An extraordinary trait that humans have, one that separates us from all other living beings, is our “unique symbolic cognitive style” (Tattersall, 2017). As the philosopher Ernst Cassirer noted, humans are not the animal rationale but the animal symbolicum (Cassirer, 2006, p. 31). Although other animals are capable of challenging cognitive behavior — for instance, the crow’s ability to make stick tools (Bluff et al., 2007), and the apparent symbolically mediated behavior of late Neanderthal populations (Jaubert et al., 2016) — the human capacity for symbolic thinking is immeasurably greater and qualitatively distinct, so much so that Charles Darwin himself commented, “the difference between the mind of the lowest man and that of the highest animal is immense” (Darwin, 1871, p. 100). Alfred Russel Wallace, a co-discoverer of evolution by natural selection, was particularly puzzled because he did not see tangible evolutionary advantages to the products of this unique capacity for symbolic thinking, such as music and the arts (Wallace, 1870). Assuming that early human symbolic behavior can be read from the archeological record, we explore the emergence of cave and rock art in human evolution and assess its relation to the development of human language.

When did we acquire this cognitive capacity for symbolic thinking?¹ The answer to this question must necessarily be based on indirect evidence, since we do not have access to facts about the variability and heritability of this trait (Lewontin, 1998). Suppose we equate high cognitive ability with brain size. The hominid brain has been growing for 2 million years, doubling in size twice during that period (Holloway et al., 2004), with modern humans at the end of the line showing the highest encephalization quotient (Finlay, 2009). However, Neanderthals had a brain that was larger in volume than humans (Holloway, 1981). Even after discounting for the disproportionate size of the area dedicated to visual perception (Pearce et al., 2013), their brain size was still comparable to that of modern humans. Yet they did not develop the kinds of behavior, such as agriculture and language, that we associate with high cognitive ability (Tattersall, 2008, 2010).

Is there something else in human evolutionary history that would indicate some drastic and qualitative change in behavior, signaling the emergence of symbolic thinking? Tattersall (2008, 2012, 2016a,b, 2017) makes an intriguing observation about the pace of technological innovation. The first stone-tool technology appeared 2.5 million years ago (Semaw et al., 1997), and it stayed basically the same for a million years before innovation was introduced in the form of the Acheulean handaxe. Another million years went by before a significant innovation took place, in the form of core preparation. In other words, innovation was rare and interspersed with long stretches during which hardly any change occurred. But toward the end of the Pleistocene, a profound shift occurred: technological innovations began to appear in rapid progression, and this marked a “relatively abrupt and qualitative change in mental information processing” (Tattersall, 2017, p. 5). This era of rapid change corresponds approximately to markers of symbolic thinking, such as the pieces of engraved ochres (Henshilwood et al., 2002) and the marine pierced shells (Henshilwood et al., 2004; d’Errico et al., 2005) found at Blombos Cave, which have been dated back to around 70,000–100,000 years ago.² These constitute the first archeological proxies signaling the fixation of the human language faculty (Huijbregts, 2017).

An often-noted early piece of evidence for symbolic thinking is the two slabs of ochre from the Blombos Cave in South Africa. Ochre is an iron-rich mineral that served several roles, including body decoration, along with more utilitarian roles (Watts, 2009; Hansen, 2011). In Blombos Cave, more than 8,000 pieces of ochre-like material have been found (Rosso et al., 2017). Some, like those in Figure 1, have geometric engravings and incisions.

It has been suggested that these regular patterns are a proxy for symbolic thinking (Henshilwood et al., 2002; Tattersall, 2009).³ The idea is that the ochre engravings are an external, abstract representation of internal high-order cognitive processes. This is similar to spoken language, which is an external form of highly complex internal cognitive representations and computation. Although Neanderthals produced etchings (Rodríguez-Vidal et al., 2014) and geometric structures (Jaubert et al., 2016), they apparently did not possess the cognitive capabilities that modern humans do (Tattersall, 2008, 2010; Mendez et al., 2016; Sankararaman et al., 2016; Vernot et al., 2016).⁴ The mechanisms for this heightened cognition involve computational processes that may also occur in other animals but that in humans “are uniquely powerful in their range, capacity and flexibility” (Heyes, 2012).⁵

The rapid innovation in tool making and the production of the Blombos Cave ochres and pierced shells suggest that modern humans by around 100,000 years ago were able to tap some cognitive resource that had not existed before. We will look at a well-known phenomenon that heretofore has been sparsely considered as exemplifying symbolic thinking. This is the phenomenon of cave and rock art, which is found on every major continent occupied by modern humans (Bahn and Fossati, 1995, 2003; Bahn et al., 2008, 2012). We wish to understand the nature of the expressions of symbolic thinking inherent in these artifacts as a way to begin to understand the evolutionary process that led to a fully developed symbolic species. We will show that our findings about these artifacts parallel aspects of human language.

We propose that the phenomenon of cave and rock art plausibly indicates how an internalized “system of thought” (Chomsky, 2011, 2013, 2015, 2016), which presumably evolved with the speciation of modern Homo sapiens around 200,000 years ago (Huijbregts, 2017), may have taken shape into concrete, externalized language. If this turns out to be true, the often-stated idea that “language does not fossilize” (e.g., Deacon, 1997; Fitch, 2010; Berwick and Chomsky, 2016) is not quite true: pieces of externalized language may turn out to be hidden among the art forms produced by our early modern human ancestors.

Some of the most striking artifacts from the life of early humans are the art forms found in caves throughout the world. Some of the most well-known are the Upper Paleolithic examples found in France and Spain. There are a number of puzzling features of this cave art that until recently escaped any rational explanation. These pictographs and petroglyphs are often found deep inside a cave, frequently in inaccessible locations. They tend to cluster narrowly in one location, ignoring nearby surfaces that appear to be just as suitable. And over ninety percent of the figures consist of hoofed animals (Gourhan-Leroi, 1967, 1982; Waller, 1993a, 2006).

A subfield of archeology, called archeoacoustics, has produced the idea that cave paintings are intimately related to the acoustic nature of the cave chambers (Reznikoff, 1987; Reznikoff and Dauvois, 1988).⁶ Hoffman (2014), for example, used a laptop and loudspeakers “to sweep a sine wave tone through all audio frequencies, recording the results to capture the acoustic fingerprint of each space.” Such detailed studies of prehistoric sites support the idea that the subject matter and location of the pictures relate directly to the acoustics of the cave structure. Waller (2002) points out that the pictures often cluster in areas with enhanced acoustic properties. For instance, in the deep caves of Font-de-Gaume and Lascaux, pictures of hoofed animals such as bulls, bison, and deer appear in chambers in which the echoes, resonances, and reverberation created percussive sounds that resemble hoof beats⁷, as illustrated in Figure 2. In contrast to this, in chambers that are acoustically quiet, one finds pictures of felines (Waller, 1993a) or simple dots and handprints (Hoffman, 2014).⁸

Thus, acoustics offers a compelling explanation for the location of paintings in chambers deep inside caves, because these chambers have special acoustic properties; the clustering of paintings in certain areas of the cave wall, because they are acoustic “hot spots” (Blesser and Salter, 2009; Mattioli et al., 2017); and the predominance of hoofed animals as the subject matter of the paintings. Additionally, stalagmites and stalactites that ring like a musical instrument when struck have been found to be marked with paint (Hoffman, 2014).

The rock art of the San people constitutes evidence that rock art with CMIT properties may have existed in Africa prior to the migration of humans from the continent.

Anatomically modern humans appeared in central Africa 200,000 years ago. Gronau et al. (2011) carried out whole-genome sequencing on six individuals from different regions of the world: a European, a Yoruban, a Han Chinese, a Korean, a Bantu, and a San. The research shows that the San population was the first to split from other populations, and this occurred 108,000–157,000 years ago. They moved to southwest Africa, where they continue to reside to the present. Gronau et al. (2011) found that the Eurasian population diverged from the rest 38,000–64,000 years ago, which marks the time that H. sapiens began to migrate out of Africa.¹⁴

Along with the genetic evidence for the San’s early divergence from other human populations, there is also linguistic evidence. The San’s languages belong to the Khoisan family. All biologically Khoisan groups speak a language with phonemic clicks (Güldemann and Stoneking, 2008), which are consonants with a distinctive popping sound. Khoisan can be seen as one of the only language families in the world with clicks (Huijbregts, 2017). The only other language family is Bantu, but only in areas of contact and intermarriage with Khoisan populations, indicating the borrowing of consonantal clicks into Bantu languages (Herbert, 2002; Maddieson, 2003; Sands abd Güldemann, 2009).¹⁵ As Huijbregts notes, this suggests that the San, once they split off from the rest of the human population, stayed relatively isolated, something also supported by the genetic research (Gronau et al., 2011). This means that anything we find in the population may very well have been there to begin with, possibly even before they split from other human populations.

The San produced rock art that has been dated as far back as 70,000 years ago (Thackeray, 2005). The rocks were decorated because it was believed that a spirit world existed beneath the surface (Lewis-Williams and Dowson, 1990). We find this type of rock art in other regions of the world as well, typically those with an animistic tradition (Bahn and Fossati, 1995, 2003). The idea of a spirit world behind the surface of the rock could come from the acoustic property of echo: an acoustic signal is detected despite the absence of a direct source for it at the point of the sound.

There are two important points about San rock art and its relationship to symbolic thinking. First, the fact that some of the rock art predates the migration out of Africa gives credence to the hypothesis that CMIT is an example of the expression of symbolic thinking and even a factor enhancing this capacity’s spread throughout the human population. The second point relates to the apparent relative isolation of the San population from others in Africa and beyond, as indicated by the unique existence of clicks and by the genetics of the San population. The point Huijbregts (2017) makes is that the seeds of human language must have been there prior to the first genetic split, ∼125,000 years ago. This is because other populations developed a language as well. So, some cognitive property that preceded the development of language existed prior to the first human lineagesplit. If we assume that this cognitive property included symbolic thinking, the cognitive underpinnings underlying CMIT had taken root earlier than the split of the San population from the rest.¹⁶

Cave and rock art in general and language have a number of striking similarities:

The first three points relate to the observation that artistic artifacts, either deep in caves or closer to the surface, occur on every continent occupied by modern humans (Bahn and Fossati, 1995, 2003), and the oldest occur in Africa (Henshilwood et al., 2002). The fourth point has to do with the function of art and of language: both are used for communication.¹⁷ The fifth point has to do with the content of artworks and of language: both may indicate actions (i.e., predicates), objects (i.e., nouns), and modification (i.e., adjectives).¹⁸ The final point, that art and language are external symbolic forms of internal mental states, is an obvious one, and it also may connect the two to genetic studies. The FOXP2 gene is implicated for speech in humans and for other externalized communication forms in mice (Groszer et al., 2008; Castellucci et al., 2016; Chabout et al., 2016) and songbirds (Haesler et al., 2007), but only modern humans have art and language. Is there a genetic basis for this? FOXP2 underwent change in modern humans (Vargha-Khadem et al., 1995; Lai et al., 2001) that affected a binding site for the transcription factor POU3F2 (Maricic et al., 2013; Huijbregts, 2017). The POU3F2 variant only occurs in modern humans, being absent from Neanderthals and Denisovans. As Huijbregts (2017) notes, this change could be seen as leading to the acquisition of full speech. Given the similarity with art, we can speculate with Huijbregts that a similar genetic change may have given rise to the multi-modal art that occurred all over the world alongside language.

The symbolic thinking that developed in humans led to rapid technological innovation, sophisticated visual arts, and language. This newly formed cognitive capacity may have had another, unexpected result. After continuously growing in size over the span of the Pleistocene, our brain has contracted in size by 13% in the past 20,000 years or so (Hawks, 2011 and references therein). One possible explanation is that the symbolic thinking that developed in modern humans led to a fundamentally different way to compute data, one that extracts only the essence required for abstract representation instead of computing the entire set of incoming raw data (Tattersall, 2017). Our brain membrane is metabolically expensive, so the newly formed algorithm that requires less data led to shedding of the unneeded membrane, resulting in brain diminution in recent evolutionary time. Our proposal is that the symbolic thinking pervasive in humans that led to brain diminution is exemplified, and was even enhanced, by the CMIT that we see in the cave and rock art of Africa and elsewhere in the world and by the development of language. Thus, contrary to Wallace, the development of the arts gave the modern humans a powerful evolutionary advantage.

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.