It’s reasonable to assume based on our experience on Earth that there is a direct correlation between intelligence and complexity; the average human brain weighs about three pounds and contains about 86 billion neurons (about one-quarter of them in the neocortex), each neuron a distinct cell of less than 100 microns that rivals an average terrestrial microorganism in complexity. And while we have yet to find anything in the natural world that’s quite as neurologically complex as a human brain, we’ve seen some evidence that microbes working together in large numbers can demonstrate swarm intelligence—most notably the bright yellow slime mold physarum polycephalum, which consists of microscopic amoebae that are really good at solving mazes.
Since intelligence on Earth implies complexity of some kind—either cells organized into tissue, or single-celled organisms working together to achieve swarm intelligence—we tend to associate intelligence with brain size. This is a mistake when we’re dealing with a completely unfamiliar ecosystem of unknown scale, because a single cell in another ecosystem might be just as complex as a single cell in ours, but significantly smaller. You could have civilizations made up of tiny, intelligent creatures the size of brine shrimp—a real-life example of those creepy Sea Monkey ads you might have run across in old comic books. Even microbes, operating in tandem, could potentially develop swarm intelligence that rivals the complexity of the human brain.
All of this to say: we shouldn’t assume that creatures without large brains aren’t intelligent. There could be some amazing little minds swimming around in the oceans of Europa and Enceladus—or growing on the surface of rocks at the bottom. We don’t know what kind of shape an unearthly brain might take. All we can be reasonably sure of is that it probably won’t look very much like ours. It isn’t enough, in other words, to simply discover extraterrestrial intelligence; we also need to be able to recognize it when we see it. That might turn out to be harder than we’d anticipated.