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They don't necessarily mean little green people or little grey people with big eyes, but in more basic form (head, legs, eyes, big relative brain size, and so forth). I've heard the same claim a few times before, and I want to dissect it a bit. The basic idea is that life can only show up in a limited set of environment types, and that these environments will push developing life on any planet toward the same basic solutions, and along the same developmental path, until they reach the optimum form.
I'll relate one such argument below:
All organic life begins in water--salt water--and would evolve and split into many ridiculous and odd patterns (like on our planet). Evolution is interesting stuff. Somewhere along the line a creature (this, being on planet "X", not Earth) would evolve that was able to "swim" by constricting its body side to side (or up and down). Logically the "duality" of this movement would increase speed and the pattern proceeds from there.It goes on, but there it is: the history of life and its inevitable progression toward a particular optimum form in a few short paragraphs. Unfortunately, this type of story can be really convincing because the arguments make great intuitive sense, even if they turn out to be wrong. Some of my own thoughts:
The need of a mouth at the front to gather food from water flowing in is logical. The tail would evolve to flap (most likely) side to side. Fins on either side (up front and in the back) would also rise as a part of the mutative process. Since this creature swims forward, the visual, olfactory, and auditory functions would develop at the front where the most stimulus would enter-- the ganglial nodes and development would also occur close to the input-- thus the brain would be at the head.
Since the creature undulates side to side to swim forward, one sensory organ on either side would also be the evolutionary logical step. The mouth in the front-- eyes and ears on the side. Since taste and scent are intimately connected (both sample chemicals in the environment) they would probably develop in the same way (nostrils are quite unique in that they are not totally necessary for scent-- see: snakes).
Over time, these organisms' superior biology would cause them to become dominant. Notice the proliferation of these types of creatures on our planet: sharks, indeed all fish, dolphins, eels, lizards, frogs, dogs, birds, horses, apes, humans. Practically all life that grows beyond six inches (yes-- there's sea stars, mollusks and whatnot) follows the rule of TWO (bi-this, bi-that; i.e. bi-pedal).
When, one day, these creatures on planet X started spattering up onto muddy land, they would probably use their fins on either side to pull them. Over time the hind fins, or tail, would split into the two real limbs. Eyes, mouth, ears and brain forward, body in between, and feet firmly planted on the ground.
Further into the future, the limbs closer to the eyes and nose, growing more and more prehensile, would be used to pick up and manipulate objects and prey. As development continued, these forelimbs would become more and more articulate and eventually end in hands.
They would be endoskeletal because exoskeletons are extremely inefficient over a few inches and make growing, moving and manipulating objects very difficult. Compound eyes, though great for small creatures, are horribly inefficient and incapable of guiding large, complex creatures. Internal lungs, closed circulatory blood system, and endothermia are all needed to grow large and mobile as well -- any complex land roaming creature would have these attributes as well.
Even if life has to begin in water (and astrobiologists are far from sure of that), who says it will develop as laid out above? Jellyfish, e.g., don't have a mouth at the front where they swim forward -- what if another form of locomotion works just as well for getting around in water (especially the solutions we haven't even imagined)? Maybe alternate solutions could be even more effective not just in water, but eventually when adapted to a land-like environment.
For that matter, who says life will end up on land before it becomes intelligent? In the one data point we have (Earth), tool-using, linguistic intelligence arose on land and not yet on water, but that is far from evidence that it has to be so. Some people have argued that dolphins never developed the way primates did because of their lack of opposable digits - they need streamline flippers to continue swimming well. Ah, but consider the octopus, which is well-studied for its abilities to manipulate objects. For all we know, oceanic forms could have all the necessary prerequisites to develop whatever it is (tools, culture) that leads to or coevolves with particular forms of intelligence.
Further, maybe the so-called "rule of two" noticed among large creatures on Earth (and ignoring all the myriad exceptions) does not, in fact, suggest a universally superior form for life-supporting environments. An alternate explanation is that all the bi-this and bi-that creatures we see on Earth have a common ancestor that happened to take one of multiple possible (and equally viable) evolutionary paths, and all these creatures have the "rule of two" pattern because of their shared history, not because of convergent evolution toward the one best solution.
Granted, there are going to be some mechanisms or forms that probably just would not work very well in *any* environment that's likely to be able to harbor life-like entities, so some restrictions will exist, but I think we don't have *near* enough information to conclude that any environment that can support life-like entities is going to have basic solutions that will likely be shared on most life-supporting planets. I mean, we have as little as one data point (shared ancestor's development here on Earth) or at best multiple convergent-evolution data points but for just one planetary environment. But even in the latter case, maybe life or life-like things can show up on a much wider range of planetary environments than we think. Not to mention, a very small change in gravity or atmospheric pressure or some other variable might lead to bigger changes in form than we predict.
Personally, I have trouble finding anything on which to base speculation about what intelligent life on other planets would look like and how intelligence might evolve there. We could look at encephelization history in primates or cetaceans here on Earth, but let's face it, we don't know if an alien species' processing functions will occur in anything resembling a brain-like structure at all, nor if our brain-like solution is the optimum one or just 'good enough'. Natural history is full of examples of evolutionary adaptations that seem to be 'good enough' solutions, but far from perfect, not to mention far from what one would logically and intuitively suspect before-the-fact. In other words, life does not develop as we would expect it to, and it does not always find what we consider the 'ideal' form -- it need only find a good enough form to stick around a while in that niche.
But, more importantly, if we're talking about creatures that are traveling between solar systems, I suspect they are going to be technologically developed to the point where any environmental natural selection pressures would not be the dictating factor of their forms any more.
Even our young species is on the verge developing technology to take control of our own form (gene therapy and other biotech, along with nanotech and other nascent biomedical technology). We likely won't drastically change the basic form very fast (for social reasons), but eventually we won't be anything resembling Homo sapiens anymore, and maybe not even resembling any previous life on Earth at all.
Especially if we are getting to the point in our civilization where we don't consider Earth home (it is going to die, after all, when our sun dies in a few billion years) but start going to other, younger star systems, we will likely find much more versatile forms for ourselves. So why would we expect intelligent alien species, especially those that have mastered interstellar travel, to have kept the form-solution they evolved on their original planetary environment?
But all this alien stuff aside, the bigger lesson here is in how easy it is to tell a just-so story of evolution to explain the life we see around us here on Earth. We can offer stories like the one quoted above in order to speculate on some possible hypotheses, but it takes a much deeper and more careful analysis just to find out if a particular claim about creatures here on Earth makes sense, let alone to prove that a solution happened because it had to as opposed to it being one possibility among many.
It is not just the critics of evolution who have a bone to pick with evolutionary explanations. We all have to remain skeptical and vigilant against too-easy explanations that intuitively sound right but are far from proven. We have to consider alternate explanations, look more deeply at the actual data (i.e. the vast examples of life we do have on Earth, and more indirect evidence), and test claims even if they already seem to have some evidence for them.
It is crucially important to increase scientific literacy and knowledge even among non-scientists, but the danger in popularizing science comes from misunderstandings and mistaken assumptions by people who only know a few pieces of the puzzle. Scientists themselves have a duty to put every theory through the ringer and constantly reexamine it in light of new evidence, but non-scientists, I think, also have a duty to remain on guard against accepting merely intuitive stories based on partial information or partial understanding. It may sound good and fit with what you know, but that does not make it true.
I don't know, at some point -- given that none of us can be an expert in all areas of human knowledge any more -- perhaps we have to concede to the experts to answer the hard questions. It doesn't mean we don't attempt to understand things ourselves, but we have to be careful about assuming our own understanding of things is complete when it is not. In other words, remain open to the possibility that even the most logical-sounding argument might be wrong.
"There are many hypotheses in science which are wrong. That's perfectly all right; they're the aperture to finding out what's right. Science is a self-correcting process. To be accepted, new ideas must survive the most rigorous standards of evidence and scrutiny."
"Science is not the affirmation of a set of beliefs but a process of inquiry aimed at building a testable body of knowledge constantly open to rejection or confirmation. In science, knowledge is fluid and certainty fleeting. That is at the heart of its limitations. It is also its greatest strength."
"I was gratified to be able to answer promptly, and I did. I said I didn't know."