Some UFO skeptics suggest that terrestrial life is the only life in the cosmos, therefore, whatever UFOs are, they can not have anything to do with aliens. They advocate not just a 'rare Earth' hypothesis, but a 'unique Earth' hypothesis. That's despite the fact that there are 'billions and billions' of possible sites in the Universe where life could take hold, evolve and ultimately boldly go.
A few UFO skeptics do tone down that argument by acknowledging that extraterrestrial life forms exist as in extraterrestrial microbes, plants and multi-cellular animals but that extraterrestrial intelligences don't exist. That still means that UFOs have bugger-all to do with aliens. Even if extraterrestrial intelligence exists, only humans have invented technology, and even if aliens have invented technology, well those dumb alien bastards exterminated themselves within a short time frame after discovering chemical, biological and radiological warfare technology. The upshot is that extraterrestrial intelligences with a suitable 'boldly going' technology that have in fact survived to 'boldly go' have a mathematical value of zero; UFOs as a manifestation of these aliens therefore can not be and other more prosaic explanations must be found.
So are we alone in the Universe? That's a question that's been asked by millions of philosophers, scientists and the general public over the eons, without, to date resolution. Of course the word 'alone' implies alone in the sense of whether or not there exists elsewhere in the cosmos our rough equals; more likely as not our betters since humans have been around for only a tiny fraction of cosmic time. We want to get to know our neighbours across the street, not their pets, or their plants. The standard gut-feeling answer to the question 'are we alone' usually revolves around how vast the Universe is, and surely, given the billions and billions of stars in our galaxy and the existence of billions and billions of galaxies each with billions and billions of stars, coupled with the vastness of cosmic time, surely we can't be the proverbial 'It'.
There's unfortunately one slight flaw in that statistical approach. There's a rather long chain of events that have to happen, hurdles to be jumped, in order to get from the chemical elements of star-stuff which we're made of, to our hypothetical biological cosmic neighbours. Depending on whom you talk to, that chain can be extremely long indeed. The point is, if any one factor in that chain of causality has a very low probability of coming to pass, it matters not one bit whether or not all the other factors are extremely probable, the overall result is going to be low. If any one factor is as close to zero as makes no odds, then the overall answer will also be as close to zero as makes no odds. Certainty multiplied by certainty multiplied by certainty multiplied by certainty multiplied by zero multiplied by certainty multiplied by certainty multiplied by certainty ultimately equals zero!
It's been pointed out by others, and I tend to have to agree, that astronomers (being physical scientists) tend to be much more optimistic and supportive of the notion that advanced life forms in the Universe - extraterrestrial intelligences - are a dime-a-dozen. That's relative to biologists (being life scientists), who considerably hedge their bets and who it must be said are presumably better qualified to pass judgments. So, taking things from a more biological perspective, what's what?
With 13.7 billion years to play with since the origin of our Universe (that Big Bang event); with billions and billions of stars in our own galaxy alone; with billions and billions of galaxies scattered throughout the cosmos each with billions and billions of stars therein, with extra-solar planets (potential cosmic real estate for E.T.) being discovered around many of those stars in our own galaxy at a rapid rate of knots, (and by implication planetary systems should exist in other galaxies as well); with the chemical elements required for life commonplace throughout the Universe; with the principles of Darwinian evolution given as universal, what odds that we are really the proverbial 'It'? And what are the implications for UFOs being the manifestations of extraterrestrials?
When it comes down to the UFO extraterrestrial hypothesis (ETH), it's only our own Milky Way Galaxy we need concern ourselves with. Even I acknowledge that though extraterrestrial civilizations probably exist in other galaxies, travel times between galaxies quickly exceed any logical transit times available, even when invoking a "Star Trek" warp drive. Interstellar travel, travel within the confines of our own galaxy however, is quite another matter. Confining our 'are we alone' search to our own galaxy still gives us some ten billion years to play around with; billions and billions of stars and no doubt planetary real estate, all those abundant chemical elements, and Darwinian evolutionary principles. It would be a very brave scientific soul to suggest, given those sorts of statistics, that we are, even in our own galaxy, the proverbial 'It'; not just the new kid on the block, but the first and only kid on the block.
Not even a UFO ETH skeptic like a certain SETI (Search for ExtraTerrestrial Intelligence) scientist of my acquaintance would argue we're the proverbial 'It' - it would make a mockery of his own chosen career path.
So in summary to that first objection that only terrestrial life exists: 1) The Universe is a bio-friendly Goldilocks Universe - we're here after all. 2) There is plenty of real estate in the cosmos that could give rise to and host hardy microbial life forms to kick-start things off. 3) The appropriate chemicals, organic chemicals, and biochemicals; appropriate life producing and sustaining chemistry full stop, are present throughout the cosmos. 4) There's been a massive amount of time for life to originate, evolve, survive, thrive and migrate.
Ultimately, what this all boils down to are three key points: 1) whether or not extraterrestrial intelligence exists, and if so, 2) does extraterrestrial technology, technology that can get E.T. from there to here, evolve of necessity? 3) Lastly, having evolved a sophisticated advanced boldly going technology, how long do you have it for? Translated, we need to answer whether or not Darwinian evolution, natural selection, will favor intelligence, technology and long-term survival.
Intelligence First
Some people object to the UFO ETH on the grounds that we (humans) are the proverbial be-all-and-end-all of the cosmos in terms of overall IQ smarts and being tool makers - there are no other advanced extraterrestrial civilizations, therefore UFOs can not have anything to do with extraterrestrial intelligence. Translated, they adopt the more religious point of view that humans (and human intelligence) were created in the image of God and therefore no other intelligences can exist. No alien intelligence means no-go to the UFO ETH.
An answer to that issue, that extraterrestrial life exists, but not extraterrestrial intelligence (for religious reasons or otherwise), is yet again that not even respectable SETI scientists would propose this as an objection to the UFO ETH since again that would undermine their own work. Clearly the evolution of intelligence, albeit being just one of many competing traits for biological survival-of-the-fittest, does have ultimate survival value. The Earth provides a practical example of that. Many species can be attributed to having a reasonable degree of ability to figure things out. It is possible to evolve extremely high levels of intelligence. The proof of that pudding is witnessed by we humans existing. If Mother Nature can evolve one biological highly intelligent species, She can do it again, and again, and again on other worlds.
Now based on a statistical sample of one (terrestrial life), it's been a long tough road to get from microbes to ferns to jellyfish to sharks to newts to crocodiles to crows to cattle. Once you have multicellular critters (like ferns and cows) that have survived and thrived in a reasonably stable part of the Universe over many generations, will they evolve intelligence? I mean finding an extraterrestrial equivalent of a trilobite is all well and good, but we want to find neighbours more like ourselves. Again, no alien intelligences translate into UFOs having zip to do with aliens.
The issue now is having evolved to a multicellular stage (like trees and magpies and buffalo), will organisms develop some higher brain function? Is there any further evolutionary advantage towards increasing one's intelligence? By going back to our sample of one, if Earth is any guide, the answer is roughly 'not likely'. There are millions of multicellular species that have existed, and do exist, on Planet Earth. There are apparently only a very few species that have evolved something beyond the minimum level of brain power required for their day-to-day survival. That doesn't inspire confidence that intelligence has inevitable value as a means of survival.
By far and away, most multicellular critters just operate on pure instinct and don't (can't) stop to figure things out (far less stop to smell and appreciate the roses) - but, there are a few exceptions. Many wild birds would put our everyday companion animals like dogs and cats to shame in the IQ department. I mean I adore my cats, but neither is a little furry Einstein. Whales and dolphins have also been credited with being in the higher IQ bracket; ditto our close primate cousins. In the invertebrate kingdom, the octopus is pretty smart - by invertebrate standards (and then some if one is honest). However, on balance, most multicellular critters put their evolutionary strategies into something other than higher brain functions. Take my cats. Is it to their survival advantage to 'figure things out' or to just be a bit faster afoot; have a bit more acute hearing; have sharper vision? Nearly all organisms put their survival abilities into something other than pure brain-power. Clearly brain-power has survival-of-the-fittest attributes. But, intelligence is not the only game in town, and therefore doesn't have what I'd call any evolutionary 'certainty' or destiny. However, it would be illogical to say that developing intelligence, the ability to figure things out, isn't valuable and doesn't have any survival value; it's just that if you were to list all the multicellular animal species on Planet Earth, very few would have an IQ of even one (the human average is 100). So, let's say evolving intelligence on another planet once having reached the stage of becoming multicellular is somewhere between near certainty and highly improbable. That's a rather 'have your cake and eat it too' position.
IMHO, the bottom line is that intelligence, the ability to figure things out, has evolutionary survival value and will tend to be selected for, and thus over time, there will tend to have life forms that have evolved ever higher IQ's. Here on Earth, just about all mammals and birds, and some exceptional invertebrates (the cephalopods like squid and the octopus), have reasonable IQ's at least when compared to bacteria, plants, insects, fish, etc. Of course just as some kinds of organisms are faster than others, or have keener senses of sight or smell or hearing, not all advanced organisms are going to end up equal in the IQ stakes. But, the fact remains, the ability to think, to figure things out, can only increase your odds of survival and leaving behind more offspring.
Technology Second
Okay, we have lots of widely separated planetary abodes throughout the cosmos that have an intelligent alien species on them. Since we assume your intelligent alien neighbours are fairly far away and you want to discover them, and then maybe communicate with them, that poses a problem. If you want to find them, you or your surrogate, has got to go to them, and/or they or their surrogate have to come to you. In a terrestrial analogy, you have a barrier like an ocean or vast desert or mountain range separating you from them so it's difficult to hike or swim the distance. The surrogate mentioned earlier could be a smoke or radio signal or laser beam, but if you want something more up-close-and-personal then you tend to need boats or planes or four-wheel drives or covered wagons, or in our interstellar scenario, rocket ships. Once you do establish 'first contact', you'd like to keep in touch. On Earth, the usual means of keeping in touch other than by face-to-face communication is by snail mail, phone, or email - snail mail apart, its radio or electromagnetic communications in general that's usually employed (even smoke signals use reflected light waves to deliver the message).
That introduces one additional complication for the UFO ETH; it's not enough to just be intelligent. You need to have technology (and even snail mail as noted above is still a form of technology). Then, and only then, will the 'are we alone?' question be answered to our absolute satisfaction. That's initially what SETI is all about. We need technology if we are to find (maybe communicate with) extraterrestrial intelligence(s); and/or extraterrestrial intelligence(s) will need technology to find us. One or both of us has to have invented engineering to a somewhat sophisticated level - maybe rocket ships, maybe radio telescopes, but something technological is required.
There's also a hidden assumption here - you actually want to seek out new civilizations. It matters not one jot if you have all the required technology but care not to use it for the purpose of answering that question - 'are you alone in the Universe?' I'll assume here that if you have intelligence, and you've been able to construct appropriate technology, then part of your intelligence is devoted to being a curious critter who wants to know and find out things - so that's a certainty of one. Curiosity and associated intelligence, or intelligence and associated curiosity are two sides of the same coin.
But what's the level of certainty of developing appropriate technology in the first place? Rather poor judging from all those terrestrial species that have a somewhat reasonable degree of smarts to their credit. There's the human species of course, and though while we're not quite a sample of one, there having been other hominoid species with some IQ capacity (like Neanderthals), its pretty close to being a sample of one. There are documentary observations of some animals (primates mainly) not so much manufacturing, but making use of existing 'tools' to assist in their survival. Alas, most intelligent species lack the anatomy and/or the right environment to manipulate objects. In the case of dolphins and whales, their ocean environment stymies any way and means of constructing things and making use of fire, for example. So, developing technology has to be rated, judging from our terrestrial sampling, as rather low; otters using rocks to break open clams not withstanding.
So, for UFOs to be alien spaceships, one needs an extraterrestrial intelligent species to develop appropriate technology, and here's where I see a bit of a bottleneck. The evolution of technology isn't inevitable and have a lot of just-so factors attached.
Firstly, your home planet has got to come equipped with the right sorts of materials like oxygen and metallic ores and other objects (rocks, wood, etc.) than can be turned into useful tools, and of course most important a suitable supply of energy sources. That you'll have at your disposal all the required material and energy resources is not a given.
Water worlds are out of the running since it's difficult to discover and utilise fire in that sort of environment.
You can't have all your required locomotive appendages (legs) in contact with the ground - some limbs have to be free to manipulate objects in your environment. Birds have wings that are off the ground, but since wings aren't good at making tools, that seems to rule out wings, and all birds of a feather, pretty much as well as tool makers.
So, I've already ruled out dolphins and whales and the cephalopods (like the octopus) being water based creatures; the birds with their useless wings as far as building things is concerned; and all the four-footed walking mammals (or reptiles or amphibians).
It might be conceivable that you can build up a technology using your mouth parts and/or using a tail (if you have one) to manipulate and build things, but we don't have obvious terrestrial case studies, although you might argue that bees, wasps, termites, ants and birds can build elaborate structures using just their mouths. So that's in the 'maybe' basket.
Technology is also a double-edged sword. The use of technology has had obvious survival value for the human species. You wouldn't be hard-pressed to come up with dozens of technological inventions that have enabled us to survive longer and thrive better and be ever more fruitful and multiply. But, our technological genie is also out of the bottle, and unless you're a hermit, you will have noted by now that technology can also reduce our quality of life, and no doubt you wouldn't be equally hard-pressed to give dozens of examples, from handguns to the automobile - which leads nicely into the last consideration.
Longevity Third
Lastly, there's the issue of longevity. If your neighbours buy up and move in, but then sell up and move out again in less than 24 hours, that doesn't allow much time to meet and greet them and gossip over an afternoon cup of coffee - blink, and they're gone. But if you're both on the block for twenty years, that allows lots of time for afternoon teas, philosophical chats, bridge games, etc. So, how long do technological civilizations last?
Well, the pessimist will look around and note global warming, probably antibiotic resistant germs giving rise to pandemics, chemical, biological and radiological warfare and/or terrorism, the extinction of biological species, rampant industrial pollution, and in general an overall quality of life heading rapidly down the drain, right down to the point that the human race will probably go extinct by our own hand. But if you're an optimist, then the sky's the limit.
It's not all that obvious that technology actually adds all that much value towards ultimate long-term survival. Lots of technological advancements have, like controlling energy sources such as fire, developing a sustainable food supply via agriculture, the rise of modern medicine and food preservation technologies. But then lots of modern technological wonders, the automobile, CDs, sofas, microwave ovens, and thousands of other consumer products don't really contribute much to our overall survival - certainly cars don't when considering the road toll! That brings up the fact that things technological can sometimes work in the opposite direction. Toxic this, pollutant that, nuclear the next thing; then throw in a bit of global warming; the rise of urban city living with overcrowding and in general overpopulation; chemical, biological and radiological warfare/terrorism; instruments of warfare in general, like guns; the overuse of antibiotics hence the rise of antibiotic resistant germs; exposure to electromagnetic fields - well, the list of horrors or potential horrors keeps on keeping on and on.
It makes for an interesting question: would mankind ultimately survive longer had technology never entered the equation, or not? It's an unanswerable question in that 1) we can't run the contrary as a controlled experiment, and 2) that the genie is well and truly out of the bottle and there's probably no turning back now.
Assuming humanity as a collective whole doesn't end up going the way of the Dodo within the next several generations, even centuries - whether it actually morally deserves to go extinct is another question - then what?.
It's hard to imaging what human civilization, what humans themselves will be like 1000 years from now. If you could come back 1000 years hence, would you indeed find a human civilization, indeed find recognizable 'humans' at all? Once you have evolved to the stage of being a multicellular critter with intelligence and advanced technology, then physics, chemistry and plain everyday evolutionary biology are no longer in control of your evolution. You are now in control! You are in control not only over the future evolution of other species (artificial selection instead of natural selection) but of your own evolution. The age of the designer baby is already here, albeit still in its infancy (pun intended). What will another few decades bring to this now embryonic field; obviously one with an ever ongoing and continuing maturity?
Humans will probably go kaput within 1000 years, not because of any global nuclear war, or pandemic, or asteroid strike, but because humans have by their own free will evolved themselves into something else, and the process has already started. In fact, it's possible that in 1000 years time there could be two humanoid species on Earth. One will be an amalgamation of flesh-and-blood plus 'iron-and-silicon'; the other pure 'iron-and-silicon' (artificial intelligence, perhaps in the form of robots).
The first is not too difficult a swallow. Just replace or augment flesh-and-blood bits with 'iron-and-silicon' bits (or wood bits, or ceramic bits, or plastic bits, etc.). Look at most pirate films and you'll see those beloved peg-legs and hook-hands. Do you wear glasses or contact lenses? What about a hearing aid? Perhaps you have an artificial joint(s) or a heart pacemaker. You surely have a dental filling (or two), maybe even dentures. Then there's artificial skin and all manner of other internal or external types of technology that have replaced your failed flesh-and-blood - like kidney dialysis. There's now serious talk about the development of a bionic eye within a few years (to go alongside the bionic ear). What further artificial bio-bits will be available in another 20 years, another 50 years, or another 200 years? The era of "RoboCop" or a real life "Six-Million Dollar Man" (and "Woman") is getting close to fruition.
Within 500 years or so, maybe less, I can envision that someone will be able to download the contents of their brain (their mind) into an 'iron-and-silicon' equivalent. Why? Well, does the word 'immortality' (or as close to immortality as makes no odds) suggest a possible reason? You don't think anything of endlessly replacing worn automobile parts for new parts to extend the useful lifetime of your car. Why not endlessly replace your worn parts? Your mind, that 'inner you' housed within your brain won't last forever. Replace it - transfer it to a more durable technology. Do it again, and again and again as is necessary. In fact, one might create a mega-mind or super-mind by merging into an 'iron-and-silicon' body containing a lot of minds (in much the same way as computer hardware can have a lot of operating software programs. By merging the minds of say a cosmologist, general relativist, quantum physicist and mathematician, one might speed up the development of the Holy Grail of physics, the Theory of Everything (TOE) - which is as currently conceived, a theory of quantum gravity.
Once your mind is contained in an 'iron-and-silicon' 'head', just attach that to an all 'iron-and-silicon' 'body'. Then boldly go where no 'iron-and-silicon' human has gone before. Immortality indeed!
All of which leads to a future Earth inhabited by a humanoid robot species, artificially evolved from today's human species. That process too has already started - robotic appliances, even artificial 'iron-and-silicon' 'pets' are now on the market. Research into artificial intelligence is ever ongoing. Watch that final minute of the final episode of the TV revision of "Battlestar Galactica"! How about those sci-fi "Transformers" or "Terminators", or Data (from "Star Trek: The Next Generation")? Then there's "Doctor Who's" Cybermen or his main enemy, the Daleks (though Daleks are part machine; part organism).
Think of those robots from "Westworld" or the "Futureworld" sequel where nothing can go wrong, go wrong, go wrong, go wrong, go wrong... Then there's "The Stepford Wives", "Cherry 2000", the original Cylons from the original "Battlestar Galactica", and many more. It might be just science fiction today - could it be science fact tomorrow? There doesn't seem to be any violation of physics involved. In physics, everything not forbidden is compulsory! However, some of those sci-fi scenarios suggest that perhaps ultimately there might be a conflict between the (part) machines we become, and the (artificially) intelligent machines that we create!
We're ultimately in control, so a quasi-robotic future isn't of necessity compulsory. But I suspect it will happen. Why? There are rational reasons for humans deliberately abandoning their flesh-and-blood existence and evolving themselves, if not 100% into 'iron-and-silicon' then at least into something part flesh-and-blood coupled with part 'iron-and-silicon' - sort of like we have today (recall those now primitive dentures and peg-legs).
Quite apart from immortality (well quasi-immortality anyway) arguments, its nice having more indestructible bodies and bodies that can be more easily repaired. Death won't go away of course, not totally - accidents will still happen. Presumably, your mind will be able to absorb 10, 100, 1000 times the amount of experiences, memories, knowledge, etc. than is currently the case. You might be able to explore environments now closed to you, like taking a stroll across the sea bottom - many kilometres down - in your 'iron-and-silicon' robotic 'birthday' suit.
All of which then opens up the entire 'boldly going' experience hinted at above. What's the hardest part of going to Mars? - it's the flesh-and-blood frailty of the human body - the need for gravity, oxygen, organic food, water, space suits, and that you can't carry spare flesh-and-blood parts along. Extrapolate to our exploration of the entire solar system, then our stellar neighbourhood, eventually the galaxy. Even if you don't want to go yourself, well, there's artificial intelligence housed in perhaps nanotechnology bodies, spreading throughout the cosmos like so much a cancer analogy.
The ultimate point of all of this is that if eventually us (humans), why not them (extraterrestrials) now? Translated, after a relative short period of biological development, a civilization can obtain longevity that evolutionary development into 'iron-and-silicon' provides, coupled with far easier expansion into the realm of outer space.
So, overall, UFOs might not be alien spaceships right here and now, because it's 1) somewhat relatively hard to evolve multicellular organisms (but obviously not impossible); 2) will intelligence tend to have evolutionary survival? 3) Associated advanced technology isn't inevitable and might even be counterproductive. 4) If counterproductive, longevity is at risk. Thus, Earth, with its multicellular critters and humanity with its technology, might be quite the rare planet within the Universe - according to some.
But, there's a catch. There's always a catch. What ultimately undermines the UFO ETH sceptics is that all you need is ONE boldly going, intelligent, advanced technological and long-lived extraterrestrial species and the galaxy is theirs for the taking and we're in their sights. Few pundits would like to bet against that ONE, given, in the immortal words of the late Carl Sagan, a statistical possibility of 'billions and billions'.
