"Some say they see poetry in my paintings; I see only science." -Georges Seurat



Thursday, January 17, 2013

Is There Alien Life in the Universe? Probably, But Not What You Think

There are billions or even trillions of stars in every galaxy, and there are billions of galaxies in the universe. Around many of these stars are probably planets, with many of these planets being Earth-like in regards to size and distance from the parent star. Surely, then, given the sheer magnitude of the numbers, there must be alien life out there somewhere, right? Yes, there probably is alien life, but it's probably not what you are thinking. Alien life, if it exists, is almost certainly stupid goo.


It seems likely based on the current understanding of biology and astronomy that life may be common throughout the universe; maybe even very common. But I doubt we would find alien life forms to be anywhere near as complex as life on Earth, and even if they were, the chances of them being "intelligent" are basically nil. I'll explain why, but first I need to give a brief explanation of the Drake Equation before getting to my points.

The Drake Equation is a formula used to estimate the number of technologically-advanced alien civilizations in our galaxy capable of interstellar communication. It was invented in the 1960's by astronomer Frank Drake and used to argue for a concerted effort (SETI) in searching for possible radio communications coming from alien life. The equation consists of several factors that get multiplied together to arrive at a final estimate. Some of these factors include: the number of stars in the galaxy, the number of planets per star, the number of habitable planets, the proportion of habitable planets on which life arises, and the proportion of habitable planets on which intelligent life arises. It is this last factor (term "FI" in the equation) that I will focus on. We will perform our own estimate for this term, and then compare our final results to Drake's initial calculation, and to a more recent calculation that utilized the "best" available current estimates for the terms.

Drake and colleagues performed an initial calculation in 1961 and came to the conclusion that there are probably between 1,000 and 100 million technologically-advanced civilizations just in our galaxy. So the universe should be teeming with intelligent life, according to their calculation. But his FI term they used in the equation (the proportion of planets with life that develop intelligent life) was 1, meaning Drake was assuming that 100% of the time life arises, it ultimately leads to intelligent life. Even my personal hero, astronomer Carl Sagan, estimated this term to be high for his calculation. I really truly hate to disagree with the esteemed scientists Drake or Sagan, but this idea is just wrong.

First, it assumes that life irrevocably "leads" to intelligence, as if evolution through natural selection is a purposeful or directional process. It is not. Evolution is not a ladder of complexity or intelligence or perfection that we are slowly climbing and that we will someday reach and declare ourselves "All Done!" Evolution depends upon and thrives upon utter randomness, and is a totally directionless process. There is no direction or ultimate purpose or endpoint of evolution. Today's advanced models are tomorrow's fossils; a changing environment (which happens constantly) can easily make the best and most clever organisms' designs obsolete. It is estimated that 99.9% of all species that have ever existed have gone extinct. We are the lucky few that remain, and our continued survival should always be in doubt.

One counter to this argument is that there seems to be several examples in nature of convergent evolutionary traits, like eyes or wings. This means that nature has independently discovered or stumbled upon these solutions (often multiple times), so they must be generally useful classes of solutions to the problems of survival and reproduction. If given enough time and appropriate variation, eyes or wings are almost certain to arise on our planet, and in life on any planet, or so the thinking goes. In the same vein, intelligence is often assumed to be a convergent evolutionary trait. People typically just assume that intelligence is "generally useful" and so is certain to evolve. Most of the time, this assumption is implicit, and the arguers are not even aware they are making it nor are they aware that it is a highly questionable assumption.

Even if eyes or wings are convergent traits, I would still argue there is no general trend for organisms to get smarter. Think of intelligence as a particular evolutionary adaptation to a particular problem (or set of problems) faced by our ancestors. A good analogy is the peacock tail. Peacocks have huge, ungainly, ornamental tails in order to attract mates (it demonstrates good health, nutrition, genes, etc., to a prospective mate). But such an adaptation is obviously costly. Peacock tail size and beauty developed in a sort of runaway arms race, in which each generation needed to have a fancier and larger tail than the last in order to stay competitive in the mating game. The only thing that ultimately stopped the development of larger and larger tails seems to be its sheer size; too big of a tail and the peacock could not move effectively through its environment. Things just cannot grow exponentially forever.

Advanced intelligence in humans is the equivalent of a peacock tail; it's extraordinarily effective for our particular physical and social environments, and several unique environmental factors seem to have led to a runaway development in greater and greater intelligence. But this is true only for our particular species, in our unique environment. We would never dare to argue that alien life is likely to develop fancy ornamental peacock-like tails just because tails and/or wings seem to be a convergent evolutionary solution to the problems of flight on Earth. Analogously, we should not dare to argue that alien life is likely to develop fancy expensive human-like intelligence, even if some rudimentary level of intelligence were convergent in evolution (and this assumption is questionable as well).

Intelligence is difficult to evolve because big brains are costly, in a variety of ways. First, brains are metabolically-greedy. The human brain consumes 20 to 25% of our food and oxygen intake. This is a very risky avenue of survival for evolution to pursue, because such high metabolisms require large and regular high-calorie meals. Any variation in the environment that effects steady high-caloric food intake could easily spell doom for a smart animal. Being smart is a huge gamble.

Second, big brains are also big. I don't mean to be circular, but large brains means bigger skulls are required. However, heads cannot just grow astronomically larger forever. There are limiting factors, such as the ability to remain quick, mobile, and agile enough to hunt high-calorie meals; the increased risk of missteps or falls with a larger unwieldy head; and the ability for newborns to fit through the birth canal without injuring or killing their mothers in the process of birth. Even today, giving birth is a serious medical risk for human mothers due to the massive size of infants' heads. Interestingly, childbirth is primarily dangerous only for humans, but not other animals.

Although I had to part ways with Drs. Drake and Sagan on the topic of intelligent alien life, I am not alone in my thinking. In a famous 1995 article by the esteemed scientist Ernst Mayr, he argues that high intelligence is probably rare for life because (1) being really smart may not be as advantageous for life as is commonly assumed, and (2) it may be unlikely and/or difficult for nature to hit upon this solution. Only one species has ever done it, and it took much longer than one might expect if intelligence was a convergent evolutionary trait. Mayr argues that "high intelligence seems to require a complex combination of rare, favorable circumstances" to have evolved, implying that it is very unlikely for alien life to do so.

Given these concerns, we can see that while intelligence has its obvious benefits that most can agree on, most people gloss over the extreme costs and risks and unlikelihood of developing intelligence for an organism. And subsequently, I believe this results in people drastically overestimating the probability that life on other planets will be intelligent, particularly high intelligence capable of producing advanced technologies and advanced civilizations that can communicate across interstellar space. Next, let's explore some more reasonable estimates for the FI term in Drake's equation to see where it leads in comparison to a modern Drake equation calculation.

The FI term in the Drake equation, again, is the proportion (ranging from zero to one) of planets containing life that go on to develop intelligent life. By intelligence, it is meant high or advanced intelligence capable of developing technologies that can communicate across space. Drake originally estimated this term at one, while Sagan used a number near one. Using more current data and thinking, a recent calculation used an FI value range of 0.01 to 0.025. The author concluded that the number of advanced alien civilizations in our galaxy, right now, is somewhere in the range of near-zero (0.00127) to 245 civilizations. This calculation alone is a sobering estimate, as it suggests that even if the high end of the estimate were accurate, only 245 advanced alien civilizations exist in our galaxy. Since the Milky Way is 100,000 light years in diameter, this means if these civilizations were roughly evenly distributed (along a straight line), our closest alien neighbor would be over 400 light years away. If we consider the distribution throughout the galaxy as falling within a circular area (which would be more accurate), the distances apart are even greater. Searching out and finding such life would be effectively impossible given such huge distances; just communicating would be extraordinarily difficult since it would take 800 years or more just to hear a reply to one of our own messages.

I think a better estimate for FI, also suggested by Dr. Mayr, is the  number of species on Earth that have developed intelligence, out of the total number of species that have ever existed on Earth. Given that there are about 3 million to 30 million different species alive today, and that 99.9% of all species that have ever existed have gone extinct, then a high estimate for FI = 1 out of 3 billion, and a low estimate for FI = 1 out of 30 billion. If we use the high estimate, just to be optimistic, and apply this new term to the more recent calculation described above using their optimistic estimates, then the number of advanced civilizations right now in the Milky Way is 0.00000326, or 3.26 advanced alien civilizations per 1 million years. This is not good news for alien hunters.

Of course, I would be more than happy to be completely wrong about this entire analysis, and it's fun to imagine what it would be like to find out we are not alone in the universe. What an awe-inspiring, incredible idea. What might be more awe-inspiring and incredible, though, is the idea that as far as intelligence goes, we just might be alone in this vast, mystifying universe.

17 comments:

  1. I think you underestimate how inevitable high intelligence is. Evolution is all about adaptive value and intelligence can be defined as the capacity for adaptive behavior: problem solving; so the trial and error of natural selection would favour intelligence over long spans of evolutionary time. But high intelligence is complex so it takes a long, long time to evolve, but many planets may have had life for far longer than we have.

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    1. Thanks for commenting. You have demonstrated exactly my point, that people love to list off the benefits of intelligence ("it has adaptive value") but fail to consider its costs. Evolution is a balancing act and every adaption is a trade-off. There is no free lunch in nature, not even for intelligence. We tend to view intelligence as this magical powerful force that confers only goodness upon its holder. But it is indeed costly, as discussed above.

      This tendency to ignore the costs of intelligence results, I claim, in a vast overestimation in how likely intelligence is to evolve. You say "the trial and error of natural selection would favor intelligence over long spans of time" but this is just an unsupported claim you are making, with no real compelling reason to assume it is true.

      If intelligence is so likely, why did it take 4 billion years to evolve it on this planet? If intelligence is so likely, why did only one of the millions upon millions of species evolve high intelligence on Earth? If intelligence is so likely, why is the universe not teeming with smart alien life?

      Where are all the aliens (the Fermi Paradox)? We can come up with all sorts of wild ideas about why aliens aren't here, but a really simple and elegant explanation that most people won't even consider is that intelligence is unbelievably rare and difficult to evolve and costly to possess.

      And the claim that "many planets may have had life for far longer than we have." True, or they may not have life at all. Or maybe they only recently developed life. Or other planets may have life, but not intelligent life. We don't know, so I don't see how you can use this in support of your other claims. It's just wild speculation, and that's OK, but you probably shouldn't try to use speculation to support other speculations. Thanks again for reading and commenting.

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    2. There is no free lunch in nature, not even for intelligence. We tend to view intelligence as this magical powerful force that confers only goodness upon its holder. But it is indeed costly, as discussed above.

      Every trait has costs, but intelligence is fundamentally different from virtually all other traits in that other traits are just adaptations, while intelligence is an ability to adapt, so it's a logical conclusion of the evolutionary process.

      If intelligence is so likely, why did it take 4 billion years to evolve it on this planet? If intelligence is so likely, why did only one of the millions upon millions of species evolve high intelligence on Earth? If intelligence is so likely, why is the universe not teeming with smart alien life?

      Because it's so complex, it requires an enormous amount of evolution to occur, and humans just happen to be the first known species to reach that degree of evolutionary progress, but there's a scientist who argues that if dinosaurs hadn't gone extinct, they too would have evolved into large brained bipeds and were already on that path. So intelligence is rare in the universe because it takes so long to reach that level of evolution that odds are a meteor type catastrophe is going to kill you off first, but given the vastness of the universe, even extremely rare events occurs repeatedly.

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    3. You are just engaging in wild speculation again: "intelligence is fundamentally different from virtually all other traits". What reason do we have to believe this? And NO, intelligence is not a logical conclusion of the evolutionary process. Evolution is not a directional process. There is no ladder of complexity or intelligence that it is climbing or headed towards. You are just wrong on this point. To your credit, it is a common misunderstanding of evolution, one that I am trying to get people to reconsider because it is not accurate.

      Dinosaurs had more than enough time to development intelligence, millions of years, but their brain sizes weren't exploding. In their physical, social, ecological niche, intelligence was not evolutionarily adaptive, probably because of the enormous costs of being smart.

      Which is the point I have been trying to hammer home. I'll say again, intelligence is not a free lunch. There are huge costs, it is risky, it is needy, it doesn't always help. It's only advantageous in specific, probably rare environments. Bacteria don't need intelligence because it confers them no benefit over the costs it would require. Neither do plants, or mold, or most of life. Smarts are costly and rare, that's the point.

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    4. As I said, the reason we have for believing intelligence is fundamentally different from other traits is other traits are adaptations, while intelligence is an ABILITY to adapt. Other traits are solutions to problems, while intelligence is an ABILITY to problem solve.

      I agree with you 100% that there is no grand purpose to evolution; no higher power directing it. However BILLIONS OF YEARS of random trial and error arguably does produce a ladder of evolutionary "progress". E.O. Wilson proposed four major steps on the evolutionary ladder: (1) primitive prokaryotes with no nucleus, (2) eukaryotes with nucleus and mitochondria, (3)large multicellular organisms with complex organs like eyes and brains, and, lastly (4)the emergence of the human mind.

      Princeton professor John Bonner asked "why has there been an evolution from the primitive bacteria of billions of years ago to the large and complex organisms of today" and noted that the later an animal emerged in earth's history, the greater its brain size and culture, and even proposed ranking plants on an evolutionary ladder citing slime mold vs angiosperms.

      And while it's true that dinosaurs probably never approached anything close to human levels of intelligence, there was a clear trend of increased brain size relative to body size, and by extrapolating the trend into the future, paleontologist Dale Russel argued that some dinosaurs would have becomes as smart as people had they not gone extinct.

      The evolution of intelligence is not unique to humans, we just happen to be the first known animal to take it this far; however the trend itself is quite pervasive, as the fossil record shows an explosion in relative brain size all over the animal kingdom during the past 65 million years. Even animals as distant from us as crows appear to be ape-like in their intelligence and proportional brain mass.

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    5. Thanks again for sharing your thoughts. EO Wilson's step 3 to 4 is quite a leap, conceptually and otherwise. We can easily point out where other animals have achieved step 3, and basically argue convincingly that it is a convergent trait. But not so with step 4 (high intelligence). And yes the evolution of technological intelligence IS unique to humans. I'm comfortable with your argument that maybe we are the first ones to be intelligent (someone had to be first), but I don't see it as being very compelling.

      Extrapolation can be a dangerous game to play, and I think the dinosaur example may demonstrate why. But if you can provide any links or citations on it, that could contribute to our discussion here. I'll check out the history of brain size development in animals across time, for some comparison, this might help us out.

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    6. I did a bit more research and I think you're correct to emphasize the uniqueness of human intelligence. I still think that evolution eventually leads to high intelligence, but the degree of intelligence humans have appears to be a fluke. Wikipedia lists the encephalization quotient (EQ: brain size relative to body size) of various mammals, and they are roughly as follows:

      Humans 7.6
      Botlenose dolphins 4.14
      Orca 2.94
      chimp 2.35
      rhesus monkey 2.1
      elephant 1.75
      dog 1.2
      cat 1
      horse 0.9
      sheep 0.8
      mouse 0.5
      rat 0.4
      rabbit 0.4

      Excluding humans and near-humans (other primates) from the sample, it looks like the distribution of mammal EQ has a mean of 1.4 with a standard deviation of 1.2. That makes human EQ 5.17 sigma above the mean, implying only one in 8 million species of mammals should be as smart as we are. Considering that there are currently only 5,416 mammal species on the planet, the odds of there being a mammal as smart as humans on the planet are only one in 1,477.

      According to physicist Michio Kaku, there are 500 million planets in our galaxy that can support life. The average age of earth like planets appears to be around 6.34 billion years with a standard deviation of 2.73 billion years. How old is the life on these planets? We have no idea if they even have life, but based on Earth as our only sample, life on an Earth type planet appears to occur only once every 4.54 billion years, so on average these planets have only had life for 6.73 - 4.54 = 1.8 billion years (SD still 2.73 billion). Given this distribution, only about 16% (80 million) have had life as long as we have, and thus can be expected to have Earth's level of evolutionary development and diversity. However as mentioned above, even on Earth, the odds of there being a species as smart as humans is only one 1,477, thus there should be about 54,164 intelligent species in our galaxy.

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    7. Thanks for sharing, that's some nice data I hadn't considered. It does seem to support the idea that "high intelligence" or technological intelligence or human-like intelligence, whatever we wish to call it, is almost like a fluke, as you said. Rare and probably difficult to evolve, is how I am interpreting it.

      Here's some additional data to consider:
      http://thebrain.mcgill.ca/flash/capsules/histoire_bleu04.html

      The important thing to note at the linked page is that a few times humans have had explosive brain growth, in which brain size increased dramatically in a very short time period (short in terms of evolutionary timescales). If there was a "general trend" to get smarter, we probably wouldn't expect to see explosive changes, but more gradual increases, and across other species too. Explosive changes suggest a strong selection pressure for intelligence at particular periods of time, but that were apparently only limited to humans (and specifically homo sapiens). Note again that this does not seem to indicate a general trend for animals to get smarter.

      I very much like your calculation, and of course your result is intriguing because it is much more suggestive of intelligent life being common in our galaxy. You should try plugging your own 1/1477 FI term into a popular Drake Equation calculation to see what pops out.

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    8. I'm glad you liked my calculation. I think looking at the bell curve distribution of (non-primate)mammal encephalization is a good way of estimating how probable human intelligence is, because the data does not depend on the fact that humans happened to evolve. Other estimates of FI by contrast assume that since humans just happened to evolve on one of the 5 inhabited continents, the odds of intelligence evolving on an inhabited planet (analogous to a continent) are 20%. But since humans are the only example of intelligence, such logic can't tell us whether we are a fluke, and maybe if there were a thousand inhabited continents, instead of just five, we'd still be Earth's only intelligent life (as my one in 1,477 estimate suggests).

      Your link is interesting and we agree that high intelligence is almost a fluke, but I don't want to overstate the case by denying a general trend for animals to get smarter. It's certainly true that there's been no increase in fish/reptile encephalization in the last 225 million years, however the encephalization of mammals has QUADRUPLED since they first appeared on the fossil record. The encephalization of birds has increased TENFOLD! So it seems that over time, animals just get smarter or stagnate; it's rare for them to get dumber, so on net, the average intellect of living organisms seems to keep going up.

      Now as you say, this may be just due to rare cases of explosive growth, rather than any steady gradual trend, but over hundreds of millions of years, rare cases occurring repeatedly form a general pattern of rising intelligence across most types of animals. So intelligence was arguably inevitable; just not this much, this soon. It probably should have taken another 600 million years.

      I found a Drake Equation calculator online:

      http://www.classbrain.com/artmovies/publish/article_50.shtml

      It wouldn't allow me to enter an FI as specific as 1/1477; the closest it allowed is 1/1000. It calculated only 12 communicative civilizations!

      I think the biggest uncertainty we have with all this is not the frequency of intelligence which is relatively knowable, but the frequency of life itself. Many scientists assume that since life emerged relatively early in Earth's history, that it occurs easily, but if it's so easy, why did bio-genesis occur only one known time in Earth's 4.54 billion years? If we assume bio-genesis occurs just over 10% of the time on an Earth type planet, as one study suggests, then that calculator gives me just over 2 communicative civilizations.

      The other is wild card is the date when life starts on a planet. Most Earth type planets are actually older than Earth, but life may have started exceptionally early on Earth, and if so, intelligence had far more time to evolve here. My 1/1477 FI only makes sense if life on other planets has had as much time to evolve as we have.

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    9. Sweet calculator, thanks for the link! I'm getting slowly persuaded to your argument, that there just might be a teensy tiny trend for a few species (primates) or groups of species (mammals, birds) to get smarter. If it is there, it seems to be agonizingly slow for most of life, and basically absent for huge swaths of the tree of life (bacteria, viruses, single-celled organisms, insects, mold, etc. etc).

      Googling "evolution of complexity" brought up some interesting points, namely that the maximum complexity (though they didn't say intelligence, just complexity) of life on average seems to be very slowly getting higher, but that the median value is even smaller, and the mode basically hasn't changed throughout all of life on Earth. Most of life has stayed dumb and dumber.

      The interpretation, interestingly, is that there then isn't a true "trend" towards greater complexity in life, per se, but that through utter randomness the tail of the distribution just sort of gets a bit fatter and fatter every now and then, creeping out to the extremes.

      But since we are only interested in the tail-end anyway, assuming it encompasses complexity high enough to include "intelligence", this is a pretty good argument. It would be interesting to know if such a finding is a fluke limited to Earth (since the leap from single cellular life to multi-cellular life took so incredibly long, implying it might be difficult to do so and not a general trend either).

      I would feel more comfortable using a smaller FI term than the mammal one you calculated, and instead open it up to other groups as well (fish, birds, etc., etc.). Because as of now we are just assuming that once life arises on a planet it will automatically stumble upon living creatures as complex and smart as mammals.

      You point out, interestingly, that the real bugger in the Drake Equation is the frequency of life itself. It's exciting to know that this is actually one of the terms that, with future data and exploration and analysis, humans might soon be able to get some accurate empirical estimates of this number by exploring nearby worlds. The Drake Equation has taken a lot of shit over the years in terms of criticism, but I think it admirably accomplished its goal by spurring research, inspiring future scientists, and helping point out to the general public why space exploration and basic science are so important: they can help answer mind-bogglingly Big questions.

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    10. I believe evolution does form a ladder of "progress", for lack of a better word. While most of life still lacks even the most rudimentary intelligence, even after billions of years, what I suspect you'll find is that the least intelligent life forms (i.e. reptiles, bacteria) branched off the evolutionary tree prematurely, while the most intelligent life forms (mammals, primates) branched off more recently and then within primates, humans are the most recently emerged. So while there's still huge numbers of dumb organisms, those occupying the highest branches of the evolutionary tree, the most evolved if you will, are relatively smart, suggesting that intelligence is the direction evolution has been heading, even if a ton of dumb relics from more primitive times continue to exist.

      I doubt the trend towards complexity is fluke limited to Earth, but rather evolution works through trial and error, so it's perfectly natural to expect such a process to "improve" things over time.

      I understand your concerns about my FI being limited to mammals (that just happened to be the brain size data I found) but it shouldn't make much difference. As you recall, I calculated that human intelligence had a statistically expected one in 8 million rarity among mammal species, and since only one out of every 293 species on Earth are mammals, then we multiply that one in 8 million rarity by 293 to determine how statistically rare human intelligence is with respect to ALL extant species. This tells us that only one in 2.348 BILLION species should be as smart as humans. But there are only 1,589,361 living species on Earth:

      http://www.factmonster.com/ipka/A0934288.html

      Thus technological intelligence should only occur on one in 2.348 BILLION/1,589,361 = 1,477 Earth-like planets (inhabited for several billion years). So the way I did the math, FI remains one in 1,477 regardless of whether the reference population is limited to mammals or all life on Earth.

      Ideally I would want the initial encephalization distribution I calculated to be based on a random sample of all living organisms, but even if I found such data, the fact that many species lack brains completely would skew the bell curve through a pile up of zero scores, making the data statistically useless. Thus a good compromise, might be to find a random sample of all animals (not mammals only), with the understanding that animals are just a subset of all life, and see if I get a similar FI.

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  2. I think a better estimate for FI, also suggested by Dr. Mayr, is the number of species on Earth that have developed intelligence, out of the total number of species that have ever existed on Earth. Given that there are about 3 million to 30 million different species alive today, and that 99.9% of all species that have ever existed have gone extinct, then a high estimate for FI = 1 out of 3 billion, and a low estimate for FI = 1 out of 30 billion. If we use the high estimate, just to be optimistic, and apply this new term to the more recent calculation described above using their optimistic estimates, then the number of advanced civilizations right now in the Milky Way is 0.00000326, or 3.26 advanced alien civilizations per 1 million years. This is not good news for alien hunters

    But this analysis only holds if you assume there is only one species per habitable planet; an untenable assumption given that on Earth there are currently as many as 30 millions species. Remember FI is "the proportion of habitable planets on which intelligent life arises" and not "the proportion of life that is intelligent".

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    1. Technically speaking, FI is the proportion of life that is intelligent, see here: http://en.wikipedia.org/wiki/Drake_equation#The_equation

      In other words, FI is the proportion of habitable planets on which intelligent life arises, out of the number of habitable planets on which life arises. I maybe didn't explain this as clearly as I could have, sorry for the confusion.

      The trick is estimating this term given incomplete information. Do we include all species in our sample? Or just the species that currently exist? Or maybe just mammals? What about considering life on each continent as being independent samples (as others have done before)? These are all interesting discussion points, and unfortunately there is no clear, definite answer. But I think what is perfectly clear is that this proportion is not 1, or even near one. That's my take-away point I want people to consider.

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    2. Technically speaking, FI is the proportion of life that is intelligent, see here: http://en.wikipedia.org/wiki/Drake_equation#The_equation

      In other words, FI is the proportion of habitable planets on which intelligent life arises, out of the number of habitable planets on which life arises. I maybe didn't explain this as clearly as I could have, sorry for the confusion.


      One of us is still confused, because "the proportion of life that is intelligent" is completely different from "the proportion of habitable planets on which intelligent life arises, out of the number of habitable planets on which life arises".

      To illustrate the difference, let's make the absurd assumption that Earth is the only planet where life of any kind emerged. Then the proportion of life that is intelligent, is currently as low as one in 30 million (under the assumption that there are 30 million extant species on Earth and no life anywhere else). However if earth were the ONLY planet where life arises, then F1 = 1, because on the one planet where life emerges, intelligent life has emerged.

      So the point is, even though the odds of any one CURRENT species being intelligent are say, one in 30 million, there might be roughly 30 million living species per inhabited planet, so virtually every inhabited planet has intelligent life.

      The question I would ask is how many habitable planets in our galaxy are likely to currently have life that has been continuously evolving for at least 4 billion years (the time it roughly took for just one intelligent species to evolve on Earth). That's how I would estimate the number of intelligent species in our galaxy.

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    3. This last point might make a nice modification to the Drake equation (taking time of evolution into account). I know some forms of the equation take into account the "estimated" lifetime of a technological solution. But have never seen the other, could be interesting to consider, thanks.

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  3. I am bringing nothing to the table aside from saying that i read your article and everyone's responses. It was a really fun read.

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    1. Thank you very much for reading, I appreciate the kind words.

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