Here’s a somewhat related question that just occurred to me. I know that interplanetary missions are launched at the times when they can take advantage of the motion of the target planet to shorten the journey. Would an interstellar mission take the relative motions of stars into account as well, or would the power needed to even contemplate such a mission make that calculation more or less irrelevant?
Eucaryotic cells probably did evolve just once, but that doesn’t mean that the eucaryotic cell as we know it on Earth is the only possible path to complex cells and multicellular life. And as has already been pointed out, sexual mechanisms exist in many procaryotes and long precede the origin of eucaryotic cells.
Granted. It’s just the only way it’s happened on Earth.
True, but it’s a pretty different kind of sex. In eukaryotes, sex dictates different roles for what each parent contributes to the offspring. In prokaryotes, it simply defines a dichotomy of plusses and minuses. The dichotomy may improve distribution of beneficial alleles (in either case), but for prokaryotes, that’s about all it accomplishes.
If I’m not mistaken, with prokaryotes, an individual is either F+ or F- separately for each sex-discriminating plasmid, unlike prokaryotes, where the entire organism is either male or female (with the usual caveats for hermaphrodites, of course – but in any given mating, the organism is either contributing as male or as female).
So, my point is that the sex that eukaryotes exhibit is (as argued by Lane) intricately linked with the fact that it has mitochondria (and possibly other organelles with their own DNA), whereas the sex of prokaryotes is, well, it’s all over the map, like so many things with prokaryotes. Heck, we don’t even know where one species ends and the next begins. Many nice neat definitions that work so well (most of the time) for eukaryotes don’t really apply to stuff like bacteria. Regarding sex, bacteria aren’t limited to 2 of them. IIRC, there are some plasmids with over a dozen “sexes”.
Even if all this is true (and there are some points I would dispute), why is differentiation into two sexes such an evolutionary advantage? The procaryotic system (or systems) are more varied and flexible, and permit very rapid evolution.
Yeah, that’s an unsettled question.
Here’s one fairly easy to grasp article covering a number of arguments, none of which is quite compelling, and which doesn’t mention Lane’s hypothesis. You’d have to read Lane’s book to get the benefit of his argument, which I would be able to do justice to. The paper I cited pretty much ignores prokaryotes, and mostly applies to eukaryotes which either reproduce sexually or asexually without genetic exchange. The bacteria model avoids the cost of the sexual model and gets many of the benefits of the eukaryote sex model without most of its disadvantages. Perhaps that’s why it dominates, for bacteria. 
Part of Lane’s point is that you don’t get multicellular organisms of any significant complexity without eukaryotes. I don’t recall his reasons for that. IIRC, it’s a bit of a chain argument: for complexity you need apoptosis; for that you need mitochondria, and the latter work out only in the context of eukaryotes with sexual repro. Sorry Mr. Lane for my clumsy job relating your carefully constructed chain.
Sure, this is absolutely true, and there is some evidence that this may even be occurring in multicellular species via natural viral transfer of compatible gene sequences. But horizontal transfer is not as stable as the variability provided by sexual reproduction, where combinations of specific nucleotide sequences which provide beneficial phenotypical characteristics can create discrete new genes which are then relayed in toto to future generations and possibly reinforcing other phenotypes. This allows for high fidelity reproduction of complex forms, such as feathers plus gliding appendage surfaces forming wings. The fact that there are no complex animal or plant forms which are not derived from sexually reproducing ancestors argues that such a regulated manner of discrete variability is a necessary condition for complex life, at least as it exists on Earth. This may not be true of a genuinely alien form of life, but there is an argument to be made for considering it as a likely condition
See Bernstein, H. and Bernstein, C, “Evolutionary Origin of Recombination during Meiosis”, BioScience, July 2010. The commonality in how meosis occurs in all sexually reproducing species is similar enough that it is extremely likely that all sexually reproducing species have a common ancestor. The advantages of sexual reproduction for relatively simple eukaryotes, however, don’t seem to justify the complexity and the fact that you essentially have two codependent forms of a species. That, and my personally difficulties with dating, would seem to argue that sexual reproduction shouldn’t be necessary enough to have evolved and survived in the aggressive competition among eukaryotic species, and yet this and the subsequent differentiation of highly specialized cells serving a common organism, which is equally puzzling, have obviously been highly successful evolutionary paths, though clearly single cell organisms have also been successful, just not at building skyscrapers and manufacturing shoes with glowing lights in the heel.
Stranger
I’d say given the size of the universe, the probability that a planet other than Earth has evolved intelligent life is virtually 100%. Unfortunately, as far as we know, light-speed is the universal speed limit, and anything physical can’t hope to get anywhere near that. So unless we discover mass effect technology, hyperdrive, warp speed or some other sci-fi tech, it seems unlikely we’ll ever actually encounter extraterrestrial life.
If you look at the original question and analyze it reading closely, it asks, ‘is there off-world intelligence?’ (besides Earth).
To answer this we have to ask ‘what is intelligence’ i.e. what meets our standard and what causes ‘intelligence’ as a trait.
Generally, it’s assumed that something is intelligence if it is self-aware, and is guided by more than instinct. But it’s probably a safe assumption that the OP is asking if there is ‘human-like’ intelligence on other worlds.
Therefore we have to consider how does ‘intelligence’ evolve? Does nature select for it as a trait.
If you look at various life forms on Earth, you have to come to the conclusion that in general evolution selects for size, and bigger teeth and claws over intelligence. It was only when we had a near extinction event that small mammals were able to exploit niches left by the extinction of larger dinosaurs. And to best exploit these niches, mammals seemed to evolve traits that lead to intelligence.
So, if nature doesn’t naturally select for intelligence (unless there’s something catastrophic like a near extinction event), it means that human like intelligence is probably a rare event.
Now, we have to consider what a ‘rare event’ is in a Galaxy that has about 300 billion stars. If 1% of those stars have planets in the ‘goldilocks zone’, i.e. favorable for development of life, then there might be 3 billion planets which could produce at least microscopic life. If 1% of those had favorable conditions that lead to the development of higher forms, you might have 30 million. If 1% of those remained habitable for long enough for mammals (or creatures which could possibly develop intelligence, given the right conditions), that’s 300,000 worlds. If only 1% of those had a near-extinction event then you might have 3,000 possible worlds where intelligence could be found. But then there’s other pressures and events and possibly random chance. You might end up with fewer than 30 worlds in a Galaxy the size of the Milky Way which could end up with life that possesses intelligence to rival humans.
Now, there’s one other factor and that’s ‘time synch’. In the 14 billion years that the Universe has existed, could the life that might develop on these handful of planets still exist during the time that is contemporary to our current time. To me it’s probably very unlikely that any intelligent or sentient civilizations are extant in the MW galaxy at this present time. In addition, in order for us to meet, given normal travel time, it’s also pretty unlikely that we would be able to meet in a normal life-time.
Given all that, the best answer is probably, ‘no, it is unlikely that there exists human-like intelligence in the MW Galaxy concurrent to our time, or capable of meeting us within a life time’.
At least that’s one way to make an analysis. It’s always possible that there are ‘outliers’ to any predictions.
Not really. The most successful creatures on the planet are the smallest, and lack teeth and claws entirely. And relatively high intelligence has evolved independently multiple times: Many mammals are fairly intelligent, as are the octopodes, but the common ancestor of mammals and octopodes barely even had a brain. This is not to say that everything will evolve to be more intelligent-- Evolution goes with whatever works, and there are plenty of stupid things that work just fine. But it’s clearly a strategy that evolution tries out every now and again.
And not only tries out, but even to the point that the requirements for high intellect (high nutrient demand, large cranial diameter making making childbirth difficult and painful) would seem to otherwise compromise the reproductive viability. Clearly, intelligence works at least some of the time, and it would be unexpected for a foreign ecosystem which is of comparable complexity to that on Earth to not evolve something akin to conceptual intelligence. However, what we think of as the hallmarks of higher intelligence, such as philosophy, literature, rhetoric, mathematics, et cetera may not have the same definition in some other form of life. Their poetry may be in the form of mathematical proof, or philosophy may be irrelevant. Rhetoric might be a particular configuration of colors and sound which compels other members of their species to automatically obey. The likelihood that we would be able to communicate with an alien intelligence on any but the most superficial levels is probably remote.
Stranger
I’d say the fact that it’s evolved independently more than 25 times (pdf link) kind of does make it a foregone conclusion…
One thing that’s always bothered me upon reading about the SETI program and the Drake equation, is that. . . basically, as far as we know, the Earth is the “Garden of Eden” and yet, we can’t really communicate in any substantial way with any other species on Earth. So, on the Earth, we’re only one of millions of species, to develop “advanced communications”. And when you combine that with the vastness of time and space, as Stranger mentions above, I think the likelihood of finding “intelligent” life out there is very small. But, I think animal might be much more common.
Exactly.
Not exactly. There have been at least three species that have evolved on Earth with intelligence comparable to ours. Why aren’t we communicating with the other two? Because we wiped them out long ago. Whether by deliberate warfare, or just plain outcompeting them in some other way (I’m fond of the idea that we beat the Neanderthals by being sexier than them), we decided that the planet just wasn’t big enough for the three (or even two) of us. Apparently, apex intelligence is a fairly narrow niche.
The Drake equation is a pretty well thought out version of your line of reasoning here, which applies to the galaxy and not the universe, and for one intelligence to detect the other. I wondered why the age of the universe didn’t appear, but it’s because the universe is way old enough not to matter with regards to light reaching from one end of the galaxy to the other.
If you tried to revise the Drake equation for the universe, you’d have to add a few complicating parameters, including the age of the universe and the time it takes to form compex elements (assuming life is chemical, which isn’t necessarily the case, but so far it’s all we know.) The Drake equation cleverly includes rate of star formation, and also cleverly eliminates the speed of light and age of the galaxy (assuming 1-way “communication”). Again, it would be more complex for two-way communication, and I believe the speed of light and a density function of the galaxy would be needed.
Regarding near extinctions, that’s not such a terribly rare event, having happened several times in Earth’s past.
The rapid variability of climate on Earth may have been a big factor promoting intelligence as a way of coping with rapid change, rather than relying only on good general strategies plus evolution. The Holocene period (in which agriculture and civilization arose) seems to be an unusually stable period, climactically. I guess we’ll find out whether intelligence is really very good at adapting to dramatic climate changes in the future. 