It's been pointed out by others, and based on my readings I tend to have to agree, that astronomers (physical scientists) tend to be much more optimistic and supportive of the notion that advanced life forms in the Universe (extraterrestrial intelligence) are a dime-a-dozen relative to biologists (life scientists), who 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?
For starters, our Universe is a Goldilocks Universe in that the fundamental laws, relations and principles of physics unite in such a way as to be ultimately bio-friendly. If the Universe wasn't bio-friendly, we wouldn't be here to comment on that. That's not to say however, in relative contrast, that many (most) parts of the Universe aren't overly bio-friendly. You'd be hard-pressed to survive and thrive in the depths of a stellar core, heading down a Black Hole, vacationing on the surface of a White Dwarf or in the hard vacuum of space itself. So, overall the physics of the Universe displays the physics of a Goldilocks Universe, but actually very few addresses within an overall Goldilocks Universe are really, by our definition, Goldilocks. However, starting with bio-friendly physics, where do we go from that point? Well, physics begat inorganic chemistry. That's step number one, and clearly that's easy because there's an awful lot of inorganic chemistry in our Universe.
Going from inorganic chemistry to organic chemistry isn't difficult either. Interstellar space is full of dust and gases made up of organic chemicals; ditto many of the planets and moons within our solar system (i.e. - Titan, a moon of Saturn) have organics being part and parcel of their composition, and comets, asteroids and meteors too can contain organic compounds.
Judging by how quickly organic chemistry turned into biochemistry (the origin of life) on the early Planet Earth, it's not difficult to generate simple proto-cellular to unicellular life forms if the conditions (adequate energy, temperatures, environments) are Goldilocks conditions.
Yet life, even microbial life, is still very, very complex (try making a microbe from scratch if you doubt it). The fact that life arose from scratch on Earth within a very, very short span of geological time after the planet formed is a bit suspect IMHO. But what if Earth were seeded by microbial life forms already in existence from space (or deliberately seeded by extraterrestrials as the Nobel Prize winner Francis Crick has proposed)? Now I realize that just puts off the origin of life question to another time(s) and place(s). However, given the vastness of the cosmos is far greater than that of our finite globe, and given that the cosmos existed for vastly longer periods of time before our sun, solar system and home planet came into existence, such additional time and space easily turns the improbable into a near certainty. And once established somewhere, then life could spread throughout that time and space, until it reached our young planet.
Earth arose billions of years after the universe and our galaxy had evolved, ample time for life to have arisen elsewhere, and seed the early Earth. This is the concept of panspermia. We know that comets, meteors, and the cosmic dust of outer space are chock-o-block full of complex organic molecules. We know that simple terrestrial life can survive the outer space environment if suitably shielded - and it doesn't take much to do the shielding. We know that surface bits from planets/moons can be ejected into space, carry a cargo of microbes, and land on another planet, even eons later with the microbes still viable. Of course 99.999% of all such microbial life will be doomed to forever wander in space or crash onto a cold, surface of a planet with no atmosphere or water, or plunge into a star, etc. But, sheer numbers will insure that now and again some microbes will land on a hospitable abode and be fruitful and multiple and evolve. The interesting bit is that if then, then now. And thus panspermia will be happening today. Certainly some meteorites which have impacted Earth have inside them 'organized elements' suggestive of microbial structures - the Murchison Meteorite from Australia is one such stone. The problem is terrestrial contamination as there are often lengthy time periods between their fall and subsequent discovery. As an aside, if Fred Hoyle & Chandra Wickramasinghe are correct (and I believe they are), microbes (bacteria and viruses) impacting Earth today are largely responsible for some select or various disease epidemics and pandemics, past present, and no doubt future.
On Earth, microbes rule, OK? The biomass of all the bacteria, etc. put together easily equals the biomass of every other multicellular plant and animal added together. And microbes can live in environments where multicellular critters fear to tread and often can't: from the coldest terrestrial environments, up to the near boiling temperatures, from deep underground to the heights of the atmosphere, from inside water-cooled nuclear reactors and the interior of rocks, to intensely saline, acidic and alkaline environments, to ecosystems where the sun never shines, like the abyssal depths.
They can even survive outer space. Bacteria survived on the surface of the Moon - on Surveyor Three. This was possibly the most significant discovery of the entire Apollo Moon program and it hardly even rated a mention. Astronauts from the Apollo 12 mission brought back to Earth parts of the unmanned Surveyor Three Lunar Lander. Terrestrial bacteria on those parts survived the lunar vacuum, solar radiations (UV, etc.), the massive temperature extremes, and lack of water and nutrients. Experiments since then in low earth orbit have confirmed that given just minimal shielding, bacteria can boldly go!
You'd be aware of how difficult it is to totally sterilize something, be it hospital equipment or a spacecraft bound for a Martian landing. They're tough - have you ever read about a mass extinction event where a bacterial species, unlike say the multicellular dinosaurs, went poof? Microbes are easy to transport. They can be blasted off the surface of the Earth, shielded from radiation by the debris, and survive to land on another world and be fruitful and multiply. There's little doubt that somewhere way out there, terrestrial bacteria have hitched a ride to the stars, bolding going where lots of microbes have gone before! Translated, I firmly expect that the universe is teaming with life in all sorts of places. The less than glamorous catch is that LGM is not going to stand for Little Green Men, but Little Green Microbes.
But now we come to our first and major bottleneck.
It took 0.5 billion years for the unicellular origin of life on Earth, or for microbes from space to take root on Earth, but then it then took nearly another 3.5 billion years between the appearance of that 'simple' single proto-cell life form and the eventual evolution and the resultant Cambrian explosion of multicellular (complex) life forms. For some reason(s), it appears to be biologically difficult to go from the simple single cell to a complex multicellular organism based on the only example we have to judge such things. Probably the overall obstacle to the early quick-smart establishment of multicellular (complex) life is that heretofore microbes (single cells) were 100% self-sufficient generalists. Complex organisms require the evolution of single cells to give up being generalists, become specialists, and work as part of a team. That degree of organization apparently takes lots of time, especially to the stage of where it can leave a large fossil presence, if it takes place at all.
Once you get to the multicellular stage, and in order to evolve further, well the trick is to survive, for the Universe is a dangerous place.
Planetary environments tend to be dangerous and rarely stable, and thus you need a lot of factors in place to ensure that even simple life even survives the long term and get the chance to evolve into multicellular life. Or, if you have evolved multicellular life, the odds are pretty good it's going to get the Big "E" - Extinction. I mean complex life is very vulnerable to environmental forces. A tornado probably isn't going to bother bacteria, but it sure could tear you apart. That's not to say bacteria can survive everything the cosmos can throw at them, but when nasties come, you stand a better chance of survival if you're a microbe. So, in order to get to the first proto multicellular critter, and from that to us, you need a lot of Goldilocks factors operating in your favour.
But there are apparently just too many planetary Goldilocks factors at play to grant the probability that complex, multicellular, animal, life is common throughout the cosmos. You need to be in a quiet part of the galaxy - no nearby supernovas, black holes to suck you in, gamma ray bursters, etc. You need a long lived stable single star. You need a gravitationally stable solar system so that planets are not gravitationally disturbed out of their orbit and either plunged into the parent star or cast out of the parent system altogether. You should have a good Jupiter(s) to absorb and/or gravitational deflect comets and asteroids that would otherwise slam into your otherwise environmentally-friendly planet causing havoc to established life forms. You need a planet that's in a pretty circular orbit, one that doesn't stray too close or too far from the habitable zone surrounding the parent star. The planet must have a fairly stable temperature range over geologic time periods, and so you must have an atmosphere, and thus has to be massive enough to retain an atmosphere, without being so massive that you end up with a brown dwarf - a quasi-stellar body. The axis can't have an extreme tilt, and it would greatly assist if the planet had a large moon around it to assist its long term stability. You need some sort of plate tectonics to ensure land building and the recycling of materials. If it's intelligence, with technology you seek, the planet can't be a water world. The planet must have formed in a region abundant in the heavier chemical elements (oxygen, sulphur, carbon, silicon, nitrogen, various metals, etc.). When you take all those factors (and more) into account, the number of suitable abodes where simple life can slowly evolve into complex life decreases rather quickly. And there's no guarantee that there is really any directed purpose to evolution in that evolution doesn't require simple life to become complex life. Survival and leaving offspring is what it's all about, and if single cell critters do that what more is needed?
One note about planetary disasters or catastrophes is that these cut both ways. They can't be frequent enough and/or large enough to wipe out the entire biosphere in total, especially the biosphere comprised of complex life forms, but on the other hand, infrequent small disasters can spur on evolutionary change by opening up environmental niches, but depending on who or what you are, when you are, and where you are, a disaster can be a double edged sword. I mean if you're a T-Rex sixty five million years ago, its bad news. On the other hand, without the bad news for T-Rex, we wouldn't be here, so for us, an asteroid impact 65 million years ago turned out to be good news!
It's also difficult to naturally transport complex life around the galaxy, unlike microbial life. If a meteor hit Earth and blasted a chunk of terra firma off towards Mars, pity the poor cockroach going along for the ride. Cockroaches are tough, but not that tough. And even if a cockroach did make it alive to the surface of, say Mars, it wouldn't survive long.
Anyway, once you have multicellular critters 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 beings more like ourselves.
IMHO, 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 evolved life forms with 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.
Finally, for SETI to succeed, for UFOs to be alien spaceships, one needs our intelligent species to develop technology, and here's where I see bottleneck number two. The evolution of technology isn't inevitable and has 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 metal ores and other objects than can be turned into useful tools, and of course a suitable supply of various energy sources. That's 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 in contact with the ground - some 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 birds of a feather pretty much as well
So, we've already ruled out dolphins and whales and the cephalopods being water based creatures, and the birds with their useless wings as far as building things is concerned, and all the four-footed walking mammals.
It might be conceivable that you can build up a technology using your mouth parts and/or using a tail to manipulate and build things, but we don't have obvious terrestrial case studies, although you might argue that bees and wasps and termites and ants and birds can build elaborate structures using just their mouths.
It's not all that obvious that technology actually adds all that much value towards ultimate long-term survival. Lots of technological advancements do, like controlling fire, developing agriculture, the rise of modern medicine and food technologies. But then lots of modern technological wonders, the automobile, CDs, sofas, microwave ovens, and thousands of other consumer products don't really contribute to our overall survival - certainly cars don't when considering the road toll! Then 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 and the rise of antibiotic resistant germs; exposure to electromagnetic fields - well, the list of horrors or potential horrors goes 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.
So, overall, SETI might not be able to detect our technological and communicating kin out there, and UFOs might not be alien spaceships right here, because it's 1) hard to evolve multicellular organisms, and 2) technology isn't inevitable and might even be counterproductive. Thus, Earth, with its multicellular critters and humanity with its technology, is quite the rare planet within the Universe - according to some.
The main proponents of what is now called the 'Rare Earth Hypothesis' are the scientists Brownlee and Ward (see below), and they have certainly stirred up an astrobiological hornet's nest with the idea. That's good for science in the long term. The belief in an unproved but accepted scientific proposition, in this case that that there are lots of complex alien critters out there, needs to be challenged if fields of inquiry aren't going to stagnate. However, make no mistake, it is the 'Rare Earth Hypothesis', not the 'Unique Earth Hypothesis', so religious fundamentalists who have taken this hypothesis to their hearts; who still need Planet Earth, and human beings, as some sort of religious special creation, should really not take comfort from these ideas. The Copernicus Revolution is still alive and well.
Further recommended readings:
For starters, our Universe is a Goldilocks Universe in that the fundamental laws, relations and principles of physics unite in such a way as to be ultimately bio-friendly. If the Universe wasn't bio-friendly, we wouldn't be here to comment on that. That's not to say however, in relative contrast, that many (most) parts of the Universe aren't overly bio-friendly. You'd be hard-pressed to survive and thrive in the depths of a stellar core, heading down a Black Hole, vacationing on the surface of a White Dwarf or in the hard vacuum of space itself. So, overall the physics of the Universe displays the physics of a Goldilocks Universe, but actually very few addresses within an overall Goldilocks Universe are really, by our definition, Goldilocks. However, starting with bio-friendly physics, where do we go from that point? Well, physics begat inorganic chemistry. That's step number one, and clearly that's easy because there's an awful lot of inorganic chemistry in our Universe.
Going from inorganic chemistry to organic chemistry isn't difficult either. Interstellar space is full of dust and gases made up of organic chemicals; ditto many of the planets and moons within our solar system (i.e. - Titan, a moon of Saturn) have organics being part and parcel of their composition, and comets, asteroids and meteors too can contain organic compounds.
Judging by how quickly organic chemistry turned into biochemistry (the origin of life) on the early Planet Earth, it's not difficult to generate simple proto-cellular to unicellular life forms if the conditions (adequate energy, temperatures, environments) are Goldilocks conditions.
Yet life, even microbial life, is still very, very complex (try making a microbe from scratch if you doubt it). The fact that life arose from scratch on Earth within a very, very short span of geological time after the planet formed is a bit suspect IMHO. But what if Earth were seeded by microbial life forms already in existence from space (or deliberately seeded by extraterrestrials as the Nobel Prize winner Francis Crick has proposed)? Now I realize that just puts off the origin of life question to another time(s) and place(s). However, given the vastness of the cosmos is far greater than that of our finite globe, and given that the cosmos existed for vastly longer periods of time before our sun, solar system and home planet came into existence, such additional time and space easily turns the improbable into a near certainty. And once established somewhere, then life could spread throughout that time and space, until it reached our young planet.
Earth arose billions of years after the universe and our galaxy had evolved, ample time for life to have arisen elsewhere, and seed the early Earth. This is the concept of panspermia. We know that comets, meteors, and the cosmic dust of outer space are chock-o-block full of complex organic molecules. We know that simple terrestrial life can survive the outer space environment if suitably shielded - and it doesn't take much to do the shielding. We know that surface bits from planets/moons can be ejected into space, carry a cargo of microbes, and land on another planet, even eons later with the microbes still viable. Of course 99.999% of all such microbial life will be doomed to forever wander in space or crash onto a cold, surface of a planet with no atmosphere or water, or plunge into a star, etc. But, sheer numbers will insure that now and again some microbes will land on a hospitable abode and be fruitful and multiple and evolve. The interesting bit is that if then, then now. And thus panspermia will be happening today. Certainly some meteorites which have impacted Earth have inside them 'organized elements' suggestive of microbial structures - the Murchison Meteorite from Australia is one such stone. The problem is terrestrial contamination as there are often lengthy time periods between their fall and subsequent discovery. As an aside, if Fred Hoyle & Chandra Wickramasinghe are correct (and I believe they are), microbes (bacteria and viruses) impacting Earth today are largely responsible for some select or various disease epidemics and pandemics, past present, and no doubt future.
On Earth, microbes rule, OK? The biomass of all the bacteria, etc. put together easily equals the biomass of every other multicellular plant and animal added together. And microbes can live in environments where multicellular critters fear to tread and often can't: from the coldest terrestrial environments, up to the near boiling temperatures, from deep underground to the heights of the atmosphere, from inside water-cooled nuclear reactors and the interior of rocks, to intensely saline, acidic and alkaline environments, to ecosystems where the sun never shines, like the abyssal depths.
They can even survive outer space. Bacteria survived on the surface of the Moon - on Surveyor Three. This was possibly the most significant discovery of the entire Apollo Moon program and it hardly even rated a mention. Astronauts from the Apollo 12 mission brought back to Earth parts of the unmanned Surveyor Three Lunar Lander. Terrestrial bacteria on those parts survived the lunar vacuum, solar radiations (UV, etc.), the massive temperature extremes, and lack of water and nutrients. Experiments since then in low earth orbit have confirmed that given just minimal shielding, bacteria can boldly go!
You'd be aware of how difficult it is to totally sterilize something, be it hospital equipment or a spacecraft bound for a Martian landing. They're tough - have you ever read about a mass extinction event where a bacterial species, unlike say the multicellular dinosaurs, went poof? Microbes are easy to transport. They can be blasted off the surface of the Earth, shielded from radiation by the debris, and survive to land on another world and be fruitful and multiply. There's little doubt that somewhere way out there, terrestrial bacteria have hitched a ride to the stars, bolding going where lots of microbes have gone before! Translated, I firmly expect that the universe is teaming with life in all sorts of places. The less than glamorous catch is that LGM is not going to stand for Little Green Men, but Little Green Microbes.
But now we come to our first and major bottleneck.
It took 0.5 billion years for the unicellular origin of life on Earth, or for microbes from space to take root on Earth, but then it then took nearly another 3.5 billion years between the appearance of that 'simple' single proto-cell life form and the eventual evolution and the resultant Cambrian explosion of multicellular (complex) life forms. For some reason(s), it appears to be biologically difficult to go from the simple single cell to a complex multicellular organism based on the only example we have to judge such things. Probably the overall obstacle to the early quick-smart establishment of multicellular (complex) life is that heretofore microbes (single cells) were 100% self-sufficient generalists. Complex organisms require the evolution of single cells to give up being generalists, become specialists, and work as part of a team. That degree of organization apparently takes lots of time, especially to the stage of where it can leave a large fossil presence, if it takes place at all.
Once you get to the multicellular stage, and in order to evolve further, well the trick is to survive, for the Universe is a dangerous place.
Planetary environments tend to be dangerous and rarely stable, and thus you need a lot of factors in place to ensure that even simple life even survives the long term and get the chance to evolve into multicellular life. Or, if you have evolved multicellular life, the odds are pretty good it's going to get the Big "E" - Extinction. I mean complex life is very vulnerable to environmental forces. A tornado probably isn't going to bother bacteria, but it sure could tear you apart. That's not to say bacteria can survive everything the cosmos can throw at them, but when nasties come, you stand a better chance of survival if you're a microbe. So, in order to get to the first proto multicellular critter, and from that to us, you need a lot of Goldilocks factors operating in your favour.
But there are apparently just too many planetary Goldilocks factors at play to grant the probability that complex, multicellular, animal, life is common throughout the cosmos. You need to be in a quiet part of the galaxy - no nearby supernovas, black holes to suck you in, gamma ray bursters, etc. You need a long lived stable single star. You need a gravitationally stable solar system so that planets are not gravitationally disturbed out of their orbit and either plunged into the parent star or cast out of the parent system altogether. You should have a good Jupiter(s) to absorb and/or gravitational deflect comets and asteroids that would otherwise slam into your otherwise environmentally-friendly planet causing havoc to established life forms. You need a planet that's in a pretty circular orbit, one that doesn't stray too close or too far from the habitable zone surrounding the parent star. The planet must have a fairly stable temperature range over geologic time periods, and so you must have an atmosphere, and thus has to be massive enough to retain an atmosphere, without being so massive that you end up with a brown dwarf - a quasi-stellar body. The axis can't have an extreme tilt, and it would greatly assist if the planet had a large moon around it to assist its long term stability. You need some sort of plate tectonics to ensure land building and the recycling of materials. If it's intelligence, with technology you seek, the planet can't be a water world. The planet must have formed in a region abundant in the heavier chemical elements (oxygen, sulphur, carbon, silicon, nitrogen, various metals, etc.). When you take all those factors (and more) into account, the number of suitable abodes where simple life can slowly evolve into complex life decreases rather quickly. And there's no guarantee that there is really any directed purpose to evolution in that evolution doesn't require simple life to become complex life. Survival and leaving offspring is what it's all about, and if single cell critters do that what more is needed?
One note about planetary disasters or catastrophes is that these cut both ways. They can't be frequent enough and/or large enough to wipe out the entire biosphere in total, especially the biosphere comprised of complex life forms, but on the other hand, infrequent small disasters can spur on evolutionary change by opening up environmental niches, but depending on who or what you are, when you are, and where you are, a disaster can be a double edged sword. I mean if you're a T-Rex sixty five million years ago, its bad news. On the other hand, without the bad news for T-Rex, we wouldn't be here, so for us, an asteroid impact 65 million years ago turned out to be good news!
It's also difficult to naturally transport complex life around the galaxy, unlike microbial life. If a meteor hit Earth and blasted a chunk of terra firma off towards Mars, pity the poor cockroach going along for the ride. Cockroaches are tough, but not that tough. And even if a cockroach did make it alive to the surface of, say Mars, it wouldn't survive long.
Anyway, once you have multicellular critters 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 beings more like ourselves.
IMHO, 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 evolved life forms with 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.
Finally, for SETI to succeed, for UFOs to be alien spaceships, one needs our intelligent species to develop technology, and here's where I see bottleneck number two. The evolution of technology isn't inevitable and has 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 metal ores and other objects than can be turned into useful tools, and of course a suitable supply of various energy sources. That's 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 in contact with the ground - some 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 birds of a feather pretty much as well
So, we've already ruled out dolphins and whales and the cephalopods being water based creatures, and the birds with their useless wings as far as building things is concerned, and all the four-footed walking mammals.
It might be conceivable that you can build up a technology using your mouth parts and/or using a tail to manipulate and build things, but we don't have obvious terrestrial case studies, although you might argue that bees and wasps and termites and ants and birds can build elaborate structures using just their mouths.
It's not all that obvious that technology actually adds all that much value towards ultimate long-term survival. Lots of technological advancements do, like controlling fire, developing agriculture, the rise of modern medicine and food technologies. But then lots of modern technological wonders, the automobile, CDs, sofas, microwave ovens, and thousands of other consumer products don't really contribute to our overall survival - certainly cars don't when considering the road toll! Then 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 and the rise of antibiotic resistant germs; exposure to electromagnetic fields - well, the list of horrors or potential horrors goes 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.
So, overall, SETI might not be able to detect our technological and communicating kin out there, and UFOs might not be alien spaceships right here, because it's 1) hard to evolve multicellular organisms, and 2) technology isn't inevitable and might even be counterproductive. Thus, Earth, with its multicellular critters and humanity with its technology, is quite the rare planet within the Universe - according to some.
The main proponents of what is now called the 'Rare Earth Hypothesis' are the scientists Brownlee and Ward (see below), and they have certainly stirred up an astrobiological hornet's nest with the idea. That's good for science in the long term. The belief in an unproved but accepted scientific proposition, in this case that that there are lots of complex alien critters out there, needs to be challenged if fields of inquiry aren't going to stagnate. However, make no mistake, it is the 'Rare Earth Hypothesis', not the 'Unique Earth Hypothesis', so religious fundamentalists who have taken this hypothesis to their hearts; who still need Planet Earth, and human beings, as some sort of religious special creation, should really not take comfort from these ideas. The Copernicus Revolution is still alive and well.
Further recommended readings:
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