[dtech]

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.

[titzer]

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

[godelski]

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.

[Alan_Dillman]

"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.

[godelski]

> 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.

[Alan_Dillman]

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.

[ncmncm]

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.

[godelski]

> 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.

[ClumsyPilot]

"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.

[godelski]

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.

[ClumsyPilot]

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?

[godelski]

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/

[d110af5ccf]

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)