(Update: As I said was probable, I’ve revised some parts below now that I’ve worked through some of the statistics in the paper.)
This story has been knocking around the blogosphere and the science news services (see, for example, this slightly too credulous piece on Universe Today). I had ignored it, hoping it would go away.
The basic idea is that intelligent life in the universe is rare or unique - the press release argues for unique - because the evolutionary steps that are necessary to evolve intelligence are unlikely.
There’s several problems with this idea, and I see now that it isn’t going away - blogs and news services are still covering it a week after release. So it’s time for a gentle takedown.
The paper is available online. There are large differences between the paper and the press release and its treatment by news outlets and interviewers, so it is hard to know which one to address. The press release is getting far more attention, but the paper contradicts the conclusions emphasized in the release and popular press stories at several points. So I’ll deal with both.
The general thesis of the press release seems to be:
- Stars have only so much hydrogen to burn. This limits the amount of time a planet can spend in its system’s habitable zone.
- Complex life, and humans, arose late in the habitable period of Earth, which may have as little as another billion years to go as a hotbed of life. This suggests that evolving intelligent life is very difficult and statistically unlikely.
- Complex life is “separated from the simplest life forms by several very unlikely steps and therefore will be much less common.”
These three premises match up very nicely to the three problems that these premises have:
- The first premise is that life has a limited time to evolve, as determined by the lifespan of its star. The problems are these:
- The limits placed on the lifespan of the star are not informed by astronomy. Not all stars are sun-like. Some of them have life spans much shorter than that of the Sun, but other dwarf stars are believed to have life spans two to three times that of the sun. So while the statistical model may be appropriate for Earth, you can’t similarly limit the odds of life on extrasolar planets, because many of these systems will last longer than ours.
- We know life can adapt to varying environmental conditions, including temperatures and pressures once thought to be impossible for life on Earth. This greatly widens our conception of what a “habitable zone” might be, which in turns increases the time that certain exoplanets will spend in those zones.
- And finally, our conception of “habitability” might be way off. Earth has, on average, a remarkably stable climate compared to what we know about certain other exoplanets. Because of that, we have no idea how life might have adapted to unstable climatic conditions in other solar systems. It is hard to even define what a “habitable zone” in an alien life-bearing planet might be - while there are some fundamental limits at which Earth-like biochemistry simply can’t work, life on Earth doesn’t use all possible amino acids or proteins, and hasn’t adapted all possible ways of ‘hardening’ such systems to deleterious conditions. Life arising on another planet seems likely to solve these problems right away. So our definition of “habitable” might have very little applicability to the biology of another planet.
- The second premise is that complex life, and humans, arose late in the habitable period of Earth, which may have only a billion years to go as a life supporting planet. This therefore suggests that evolving complex life is difficult and unlikely - otherwise it would have happened sooner. The problem is this:
- While it may be true that Earth is late in its habitable period, we are dealing with a sample size of one. We don’t know if Earth was, for some reason a “late bloomer,” or if we shot out of the starting gate at enormous speed. A sample size of one is useless - but having said that, it is more reasonable to suppose that Earth is closer to the average than a statistical edge case - because by definition, the average is more probable. That suggests that the evolution of complex, intelligent life in multiple trials would happen in more and less than 4.5 billion years, and that the peak would be somewhere around 4.5 billion years. But remember - this is a useless sample size - I’m just speculating. The point, though, is this: Given a star with a solar lifespan, or longer, the logic that Watson uses to justify his opinion of the rarity of intelligent life breaks down. It actually shows that intelligent life is fairly likely on such planets.
- The third premise is that complex life is “separated from the simplest life forms by several very unlikely steps and therefore will be much less common.” The problems are these:
- We don’t actually know that complex life is “separated from the simplest life forms by several very unlikely steps.” Nobody has done any experiments to explore the odds of the various evolutionary steps that Watson mentions. Nobody has explored the statistical probability of the origin of cellularity, or of cellular specialization, two evolutionary steps that Watson cites as being rare.
- One evolutionary step that the press release cites as rare is the evolution of multicellularity. This strikes me as false. Evolution in the lab has shown that single-celled organisms tend to colonize pretty readily, suggesting that step at least is not unlikely. And Watson points out in the paper, “…the formation of colonies from individuals seems definitely to have occurred more than once in the history of life, and can be excluded on those grounds [as being highly probable, and not rare like the others]….” So here there is a contradiction between paper and press release here.
The problems go deeper, however. Watson’s supposedly necessary evolutionary steps might be appropriate for evolving Earth-like intelligent life, but it isn’t clear to me they are inherently necessary for all intelligent life. From the press release:
Prof Watson suggests the number of evolutionary steps needed to create intelligent life, in the case of humans, is four. These probably include the emergence of single-celled bacteria, complex cells, specialized cells allowing complex life forms, and intelligent life with an established language.
Why do we need bacteria as a step in this chain? Why can’t nucleation come first, and then a secondary membrane evolve later to protect (or render captive?) the nucleus and the materials and organelles it depends on, thus bypassing bacteria-like organisms entirely? I don’t claim to know all the different ways that we can get from replicating molecules to prokaryotic cells, but I think it is a failure of imagination to presume that something like a bacterium is a necessary step in the evolution of prokaryotic cells, and onward to intelligence. Failure of imagination is going to be a recurring theme from here on out.
Similarly, do we need cellular specialization for intelligence? I grant it is hard to see how it could be otherwise, given our understanding of terrestrial biology. But is this really the only way to go about it? This requirement is presumed, not defended, and I’m not familiar with literature that defends it as a universal (as opposed to Earthly) requirement for intelligent life.
And finally, the press release says the evolution of language is a necessary step. So what about language? I’m not a linguistic specialist, so I went looking for a definition. Wikipedia offers the following:
A language is a system of visual, auditory, or tactile symbols of communication and the rules used to manipulate them.
Visual, auditory, or tactile…. That corresponds to the senses of vision, hearing, and touch, but I have a problem with limiting things that narrowly. I can imagine, even if Watson cannot, an intelligent species conducting complex abstract communication chemically. I can also imagine it being done with sensory organs (and hence senses) not represented on Earth. I can also imagine it being done through what humans might consider direct neuro-anatomical contact.
Again, there is a failure of imagination here - I’m not saying any of these methods I speculate about are probable, but it seems like they might be possible, and there are surely more ideas out there than I’m able to come up with in five minutes of thinking about the problem. To be fair, the obsession with language belongs to the press release, not the paper. We should probably just replace “language” with “communication method.” But whatever word you use, the language issue is a problem for Watson’s hypothesis. Every time we think up a possible method of communication, linguistic or not, we’re increasing the opportunities for life forms to evolve a communication method necessary for becoming an intelligent species by Watson’s criteria.
In other words, whatever odds Watson settled on for the evolution of intelligence, they are too low, because he only seriously explores an Earth-like way of evolving an Earth-like intelligent species.
Moving on, what I’m not seeing in either the press release or the paper is any discussion of the definition of intelligence. I think we’ve had a lot of intelligent life on Earth, and in widely separated taxa, as well. I own a parrot - it superficially appears to be at least as intelligent about physics and interpersonal relationships as a human three year old. I’m told that porpoises and most other cetaceans are far from stupid. I’ve seen amazingly intelligent cooperative behavior among ravens in Alaska that I might post about someday. And I’m not sure that we’ve really come up with a good definition of the term “intelligence,” nor are we good at recognizing its existence in other species, even when we are familiar with them.
I think what Watson is after is intelligent and technological species. I’ll grant that my parrot is only barely technological (he knows how to use a few simple tools for single tasks), and the rest of my examples apparently not at all technological. But they are intelligent. If you need to evolve from intelligent to technological, and you offer some odds on that happening, we can all see that those odds went through multiple trials on Earth. Parrots had their chance and didn’t make it, cetaceans had their chance and didn’t make it, chimps had their chance and almost made it but not quite, ravens had their chance, Velociraptor probably had its chance - and each time, there’s a certain percent chance of intelligence going technological.
This is another flaw in the hypothesis. The paper evaluates only the evolution from primate to human societies, which I am interpreting as evolution from intelligent to technological societies. The paper recognizes that on Earth there was only one chance of this happening, and that it had to happen among primates. But looking around, it seems to me there were a bunch of chances for this transition to evolve, and that it happened to occur in primates. There wasn’t just one roll of the dice. This would drive down the probability of any given species making evolving technology, but would seem to increase the probability that technology would arise sooner or later, since there are so many different taxa it could have evolved from.
But remember - our sample size is one. It isn’t big enough to draw conclusions from. I’m merely trying to show that the data we have to had can be used to support different conclusions from Watson’s.
Finally, one more objection - and I think a real, genuine objection, and one that is fatal to the hypothesis. From the press release:
His model, published in the journal Astrobiology, suggests an upper limit for the probability of each step occurring is 10 per cent or less, so the chances of intelligent life emerging is low – less than 0.01 per cent over four billion years.
Interesting - he’s saying that the combined odds of all the necessary steps to producing intelligent life actually happening are less than one hundredth of one percent.
Astronomers have recently determined that rocky planets are very common in the local universe. According to these results, at least 20%, and possibly as many as 60%, of sun-like stars have rocky planets. There are about 100 billion stars in the Milky Way, 10 billion of which are like the Sun. We’ll adopt the worst-case scenario from the study and say that only 20% of them have rocky planets. We’ll further assume that rocky planets are the only kind that can support technological life, and we’ll completely ignore the frequency of rocky moons orbiting “hot Jupiter” exoplanets, which is likely to bring the number of available bodies way up.
That leaves 2 billion suitable planets in the Milky Way. Watson says the chances of intelligent life evolving are less than 0.01 percent. Fine - multiply 2 billion times 0.0001 to give you the maximum number of planets that should have intelligent life, and what do you get?
Two hundred thousand.
Wow - 200,000 planets in the Milky Way alone that support intelligent life? That’s amazing. There’s some limitations on this number, however:
- Some of those planets are not going to be in the habitable zone.
- Some are going to have other problems seemingly prejudicial to the development of life.
- Only about half this number are currently 4 billion years old or older.
However, even after taking a few bites out of this huge, juicy apple, we’re going to have a lot of apple left. This should be enough to show that Watson’s cumulative odds are going to have to be considerably smaller than 0.01% if he’s going to make intelligent life unique, or vanishingly rare. As it stands, Watson’s odds for intelligent life are so incredibly high that he has utterly failed to explain the Fermi Paradox - which the paper mentions by name and apparently seeks to explain.
Look at it this way - if he’s off by an entire order of magnitude, there’s still 20,000 planets in the bin. His odds have to be six orders of magnitude off just to get the number of other planets with intelligent life in the Milky Way below one.
And that’s just in this galaxy. Extend that to the universe, and you’ve got major problems coming up with an expression of probability low enough to make Earth special in this way.
Watson is an environmental scientist, not an astronomer, so he can be forgiven for not understanding the large numbers involved here. However, perhaps we might gently suggest he include an astronomer on the research team? Or at least consult with one before writing the press release? Dare I say that he’s made the same mistake that evolution deniers make - just as they do not understand the vast amounts of time and the huge numbers of individuals through which evolution has had a chance to work, Watson does not seem to understand that the number of stars with suitable exoplanets can only be described by a gargantuan, unimaginably huge number.
Personally, I find 200,000 planets with intelligent life to be incredibly optimistic, so I’d really be more comfortable if his odds were way lower. But then, we have no reliable data here - it’s almost all speculation.
But the problem ultimately doesn’t go away - there are so many rocky planets (according to our population statistics) orbiting Sun-like stars in the universe that I have no way of grappling with numbers that big. Ultimately, I don’t find Watson’s ideas compelling for these contradictory reasons: his odds of evolving intelligent life are way too high; his conclusions about the uniqueness or rarity of intelligence are contradicted by back of the envelope calculations; and the number of candidate planets is so high that given any reasonable set of odds, intelligent life still looks highly probable.
