No matter how you spin it, this discussion is based on extrapolating from one data point (Earth) out to the entire universe. No matter how clever your thinking is, any conclusions will be shaky as hell until we go out there and look. We need more data.
Are you saying that the scanners have not detected any M class planets? Perhaps we need to revers the polarity 
Here’s Andrew LePage’s blog, which examines new ‘habitable’ planets as they are announced. So far every new discovery has come up wanting.
No doubt there are better candidates out there, hidden by the way Kepler detects planets; real Earth-like worlds would be almost invisible to this method. But that still means that true Earth-like worlds have not yet been reliably observed.
Except for the belly button on the upright reptilian.
The odds of humanoid intelligent extraterrestrials existing are not changing. They exist, or they don’t. What is changing are how many humans believe that humanoid intelligent extraterrestrials are a possibility. I believe that number is increasing.
When it should be decreasing.
Humanoid aliens are very unlikely, unless you have a very low standard of what a humanoid is.
I’d tend to think aliens would be, well, alien. Lovecraft was probably right on that, but even he had to use metaphor for existing Earth species (it’s like a crab and mushroom!). Convergent evolution is a thing, though that assumes similar environments.
With our straight, upright spines humans are peculiar creatures even on Earth. There have been many bipedal animals, but as far as I know they’re mostly cantilevered. Penguins have vertical spines. Can’t think of anything else off hand. It’s not like going from brachiating tree dweller to savannah dweller causes tons of species to evolve bipedalism, like the ocean turns both reptiles and mammals into torpedo looking fishy things. So if an alien is bipedal I’d think they’d have a stance more like a bird. Maybe you could call that vaguely human.
Elephants would be excellent candidates for non-human civilization. They’re already extremely smart, they live in large social groups, and their manipulator is both strong (lift tree limbs) and dextrous (they can paint with it, using a normal paint brush).
Here is an older thread addressing a similar question in detail: [THREAD=435424]What are the odds of humans evolving on a different planet?[/THREAD]
Stranger
As I noted above, it also assumes and *requires *similar starting points. A squid and a tuna and a porpoise both exist in not just similar environments, but exactly the same environment. They fill the same ecological niche.
Now would you consider a squid and a porpoise to be examples of convergent evolution? Of course not. they look nothing alike and have almost no analogous features. From the means of propulsion to their prey capture mechanisms to their reproductive systems they are utterly dissimilar.
We could do the same for other niches and environments. Does a dragonfly look at all convergent with a swallow? Does a tiger beetle converge with a shrew? Is a woodouse really very convergent with a snail? Identical environments and niches, to the extent of being direct competitors , but no convergence.
You only really get convergent evolution when the organisms being examined are already closely related, because that restricts the number of viable solutions available. Tuna and dolphins and icthyosaurs all ended converging on the same form because they all started out as vertebrates, and their is only one optimal design that a vertebrate can readily develop for that lifestyle. But if you look at the invertebrates that have occupied the same niche, their is no evidence of convergence of any sort. Nobody would ever consider a squid or anomalocaris to be convergent with flipper.
This is the biggest argument against humanoid aliens. Life from another planet won’t be even vaguely related to us. It will differ on the most fundamental of level of what it is made of. For example, we unconsciously think that all “animals” must have the same general “feel” as Earth animals because they will be made of proteins: made of meat. But even that is an unwarranted assumption. Alien animals may have a fibrous structure more similar to that of plants, or they may be crystalline and feel like a sandbag, or they may have basic structure that we haven’t even conceived of. Starting from such fundamentally different building blocks, the way that animals life evolves will be almost certainly be totally incomparable to anything we know. Not just in the sense of not being bipedal, but everything in the way they are constructed, the way they move and the way they breathe.
We can speculate endlessly on how such life could evolve, but we can be pretty sure that anything vaguely humanoid isn’t on the list.
But wait… A dolphin and a tuna are (somewhat) convergent. They both adapted a fusiform silhouette, fins for propulsion, etc. Dolphins started out as land animals, looking vaguely pig-like, but the environment helped shape them into something somewhat fishlike.
Convergence does happen…it just isn’t a “rule.”
That is exactly what I said. You then somehow missed the rest of what I wrote, thus missing the entire point.
No, the point is that it does happen, but it requires the animals to be closely related to start with. no matter what you do, a squid will never look like a tuna and woodlouse will never look like a snail.
Beg your pardon; it seemed as if you were saying it doesn’t happen.
ETA: I don’t agree it requires the two species to be closely related to begin with. The pig-like ancestors of dolphins weren’t all that closely related to the ancestors of tuna.
I do think it’s just one of those random/coincidental things that’s gonna happen, unavoidably, when you have a whole lot of evolutionary lines going on. You’ll get resemblances, sometimes very close ones, but without any concrete reason for them. They could just as easily diverged in appearance as converged.
Human bodies are formed by human souls, not by any evolutionary means.
Much closer than they are related to squid, or anomalocaris, or kelp. Pigs and tuna share many anolagous structures and similar bodyplan.
There are no pig, or tuna on other worlds, so this sort of convergent evolution that starts from a closely analogous bodyplan that results in sharks, tuna, icthyosaurs and dolphins is going to happen much less often there.
However, second-guessing God is seldom profitable, especially when talking about worlds that we are never going to reach in this life. The discussion is more interesting if you frame it on the basis that evolution is a thing, and leave the argument that it is not for another thread.
Human bodies are formed by complexes of amino acids, lipids, apatite mineral structures, and a small but critical complement of calcium, phossphorus, potassium, sodium, sulfer, magnesium, iron, and traces of other elements. Nowhere within or attached to the human body has there been found anything akin to a “soul”, nor has the apparent complete absence of such resulted in any gulf of explanation of how the body functions or from which it evolved.
Stranger
I don’t believe that in real life Sarek and Amanda Grayson could mate and produce a bouncing baby Spock, but the rough planform is efficient, with the CPU and most sensors located in a central place high up so the sensors can be most effective and bilateral symmetry for ease of motion without a lot of complexity. Bones or an exoskeleton rather than tentacles because they give something for muscles to work against, while reducing energy use at rest. A skeleton is also nice to have when a large creature exists outside of a supporting medium, like water.
Two or more arms with manipulation units that are not also required for locomotion. Two or more legs for locomotion–two and four are about even for energy efficiency in locomotion, but two burn fewer calories at rest. A tail for balance is optional.
All told, the humanoid pattern is a pretty good layout, with a bias towards cheapness. We should not be surprised if we were to find creatures from other planets that follow it. Parts could be added, like more arms, but it’s gonna cost ya.
:smack: How could we have missed that scientific fact? You’re absolutely correct, of course. Revise the textbooks!!
In his book Starmaker, Olaf Stapledon imagined that the humanoid form does emerge on occasion, because of the general utility you describe. He did not expect that such forms would necessarily evolve from the same lineage that humans came from- he describes humanoids descended from frog-like stock, from centaurs that lose a pair of legs by fusion, from five-legged starfish analogues and even from plant-like organisms.
None of these are impossible, but none of them would result in anything recognisably human. If you are happy to accept a bipedal starfish or landsquid as a fellow human then that is fine, but the chances are that most alien humanoids will look significantly different from anything we have yet imagined.
All of these arguments implicitly assume that the bipedal humanoid form is somehow the particularly optimum form for intelligent life and any deviation from it is some kind of deficiency. But the reality is that the most we can say of it is that it may be the optimum form for an intelligent creature evolved from brachiating apes to inhabit the grassy savannas of the Serengeti plains to use stone tools to gather plant seeds and fruits and scavenge or hunt land mammals. It says nothing about life that may have evolved in any other conditions, with other means of sustenance, using whatever precursor planform that came from its ancestors. The ways in which the human form is suboptimal and in which we would design an organism far differently are too numerous to list, but serve to indicate that the human form is as much an accident of its own history as any kind ideal adaptation to the peculiar conditions which guided its evolution.
The idea that the human form is somehow “low cost” or “high efficiency” is not born out in any objective assessment of the human form; from the complicated and arduous birthing process, the misrouting of nerves because of how the hominid form evolved, the poor heat management systems in any but tropical environments (requiring acquiring the skin and fur of other, better adapted creatures in higher climes), and the vulnerability of the brain and sense organs by putting them way on top and out in front rather than protecting them within a thorax-like cavity, the human animal is poorly ‘designed’ as an intellectual creature. In fact, the humanoid form is such an unlikely salmagundi of disparate features it seems highly unlikely that another intelligent species would evolve a similar form even under identical conditions.
Stranger