Recent years have seen an efflorescence of books on human uniqueness; indeed, they are becoming a genre (among others, Gärdenfors [2003]; Tomasello [2009]; Sterelny [2014]; Henrich [2016]; Berwick and Chomsky [2017]; Heyes [2018]; Enquist et al. [2023]). It’s Only Human can be read a concise synthesis of recent debates, as well as a more personal attempt at solving the riddle of human cognitive uniqueness. How can we best describe the suite of cognitive traits that arguably caused the human species to invade and transform our planet in such a short amount of time. Armin Schulz defends and illustrates several trends that have been gaining momentum in the ‘human uniqueness’ literature. These trends tend to go in the same direction: away from the view that human uniqueness can be captured as one genetically encoded trait.

The best known (if not the most credible) example of that view is Berwick and Chomsky’s ([2017]) ‘Prometheus’ hypothesis. In this scenario, a uniquely human capacity for recursive thought and language emerged thanks to a random and improbable genetic mutation, which subsequently spread, giving rise to human cognition and language as we know them. In direct opposition to this, a consensus is emerging from recent work that whatever makes humans cognitively unique does not rest on one single trait, that culturally transmitted traditions played a role in the evolutionary history of human cognition, that uniquely human traits evolved gradually and in a piecemeal fashion, and that this gradual evolution was made possible by feedback loops, where the growing sophistication of one cognitive trait would foster the development of other traits. It’s Only Human defends and develops these claims, which Schulz does not present as novel contributions but which he nonetheless systematizes to a higher degree than previous authors (especially the last claim).

That no single ‘silver bullet’ adaptation underlies uniquely human cognition is argued for in chapter 2. For Schulz, there is no subsuming all the peculiarities of human cognition under one umbrella, be it language, symbolic thought, tool use, or the capacity for culture. The same argument applies to each of these capacities taken on its own: neither language, symbolic thought, tool use, nor the capacity for culture can be captured as one central cognitive capacity. Instead, Schulz describes them as relatively heterogeneous and overlapping suites of cognitive traits. For instance, the capacity of humans for cultural learning may, on Schulz’s view, depend on dedicated adaptations for communication or joint attention, but it may also rest on greater social toleration, cooperation, or general learning abilities—or on all these things combined. Language, both a product of cultural learning and an extension of it, may or may not rely on specific innate adaptations for syntax; but language users profit from other cognitive capacities, like mind-reading. The evolution of human cognitive uniqueness, as depicted here, was a matter of enhancing our species’ mental toolkit across the board. The resulting cognitive traits are by no means always unique to our species—just more developed. Human uniqueness is a matter of degree rather than a difference in kind. Humans are unique; they are not unique in one single way.

How this massive (if gradual) enrichment of our mental toolkit came about is the topic of chapters 3 and 4. Schulz offers an eclectic and conciliatory synthesis of previous proposals, trying to adopt, for each account, those aspects that are the least controversial and the most compatible with other views. Like several contemporary accounts, Schulz stresses the role of cultural learning in the emergence of distinctively human cognition. But unlike most of them, he does not overstate this to the point of denying the existence of biologically evolved and developmentally canalized mechanisms. To justify this, Schulz starts by noting that the kind of cognitive task that only humans can solve requires us to explore and choose among very large sets of possible inferences. To narrow down the set of possible inferences to the handful that is actually appropriate, cognition avails itself of innately specified priors (called ‘evolved representational expectations’ in the book) in addition to culturally acquired information. Refreshingly, Schulz avoids pitting culture against biology, as though the two were playing a zero-sum game. Evidence that culture contributes to shaping a cognitive capacity cannot be taken to imply no biologically evolved constraint is at play. On the contrary, human cognition needs all the help that it can get.

Still, simply pointing at the joint contributions of culture and biology does not satisfy Schulz, who notes that humans are not merely equipped with uniquely sophisticated cognitive tools, but also capable of using these tools in spite of the costs that this entails. These, in Schulz’s view, are primarily energetic and metabolic costs, driven by cognitive demands linked to memory and executive processing. This is a recurring theme in the book, and a novel idea in this literature: it is one thing to have the cognitive wherewithal of language or mind-reading, another to routinely pay the processing costs associated with their use.

It is cognitive technologies, or thinking tools, that are supposed to explain how these costs are met. No precise definition is given for these technologies; rather, they are characterized in different ways in different places in the book. First of all, a thinking tool is a culturally transmitted way to store information outside one’s mind for the long term, by offloading it to some kind of material support (like writing, knotted strings, or accounting tablets). Second, some thinking tools also allow us to make inferences based on externalized representations—think here of calculating machines, logarithmic tables, or the abacus. Although the prototypical examples of cognitive technologies are material artefacts, thinking tools need not be objects. Social institutions are listed as thinking tools, as well as (occasionally) songs or myths.

Thinking tools, the argument goes, allow humans to pay the costs of using sophisticated cognitive abilities. That point is familiar from theories of extended cognition: offloading information storage and processing to external devices enhances the range and efficiency of thought. In so doing, cognitive technologies set up the positive feedback loop that lies at the heart of Schulz’s account of human uniqueness. Thinking tools do not simply boost individual cognition, they improve cultural transmission as well. Cognitive technologies, being cultural, increase in number and efficiency when cultural transmission gets more accurate; cultural transmission contributes to building up the human ability to carry out complex inferences on numerous abstract representations; and these reasoning capacities make it easier, in turn, to invent better thinking tools, closing the loop.

This positive feedback mechanism is central to Schulz’s account, because it explains why humans may be cognitively unique without being unique in any single way, our difference with other animals often a matter only of degree. On his account, culture supercharged the evolution of cognitive capacities across the board, without necessarily favouring any specific cognitive domain above all others. The result is closer to an enhanced capacity to handle any and all complex and abstract concepts than to an improvement in one species-defining adaptation.

The book, it is worth noting, makes two orthogonal claims about human uniqueness: first, that human cognition is distinctive on more than one dimension; second, that on each of these dimensions, human uniqueness is a matter of degree. The feedback loop hypothesis clearly fits with the second claim: human features increased gradually instead of arising from one qualitative jump. It is not clear to this reviewer how the feedback loop scenario explains or supports the view that human uniqueness consists of multiple cognitive traits instead of just one.

The rest of the book illustrates this broad framework by focusing on specific capacities: mind-reading (chapter 5), morality (chapter 6), trading (chapter 7). Before concluding, chapter 8 also covers religion, artificial intelligence, and patent laws (with a concision that might seem rushed).

Schulz’s treatment of mind-reading is fairly representative of the other chapters and of the general logic of Schulz’s account. It wisely refrains from making overly risky claims about the uniqueness of human mentalizing. It is difficult to disagree with the claim that reasoning about others’ mental states is stronger in humans across a broad range of dimensions: we alone can reason accurately about others’ mental states, with a high degree of recursivity, even when these mental states consist in highly abstract concepts. Critics may object that, on this view, what makes human mind-reading unique has nothing to do with mind-reading per se; instead, it boils down to general cognitive capacities for making complex inferences about abstract concepts. But that is exactly Schulz’s point throughout the book: human uniqueness is best seen as a general enhancement of relatively commonplace capacities.

Schulz follows previous work (Zawidzki [2013]; Heyes [2018]) in giving culture a major role in the evolution of human mind-reading, but he does so somewhat reluctantly. Extreme culturalist accounts of mind-reading liken it to literacy: a skill that is just as dependent on teaching as learning to read, and neither more nor less supported by biologically evolved capacities (Heyes [2018]). Schulz refrains from endorsing this view. As he notes, no one seriously claims to have identified societies in which mind-reading would be altogether absent—places where, say, most adults understand but fail to pass a standard false belief task. It’s Only Human’s soft culturalism rests on the view that our interpretations of others’ mental states are coloured by cultural expectations about the causes of behaviour, and on two studies showing that the development of mind-reading follows different trajectories in different places. This variation is explained by the importance of thinking tools in the evolution of mind-reading, which also coheres with the claim that the specificity of human mentalizing does not rest on one single genetically encoded adaptation.

It’s Only Human is a commendable philosophical introduction to the cognitive science literature on human uniqueness. Schulz provides a balanced, carefully argued, and insightful version of a position that is gaining ground in the literature: the sources of human uniqueness are not to be sought in one species-defining biological adaptation. His middle-of-the-road account eschews the excesses of previous culturalist accounts. That said, two of the book’s qualities, its prudence and its concision, also keep it from raising some crucial questions, making it inevitably incomplete as an explanation of human uniqueness.

First among these unanswered questions is why a positive feedback loop linking culture, technology, and cognition got started for our species, but not for others. The book makes it clear that with culture as with anything else, the human advantage is only a matter of degree: while other animals do have capacities for cultural transmission, these remain rudimentary having not been touched by the culture–technology–cognition feedback loop. But why would the feedback loop ignore other species? Schulz’s brief attempts to answer this difficult questions allude to some kind of threshold effect (for example, pp. 80, 119). For the loop to kick into motion, at least one of its three components (it is not clear which; perhaps it’s all three) has to reach some critical level of efficiency. There is no telling where that level is and what prevented other animals from reaching it. Difficult questions, obviously, but without answering them, It’s Only Human cannot make clear, refutable predictions that comparative psychologists may test.

Another pending question concerns the role of thinking tools (or cognitive technologies) and their nature. On this point Schulz’s book (as I read it) licenses two quite different interpretations. On one reading, thinking tools are a very broad category encompassing any forms of symbolic culture, from computers and books to songs, stories, and myths. A calculating machine is a thinking tool, as are The Lord’s Prayer and The Threepenny Opera (two examples cited in the book) and even some institutions like marriage and markets. On another reading, thinking tools have to be, well, tools—that is to say, external artefacts like tablets, books, calculating machines, and so on. Schulz explicitly defends the first interpretation, but in the vast majority of his examples, thinking tools are external artefacts.

If we take the first reading, it is hard to see how exactly thinking tools fulfil the purposes the theory assigns to them: to store and process information externally (Schulz does not subscribe to extreme externalist accounts of cognition, which his book criticizes). A song or a prayer known by heart is not a way to store information externally, but the very opposite: a piece of internally stored information. (As Schulz notes, songs or prayers can occasionally be used as mnemonic tools. But even then, mnemonic tools by definition stabilize internal memories; they do not store information externally.) It is equally difficult to imagine how songs, rituals, or institutions can perform inferences independently of individual minds—the other major function of thinking tools.

The second way of defining cognitive technologies thus seems more promising, but here Schulz’s account runs into a problem of timing. Most of the thinking tools he mentions—writing, books, scrolls, quipus, the abacus, accounting tablets, and so on—are quite recent on the timescale of human evolution. All these things are post-Neolithic inventions, unknown to non-agriculturalists for a long time, and they could not have influenced the evolution of cognition for most of the history of Homo sapiens. Even the idea that writing had a profound impact on the evolution of human cognition since its invention 5000 years ago (p. 81) somewhat misses the mark. Writing as it existed 5000 years ago could not do many of the things that it does today (like encoding full sentences), and only a few could use it. Even today, 15% of humans are illiterate (the figure gets higher still when taking functional illiteracy into account). Here and there, It’s Only Human alludes to other, more ancient forms of information storage, like petroglyphs or cave paintings; but it is doubtful whether such Palaeolithic images were ever used to store large quantities of information externally (let alone process it), as opposed to serving as mnemonic props to aid the retrieval of already memorized information (Morin [2023]).

Schulz’s insistence on the importance of modern thinking tools like writing or numerical inscriptions for the evolution of human cognition sometimes suggests that the real import of the book is not in its concern for universal human traits. This tendency is clearest in the chapter on trade—a practice clearly ignored by most human societies, Adam Smith’s outdated views notwithstanding (Graeber [2014]) and dependent on very recent technologies like money and accounting. Here, the book’s focus is a phenomenon that may be uniquely human, but is by no means pan-human. This, perhaps, is the best way to make sense of the book’s overall argument: not as a theory of human nature, but as a theory of the uniqueness of some human practices.

Olivier Morin
Institut Jean Nicod
alf.drummond@gmail.com

References

Berwick, R. C. and Chomsky, N. [2017]: Why Only Us: Language and Evolution, Cambridge, MA: MIT Press.

Enquist, M., Ghirlanda, S. and Lind, J. [2023]:  The Human Evolutionary Transition: From Animal Intelligence to Culture, Princeton, NJ: Princeton University Press.

Gärdenfors, P. [2003]: How Homo Became Sapiens: On the Evolution of Thinking, Oxford: Oxford University Press.

Graeber, D. [2014]: Debt: The First 5,000 Years, Brooklyn, NJ: Melville House.

Henrich, J. [2016]: The Secret of Our Success: How Culture Is Driving Human Evolution, Domesticating Our Species, and Making Us Smarter, Princeton, NJ: Princeton University Press.

Heyes, C. [2018]: Cognitive Gadgets: The Cultural Evolution of Thinking, Cambridge, MA: Belknap Press.

Morin, O. [2023]: ‘The Puzzle of Ideography’, Behavioral and Brain Sciences, 46, available at doi.org/10.1017/S0140525X22002801.

Sterelny, K. [2014]: The Evolved Apprentice: How Evolution Made Humans Unique, Cambridge, MA: MIT Press.

Tomasello, M. [2009]: The Cultural Origins of Human Cognition, Cambridge, MA: Harvard University Press.

Zawidzki, T. W. [2013]: Mindshaping: A New Framework for Understanding Human Social Cognition, Bradford Books.