I finished up last week's post on Great Filters by looking at the Galaxy according to the numbers I've cooked up. It produced the surprising figure of lifebearing planets every 50 light-years or so. In cosmic terms that's right next door. It means that in the near future we have a pretty decent chance of detecting some "biosignatures" emanating from planets orbiting other stars.
Of course, that conclusion is also kind of alarming. If life really is that common, then the real Great Filter might be looming up ahead of us, unforeseeable and unavoidable.
But there are still some obstacles between life, even complex land-dwelling animal life, and the ability to make yourself heard across interstellar distances. The biggest one is, how common is intelligence?
First, of course, there's the issue that intelligence is maddeningly hard to define. Does it mean communication, abstract reasoning, problem-solving, tool-making, social organization, all of the above, or something else? For the purposes of this essay I'm going to fall back on another functional definition: "Intelligence" is the ability to invent some method of communicating across interstellar distances or conduct activities which are detectable at that range.
I'm aware that this is incredibly reductive, but it's still useful. If the Galaxy turns out to be full of planets inhabited by beings with complex languages, abstract philosophies, and sophisticated art forms — but none of them can even conceive of the idea of building a radiotelescope or an interstellar probe, then we've solved Fermi's Paradox. If human-style intelligence is unique, then that's our Great Filter.
So . . . how common is intelligence? Obviously we have no idea. But looking at the history of life on Earth gives some hints. Complex life has existed on Earth since the Cambrian Era, about half a billion years ago. The hominins (including the genera Australopithecus and Homo), appeared about 4 million years ago. So intelligence — by a very generous definition — has existed for less than 1 percent of the history of life on Earth. Evidently intelligence is not an inevitable evolutionary strategy.
That suggests that intelligence is rare. It certainly requires a huge biological investment in energy-hungry brain tissue. But the payoff turns out to be huge! Why didn't any other species do it? There's at least one theory that there has been a selection pressure in favor of intelligence over the history of life. There's some support for the idea. Vertebrates are (mostly) smarter than invertebrates, and eventually took over. Mammals are (mostly) smarter than non-mammals, and eventually took over.
Well, maybe. Yes, I know about cephalopods. And I know about birds. But without a time machine we can't go back 50 or 100 million years to see if Paleogene-era birds were as smart as contemporary parrots and crows, or if Cretaceous cephalopods were as unnervingly intelligent as contemporary octopuses.
Ultimately it doesn't matter. If cephalopods got as smart as they are now hundreds of millions of years ago, they obviously hit a hard limit and stopped. If birds got smart tens of millions of years ago, they must have hit a wall, too (or a plate-glass window). It's possible that different methods of being intelligent have inherent limits built in, and only hominin intelligence avoided those limits. In that case we can rephrase our question of the frequency of intelligence as the frequency of "intelligence without built-in limits."
Or, if there really is a selective pressure in favor of intelligence, then it seems to have cooked up near-human intellects in a bunch of different genera all right at the same time, and winner takes all. And so in that case we can rephrase the filter as "where selective pressure acts in favor of intelligence at a similar rate."
Either way, I'm going to stick with the 1 percent figure. That winnows down the number of potential intelligent species in the Galaxy to just 10,000. That's still a lot, but it means an average separation of a few thousand light-years between worlds with intelligent beings on them. No more planet-of-the-week Star Trek episodes.
The second big intelligence-related Great Filter is tool use. Now things are going to get very fuzzy, because defining tool use is hard. Is picking up a stick to poke an anthill tool use? Is bending a wire rod into a hook to get food out of a bottle tool use? Does it matter if this behavior only happens in an animal-behavior lab when humans set up the conditions? Does it matter if there is no transmission of "technological" knowlege from generation to generation?
A key element of the tool use question is how important are hands? We've got great hands, combining strength and dexterity, but our hands are the product of a complicated and unlikely evolutionary history. Elephant trunks are stronger but not as dextrous — and they've only got one each. Octopus tentacles are fantastically dextrous, but of course they're mostly stuck in the water. Meanwhile birds have to make do with laborious combinations of beak and feet, and even apes have trouble carrying things because they need their hands to help with locomotion.
I'm going to say that hands aren't an important filter. When I was designing a handless alien species for my short story "The Alien Abduction" I had to come up with some rather implausible anatomy in order to make them truly incapable of manipulating objects. Even a jaw or a pair of toes would be enough.
Our hominid ancestors seem to have started making tools between 2 and 3 million years ago, which suggests that tool use and intelligence went hand in hand (so to speak). However, I'm going to stick a Great Filter in here, another coin-flip 50 percent cut. This is to cover all the potentially intelligent species with really inconvenient anatomy, beings which evolved in environments unsuitable for making tools, beings so physically capable they don't need tools, and those who just never quite got around to banging the rocks together.
It also covers beings who get stuck at a local optimum. They learn to make stone hand-axes and stop there. Our own ancestors did that for nearly 2 million years (although there's a huge sampling bias in that because stone hand-axes survive much better than tools made of wood and hide).
So we're down to just 5,000 species capable of making and using tools in the Milky Way, and presumably comparable figures for other galaxies. That's still an impressive number. Why haven't they said hello?
Next time: Civilization Filters.
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