iguanas of gethsemane

dragons_of_eden_coverCarl Sagan’s milieu was astrophysics and getting high, so his 1976 book Dragons of Eden stands as a somewhat incongruous musing on human intelligence and cognitive abilities. Smart people stepping outside their intellectual home can be annoying when they assume they’ll revolutionize an entire field in one fell swoop, but the good ones—like Sagan—can offer novel perspectives and ideas. Of course the book is the almost four decades old, so while it’s a good window to the mid-70s cognitive zeitgeist (the triune brain, left/right hemisphere specialization, and sign-language trained apes are featured), the outdated discussion probably makes it better for people with background knowledge rather than newcomers. Below are some thoughts about what “human intelligence” is.

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Cross-species comparisons of intelligence are inherently egocentric, with human intellect acting as a default reference point. Great apes, our evolutionary next-of-kin, are typically counted as the most intelligent animals, but we’re often skeptical when similar claims are made about crows, gray parrots, or octopodes (note: that octupus article is really good).

This could be sheer egocentrism, but this ape-predilection may reflect a limitation on our own intelligence rather than a belief in our own superiority. I can understand the feelings, beliefs, and desires of others (theory of mind is a critical stage in cognitive development), and I can see or project glimmers of human-like thought in chimps. But I can’t imagine the world of dogs, or the perceptual experience of echolocating dolphins and shrimp that see infrared. That we are skeptical about the cognitive skills of birds and octopi, and that sci-fi tends to depict intelligent alien life as anthropomorphic creatures with anger issues, might be an extension of how the skills of our close relatives are simply more accessible to us. I doubt most people think intelligence must be human-like; but it’s the only kind of intelligence we can really imagine.

In part because of our own intellectual limitations, measuring and comparing intelligence across species is a complicated and ill-posed problem. There are two general approaches to this task: examining behavior or examining neuroanatomy. I suggest simply asking the animals, but this has been rejected by most leading researchers. Those two approaches aren’t mutually exclusive, but both pose challenges: uncertainty about what behaviors indicate intelligence, uncertainty about how to measure those behaviors, and uncertainty about the anatomical underpinnings of intelligence.

For example, imagine you believe self-awareness is a defining feature of intelligence. How do you test that? Maybe if they recognize themselves in the mirror? That’s not a bad idea, but even a simple idea like that hides complexities. For instance, how do you know if they recognize themselves? In actual mirror tests, they paint a mark on the animal’s face and see if the animal tries to remove it. Dolphins, great apes, and elephants “pass” the test; dogs usually don’t. But the whole artifice assumes that the animal has good vision, and surely sight isn’t a prerequisite for self-awareness. If an “olfactory mirror” existed, we might all have a different impression of our canine brethren; indeed we might find many animals are smarter than we think.

A more commonly cited behavioral sign of intelligence is language. In the 1960s, researchers tried to teach apes to speak. As it turns out, apes don’t have the articulatory equipment to do that, so researchers next tried to teach them sign language (in hindsight, it seems to have taken a very long time to think to do that). Initially, apes seemed to pick up signing*, but that early success has mostly plateaued and Sagan’s mid-1970s optimism on apes’ language abilities would probably be diminished now. What we do know is that some apes can learn a repertoire of up to one thousand or more signs to communicate with their trainers (and, interestingly, scientists have noted individual differences in ape-language ability; Kanzi is said to be quite gifted, while Koko is a bit slow).

*In contrast to the approach taken with great apes, many researchers have attempted to establish direct communication with dolphins, whose vocalizations have the characteristics of language and whose cognitive skills and brain structure seem advanced. I’m partially convinced dolphins spend most of their time laughing at us.

silly humans

Whether apes or any other animal are capable of language is difficult to assess because “language” is a nebulous concept with no rigid definition (just like intelligence, which gives a sort of semantic jenga-tower feel to the whole thing). Two commonly cited features of real language are symbol use and the ability to describe novel concepts or actions. Their use of signs and their occasional tool use suggests that apes are capable of symbol manipulation. But instances of novel, generative signing (e.g., calling a swan a “water bird”) are rare. Whether those rare occasions constitute “real” language use or just random chance is still debated. I tend to be pessimistic on this point; the expansive and generative aspects of language suggest to me that there’s no “gray area” in language use—if there’s a debate about whether it constitutes language, it’s probably not language.

If mirror tests and debates about language are too messy, a different approach is to focus on anatomy, including brain size. Encephalization quotient is the ratio between the expected brain size and the actual brain size of an animal, given its mass. Humans top out EQ among mammals at above 7, meaning our brains are about 7 times larger than would be expected given our body size. Bottlenose dolphins come next at around 4, and chimps are all the way down around 2.5. Rabbits clock in at just 0.4, but they make up for it by being very cute.

Besides the face validity it has in placing humans, dolphins and great apes at the top, EQ is compelling because it fits with many theories about why human intelligence evolved. Big brains are costly: they use up a lot of energy and force us to be born immature and helpless. By one view, only when humans began eating calorie-dense meat (or possibly cooking it) could they afford the energy costs of such a bulbous brain. In keeping with the hypothesis that meat matters, the EQ of carnivores is generally higher than that of herbivores. Similarly, hunting animals have greater EQs than grazing animals, which makes sense given that hunting requires planning and strategy. Still another view is that big brains evolved in tandem with large social groups; supporting this, pack animals have greater EQs than solitary animals. Meat, planning, and navigating a complex social world might all be important in evolving intelligence.

But brain size alone isn’t a strong indicator of intelligence. In fact, comparing relative brain sizes only makes sense when the brains have morphological similarities or are evolutionarily close (e.g., humans and chimps). The octopus has an order of magnitude fewer neurons in its brain than humans, but also has neurons in each leg, making comparisons difficult and octopodes terrifying (that distinctiveness is in itself interesting, though: that such evolutionarily distant species as birds, octopodes, and apes each developed somewhat similar intellectual capabilities, even with anatomically divergent brains, may be an example of convergent evolution).

When we look at brain size, we’re almost assuredly looking not at a direct correlate of intelligence, but a byproduct of some underlying combination of neuroanatomical factors that contribute to intelligence, analogous to how we might look for smoke and heat to detect a fire. These neuroanatomical factors might include the degree of cortical folding (which increases surface area without increasing brain volume), nerve conduction velocity (which may speed information processing), or amount of cortical neurons. Certainly we don’t yet know exactly what those factors are or how they interact.

Another possible structural correlate of intelligence is spindle neurons (also awesomely known as von Economo neurons). “Typical” neurons (left) branch into multiple dendrites, which are essentially the point(s) where one neuron “connects” to others. In contrast, spindle neurons (right) are longer and have few dendrites.

neuron (left), spindle neuron (right)

Spindle neurons act almost like an express bus, transmitting information directly and only from one place to the next (a ludicrously oversimplified but reasonably apt analogy). There are many reasons to think spindle neurons may underlie human-like intelligence:

1. they are found in “intelligent” animals like great apes, whales, and elephants, but not in less intelligent animals. Within the great apes specifically they are found in greater numbers in apes thought to be more intelligent and/or evolutionarily closer to humans.

2. they are found only in specific brain areas (e.g., the anterior cingulate cortex) that generally underlie higher brain functions like conflict monitoring, emotion regulation, impulse control, etc.

3. their anatomy—the “express” connection—fits with the idea that conduction velocity is an important contributor to intelligence. If information is getting from point-to-point more quickly, the animal is smarter.

4. it might explain why humans have a poor sense of smell and taste compared to even close relatives: the ACC, locus of most spindle neurons, is largely responsible for olfactory processing in other species. Humans may have repurposed that area for higher cognitive skills, sacrificing sensitivity to taste and smell.

5. there is some limited evidence that the ACC is the seat of free will/self-awareness in humans.

That’s all circumstantial; ”Intelligence” is assuredly a complicated and partially unpredictable interaction of anatomical, functional, and environmental variables that we may never fully describe. Spindle neurons likely aren’t some magic pixie dust for intelligence, but they’re pretty rad.


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