This rationale is beyond simplistic. First of all, the human form can hardly be considered “low energy”, nor can we view it as some teleogically ultimate result; it’s what dominant intelligent lifeform on Earth is now, but perhaps not in the future. Second, it’s a mistake to think that any ongoing process like evolution comes to a thermodynamic equilibrium. While thermodynamics plays into evolutionary biology to an extent that we’re only beginning to quantify, it’s is a decidedly nonequilibrium (NET) situation. The plantigrade bipedal form is the result of intelligence emerging from monkeys; had, say, ursines or equines beaten us to the punch, the form could have been very different. (And please, no repeating the Larry Niven-penned simplification that it takes no intelligence to sneak up on a blade of grass; there are plenty of herbivorous species which demonstrate intelligence, and predator species which show very little.)
Humans or something like us are not an inevitable result of evolution; to believe so is the same kind of blind thinking that lead to heavenly epicycles and élan vital. We just got mostly lucky and demonstrate every evidence of being an unfinished, undesigned, imperfect product.
Elephants, octopus, and many speces of aves have all demonstrated tool use, sometimes to a sophisticated degree. (Admittedly, birds are technically bipedal, but they don’t use their other members as a gripping appendage.) Bipedalism was important for hominid development, so that we could grip and carry tools, and of course there are many species of monkeys and apes which are only intermittently bipedal which use tools to a greater or lesser extent. In fact, it was probably less tool use than the need to cross wide ranges of savannah on foot and see above grasses that is most likely to explain the progressively plantigrade bipedalism and increasing height of hominids which were derived from brachiating apes. You can make the point that the upright mode of locomotion led to tool manufacture (for better hunting prowess against herd animals) and allowed proto-humans to carry tools on migratory processions, but that’s a composite logic and not really a solid claim by itself.
In short, there’s no particular reason to except any arbitrary intelligent creature to be bipedal, unless they’ve evolved from brachiating ape-like creatures in a migratory lifestyle, and even then, it’s not a guaranteed solution; it just happens to be the one that worked for the singular data set that we have. Generalizing from one scenerio doesn’t make for a firm footing to claim that it’s the best or most likely of all possible worlds; it’s better viewed as the least worst of all options given a particular starting point. Adaptation is never an optimax solution; it’s always a compromise between what you’d do if you planned it out and the resources and initial conditions that selective pressure has to work with.
I think there are things we could expect not to be unique to Earth. Consider the number of times flight, or the eye has independently evolved. Of course lots of things are possible, but how about this (all speculative, I know):
-If you’re going to crawl about, it helps to have legs on the bottom of your body.
-If you’re going to eat things, process them, and excrete the waste, having the eating bit at the opposite side or end from the excreting bit makes a fair bit of sense.
-If you’re going to move in any kind of fast, specialised way (such as pursuit of prey), choosing part of your body to always be the front probably isn’t a bad idea.
-The front isn’t a bad place to put the visual apparatus - more than one of them if you want depth perception…
-Several factors above favour some kind of longitudinal symmetry
-If your food tends to fall to the ground under the force of gravity, your eyes are better off above your mouth.
-And so on…
I don’t think it’s at all unlikely that some kind of vaguely humanoid bipedal alien lifeforms could be out there (although I think we’d still have to be incredibly liberal with the definition of ‘humanoid’.
Sure. Bilateral symmetry is a consequence of cephalization and gravity. If you’ve got a mouth, it’s a good idea to put sensors near the mouth to facilitate getting food into that mouth. If you’ve got sensors it’s a good idea to put the central nerve clusters near the sensors. And so the “head” is born, with it’s corollary the tail. Now add in the fact that gravity pulls down, and your creature has a top and a bottom. Uncephalized creatures tend to be radially symmetrical because gravity still exists even in the water, and so we have starfish and jellyfish. If you want to move limbs of some kind are helpful. If you’re a large creature an internal skeleton makes more sense than an exoskeleton. If you’ve got 4 limbs but want to have manipulating organs, walking on two limbs and manipulating with the other two makes sense, although plenty of creatures seem to have perfectly fine hands on their front legs.
But such a creature shouldn’t be thought of as humanoid, if that’s the definition of humanoid velociraptors, kangaroos, and iguanadons are humanoid. And note that all those creatures are close relatives to humans, all being tetrapods. And note that there were dozens, probably hundreds of bipedal dinosaur species, but how bipedal mammal species can you name? Hominids and Macropods are about it.
There’s no evidence one way or the other, at this point. Which you prefer is mostly down to philosophy and aesthetics.
Quoth Stranger on a Train:
Mind you, that’s not an argument that Niven actually supported; it’s presented as an argument made by the Kzinti, who of course have their own set of biases.
Back to the OP, there are a lot of aspects of evolution for which it’s very difficult to calculate probabilities. But let’s look at a few of the places where it is possible. For instance, for an alien species to be truly compatible with humans, they would need to use the same genetic code as us. Let’s even suppose that they use the exact same sort of DNA as us, and the same 20 amino acids, and the same number of bases in a codon (all of these are unlikely, but the probabilities are among those that are difficult to calculate). Even stipulating all of that, we have to assign 64 different codons to 21 different results (the 20 amino acids plus a stop codon), such that each of those 21 has at least one codon. By my figuring, that leaves a number of possibilities ((64! / 43!) * 21[sup]64[/sup]). That comes out to 8.8*10[sup]120[/sup] different possible genetic codes. So the chance of the life on any given planet having the same genetic code as Earthly life, even if it has the same fundamental DNA-protein biochemistry to begin with, is only about 1 in a thousand billion billion googol. And that’s just to get something which has a vague resemblance to anything Earthly… The tiniest bacterium shares that much in common with us. That life would still have to go through the billions of years of evolution (in exactly the same way it happened here) to reach humans.
A good list, but it’s worth noting that depth perception doesn’t require binocular vision, and at range the vary narrow parallax of hominid eyes (and in general those of forward looking mammals) is insufficient by itself for estimating depth, which is instead done by some very complicated visual processing plus integration of other sensory information. Up close, however, that parallax definitely comes into play, and we might expect two or more eyes to be very useful for a toolmaker evolved to works on complicated tools at close range. Plus, having at least two allows for some redudancy should one become damaged or covered, and for virtually all terrestrial mammals eyesight is a primary sense that is in constant use.
Gross anatomical symmetry seems likely–with very few exceptions virtually all mobile creatures in terrestrial or aquatic environments demonstrate either some kind of bilateral or longitudinal symmetry, or have an atavistic symmetry which was modified due to some environmental pressure, like the flounders–and there are some reasonably strong rationales for why this would be more generally so, but it’s not a guarantee. Moties seem unlikely, but not impossible.
Niven repeats it elsewhere, however; at least, in my recollection, in one of the Draco Tavern stories, and perhaps in Footfall as well. In general, Niven’s grasp of evolutionary theory is weak, or at least highly simplified for the purpose of dramatic conflict.
Not really. What are the odds that our closest relatives on earth, chimps, will evolve into humans? Again, so close to zero that it might as well be zero. So, unless these aliens actually transplanted humans onto another planet, we won’t be seeing them on other planets. Remember, the OP is asking about humans evolving on other planets, not being kidnapped and planted on other planets.
Suppose that 4M years from now, chimps will have evolved into upright, largely hairless apes with the capacity for language. These new Pan sapiens (for lack of a better term) would still be more distant, genetically, from us than than chimps are today. The odds of them having the same mutations at the same place to get the be upright, largely hairless apes with the capacity for language are so close to zero that they might as well be zero.
Not to mention the fact that we will have evolved quite a bit in the next 4M years, too. So not only would Planet X have to have humans on it, it would have to have them at the same time humans existed on earth. No way. Simply now way (unless we postulate an infinitely large universe, and then everything is possible).
I’d say those arefairly humanoid, in the context of this particular question, so aliens approximately as similar to us (morphologically) as kangaroos are, I think could be quite rightly regarded as humanoid… ish.
And yet for some reason, in vertibrates, these characteristics are very rare if they have been heard of at all. I think there may be an evolutionary reason for this. No matter how many arms you think would be useful now we had to evolve from four legged creatures. Maybe six legs isn’t efficient for running. I don’t know.
But is the change we observe the ONLY one that will provide the evo advantage? There might be many of nearly equal utility, and each one will lead us on a much different road.
For those who are bringing up Drake’s Equation, since all terms are multiplied, all it takes is ONE element to be questionable to make the entire equation useless. But here ALL (or almost all) elements are unknown or highly speculative, making the equation more useless than useless. It’s only of philosophical interest; it’s not a scientific law with a more precise answer than something in the range from yes to no, zero to one.
As one professor aid when asked the same question: That squirrel shares at least 70% of my DNA and it looks nothing like me. Why would one expect an extra-terrestrial to?
Not really. Birds and theropod dinasaurs are digitigrade in locomotion, where as kangaroos are macropods that move primarily by saltation. Both would disqualify it for being classified as humanoid by virtue of being significantly different from the plantigrade motion that hominids developed. No anatomist could consider them vaguely morphologically similar to a humanoid. A bear or elephant could claim greater similitude in in that regard.
As Exapno Mapcase correctly pointed out, mutations may be neutral and still survive. In fact, I would guess that genes could be mildly detrimental and still survive.
The only genes that die out are the ones that make reproduction impossible, or at least highly difficult. Even so, there are a number of rare genetic disorders that are clearly very detrimental to humans, yet are still around.
