Let us resume our discussion of Kevin Laland’s excellent Darwin’s Unfinished Symphony: How Culture Made the Human Mind, by tackling the first two chapters of the second part of the book: 6, on the evolution of intelligence, and 7, on high fidelity (of copying behaviors). Just to give you heads up, we are then left with five more chapters, likely each deserving its own post: why we alone have language, gene-culture coevolution, the dawn of civilization, foundations of cooperation, and the arts. The best has yet to come, and we will be at this for a while. But I assure you, it is worth it.
The chapter on the evolution of intelligence (#6) opens with a brief explanation of Allan Wilson’s observation, back in the ‘80s, of an interesting relationship between an animal’s brain size and the time since it had shared a common ancestor with humans: over the last 400 million years animal brains have grown 100-fold, and the growth rate has accelerated over time, a potential indication of a feedback mechanism at work.
Wilson proposed a three-step hypothesis, the so-called cultural drive scenario, to explain the evolution of intelligence:
1. A new advantageous habit arises in an individual through behavioral innovation.
2. The new habit spreads throughout the population by way of social learning.
3. Selection favors mutations that augment either the likelihood of innovation or the ability to engage in social learning.
Moreover, the expectation — which has been since empirical verified — was that new habits generate selection for changes in the animal’s anatomy that are better suited to the new behavior. Indeed, a good portion of this chapter is devoted to fleshing out Wilson’s original hypothesis, as well as to line up empirical evidence for it. (Laland was supposed to work with Wilson as a postdoc in 1991, but Wilson tragically died a month before Laland’s arrival.)
One of the issues in this discussion is that, quite obviously, large brains are not necessary for animals to take advantage of social learning — think of eusocial insects like bees and ants. The idea, however, is that larger brains evolved in primates because they make social learning increasingly efficient, and that trait is favored by natural selection.
Better learning — for instance through copying — in turn requires better perceptual systems, so that it is easier for an animal to copy another’s behavior (e.g., a fishing technique) at a safe distance. This, in turns, mandates an increasingly sophisticated type of integration across perceptual systems, as well as a precise mapping of sensorial inputs to behavioral outputs:
“To copy a fishing technique, for instance, the observer’s brain must convert a stream of visual information about how others move their hands and arms into corresponding outputs specifying how the observer must also move its muscles and joints. … Selection for copying proficiency might plausibly favor the evolutionary expansion of circuitry linking the visual and auditory cortex to somatosensory and motor cortex regions of the brain.” (p. 129)
Moreover, effective copying might also promote the evolution of what is often referred to as a “theory of mind,” meaning the ability of an animal to form an idea of the intentions, beliefs, and desires of other members of its own species. Not only that, but the cultural drive hypothesis also predicts that large brained animals should evolve pro-social traits (like tolerance for other members of the group, and cooperation), because learning requires allowing especially the young to hang around so that they can observe and copy advantageous behaviors.
Sure enough, for instance it is known that young chimpanzees hang around their mothers well after they are physically independent, until age seven or so. Presumably so that they can learn from observing, copying, and experimenting with her behaviors. Notice also that in most animals the innovators are adults with a lot of experience, another reason for the young ones to hang around as much as possible.
Larger brains, of course, are a necessary byproduct of the need for better and better cognition at so many levels. One of these levels has to do with the need to keep track of social interactions in increasingly larger groups, and an interesting observation is that the rate of “tactical deception” of other members of the group correlates positively with brain size in primates: the smarter we get, the more Machiavellian we become.
One of the studies conducted by Laland’s group that I found particularly interesting is a test, across species, of the standard evolutionary psychology hypothesis that minds are highly modular, i.e., that our brains work like cognitive Swiss Army knives, with each module evolved very specifically for a particular task or behavior. No such thing. A study of 62 primate species showed just one principal component explaining a very large amount of the variance in cognitive measures, with species excelling at one domain (say, innovation, social learning, tool use, etc.) also excelling at all others. Not that I sense any indication whatsoever that evolutionary psychologists have taken such results on board, of course. Moreover:
“Those species that were designated ‘smart’ by our statistical measure of primate intelligence turned out to be precisely those species that had performed well in laboratory-based experimental tests of learning and cognition.” (p. 138)
And just to make crystal clear what Laland thinks this means for evolutionary psychology:
“Such findings are inconsistent with the view, widespread within evolutionary psychology, that cognitive abilities evolve independently as separate modules, and the results strongly imply general intelligence [across primate species].” (p. 139)
The range of brain size (both in absolute and relative terms) in primates is huge, going from 3 g in fat-tailed dwarf lemurs to 1.5 Kg in humans. The overwhelming majority of the evolutionary increase is due to development of the neocortex, known in humans to be the locus for problem solving, learning, planning, reasoning, and language. And — no surprise here — our species possesses by far the largest (both in absolute and relative terms) and best connected neocortex of all primate species.
Why would such an increase in brain size, and particularly in the neocortex, be favored in primate lineages? Because productivity increases dramatically with age, once an animal has had sufficient time to learn a range of adaptive behaviors, so long as there is a reliable inter-generational flow of information, from older to younger individuals. Furthermore, mathematical models have shown that selection favors lower mortality rates and longevity when productivity increases over time the way it does in primates:
“By virtue of possessing a big brain, and being able to use it to acquire all kinds of useful survival skills from others, some clever primates had apparently been able to extend their lifespans and live longer. In other words, in primates and primates alone, cultural intelligence facilitates survival.” (p. 147)
Next, let us briefly discuss chapter 7, on “high fidelity.” It opens with a darn good question:
“If cultural drive has operated on all the great apes and some monkeys too, then why haven’t gorillas invented particle accelerators? Why haven’t capuchins put a monkey on the moon, or devised a simian version of Facebook?” (p. 150)
Even though I don’t think a simian version of Facebook would be an improvement on things, and even though some people may be inclined to dismiss the very question as framed by Laland, this does get to the core of the issue of cultural evolution addressed by the book. Why indeed?
Kevin considers a number of proposed answers. Maybe it was just chance. As he admits, chance explanations are hard to rule out, but they are also unappealing and hard to take seriously, when one is talking about such a striking difference between Homo sapiens and everything else, a difference predicated on a large brain that consumes a hefty portion of our daily caloric intake. The answer must lie elsewhere.
Maybe it was demographics: once our species reached a certain population threshold cultural information became less likely to be lost, and it started to accumulate. But as Laland quickly observes, there are plenty of other species with large population sizes, and they haven’t invented vaccines or drafted declarations of individual rights.
Instead, some tantalizing clues come from mathematical modeling exploring how many “cultural parents” are necessary for stable cultural transmission. The details are intriguing, and I refer the reader to the book chapter for much more, but the bottom line is that a small increase in the fidelity of social learning makes a huge difference for the stability of cultural traits, and beyond a certain threshold of fidelity it makes them essentially immortal. In turn, high-fidelity mechanisms simply support a far larger accumulation of cultural knowledge, over time. So a species’ repertoire becomes both more reliable and far larger.
Why primates, however, and humans in particular? Because, as it turns out, most other species operate on the basis of low fidelity cultural transmission, including birds, fishes, and insects. As a result, most animal species either have no cultural traditions at all, or are characterized by so-called “lightening traditions,” i.e., patterns that persist in a population only for an evolutionarily short period of time and then are lost, possibly re-invented, and lost again. No cumulative process there.
“Across thousands of simulations with different parameter values, trait fidelity explained more of the variance in the buildup of cumulative culture than novel invention, modification, and combination combined.” (p. 156)
One of the interesting, and perhaps counterintuitive, findings here is that innovation is the least important of the pertinent cultural traits. That’s because cultural evolution takes place largely through continuous, cumulative, alterations on previous designs, reworking and further developing pre-existing technology. The myth of the heroic inventor is, largely, a myth.
Not only that, but teaching — defined by Kevin as behavior that functions to enhance the fidelity of information transmission between tutor and pupil — turns out also to be a crucial factor in ever accelerating cultural evolution. Even though there is controversy about this point, there are precious few solidly documented cases of teaching outside the human species, while the behavior is universal — and in some cases highly refined — across human populations. Moreover, the known examples of animal teaching do not, in fact, show up where one would expect them to:
“Animal behaviorists have now compiled evidence for teaching in a small and rather curious assortment of species; these include meerkats, ants, bees, and two species of birds called pied babblers and superb fairy wrens; with suggestive, but not yet conclusive, evidence in cats, cheetahs, and tamarin monkeys. The functional similarities between teaching in, say, ants and humans should not [however] obscure the fact that mechanistically, cases of teaching in other animals are entirely different from human teaching, and are reliant on entirely separate psychological and neural processes. … We were struck by the observation that no compelling evidence for teaching had been found in nonhuman apes, dolphins, elephants, or other large-brained mammals celebrated for their intelligence.” (p. 161-162)
Some findings about teaching in the animal world — based on computer modeling — are intuitive, for instance that it is favored by selection the more tutor and pupil are genetically related to each other. But other findings are both counterintuitive and enlightening: teaching is favored when it concerns tasks of intermediate difficulty, not too easy, not too difficult. Why? Because if a task is very easy then the individual is likely to learn it on its own, no teaching required — which in turn explains why a lot of intelligent species do not engage in teaching. By contrast, if a task is too difficult, then there simply won’t be enough teachers available to transmit it efficiently to the next generation. Comparatively few traits are found in the right intermediate range of difficulty, which is probably a major part of the explanation for the rarity of teaching in nature. There is an important lesson to be learned here:
“The incidence of teaching only appeared puzzling because we had the wrong intuitions. We had expected teaching to be exhibited by clever animals that are good at social learning. In fact, with some caveats that we will come to, smart animals rarely need to teach, because most of their skills can be picked up through copying or trial and error.” (p. 165)
The attentive reader, however, will have picked up on the fact that human beings seem to flagrantly violate the above rules: we teach very, very complex behaviors indeed! Laland and his collaborators began to suspect that it was cumulative knowledge that had made the difference. Interestingly, once the mathematical models allowed for cumulative knowledge, the results clearly showed that teaching was favored, as the relative fitness of teachers to non-teachers was almost always higher under those circumstances:
“Models that allow for cumulative cultural knowledge gain, however, suggest that teaching evolved in humans despite, rather than because of, our strong imitative capabilities, and primarily because cumulative culture renders otherwise difficult-to-acquire valuable information available to teach. The analyses suggest that human teaching and cumulative culture evolved together, through mutual reinforcement.” (p. 167)
Yet another factor facilitating the evolution of teaching is cooperative breeding, when young ones are raised by a group of relatives, instead of just the parents, or one of the parents. That’s because cooperative breeding makes the fitness cost of being a teacher surprisingly low, thus favoring the behavior. Finally, experimental results show that human children far outperform other primates at cognitive tasks in part because they cooperate with each other in the solution of problems, engaging in a high number of “altruistic events” during the time a group is occupied with a given task. The conclusion, so far in the story, is then that:
“Humans alone possess cumulative culture because humans alone possess sufficiently high-fidelity information transmission mechanisms, including an unusually accurate capacity for imitation, teaching, and language.” (p. 174)