Social organization and the evolution of cumulative technology in apes and hominins
Introduction
Recent studies have inferred the presence of culture, defined as multiple socially transmitted innovations, in chimpanzees and orangutans, based on geographic variation in behavior patterns or artifacts without obvious ecological or genetic correlates (Whiten et al., 1999; Boesch, 2003; van Schaik et al., 2003a; Krützen et al., 2011) and indirect indications of social learning in the field (Biro et al., 2003; Lonsdorf et al., 2004; Gruber et al., 2009; Jaeggi et al., 2010; Reader and Biro, 2010). These studies have allowed us to define more clearly what distinguishes human culture from that of the great apes, whose cultures probably closely resemble those of the last common ancestor of humans and the two chimpanzee species. Two major differences have emerged (Tomasello, 1999; van Schaik, 2004; Hill, 2009; Tennie et al., 2009): the cumulative nature of human technology, and the cumulative and normative nature of human cultural institutions. Our focus here is to explain the origin of cumulative technology, which is widely considered to represent a watershed in cultural evolution.
The prevailing explanation is that cumulative technology is absent in great apes because they cannot imitate, and thus cannot reproduce novel actions with sufficient precision to serve as a uniform foundation for subsequent addition of accumulations. Thus, cumulative technology was thought to have arisen with Oldowan flake tools (Galef, 1992; Tomasello et al., 1993; Tomasello, 1994, 1999; Boyd and Richerson, 1996). We think this hypothesis is no longer supported, for two reasons. First, great apes in experiments can reliably transmit complex techniques, although the exact mechanisms remain debated, and second, they show some evidence of cumulative technology, if properly defined.
With respect to the imitation question, despite much recent work on nonhuman primates, no consensus on the mechanisms of observational social learning has emerged (Byrne and Tanner, 2006; Tennie et al., 2009; Whiten et al., 2009). Nonetheless, great apes have now been found to copy complex skills with sufficient reliability to maintain basic behavioral uniformity in two-target experiments in captivity despite the presence of alternative outcomes (reviews: Whiten et al., 2007; Whiten and Mesoudi, 2008; Dindo et al., 2011), which would suffice to maintain systematic differences in technology between nearby populations in the wild (Boesch et al., 1994; van Schaik and Knott, 2001). Although some doubt remains (Tennie et al., 2006; Claidière and Sperber, 2010), most now agree that we should look elsewhere than to mechanisms of social learning to explain the elaboration of cumulative culture in humans (Price et al., 2009; Tomasello, 2009).
To evaluate possible evidence for cumulative technology among great apes, we need a workable operational definition of cumulative technology, i.e., cumulative technological innovations that have been transmitted socially to the point of having reached high prevalence in a given population (habitual or customary status sensu Whiten et al., 1999). Cumulative innovations have been defined as those beyond what a naïve individual could invent during its lifetime (Galef, 1992; Tomasello et al., 1993; Boyd and Richerson, 1996), i.e., outside its Zone of Latent Solutions (Tennie et al., 2009). However, this definition in effect assumes that the accumulation process has already proceeded to the point that it has become impossible for naïve individuals to invent the whole series of steps. It therefore excludes the initial steps of the accumulation process, i.e., those that may still be invented by an individual, which arguably are the very steps that historically determined the difference between cumulative and non-cumulative culture.
We therefore adopted an alternative approach. The build-up “implies the existence of superordinate representations abstracted from, and maintained over, the course of multiple subordinate events” (Stout, 2011: 1051), and is therefore usually accompanied by an increase in the size of the working memory, making the action series cognitively more challenging as it gets longer (Price et al., 2009). We accordingly defined the metric for the degree of accumulation (a.k.a. ratcheting) of a technique or learned skill as the number of distinct actions integrated as steps in a single functional sequence to reach an overall goal. One advantage of this metric is that this kind of complexity corresponds closely to that in terms of techno-units (Oswalt, 1976), which directly reflect properties of the tools themselves. It is in line with metrics developed for primate food processing (Byrne, 1995; Matsuzawa, 1996), and is also very similar to comparable metrics developed in archaeology (Haidle, 2010; Stout, 2011). However, we did not admit other criteria, such as the complexity of each individual action (which is also hard to define; Uomini, 2009) or the selectivity of the choice of raw material that is used to produce the tool. We excluded them because these aspects can be gradually improved over time through individual practice based on simple processes like associative learning, once the basic action has been put in place by ratcheting (e.g., Nonaka et al., 2010). It is important to stress that this system is preliminary and needs to be validated empirically through actual studies (for a recent attempt, see Sanz and Morgan (2010)). We will revisit this issue in the Discussion.
The paradigmatic case of ratcheting is when an individual adds an existing technique used in a different context, or an entirely novel technique, to an existing technique, and integrates them functionally. This can produce either a tool set (two or more tools used consecutively in a functionally integrated way), a composite tool (two existing tools combined directly), or a more complex tool (where subsequent actions modify an existing tool, adding functionality to it). Table 1 provides the definitions of the first ratcheting steps that can be recognized using this criterion, producing increasing technology levels (TL), and provides examples for both stick and stone tools.
Although cumulative technology defined this way is absent among orangutans, various examples have recently emerged for chimpanzees (Sanz et al., 2004, 2009; Sanz and Morgan, 2007; Boesch et al., 2009). In the Goualougo Triangle, for instance, the local chimpanzees use a tool set, consisting of a stout puncturing stick and a slender probe, to exploit subterranean termite nests. It is assumed that the probing tools were already well established, since they are found in many chimpanzee populations, before the stout puncturing stick was invented. Another example from the same site is the brush-tipped termite probe, where the regular termite probe (again assumed to be the starting point, given its common presence in other populations) undergoes an additional modification in which the tip is frayed, which makes it far more effective in gathering termites (which bite into the probe, and latch on more easily if the tip is frayed). This evidence from the wild is complemented by experimental work. Recently, Lehner et al. (2011) coaxed captive orangutans into making ratcheted innovations.
By these definitions, some chimpanzee technology in nature is cumulative, although the majority is not, whereas captive orangutans can be coaxed into making it. Thus, there is some overlap with the technology of the makers of the Oldowan (Table 1). Nonetheless, whereas all orangutan and most tools in the wild are TL1 and some chimpanzee tools are TL2 and perhaps even in one case TL3, regular Oldowan tools are TL2, but Oldowan tools used to modify wooden tools are TL3. Acheulean tools, in contrast, are TL3 or higher.
Given that great apes are now known to have sufficiently accurate powers of observational learning to allow ratcheting and that they show some evidence of cumulative technology in the wild or captivity, we need a new explanation of the major difference between humans and great apes in their technology. The goal of this paper is therefore to identify the factors responsible for cumulative cultural evolution of technology.
We begin by developing a model that correctly reproduces the known great ape patterns. Modeling cumulative technology is made easier by the presence of considerable variation between orangutans and chimpanzees, the two ape species showing extensive tool use in the wild. Most orangutan populations fail to show any systematic extractive tool use, but a few do and actually do so in multiple contexts (van Schaik et al., 2003a), and even show some variation within populations, depending on exposure to suitable role models (van Schaik et al., 2003b). All chimpanzee populations, in contrast, show at least some tool use (Sanz and Morgan, 2007), and some, as noted above, show evidence of ratcheting of technology. Having developed and tweaked the model for great apes, we then examine the hominin case by changing the model's parameter values in the direction of known or suspected changes during hominin evolution.
Section snippets
Methods
In this paper, we propose a novel simulation model to explain the process of accumulation of technology. It is built using the same basic framework proposed by van Schaik and Pradhan (2003) to model tool use in great apes, which replicated geographic variation in orangutan tool use, and found it to be a function of variation in opportunities for social learning (see also Enquist et al. (2010)). The current model simulates changes in a population's level of technology over time, as a result of
General model results
The model's output is the percentage of the population that has reached technology levels TL0, 1, 2, or 3, as a function of time (in years). In Fig. 2(a), we plot this for a hypothetical great ape population with moderate sociability (N = 501, α = 0.2, κ = 1, ε = 0.0001, μ = 0.05, λ = 15). At this level of sociability, TL1 can establish itself. In Fig. 2(b), we have increased sociability to κ = 2. Now, TL1 establishes itself first, peaks, and then gives way to TL2, which in turn gives way to
Implications of the model
By ca 2.5 Ma, hominins already had reached lithic technology levels exceeding that of most chimpanzees (see Table 1), showing a definite advance toward cumulative technology. The simulations presented here suggest that this development in hominins was induced by changes in social organization that led to higher sociability, brought about by cooperative hunting or scavenging, followed by the adoption of full terrestriality and teaching elicited by systematic food sharing and provisioning, which
Conclusions
This modeling study showed that we could explain variation among orangutans and chimpanzees in the presence and degree of accumulation of their (mainly wood-based) technology with reference to varying sociability, which affects the opportunities for social learning. The degree of accumulation of technology well into levels shown by the most complex Oldowan tools can plausibly be attributed to further increases in sociability, and the introduction of teaching, which increases the probability of
Acknowledgments
We thank Sagar Pandit for helpful discussions. We also thank the anonymous reviewers for their suggestions and questions. Supported by Swiss National Fund (grant no. 31003A-111915), the A.H. Schultz Foundation, and the Max Planck Society.
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