Cultural diffusion occurs in chimpanzees

MacDonald et al. (1) propose that Middle Pleistocene fire use signaled the onset of widespread cultural diffusion and that it was “the first clear evidence of the emergence of cultural diffusion in the evolution of humankind” and “a distinctive characteristic of the cultural behavior of Homo sapiens.” They attribute such diffusion to “tolerant intergroup encounters” that facilitate transmission. All three assertions are questionable, given extended findings by cultural primatologists studying humankind’s nearest living relative, the chimpanzee (Pan troglodytes).

Primatological interest in cultural diffusion is longstanding (Fig. 1). Twenty years ago, Whiten et al. (2) proposed four models of cultural diffusion in living wild chimpanzees and provided evidence for them from eight study sites across Africa, from Senegal to Tanzania. They identified 39 cultural variants that showed cross-populational differences.

Fig. 1.

Four models of cultural diffusion, illustrated by hypothetical communities A through F. (A) Diffusion of a behavior pattern (shading) from a single origin (B) to neighboring populations (A, C, and D). (B) Independent origins and diffusion from multiple sources (B and E). (C) Diffusion with differentiation, creating variants. On the left, differentiation occurs in a behavior pattern from B to C and from B to D, creating new types. On the right, graded differentiation occurs (e.g., tool dimensions) in diffusion of a behavior pattern from F to E (and perhaps others). Reprinted with permission from ref. 2.

These findings from living apes suggest that cultural diffusion emerged much earlier than 400 kya, perhaps as long ago as the Last Common Ancestor of living chimpanzees and humans, at 6 Mya to 7 Mya (3). Otherwise, one must posit independent emergence in divergent evolutionary lines that led to its presence today in Pan and Homo. Cultural diffusion is not a distinguishing characteristic of Homo sapiens.

The authors (1) ascribe intergroup tolerance as necessary for cultural diffusion. However, chimpanzees show cultural diffusion despite fierce intergroup competition in the form of aggressive territoriality, sometimes with fatal consequences. Their cultural diffusion appears to occur through the emigration of postpubertal females from their natal groups and their immigration into neighboring groups. Sometimes, this migration leads to immigrants acquiring new cultural variants from residents (4), while, in other cases, the residents acquire new cultural variants from the immigrant (5–7).

The authors (1) posit one criterion of cultural diffusion as a “marked, geographically widely distributed pattern” and call for “multiple proxies at multiple sites from each region and recurring within sites.” Several well-documented chimpanzee cultural traits fulfill these requirements, such as the foraging technology of termite fishing, in which an ape threads a manufactured probe into a termite mound and extracts termites that attack the tool in defense. Termite fishing occurs widely across the sub-Saharan range of chimpanzees, from Tanzania to Senegal, but takes different forms at different locations (8) Similar widespread distribution exists for other behavior patterns, for example, ant dip, leaf sponge, and hand-clasp groom.

Another distinguishing characteristic of human culture, according to the authors (1), is the existence of regional traditions, that is, technological traits and trajectories that reflect regional cultural repertoires. In chimpanzees, this regionality is exemplified by percussive nut cracking with hammer and anvil, of stone or wood. With one possible exception, all such cases occur only in far West Africa, despite there being no ecological reason for their absence elsewhere (9).

Lacking behavioral data on the cultural diffusion of humanity’s hominin forebearers, we can use extant chimpanzees’ cultural diversity as a useful model for the evolutionary origins of human culture, from collaborative research informed by archaeology, paleoanthropology, and primatology (10).