When Jane Goodall (Reference Goodall1998) first reported that chimpanzees strip leaves from twigs to fish for termites, Louis Leakey famously responded: “Now we must redefine tool, redefine Man, or accept chimpanzees as humans.” The decades of research following Goodall's discovery have convincingly rendered invalid any description of Homo sapiens that places us as the planet's only tool-making animal. Yet we remain without peer when one considers the depth, breadth, and inventiveness of our tool use. Vaesen's claim that the way we use tools constitutes a major cognitive discontinuity between our closest relatives and ourselves is staked on solid ground. Standing out in his coverage of tool-related cognitive traits is a reliance on experimental comparisons of human children and adult chimpanzees. In this a critical point is missed: Ours is the only species to have a childhood as a life stage, something that likely lies at the heart of the discontinuities Vaesen outlines.

The majority of mammals follow birth with a period of infancy (characterised by the appearance of deciduous teeth and all or some nourishment being provided by maternal lactation) that transitions seamlessly into adulthood. Chimpanzees postpone puberty and insert an extended phase of juvenile growth between infancy and adulthood whereby offspring are dependent on their mothers for about 5 years (Kaplan et al. Reference Kaplan, Hill, Lancaster and Hurtado2000).

Humans have a shorter infancy and, at least in preindustrial societies, breast-feeding is usually discontinued around the beginning of the third year (Kaplan et al. Reference Kaplan, Hill, Lancaster and Hurtado2000; Sellen & Smay Reference Sellen and Smay2001). Weaning at this young age places a great nutritional burden on offspring. Three-year-olds are not typically mature enough to prepare their own food and are too limited by deciduous dentition and a small gastrointestinal tract to consume an adult diet. In various hunter-gatherer societies, the solution to this problem is for older members of the social group to provide specially prepared foods that are high in energy and nutrients until self-care becomes possible at around 7 years (Locke & Bogin Reference Locke and Bogin2006). This post-weaning pre-juvenile stage constitutes “childhood” and corresponds with a distinct, species-specific growth curve and changes in sex-hormone patterns (Bogin Reference Bogin1990; Hochberg & Albertsson-Wikland Reference Hochberg and Albertsson-Wikland2008).

The insertion of childhood as a life stage not only lengthens the period of dependency on others, but also places responsibility of care for the child with the community. By contrast, chimpanzees transition directly from infant dependence on the mother to independent juvenility. Human childhood therefore affords an extended period of development during which offspring are provided multiple opportunities for learning from the broader community while buffered from survival pressures incumbent on juveniles and adults. These opportunities will be enhanced by the uniquely human practice of teaching. The slow somatic growth and delayed sexual maturation of childhood serve to maximize maturational differences between adult teachers and child students, differences that allow a great deal of learning, practice, and modification of survival skills (Bogin Reference Bogin1990).

Associated with adult-child instruction is the tendency of children to replicate all of the actions an adult uses when achieving an object-directed outcome, even actions whose relevance is shown to be causally redundant (see Nielsen & Blank Reference Nielsen and Blank2011). This “over-imitation” facilitates the rapid acquisition of skills, actions, and behaviors while avoiding the potential pitfalls and false end points that can come from trial-and-error learning. Indeed, children are not particularly good at innovating even simple tools (Beck et al. Reference Beck, Apperly, Chappell, Guthrie and Cutting2011). Though traces can be found in the infancy period, data collected among contemporary descendants of hunter-gatherers indicate that teaching and over-imitation become firmly established during childhood (Hewlett et al. Reference Hewlett, Fouts, Boyette and Hewlett2011; Nielsen & Tomaselli Reference Nielsen and Tomaselli2010). Other social-cognitive traits Vaesen links to tool use, such as theory of mind (Wellman et al. Reference Wellman, Cross and Watson2001) and mental time travel (Suddendorf et al. Reference Suddendorf, Nielsen and von Gehlen2011), are similarly established in this period – as is the uniquely human propensity for pretending, an endeavor in which the mind can find a rich bed to sow the seeds of invention (Nielsen Reference Nielsen2012). The emergence of childhood as a life stage therefore presents itself as a critical step in the evolution of human tool use. It might also have underpinned marked increases in tool innovation among our hominin ancestors.

Vaesen alludes to the apparent lack of cumulative culture evident in the Acheulean Industrial Complex, which appeared on the paleolandscape ~1.6 mya, exemplified by the teardrop-shaped bifacial hand axes made by Homo ergaster and Homo erectus. The production of these lithic artifacts is characterized by a regularity of design that lasted for hundreds of millennia and is thought indicative of a general lack of technological innovation (Foley & Lahr Reference Foley and Lahr2003; Hill et al. Reference Hill, Barton and Hurtado2009). There is evidence to suggest that such lack of innovation is due to the absence of a childhood period. During the last three decades, the most common method used to address questions of hominin growth has been the study of fossil teeth, with a modern human-like sequence of dental development now regarded as one of the diagnostic hallmarks of our species (Dean et al. Reference Dean, Leakey, Reid, Schrenk, Schwartz, Stringer and Walker2001). Microscopic analysis of growth patterns in fossil teeth indicates that dental development in lower Paleolithic hominins followed a chimpanzee-like timing. That is, childhood as a life stage was not present before 1.5 mya. Crucially, evidence points to a lack of childhood in hand axe–making erectus (Dean Reference Dean2000; Dean et al. Reference Dean, Leakey, Reid, Schrenk, Schwartz, Stringer and Walker2001). Conversely, and though a matter of ongoing debate (see Smith et al. Reference Smith, Tafforeau, Reid, Pouech, Lazzari, Zermeno, Guatelli-Steinberg, Olejniczak, Hoffman, Radovcic, Makaremi, Toussaint, Stringer and Hublin2010), rates of dental development found in fossilized Neanderthal teeth are suggestive of a childhood in this species (Macchiarelli et al. Reference Macchiarelli, Bondioli, Debénath, Mazurier, Tournepiche, Birch and Dean2006). This ties in with the emergence of the Mousterian tool kit around 300,000 years ago that signified an order-of-magnitude increase in technological complexity that we have not looked back from.

Human offspring are confronted with a vast array of tools they must learn to use. Childhood emerged at some point in our evolution and provided time for the acquisition of the requisite skills to do so, along with the emergence of the multifarious social and cognitive advances that make us who we are. With childhood there is no need to redefine tool, redefine Man, or accept chimpanzees as humans. With childhood we became Homo faber (Bergson Reference Bergson1911/1998).