Now to fi, the fraction of planet biotas that intelligent organisms emerge from. On our planet, at least, this emergence has turned out to be a long and complicated and winding path.
The Last Common Ancestor that can be reconstructed was likely an organism much like a present-day methanogen, an organism that lives off of combining such environment substances as carbon dioxide and hydrogen, and that makes all its biological molecules.
That limits where such an organism can live, and a way out of that limit is getting energy from light: photosynthesis. It has evolved twice on our planet:
- Bacteriorhodopsin – addition to chemiosmotic energy metabolism
- Chlorophyll – addition to electron-transfer energy metabolism
The second type is the familiar kind, and its most familiar version uses water as an electron source, releasing oxygen. It has enabled colonization of environments that are otherwise chemically difficult. It has also made it much easier to live off of biological material, because combining it with oxygen releases much more energy than reshuffling its molecules (fermentation).
The next step is multicellularity. It has evolved numerous times (The evolutionary-developmental origins of multicellularity), mostly among eukaryotes. Most sorts of multicellularity are plantlike, funguslike, or slime-mold-like. Some prokaryotes also exemplify these types: cyanobacteria, actinobacteria, and myxobacteria. But animallike multicellularity has evolved only once, judging from present-day organisms.
Bodies of water are poor places to work with fire and electricity, so we must consider colonization of land. Animals have done so several times:
- Arthropods: insects, pillbugs, land crabs, arachnids, myriapods (centipedes, millipedes)
- Mollusks: land snails
- Annelids: earthworms, leeches
- Vertebrates: tetrapods, starting with real-life “Darwin Fish”
Plantlike organisms have done so only once, however.
Something helpful for being large on land is an internal skeleton, and that’s evolved once in animals (vertebrate skeletons), and at least once in plants (wood). Curiously, most animal skeletons are either external (shells), skin-surface (arthropod skins), or just under the skin (echinoderm skeletons). So it may be hard for an animal internal skeleton to evolve.
Grasping limbs and jaws have evolved multiple times, however.
Among land vertebrates, grasping with digits has evolved at least twice, in primates and in perching birds (Passeriformes). Arthropods have evolved pincer limbs at least twice (scorpions, various crustaceans). Tentacles have evolved at least twice, in cnidarians and in cephalopods, and arguably a third time in proboscideans (the elephant’s trunk).
Turning to jaws, vertebrate ones are modified front gill bars, while arthropod ones are modified limbs. Some polychaete worms also have jaws.
About sense organs, vertebrates and cephalopods have independently evolved high-resolution lens-camera eyes, and vertebrates and arthropods have independently evolved color vision.
We now get to intelligence proper.
It may not often seem very apparent in our species, but I think that that is because we have rather high standards for ourselves by the standards of most of the animal kingdom. Thus, when we see some shoddy reasoning, we often think that its author must be terribly dumb, but that shoddy reasoning is described using linguistic capabilities that are far in advance of what just about every other species on this planet can do. Like a creationist declaring “Evolution can’t be true because dogs don’t give birth to cats.”
By comparison, (nonhuman) great apes have much more limited capabilities. They can learn lots of sign-language signs, but the most syntax that they have used is two-sign phrases, and even that is doubtful (Great ape language). Here is a transcript of an ape sign-language session: Koko.org – Koko’s World – Talk To Koko.
Most animal language is even simpler, the equivalent of single words or canned phrases in our languages. Vervet monkeys make calls in response to seeing various predators that their fellow monkeys respond to with appropriate evasion actions. So in human terms, those calls are “Leopard!”, “Snake!”, and “Eagle!” (The Semantics of Vervet Monkey Alarm Calls: Part I « Primatology.net, The Semantics of Vervet Monkey Alarm Calls: Part II – The Experiment « Primatology.net).
So that leaves dolphins and other toothed cetaceans as our only plausible competition on this planet. Dolphin language has been difficult to decode, but some parts of it are now understood, like imitations of echolocation-system echoes of objects. In human terms, that’s like calling a dog a woof-woof or a cat a meow-meow.
I went into this digression into language because that’s necessary for describing how to build a radiotelescope for interstellar communication — and also for describing why it might be worth building.
Another criterion is self-awareness, like being able to recognize oneself in a mirror. Human children become able to do that at about 18 – 24 months old, and a few other species seem to have this ability (Mirror test): (other) great apes, dolphins and orcas, elephants, and European magpies.
Most others don’t, and to a dog or a cat, the dog or cat in the mirror is another one.
The next question is what would drive the evolution of intelligence, since big brains can cost a lot of energy to keep going.
Visual perception or echolocation interpretation can require a lot of brainpower to discover a lot of details, and the larger brains are indeed of users of these abilities (Cetacean intelligence, etc.).
Another hypothesis is Robin Dunbar’s social-brain hypothesis. According to it, large brains evolved for keeping track of other members of one’s species, and among various simian species, there is indeed a correlation between brain size and social-group size. Extrapolating to our species, RD finds Dunbar’s number, about 150.
Large social groups and full-scale language can combine for transmitting complex information down the generations.
Might that also be true of cetaceans? Or might it be a side effect of large brains elaborated for interpreting echolocation? In any case, it’s evident from a lot of research that bottlenose dolphins, the best-studied species, have very complex societies.
So we have one example of human-scale intelligence, ourselves, and some species that come close – some of the toothed cetaceans.
So while some steps involved in the evolution of intelligence have happened several times, other steps have happened only once. It’s not clear whether the latter sort of step tends to pre-empt other instances or whether it does not often happen. So fi is up in the air.
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