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

I realize that no one knows the exact odds but listing the factors and an estimate of the odds is what I am asking.

My friend keeps insisting that humans are likely on other planets and I keep telling him that a zillion different random mutations would have to happen in all sorts of species for that to be so. His argument is that there are so many stars and so much time that it is bound to have happened.

I also had to try to dissuade him from the idea that evolution progresses towards intelligence as opposed to intelligence just being another adaptation.

Perhaps if I direct him to this thread he will be persuaded by more eloquent and knowledgeable posters.

The odds are close enough to zero that you might as well go ahead and just call it zero.

The more I read about our own evolutionary odyssey, as well as some of the meteoric and terrestrial cataclysms we had to overcome, the more I’m amazed we came into being on this planet.

To narrow down your question, is he positing that:

  1. An animal very SIMILAR to a human would have to evolve somewhere out there eventually.
  2. Honest to God, actual humans that you could impregnate with human DNA would have to evolve somewhere out there eventually?

Depends on where the life came from.

If you’re talking two planets completely separate, both evolving life from scratch, then effectively zero. (Although I might argue that some general similarities in form are likely – evolution suggests that things that wiggle must precede those that crawl, and crawling must proceed walking/running, so some generic similarity in form may be comparatively likely).

If the Chariots of the Gods type aliens came along and transplanted life from one place to another at some stage, then obviously the odds of human life get better.

Evolutionary biology has been trying to shed teleology ever since Darwin, and is apparently still failing judging from popular notions about evolution.

There is such a thing as convergent evolution. A fish, a dolphin, a shark, and an ichthyosaur all look very similar on casual inspection. But these animals are already very closely related, all being in the same phylum.

What non-vertebrates have evolved to look the same? The closest convergence from a non-vertebrate would probably be a squid…it has a similar shape…if the tentacles are tucked up. And of course the squid swims by water jets rather than wiggling it’s tail, which it doesn’t have. And on and on. A squid is a fast swimming pelagic predator like a dolphin, but it isn’t much like a dolphin despite sharing the exact same environment.

If a creature that is similar to a human is likely to evolve on another planet, then just about every species on earth would be likely to evolve. Why would something resembling humans be likely, but not golden bamboo lemurs, or kangaroos, or narwhals, or pandas, or naked mole-rats, or giraffes, or ostriches?

When you consider the evolutionary history of Homo sapiens, you realize what a long strange trip it’s been since the primordial ooze. We were once single-cell protozoans, then colonies, then wormy things, then fish, then amphibian-y creatures, then mammal-like reptiles, then little mousy things, then tree-climbing little mousy-things, then monkeys, then tail-less monkeys, then ground-dwelling tail-less bipedal apes. It’s easy to imagine that if one saber-tooth tiger had killed one too many of our ancestors, humans would never have evolved, we’d be extinct. And it is many times more likely for a human-like creature to re-evolve via convergent evolution here on Earth than on some other planet, even if millions of planets have complex life.

Think about bipedalism. There have been hundreds of bipedal organisms in the history of our planet, but NONE of them looked much like a human. There have been lots of large brained creatures in the history of our planet, but none of them were much like humans, except for our ancestors and sister species. An intelligent alien is just as likely to resemble a squid or an elephant or a dinosaur as it is to be a hairless bipedal ground-dwelling but former-tree-climbing ape. Which is, not very likely at all. It’s likely to resemble NOTHING ON EARTH.

And that’s not even getting into the biochemical issues. All life on earth is related, and has the same basic biochemical machinery…DNA coding for RNA, ribosomes that create protein from RNA, only 20 amino acids out of the thousands possible, phospholipid bilayer cell membranes, and on and on. We have no idea how much of this machinery is required for life, and how much is simply due to how the first living thing arose on earth. But it’s pretty likely that any alien life would have a radically different biochemistry from earth life.

I’m not sure that you can make the assumption that DNA is as random as you would presume. It seems to me, that since so little is known about abiogenesis, it can’t be assumed that early life-forms on another planet wouldn’t be exactly as they were on earth. From a chemistry standpoint, it makes sense that molecules would “gravitate” towards the lowest energy configurations.

We do know that only changes in DNA that result in an evolutionary advantage will survive. It’s likely that certain mutations are more common than others. I tend to think that evolution is fairly ordered. That is not to say that for every planet with a reasonable biosphere humans with 46 chromosomes two arms and two legs would evolve, but ultimately intelligence is an evolutionary advantage. Certainly four legs is an advantage for survival early in evolution, and this ultimately leads to two arms and two legs. Maybe there is an advantage to 46 chromosomes.

Nevertheless, with all the possible permutations, the possibility of getting something that we would define as human is pretty negligible.

Mr. Slant, I am not sure but I think he means completely human.

The energy configurations is true, but the rest isn’t. And from what you say afterward you’re making the mistake of thinking that these energy configurations must lead to a common product.

Probably not. We know that simple amino acids are easily formed, even outside of conditions that would support life. There are 20 amino acids that earth life uses, but we know of others that exist. We don’t know enough about the transition to say for sure, but there seems to be no reason at all that the progression from amino acids to proteins to reproducing molecules to DNA must take place in the way it did on earth. That’s one reason why intelligent design/irreducible complexity arguments fail. Once life does form it seems reasonable to assume that it will outcompete any newly formed alternate life, so there is no niche for that. On another planet, however, that original niche could be filled by anything that allows for reproduction of genetic material. It does not have to be like our life in any way.

Not true. Neutral changes will also survive. Most of our DNA is non-functioning as far as we now know, although some hints that the supposed non-functioning sections may have roles to play.

We don’t know any of these things are true. We don’t know how common any mutations are. We don’t know that intelligence is an evolutionary advantage (though we’re certainly disposed to think so). We don’t know that four legs is an early advantage and we don’t know that this leads to two legs. We don’t know that 46 chromosomes is better than any other number. You’re being biased by a sample size of one.

At last, this is something I can agree with.

If the universe is truly infinite, then the chance that there are other humans out there, independently evolved but fully genetically compatible with us, is 1. But our chance of ever meeting them is close to zero.

Yes, but the odds that there are independently evolved exact duplicate humans who meet independently evolved exact duplicate humans is 1. But the odds that we’ll be either one of those instances is near 0.

Of course, it doesn’t seem as if the Universe IS infinite in space and time, so it’s kind of moot.

Back to the biochemistry argument.

Sure, we have life on planet Earth, and every instance of that life on planet Earth uses the same biochemistry. But the trouble is that we’ve only got Earth life to go on, we really have no idea how likely it is that a second biogenesis would create life with similar biochemistry. Maybe it is extremely likely, maybe it is extremely unlikely, but it seems to me that we have no good reason to believe that non-Earth life would have exactly the same features. Even if it used DNA or a molecule extremely similar to DNA, I can’t imagine that the exact same codons would code for the exact same amino acids. Except every life form on earth uses exactly the same DNA code for exactly the same 20 amino acids.

I’d like to suggest that a good place to start looking for answer to the OP’s question would be the Drake equation. The intention was to calculate the likely number of extraterrestrial civilizations in our galaxy with which we might come into contact. While it doesn’t offer any definitive answers, it’s a good starting point for an informed discussion of the subject.

Another reasonable launching point would be the Fermi paradox, which argues that the overwhelming lack of evidence for extraterrestrial life suggests it just ain’t out there.

I remember reading research from one professor (I think at UC Riverside) where he had changed the DNA simply by changing the connection points from 3’,5’ to something else. I think it may have been 3,5. The result was DNA that had a larger helix. As a result, the DNA could not hydrogen bond to form the double helix. In this example, it appears that even a minor change in the structure of DNA resulted in a molecule that was a dead end. I suspect that DNA is organized the way it is, because that is the most likely arrangement that is not a dead end.

One more thing we agree on is that we don’t know. Since we don’t know, we can’t make any assumptions. I am only basing my judgments on a sample size of 1, but it is the only sample we have. I know of no land vertebrates that have evolved with more than 4 limbs. I’m guessing that there may be a reason for this, but I am just guessing. I see no reason to believe that your guess is better than mine.

It doesn’t give the odds that the beings are human, but Drake’s equation is pertinent here:

N = R* x f[sub]p[/sub] x n[sub]e[/sub] x f[sub]l[/sub] x f[sub]i[/sub] x f[sub]c[/sub] x L

The values of all those variables is almost entirely speculative. You’d have to add at least one more variable to that for the human-ness of the species.

Hey, where were you guys when I got flamed by fanboys in a “What don’t you like about Star Trek” thread (or something of that nature). I said that I couldn’t get around all the humanoid species from other planets whose only differences might be forehead ridges or elf ears. I went on to talk about even if a planet had exactly the same history and environment, the chances that a human would come about would be negligible.

Man, it was like I touched an exposed nerve. Of course, they brought up convergent evolution more than once. I just sighed and moved to the next thread.

Odds were heavily against humans evolving on this planet. Odds are against any animal, plant, amoeba or what-have-you that is selected in advance ever apprearing anywhere.

I just looked this up. It was professor Christopher Switzer at UCR. It turns out that it was RNA that was being modified. Apparently he has been somewhat more successful since I met him in 2001. Nevertheless, I still tend to believe that DNA was made the way it was, because that is the most stable and easily formed structure that exist. I don’t see why my guess would be any less valid than anyone else’s.

We are more than the sum of our mutations. There are several events during our history, much less the history of life on Earth in general, wherein things could have gone very differently. No terminal-Cretaceous asteroid? Mammals might well still be living in the shadow of giant dinosaurs (or, at least, probably not evolved to the point where we could appear). Or, if the (or another) asteroid struck in a different place / time, it could very well have wiped out the first humans.

Geological time is punctuated by numerous mass extinctions, each of which paved the way for adaptive radiation by the survivors. In order for honest-to-Og Homo sapiens to evolve on another planet, the entire evolutionary history of life on that planet would have to play out virtually the same as it did here. Even the structure of our planet has played a role in our history, as the motion of the plates has resulted in changing oceans, which in turn has affected the climate. So, we’re talking a planet that pretty much has to be Earth in order to evolve the same life forms.

you can also take the point of view, that no matter what order or sequence these events occured, evolution would ultimately lead to the “low energy” result. Ultimately, an intelligent bipedal mammal may be a very likely possibility no matter how events occured.

Even then, if the Great Green Google-Eyed Ones raced down on their Cosmic Hot Rods, scooped up some primitive life, and dropped it on another more-or-less Earth-like world, we’d still not expect something reproductively compatible, or likely even more than crudely similar, to humans. Even saying that it would require “a zillion different random mutations” understates the unlikelyness of this; these mutations (and the external pressures to make them selectively optimal) would have to occur in order. The odds of the same sequences of DNA–including non-nuclear DNA in mitochondria and elsewhere–being compatible are beyond astronomical, even above and beyond the presumed rarity of worlds which went through an Earth-like development and ecosystem to begin with.

Citing convergent evolution is a parlor trick; convergent evolution gives similar end results for the same problem–say, vision or aquatic mobility–but these aren’t necessarily the only solutions, nor is the result the same in the details. The vertebrate camera eye and that of sophisticated cephalopods like squid and octopus are dramatically different in construction, even though they fulfill the same essential function. And these are creatures that evolved in the very same environment; unrelated species evolving on different worlds, with different competitor and prey species are likely to have very different solutions even to similar problems.

Here’s what we can say about possible intelligent, corporeal alien life: they’re likely but not assured to[ul][li]be homothermic, and capable of adapting to a variety of environments,[/li][li]be tool users with some kind of gripping appendages,[/li][li]have the senses of vision, touch, and taste/smell,[/li][li]use respiration (rather than fermentation) to process energy,[/li][li]have a cellular physiological structure, organized into discrete organs for various bodily functions,[/li][li]have a complex central nervous system with a substantial portion dedicated to signals processing, communication, and conceptual thought,[/li][li]and have a vast library of sensory information dedicated to the preparation of food and table settings.[/ul]There’s no guarantee that they’ll be bipedal, even if they are something like a mammal that evolved on savannahs, and indeed, bipedalism has some substantial drawbacks, the offseting advantages of which are probably unique to human development and unlikely to be arbitrarily reproduced. [/li]
The only reason aliens depicted in science fiction television and movies are largely humanoid is because it’s both far cheaper, and far easier for viewers to relate to. This has absolutely no bearing on the likelyhood of human-like life evolving elsewhere.