Given how rare humanoid shape is on Earth, it doesn't seem especially sure that humanoids will be dominant among even intelligent life.

What are we referring to as humanoids though? And what are we about as intelligent? Crows? Crows aren't going to build spaceships and don't really have the means to because a beak isn't a great manipulator, even though they use tools. I don't think the person above is saying that quadrupeds would be out of the realm of possibility (maybe they are. IDK I'm a computer scientist, not a biologist. I could totally see body size, caloric intake ratios, and brain mass being related in a way that would make these unlikely), but rather things like fractaling appendages would be expected because intelligent life would need to have fine motor control. (Is a centaur humanoid?) There are just more efficient ways to do things than other ways. Yeah, maybe alien life will have a beak. Maybe their pupils will look different but I'd still expect them to have eyes with squishy lenses because that's pretty much required to be able to focus and intelligent creatures need to be able to sense things both near and far (without technological enhancements). Light sensing isn't enough if you want to build things, you need depth perception. I think if you follow this line of reasoning you'll find that a lot of attributes humans have would be pretty likely for intelligent alien life as well.

Can echo location be used for depth perception?

Yes but it is very expensive. Here's some more on bats[0] (mentions expensive echolocation around 5:45 and moving from sonic location to echolocation evolution). Eyes do also convey more information. You can get colors in addition to depth and shape and that's a good reason and easy path for eyes to develop since light sensing is a pretty easy thing to develop. Also, bats aren't blind, despite common belief.

[0] https://www.youtube.com/watch?v=zWeYCULC0UQ

It can. One eye and echolocation gives you everything you get with two eyes except redundancy.

Cyclops got by with one eye and throwing rocks.

Arms+hands and a head with a similar face though.

A head is pretty universal because eyes are so important and a feeding hole near your sensory organs is a massive advantage.

Having hands seems almost a pre-requisite to become truly intelligent and tool-using.

Octopi have all the things you mentioned (except hands). Their eyes are even better than human eyes. No one would describe them as "humanoid".

Also consider an octopi's environment. That environment is not well suited for being able to build lots of things. How do you get to the bronze age underwater? How about even earlier. Land based creatures have pretty similar forms. Snake like, bipedal, quadru/multiped, and flying/winged creatures. Snakes don't have arms and so aren't going to have fine motor control. Winged creatures are going to have a hard time dealing with heavy objects (they have hollow bones to fly and wings are incredibly fragile and I don't see nature creating super materials, plastics, or metal structural systems for animals). Quadrupeds probably aren't going to develop arms like centaurs (more likely legs become arms). So if we want a creature that can have fine motor control, live in an environment that can easily create metals, not be extremely fragile, and is able to devote a significant amount of its energy intake to its brain, you're probably going to come up with something ape like (bipedal). There are other forms here, but we can see that there are some pretty good hints that we learn just from mechanics and aren't so much dependent on specifics of Earth or humans.

"Also consider a human's environment. That environment is not well suited for being able to build lots of things. How do you get to the <untranslatable> age on earth?"

Earlier, you mentioned that beaks can't make spaceships. Well, maybe neither can hands. What we call a spaceship might be laughable nonviable. In fact, we know it is.

We're struggling at limits that some other lifeform might not have. We're hill climbing for tools that will let us do things we really want to do. Quite likely a lot of our progress is orthogonal or even oppositional to what we need to do to get off the planet.

We have no idea if some of our progress might be backsliding or entirely halting our future progress. Due to our natures. Due to our beaks.

> Earlier, you mentioned that beaks can't make spaceships. Well, maybe neither can hands.

Except hands did create spaceships...

The "beaks can't make spaceships" is a comment about crows not having hands. Crows use their beaks to manipulate objects. This does not imbue cows with great motor control. The use of tools is very different than the ability to finely manipulate tools. Aliens could have beaks, but these would not be their way to build spaceships. Similarly they wouldn't have pincers. Such grasping mechanisms just are extremely inefficient and don't allow for certain tasks.This excludes a lot of classes of tools from crows. This isn't biology, this is physics.

Our hands are like beaks to some hypothetical species, and it is hubris to assume we're great manipulators. Our fingers could be closer in utility to cow's hooves, than the manipulation techniques of some unknown species are to our own gesticulations.

And they would have tools as multipliers for their natures, just as we use tools to extend ours.

Our wonderful inventions might be another species child's play. Our herculean efforts to reach the stars might be a triviality to a species that can synthesize novel compounds in their bodies, and/or those of their stock animals. We're just at the early stages of doing that with algae, after a very long climb from first making fire.

Just as I could train and arrange to walk to Mexico, they might be able to self modify and reach the stars. No spaceship needed.

Our great ideas and dreams might be trite to bigger minds out in the galaxies. They could have easily skipped over what we struggle with.

You call those cans "spaceships"? They can barely get to your moon and back.

"Humans can't even excrete flexible ceramics. They need air. They need heat. They can't stand up to ionizing radiation. This isn't biology, this is physics!"

Cephalopods have got onto land at least twice. Nothing says they can't do it again.

> Cephalopods have got onto land at least twice. Nothing says they can't do it again.

Sure? But isn't that my point? They sure looked different and had different physical attributes when they lived on land.

> They can't stand up to ionizing radiation. This isn't biology, this is physics!

I wouldn't compare this to the ability to use a screwdriver. Or in a more abstract way, be able to grasp, twist, and push.

"Quadrupeds probably aren't going to develop arms like centaurs (more likely legs become arms)"

They already have, it's called praying mantis.

May insect forms are very non-humanoid and are very viable.

But this form is also extremely rare. You're also ignoring the main component of the argument, which is where calories are going, not that arms can come out of a body.

Insects are all not quadrapods, they are not rare, their combined biomass is much greater than mamals

I am not clear on calories - are insects inefficient?

The calorie part is about where you're spending calories. Brain or body? This gets complex real fast though. One easy part to understand is surface area and volume (but there's also things like metabolic rate and many others that go in here). But consider that brains are pretty resource intensive. In a human it's 2% of the mass but 20% of the energy. There's a linear relationship between neurons and calories but humans are an outlier in percent of energy that goes to brains. Additionally, it is the third most resource intensive system (for humans), behind skeletal muscle and the liver[0]. Which those last two parts matter a lot. Skeletal muscle scales with volume. So if you're bigger you have to eat more. The more you have to eat the more time you have to spend eating and your metabolism is probably going to slow. So it gets really complicated here really fast (and I'm admitting to my naivety to the subject matter) but basically it's not hard to see here that if you're pretty big you're probably not going to have the resources (or metabolism) to spend time thinking, which means a downward selection pressure on brain and thinking. There's going to be a Goldilocks zone here (like most things).

So basically if you can get food and don't have to spend significant time thinking to do so, there isn't going to be a selective pressure to encourage brain growth. And if you have a selective pressure to devote energy to the brain you're also probably going to have a selective pressure to reduce energy from elsewhere.

Additionally, I'll add that humans both lost fur and stood up because it is better for thermal regulation (there are other pressures too, there's never _one_ reason). So this also helps with the surface area-volume issue.

Basically what I'm saying is that there's these locally optimal conditions that need to exist at the same time to produce intelligent, tool using, civilization making creatures. (I mentioned in another comment that environment matters too. Can't get a bronze age underwater. But I also didn't mention you can't get a bronze age if you don't have a social structure, something, for example, octopi lack). Basically what astrobiologists are looking for are the combination of these optima necessary to produce certain traits. What many commenters here are missing is the combination part, mostly thinking that only intelligence matters to create a space fairing civilization, but you can't get to space if you can't create advanced composites, explosives, and many other things. Or in other words, the astrobiologists aren't idiots and probably know what they are talking about despite what it looks like. It only looks like they are dumb because people are overly simplifying the problem.

[0] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3046985/

Not disagreeing with you, but aren’t these local minimums/maximums quite dependent on eukaryotic cells, and/or protein-based life? Wouldn’t the resources required by the brain vs muscle be vastly different than what we have know?

Also, what if a non-protein based molecular machinery can be so energy efficient and powerful that species having that can skip some seemingly necessary part of development? But I’m by no means an expert on the topic, just asking. If the reasoning was about absolute limits, I would be much more accepting of them (eg. Energy required by the simplistic work done, like a given skeletal structure picking up a weight with muscles only consuming the minimal physical necessity)

Most of what I've mentioned is more physics based rather than biologically based. Even if your muscles are more efficient your skeleton is going to grow with volume, which means non-linearly. Given this, all those same things will still apply.

Brains are resource-intensive only if you are in a massive hurry. Starfish do fine, entirely without a structure you would call a brain. Packing it all into one noggin is risky, for no benefit. If you're not in a hurry, you don't need nearly as much of it, because it doesn't have to do everything at the same time.

There is no natural speed that life has to operate at. Any aliens we encounter are much more likely to be very, very slow, because it is they who can stand the vast chill void between stars. If they visit, and you dare to chop one down, you wouldn't find anything recognizable as brain inside.

I largely agree with the general idea here.

> metal structural systems for animals

Something resembling this has actually been observed in a species of deep sea snails. Iron sulfide is incorporated into the hard bits (shell, foot armor). (https://en.wikipedia.org/wiki/Scaly-foot_gastropod)

Hm. We have what, a dozen or more on Earth? Apes, monkeys, homo etc. Plus dozens of dinosaurs that stood on 2 legs. It seems pretty common.

That's a wrong way to think about it, since all modern "humanoids" are descended from the same ancestor not that long ago.

And if you consider dinosaurs humanoid, then so is that meme cat that can walk on its two back legs.

If there was an apex predator in every tree of life, in the "humanoid" shape, and I'm talking "humanoid ants" here, then it would be reasonable to assume that there's something beneficial in the "humanoid" shape that helps long-term survival.

And that's not even taking the majority of lifeforms into account - those in the seas, and greatly discounts birds. We still can't agree on if dolphins and crows are intelligent.

So, no, chances that the "humanoid" shape is somehow special out there are basically 0, since it's not even special here on Earth. We're an accident. Insects, mushrooms, plants - all those life forms are better adapted for mere survival than us (judging by cumulative mass of live organisms).

That's the wrong way to think about it. Nothing is 'better adapted' than anything else. Each is adapted to its own niche. And no accidents really - convergent evolution shows that similar ecological pressures very very often result in similar forms. Witness fish ("there's no such thing as a fish") or trees ("there's no such thing as a tree").

And that dinosaur comment - I don't know what to do with that. Dinosaurs only walked on two legs in internet memes? That's so far wrong I can't comment.

a dozen out of millions of species is not "pretty common"

Many convergent evolution paths led to the same similar form, in time. That was the point. If the environmental pressures are similar, the forms may be similar.

100.0% of spacefaring earth creatures are bipedal /s

Dogs have been send into space

They were sent, not by choice = not spacefaring! /s

> convergence between fish and dolphins

The convergence between dolphins and ichthyosaurs is even more remarkable. Ichthyosaurs were marine reptiles; air breathing tetrapods that, like dolphins, had ancestors that walked on land but eventually returned to the sea to kick fish ass.

https://en.wikipedia.org/wiki/Ichthyosaur#/media/File:Ichthy...

Bony fish evolved in rivers, and eventually returned to the sea to kick... what? Analogies fail.

Those fish are overwhelmingly more closely related to horses than to sharks.

Sure, organisms that already share a massive amount of commonalities can diverge and then converge again.

What about the branches that happened early on? We essentially have only two lineages of macroscopic organisms that are actually fundamentally different: Plants and animals.

I would expect any kind of macroscopic extraterrestrial life to be at least as distinct from Terran plant and animal life as they are from each other.

> We essentially have only two lineages of macroscopic organisms that are actually fundamentally different: Plants and animals.

That's not true, we also have fungi as a macroscopic lineage.

Or maybe you put them in the plant group? If so, that's a mistake, as they are more closely related to animals than they are to plants.

I think it's fine to lump fungi together with plants for the spirit of this discussion. The distinction between plants and fungi are made on the basis of metabolism but in this thread people are clearly talking about external behaviours observable with the naked eye.

Metabolism influences external attributes (I think behavior isn't the right word you're looking for). A larger creature requires higher caloric intake than a smaller creature (and this process isn't linear). A larger creature has different thermal regulations than smaller creatures because surface doesn't scale with volume linearly. The bigger the creature the more it calories it has to expend on the body vs the mind. There are equilibriums here that are physics based.

And then consider the selective pressures from plants vs fungi. A fungi gets its environment. Plants do some, but also need to perform photosynthesis. These have very different selective pressures for these lifeforms.

If you're talking about lineages you should actually lump Fungi with animals. Also, how are fungi behaviours comparable to plants?

If their behaviours are not comparable, then why were fungi lumped together with plants up until the 1960s?

DNA is extremely flexible, there's no macroscopic form or shape it can't take, as various insects camouflaging themselves as sticks and leaves and what not shows.

So the idea we'll see some vastly different concepts with different starting blocks is possibly unfounded.

Alien life might be very different at low level depending on their environment, but in terms of macroshapes, things like the formation of a head with eyes and mouth, upper and lower limbs, bilateral symmetry and so on will repeat over and over.

We'll see (in another life probably).

Yeah, but so what? The geometric shape isn't very interesting. The insect camouflaging itself as a leaf still functions as an insect. It doesn't perform photosynthesis.

Considering that we have a whole class of lifeforms that have nothing like a head with eyes and mouth or limbs, the idea that this would evolve independently more likely than something completely different, is also possibly unfounded.

Having two eyes, as a simple example, is arguably the cheapest way to perceive 3d at things-may-want-to-eat-me distance: just two points that enable triangulation. Similar constraints reduce the configuration space a lot. It still remains huge, but I don’t think someone that believes about certain convergence to be necessarily naive.

Having three eyes, as a simple example, is arguably the cheapest way to percieve 360-degree view of things-may-want-to-eat-me.

Plenty of animals that have 2 eyes and no 3d vision, they try to get as close to 360 coverage as possible.

> DNA is extremely flexible, there's no macroscopic form or shape it can't take, as various insects camouflaging themselves as sticks and leaves and what not shows.

That feels like a really bold claim given the evidence.

Let me know when you spot an animal that drives around on wheels.

Physically you could have wheeled animals, but wheels just won't evolve. They are perfect for somewhat smooth surfaces where you want to travel in a somewhat straight line. Organisms just cannot confine themselves to that and survive.

Unless we find a planet composed entirely of solid smooth rock we won't find wheeled animals.

How would wheels get fed with nutrition? Would they have tiny mouths?

Gears have evolved, though, which is equally cool IMO.

Half-gears.

rotifers

Rotifers do not, in fact, have wheels.

... on planets. That have a precise similarity to Earth. Sure, I can provisionally accept that.