What are the odds of humans evolving on a different planet?

Factual Questions
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If you read through the whole thread, you’ll see that several people have already said that the odds are not calculable. However, much of the discussion was on what factors would be needed to make the odds calculable, starting with an analysis of what happened on earth to reach our current point. Hardly masturbation if you understand the science involved.

Admittedly, if you don’t understand any of the underlying science, I can see how it would seem worthless, but that would be true for any science thread, not just this one.

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:stuck_out_tongue:

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In my experience, everything always takes the path of least resistance in most sciences. I see no reason to expect that the rules change for evolution.

Ironically, I would consider something that promoted reproduction to be the easier pathway so your are proving my point. Obviously, you have a different definition of easiest pathway. I would also consider the paths that are more statistically probable to be easier.

You are right, I am not talking about genes that are specifically detrimental to reproductive success.

Insects and other non-vertibrates have an entirely different building structures from vertibrates. It’s valid to take the position that all vertibrates evolved from tetrapods by random chance. I don’t think it was random at all.

I’m not talking about planning ahead. I’m saying that random chance is likely give you something relatively close to what we already have. A bird is not likely to develop a large brain size due to the weight constraints of flight and a forest and prarie dwelling omnivore would probably be bipedal to make tool use efficient, but that would strike me as being rather close to what we have. Octopusses are an interesting option, just because I haven’t found a reason they aren’t likely to develop intelligence doesn’t mean a reason doesn’t exist.

To say that it isn’t more likely is equally unsupported by reason or a useful body of data. I had to change “superior” to something I’ve actually said.

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OK, then you’re not talking about evolution.

Different structure, but the genes are remarkably similar, as **DF **mentioned upthread.

All vertebrates did not evolved from tetrapods. If you have a goldfish at home, that’s a vertebrate and it is not now a tetrapod nor was it ever one in its evolutionary history. Tetrapods got their 4 limbs from lobe finned fish which got their lobed fins from even more primitive fish which had the basic 4-lateral fin body plan we see today even in sharks (two pectoral and two pelvic fins). This body plan would seem to go back more than 400M years. Why only 4 of these fins, we do not know. But that is why we, today, have 4 limbs. As **DF **noted, it’s a conservation of an ancient body plan that had nothing to do with walking on land.

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And here is what I think is the nub of the discussion.

Life itself is not the path of least resistance. Living organisms are an extreme example of chemical disequilibrium. Of course, the reason living organisms are in such a state is due to the constant input of energy to create that disequilibrium, and when an organism reaches chemical equilibrium we biologists call that organism “dead”.

And it’s the same with evolution. It just doesn’t make sense to argue that the creatures we see living today are there because they are the most likely sorts of creatures to evolve. Just look back 65 million years ago to the Cretaceous and you’ll see a vastly different suite of animals and plants. If a body from space hadn’t hit the Earth back then, it’s very likely that the suite of animals and plants on Earth today would be much more similar to that Cretaceous biosphere. And go back 225 million years and you’ll find that the Permian had a much different suite of organisms compared to the Triassic, and if the Permian extinction that wiped out the mammals hadn’t occured, the dinosaurs never would have taken over.

So are the fish and marine mammals that dominate the oceans today more thermodynamically likely than ammonites and marine reptiles that dominated the oceans in the Mesozoic? I don’t think so.

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First of all, you’re applying a simple mechanical principle to a complex, non-equilibrium process. “The path of least resistance”, thermodynamically speaking, wouldn’t lead to any kind of life. Indeed, life can be seen as an organized resistance to entropy, an attempt to graduate the flow of energy through a variety of self-organizing and self-replicating processes which defy–or at least defer–the normal workings of equilibrium thermodynamics and chemical reactions. The entire process of respiration and the Krebs cycle is a very strong and complex extropic process which can’t in any way be described as “the path of least resistance.” Clearly, evolution doesn’t not favor “least resistance”, but rather optimum extraction of energy from the local environment with acceptible risk.

You keep throwing up “random chance” as a kind of straw man which you can hang from the gallows. I don’t believe anyone–certainly no one who has a more than superficial understanding of natural selection–views the progress of evolution as being “random chance”. Indeed, it is very much the opposite; it proceeds based on the overall strategy of maximal success in competition, and is decidedly nonrandom in pruning out unsuccessful strategies.

But it does not automatically follow, nor have you demonstrated in any qualitative or objective sense, that the bipedal form is somehow necessary or even overwhelmingly beneficial for the development of conceptual development and tool use, nor that there is any reason to believe that intelligent life would converge upon the bipedal form as optimal to support intelligence. Of the most intelligent non-primate-derived animals, only ursines and procyons come even close to the human form, being capable, albeit slowly, of plantigrade bipedalism. (They of course prefer movement on four legs, but can and so stand upright to use their forelimbs for manipulation.) Other animals of comperable conceptual or problem solving intelligence, learned behavior, and/or social complexity to the great apes are equines; in particular, Equus asinus (donkey), the aves of Corvus and Psittacinae (ravens and parrots), the more complex cephalopods (octopus), many members of order Cetacea, and some other litoral marine mammals like sea otters. With the exception of the birds, none are bipedal, and for these birds bipedalism is not the primary means of locomotion.

It is a mistake to view adaptation as being either an open process that finds an optimum engineering solution to any problem, or one that seeks the least energetic solution to a challenge. Natural selection merely favors the solution, however ad hoc the expressed phenotype (and it’s clear from even a superficial study of animal anatomy and molecular genetics that ad hoc, stopgap solutions are the nature of the business) that gives the greatest advantage to the carrier of the gene in the current environment, against existing competition, and built upon cummulative genes and expressed forms. Convergent evolution is a result of common challenges, but the routes and specifics of parallel adaptations are often strikingly different; for instance, the structure of wings on insects is very different than on birds, and different again for bats. Eyes, which have been independently evolved an estimated 40 times are very different in structure between independent approaches. The propulsive tails of cetaceans and other marine mammals that have them are horizontal (deriving from extension and modification of previous quadrapedal structure) while that of sharks and fish are vertical, even though both perform the same function.

Neither convergent evolution nor “path of least resistance” are strong principles adhered to by natural selection. Moderationg of energy gradients and accumulation of adaptations are. Please try reading Ernst Mayr’s What Evolution Is or Richard Dawkins’ Climbing Mount Improbable to get a better understanding of how evolution proceeds and why the easy or obvious solution–the one that is “the path of least resistance”–is often not the one selected.

Stranger

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It seems we have two fundamental philosophies here. One suggests that evolution is just as likely to come up with an intelligent six legged flying hippopotamus as an intelligent humanoid. The other philosophy suggests that evolution is controlled by rules that ultimately led to an intelligent humanoid. It’s true, the latter philosophy is more difficult to prove as it requires one to study how evolution works to prove that rules exist. Nevertheless, it is what I believe.

My suggestion of bipedal intelligence comes from the fact that no vertebrate with more than four limbs has ever been discovered. Tool making is much easier with two free hands, and that leaves two legs to walk on.

Of course, my belief requires the burden of proof, and assumes a more complex system. Occam’s razor is scientific folly.

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I get the feeling that few people actually read what I write these days. Not least of all because I made a pretty glaring error in my previous post about constraints…namely, I said “phalanges” when I meant “digits”. Anyway…

You’ve got the sequence backwards: tetrapods evolved from vertebrates, not the other way around. And I made a case for why the four-limbed status quo among most terrestrial vertebrates is what it is. Not because of random chance, not because it’s adaptive, but because that pattern has been “locked in”. Granted, early on, when body plans were still being experimented with during the Cambrian, that might not have been the case. But once developmental complexity reaches a certain point, there’s not as much “wiggle room” to modify that plan. Insects remain six-legged, arachnids eight-legged, vertebrates 4-legged, etc. You can reduce the numbers of limbs / digits / phalanges / whatever by shutting off gene expression, but you can’t go up from the bauplan defaults.

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And I’m saying that the number four may have been picked because thats the only way (or at least the most likely way) that it could work. I don’t have it backwards, of course tetrapods evolved from vertebrates.

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About the “path of least resistance”… Things in other sciences don’t follow the path of least resistance. What they do is follow multiple paths, and follow those paths which have less resistance more often. As an example: Suppose I have a network of parallel resistors. I’ve got, say, one 1-ohm resistor, one 2-ohm resistor, and one .5-ohm resistor. I then hook this network up to a voltage source, so a current flows. Most of the current will end up flowing through the .5-ohm resistor, since it’s the path of least resistance. But some will flow through the 1-ohm, and a lesser amount through the 2-ohm.

But now suppose that the voltage source I use is very low voltage. It’s so low voltage, in fact, that the total current is only 1.6*10[sup]-19[/sup] amps. And I only leave the voltage source turned on for a single second. In other words, I’m only sending a single electron through my network of resistors. Which one will it go through? Most likely the least resistance. But it might go through one of the others instead.

And this is the situation we have here. All of the terrestrial vertebrates descended from a single ancestor, which happened to have four limbs. Maybe four limbs was the path of least resistance, or maybe it wasn’t. If we had a great many examples of independant evolution of terrestrial vertebrates to study, we might be able to say which path offered the least resistance. But with only one case to study, all we can do is make feeble guesses.

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No one has suggested that.

No, the rest of us understand the how evolution works and that is why we are saying that bipedalism is no prerequisite for sentience. You are setting up a false dichotomy.

And yet tool using preceded bipedality. Besides, your observation is limited to what happened to have occurred on earth, with it’s unique history and physical characteristics. A world covered almost entirely with water might well develop intelligent species that never became terrestrial and might have 50 limbs or 8 or 1.

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Actually, you left out the more fundamental philosphy, which I’d bet the majority of posters here subscribe to: that evolutionary pathways are contingent on both previous history and current environmental variables (in addition to more esoteric notions, such as “degrees of freedom” for mutations; not every mutation is equally probable). We got to where we are not because of predestination or fixed rules which dictate that a bipedal intelligent form must arise, or even from infinite plasticity and we just happened to win the mutational lottery, but because of our contingent history. If you change that history, life as we know it would be very different. As such, since no planet is exactly like Earth, we cannot reasonably expect to find alien lifeforms which look like those found on Earth. Sure, there may be similarities, based on convergent evolution (comon solutions to common problems, and all that), but the end results will still very likely differ in substantial ways.

No vertebrate with four limbs will ever be discovered. If we do find aliens with spines on some distant planet, they won’t be vertebrates (as in, members of Vertebrata).

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Consistent with that analogy Chronos we end up with multiple species. What we don’t see is current jumping between the parrallel wires. The “path of least resistance” analogy is not perfect, but it demonstrates the idea.

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This is a grotesque and hyperbolic misrepresentation of what has been stated, to the point of invoking argumentative fallacy. No reasonable person would suggest that an “intelligent six-legged flying hippopotamus” is likely or even possible; evolutionary processes obvious favor forms that are realiziable within the allowances of basic physical mechanics, and there are certain general preconditions that we can estimate as being necessary for the development of intelligencel see lists in previous posts of this thread for examples thereof.

However, you are stating that a very specific form–a bipedal, human-like anatomy–is either a necessity for or a consequence of intelligence. You haven’t demonstrated this in any factual or rational way, other than to assert that gripping appendages like hands are necessary. Even this claim is circumspect–an octopus, while having a very articulate manipulative capabiltiy, has nothing like a vertebrate hand–but even if we accept this to be true, it does not follow that any intelligent, extraterrestrial species would be derived from a four-limbed ancestor and would therefore devote two of the formerly locomotive limbs to manipulation and tool use to become a biped, much less humanoid.

There are no vertebrates on Earth with more than four limbs (and for that matter, none that have fewer except for those who have deprecated limbs, and even these still show up in embryonic forms). This is a consequence of cumulative adaptation from an original terrestrial vertebate form with four lobes/protolimbs. Fantasy creatures aside, adding more limbs to a complex anatomy based on an internal skeleton would require a massive retooling of the vertebrate body form which is beyond unlikely. A “hopeful monster” with six or more limbs is hardly likely to be reproductively successful for any number of anatomical and competition based reasons, even assuming such a mutation would breed true, and the addition of nascent limbs (the predecessors of which likely existed for functions other than locomotion) in a complex form is unlikely to offer any significant survival or reproductive advantage long enough to bring it to functionality.

If it had been a six-lobed fish that crawled up on shore (metaphorically speaking…the evolution of terrestrial vertebrates is much more involved than that, of course) then we’d expect to have six-legged creatures which might deprecate or modify two or more locomotive appendages due to adaptive pressure, and thus we might anticipate a centuar-like intelligent species rather than a biped. Or a four-legged creature might have used something else–say, lips, or a prehensile tongue, or an extended probiscus–for manipulation and tool-use. Arguing that the biped form is an inevitable consequence of or requirement for intelligence and tool use because all examples we have of this are bipeds, even if it were true (which it’s not) is making a claim from a highly restricted set of data without a definitive and uniquely sufficient causal link; a combination of arguing the general from the specific and the cum hoc, ergo propter hoc fallacies. You states that “evolution is controlled by rules”, but except for the rules of physics and biochemistry there are no “rules” or “guidelines” by which evolution progresses in terms of selecting form; it simply adopts the most reproductively successful adaptation or modification of current form.

Stranger

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Ever seen an elephant? Here’s a highly social, highly intelligent quadripedal herbivore, that doesn’t use it’s front paws to maniuplate objects, but rather a specialized organ derived from the muscles of the nose and lip. Elephants have brains as large as a human. It’s easy to imagine a civilization of human-intelligent elephants a million years in the future. But those elephants wouldn’t be bipedal, and they wouldn’t use their front feet to manipulate objects like Babar.

No land vertebrate is going to grow six legs, sure. But while an elephant’s trunk isn’t a fifth limb, it functions as one, just like the Panda’s thumb that isn’t a thumb.

And even if we look at creatures that use their front paws to manipulate objects, how many of those are bipedal? And when we look at bipedal animals throughout evolutionary history, we see that although bipedalism has evolved many times, our sort of bipedalism is unique. Any quasi-vertebrate intelligent biped is much more likely to resemble a bipedal dinosaur rather than a tail-less ape with its torso cranked up 90 degrees.

One more thing to consider. Evolution DOES tend to find optimum solutions for engineering problems. But the caveat is that these are local optimums, rather than global optimums. So if you’ve got bony animals, evolution can find all sorts of novel ways to reduce weight and increase strength of bones…but no animal that depends on a strong skeleton is going to ditch calcium phosphate bones and start using aluminum, even if aluminum bones would be much stronger. Once an organism is dependent on a particular structure evolution will refine and refine that structure, but the organism can’t throw the structure away and start from scratch. So giant arthropods from the Paleozoic couldn’t evolve internal skeletons, even though internal skeletons are much more effective than exoskeletons for large animals. Animals are often stuck with their evolutionary legacy, with no way to improve it, even though obvious improvements might exist, because every animal has to exist and reproduce as a whole animal.

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Actually, polydactyly (having extra digits) does occur in humans and other species: The condition has an incidence of 2 in every 1000 live births, although the frequency is higher in some groups (an example is the Amish in the United States, due to the founder effect).

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Setting aside for the moment the point that argument by analogy is a dreadful way to extend a logical argument, I fail to see how this advances your case at all. We certanly don’t see species jumping from one order to another (except in the case of misclassification, which is an artifactural problem with taxonomy, not the theory of natural selection) it reinforces the argument that your end result is predicated by the path you’ve previously taken; four legged terrestrial vertebrates are a consequence of the ancesterial anatomy having four motive limbs, and the bipedal stance with forelimbs being rededicated to manipulative digits is a consequence of that plus what comes after, specifically brachiating primates. If intelligence had instead advanced to tool manufacturing stage in aves, we’d anticipate a more complicated manipulative beak and tongue structure (which in fact can be seen in the birds with high intelligence and the propensity to make use of “found” tools).

Evolution isn’t about “least resistance” or even optimal parsimony; indeed, the most generally adaptive creatures are ones that retain extra or seemingly unnecessary capabilities which come in handy when survival pressures change. Having a “bag of tricks” up your genotypical and phenotypical sleeve is a credible evolutionary strategy up to the point that carrying them around fatally compromises your ability to survive.

Stranger

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This isn’t an adaptation to external sitimuli but a mutation that is generally anatomically inconsistant, i.e. the extra digits are (generally) only partly functional or impair normal use, though it is possible that such mutations could be selected for if they offered an reproductively advantagous capability. (Please, no jokes about triple-breasted whores.) At any rate, adding additional fingers or toes isn’t comperable to the complexity of adding whole limbs, particularly motive implements, which would require a massive restructuring of the skeletal system to accomodate. At any rate, no vertebrate species has evolved to add additional limbs as part of the species anatomical archetype.

Stranger

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It doesn’t seem that anybody is really presenting any proof of any kind. People say that six legged creatures could have been the first to walk on land, but they don’t offer any proof of that. An elephant can be held up as an example of something that is a quadruped that uses tools, but an elephant is pretty limited in what it can do wth them. As I mentioned earlier, an octupus is an interesting example. I don’t know why octupuses haven’t developed intelligence. It could be that an ocean environment is bad for the development of language. Maybe the mutations that would lead to intelligence are only feasible in vertebrates.

Playing like your giving me information I haven’t already considered isn’t going to convince me of anything. Tell me something new.

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However, as I note in this post (from a different thread), polydactyl digits are not “new inventions”; they are the result of one or more genes being expressed multiple times during development. The extra digit / phalanx retains the developmental identity of the digit / phalanx it is copied from.

While the genes which control expression would be heritable (thus increasing the likelihood of polydactyly in certain groups), that extra finger or toe is, in itself, not.