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.
They already have, it's called praying mantis.
May insect forms are very non-humanoid and are very viable.
I am not clear on calories - are insects inefficient?
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.
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)
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.
> 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)
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.
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.
"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.
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.
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.