having three eyes, and the resource and computational requirements that come with them, plus the extra complexity of the brain having to communicate between its sections, just to gain the benefits of a third eye with complexity on par with—or surpassing—the two eyes of a bisectional species, is yet another situation requiring more resources and more computational power.

With just two eyes, you already achieved depth perception and the ability to compute three-dimensional space. So you are getting nowhere near the leap in capability that two eyes have over one eye.

It becomes a situation of diminishing returns.

When you could just spend those resources on two REALLY, REALLY GOOD eyes far more efficiently while achieving all necessary functional baselines.

(Think of it as designing your improved car to have a bigger engine, rather than designing it to have two engines, thus wasting space, duplicating complicated components, etc. Silly and not worth it, and likely to create design issues that result in diminishing returns. As previously mentioned, there's a reason sports cars don't just "add another engine"; they improve the one they already have.)

So, in my humble opinion, my money is on bisectional symmetry ALWAYS winning out over unisectional (unidirectional? idk) and trisected forms, and that bisectional species would drive trisected species into extinction in mere moments.

I think a bisectional invasive species would murder a trisected ecosystem like the Juggernaut running through rice-paper walls.

Second Point: Being Bipedal With Just 4 Limbs

Note: this one I feel is FAR, FAR, FAR more likely to have deviations than creatures with trisected formats. I'm pretty solid in my expectation on everything being binary just for efficiency's sake.

However, four legs, at the end of the day, are a lot easier to make similar to each other and operate in conjunction than three. One reason being that two limbs, one limb, or six limbs on one half of a symmetrical body plan can all be dedicated to one half of the brain portion dedicated to the function of those limbs, and only has to communicate in conjunction with one other half that controls one, two, or six limbs.

Lending to that binary efficiency I was suggesting, given that they would also be operating extremely similarly, with less concern needed regarding the total amount of necessary angle capability in every limb.

However, the more you add, even if you can standardize things better in this fashion, it still ADDS to computational needs, and thus resource needs.

As well as the resource needs to maintain those limbs themselves.

One of the reasons it is speculated we have the large brains we have, for example, has a lot to do with the reason we have the shortest guts of all the apes.

Once our food, through developing omnivory and cooking, became more varied, plentiful, and digestible (comparatively), our guts shrank—which are also relatively expensive organs as far as calories and other biological resources are concerned—and we were able to dedicate those resources to our big ol' super-powered brains.

This has also been found in some experiments I read about years ago involving selectively breeding fish for intelligence by breeding those that finished mazes the quickest.

Their guts shrank, and their grey matter increased slightly. (Given that we could provide them regular high amounts of food to make that possible.)

So, even as bisectional creatures, if you have numerous limbs and all the supportive expensive components necessary to support such a situation (which includes more gut and brain dedication), you require prohibitively more resources to support your system.

Thus somewhat incentivizing a standard bipedal, two-arm, two-eye format for intelligent, technologically developing species.

The bipedal two-arm form seems to, logically, have the most resource-efficient blueprint and require the least amount of complexity necessary to achieve all the basic environmental manipulation requirements, while leaving the most room, resource-wise, for the development of the brainpower necessary to become a technological species in the first place.

That is my story, and I am sticking to it. lol.

That is the reason I am on Team Bipedal Humanoids Are Everywhere.

Though likely FAR, FAR, FAR more different than a simple Star Trek makeup job, lol.

At least, I think it's a matter of probability.

All the others are possibilities, and the universe is huge. Thus, naturally, they have a good chance regardless.

I just happen to think the humanoid-ish, but still very different from sci-fi TV shows, form is, in my opinion, probably extremely common, and that our form is probably not at all that special.

Many argue that it is anthropomorphic bias to assume other things are like us in this circumstance, void of any connecting variables. You know, because they're aliens.

I think it is just as viable to argue that it is a bias to assume we are specifically special in our layout plan, and that we may find out it's pretty run-of-the-mill as a system that is simplest for achieving the necessary goals.