dental development [but this] period of dental immaturity is particularly prolonged in modern humans” (Smith et al. 2010:20923). Similarly, in a detailed review of studies of Neanderthal somatic growth, Thompson and Nelson (2011) conclude that Neanderthals experienced a compressed adolescence and growth spurt compared to modern humans. Additional evidence for faster maturation rates and thus shorter childhoods in Neanderthals derives from studies of brain growth patterns. Neubauer and Hublin (2012); see also Coqueugniot and Hublin (2007, 2012), estimate that Neanderthal neonates had brain sizes that slightly exceeded those of modern human neonates. This is indicative of greater pre-natal brain growth than is found in humans. Post-natal brain growth in Neanderthals was more accelerated. While shorter in duration, this accelerated brain growth led to larger overall adult brain sizes in Neanderthals and to the attainment of adult brain size more quickly (Neubauer and Hublin 2012). These findings concerning growth rate and duration are particularly important because “morphological development of the brain occurs simultaneously with cognitive development while the young individual learns and interacts with his or her environment” (Neubauer and Hublin 2012:568).

cerebellar cortex (CBC), a region associated with language, attention and manual abilities (Liu et al. 2012). What this research suggests is that even when corrected for differences in lifespan, humans have much more time to form synapses and that synapse formation is particularly influenced by experiences garnered during the first five years of life (Cohen 2012; Liu et al. 2012).

It is likely that the extended dependency period in human children evolved in parallel with the extension of synaptic development in the PFC because of this region’s involvement in higher order cognitive processing (Liu et al. 2012). Thus, it is significant that it is during early and late childhood that milestones in social and cognitive learning are reached (Casey et al. 2000; Neubauer and Hublin 2012). The results of Liu et al.’s (2012) study further suggest that the timing of synaptic differentiation and synaptic pruning is similarly extended. These results support the findings of other researchers who have noted that through late childhood and into adolescence there is a gradual decrease in synaptic density in the pre-frontal cortex with concomitant strengthening of the remaining synapses (Casey et al. 2000). This plateauing and pruning of synapses in the prefrontal cortex likely represents “the behavioral, and ultimately, the physiological suppression of competing, irrelevant behaviors” (Casey et al. 2000:246).

Cognitive Development in Childhood

In the context of the above studies, the fact that Neanderthals experienced shorter childhoods (and adolescence) is salient because learning, and in particular learning through play, may be key to understanding cognitive differences between them and modern humans. Liu and colleagues (2012) studied post-mortem brain samples from humans, chimpanzees and macaques at all stages of life (pre-natal to senescent stages). They found that there were significant differences in the developmental trajectories of synaptogenesis between these three species (Cohen 2012). Specifically, in the pre-frontal cortex (PFC), a region of the brain implicated in social behavior, abstract thinking and reasoning, the genes responsible for synaptogenesis are turned on in humans slightly after birth, peaking at five years of age. By contrast, in non-human primates the expression of these genes peaks during the last few months of fetal development and are turned off just after birth. Furthermore, there are twelve times the number of genes involved in the development and functioning of synapses in the PFC in humans than in chimpanzees. Similarly, researchers counted more than 7000 synapses in the three species at different ages and found that in non-human primates they dramatically increase in number just after birth whereas in humans they peak around 4 years of age. Similar results were found when the researchers studied gene expression and synapse formation in the lateral

The Evolutionary Importance of Play

Humans, like most mammals, spend a great deal of their dependency period in play. Play increases steadily as “young become more mobile and begin to interact with litter mates and other young ones in a group or herd and then decreas[es] as sexual maturity approaches, stable dominance positions are being acquired and the animal is typically engaged in serious competition for mates and resources” (Smith 2010:54). Play forms part of a bio-behavioral package involving prolonged immaturity, opportunities for learning and parental investment in such learning and is characteristic of more encephalized species, particularly k-strategists (Konner 2010; Smith 2010). Among invertebrates, play has been documented only among captive octopuses who, after a period of object exploration, seem to enjoy object play such as squirting water bottles at objects (Smith 2010) and playing with Lego (Kuba et al. 2006). This is perhaps not surprising given that octopuses are the most highly encephalized invertebrates. There is very little, if any, credible evidence for play among fish with the exception of sharks, or among reptiles and amphibians (Smith 2010). The frequency and variety of play behaviors increase signifi- cantly among bird species who have been documented engaging in locomotor, object and social play (Smith 2010).

Among mammals, play behavior is truly widespread. In fact, play is an almost universal mammalian characteristic with seventeen of nineteen orders of placental mammals engaging in play (Burghardt 2005). These behaviors include locomotor, object, social, and sexual play. Of all the mammals, primates engage in play the most with the apes adding ‘play mothering’ to the mammalian repertoire of play behaviors. Play mothering mostly refers to juveniles who pick up and ‘mother’ infants (Smith 2010) but there may be instances where this behavior involves the transference of species-specific mothering behaviors to inanimate objects such as sticks or logs (see, for example, Kahlenberg and Wrangham 2010). This particular form of play will be discussed further below in relation to the significance of fantasy play.

Evidence suggests that experiences garnered through play have a dramatic impact on synaptic formation, differentiation and pruning. For example, in a study involving weanling rats, one group of young rats was housed and reared with one peer, another group had access to three peers, and a third group had only an adult female for company (Pellis and Pellis 2009). All three groups of weanlings were exposed to normal socialization except that the latter group did not engage in play because it is rare for adult rats to play with juveniles – even with their own offspring. Post-mortem studies conducted on the rats’ brains revealed that the major difference between the three groups was in the development of the prefrontal cortex. Specifically, there was greater synaptic density in the medial prefrontal cortex of rats that were prevented from interacting with members of their peer group than in the groups of rats that were permitted such interaction (Pellis and Pellis 2009). In other words, that all important pruning did not take place. This study provides the first direct evidence that play deprivation actually altered the “anatomy of the neurons”, leading Pellis and Pellis (2009:92, 94) to conclude that “the brain not only shapes play but that play shapes the brain.” In fact, they observed that play deprivation reduced the ability of animals “to formulate and engage behavioral options dependent on the executive functions of the prefrontal cortex – the same kinds of problems that animals have when they are reared normally but are subjected to experimental damage of their prefrontal cortex” (ibid).

What Is Play?

While approximately 60 attributes of play have been identified (see Pellegrini et al. 2007), following Smith (2010:5), play is defined here as being comprised of familiar behaviors such as running, climbing, and manipulating objects but that

are fragmented, repeated, exaggerated or reordered in some fashion. Behaviors are normally identified as play if they have positive affect (i.e., there is enjoyment as indexed by play signals such as laughter), are flexible in form and content; are intrinsically motivated (i.e., performed for their own sake); if they are a means to an end (i.e., children are often more interested in the performance of the behavior than its outcome) and non-literal (this is most relevant for fantasy play which is discussed below) (Smith 2010:6). The importance of play is supported by the fact that it will rebound in frequency and intensity in offspring deprived of play (Smith 2010) and the lack of play experience can seriously disadvantage an animal later in life (Bateson 2005).

Costs and Benefits of Play

There are both costs and benefits to engaging in play behavior. First, a great deal of time is spent in play that could be put to other uses such as resting, eating, watching or exploring (Smith 2010:64). Second, play is often quite vigorous and energetically demanding (Bateson 2005; Smith 2010). According to Konner (2010), play increases food requirements by 10–20 %. Third, in bouts of rough and tumble play, the likelihood of injury increases (Bateson 2005; Smith 2010). Finally, there is risk associated with neglect of predator danger. Individuals absorbed in play are often less vigilant while at the same time their behavior can make them more conspicuous (Bateson 2005; Smith 2010). For selection to take place, however, the benefits of play must outweigh these costs. The apparent benefits of play include the attainment of knowledge, skills and experience through engagement with the environment (Bateson 2005), physical fitness, development of technical and social skills, cognitive development, behavioral plasticity, enhanced problem solving abilities and increased ability to innovate.

During play, animals are able to practice behaviors that they will use once they reach adulthood and to learn from their mistakes safely (Bateson 2005:17). According to researchers, this has the greatest effect on adult behaviors characterized by the greatest risk such as fighting, mating (when there is serious competition), catching prey, avoiding being prey and moving efficiently in familiar environments (Bateson 2005:17). Play also increases physical fitness. High intensity bouts of play are good exercise for animals, while low intensity bouts build general physical capacity (Smith 2010). Furthermore, there is a positive relationship between the frequency of social play and cerebellum size across species. As Smith (2010:72) observes, the fact that “the cerebellum is strongly implicated in the coordination and

control of motoric activities […] suggests that some aspects of social play, much of which is locomotor, have been selected for aspects of motor development,” the tuning of musculature and sculpting of the nervous system. One critical feature of the mammalian nervous system is its excess number of neuronal connections (Bateson 2005). As an animal develops, unused connections are lost, thus the “sculpting of the nervous system reflects the steadily improving efficiency of the body’s classification command and control systems” (Bateson 2005:16). Furthermore, these changes are reflected in behavior. Bateson (2005:16) argues that movements practiced during play become more efficient and better coordinated.

Animals also develop technical and social skills through play. Object play is well documented among primates (Smith 2010) as they use a large number of objects in their natural environment for food extraction, prey catching, agnostic displays and for a variety of other tasks. Thus, during play these animals are able to practice and explore tool behavior (Smith 2010). Social play provides opportunities for animals to cement social relationships (Bateson 2005), evaluate competitors, develop behavioral flexibility and coping skills and improve motor skills necessary for fighting (Bateson 2005; see also Ragir and SavageRumbaugh 2009). Furthermore, during social play, human children socialize and enculturate each other and this may be at least as important if not more so than parental nurturing. In fact, researchers argue that the “vertical transmission of knowledge from older to younger children parallels, complements or undermines adult-child transmission” and thus children can be seen as significant “agents of cultural change” (Konner 2010:661). Furthermore, peaks in social play activity correlate with the development of neural networks that serve as the basis for shared systems of communication (Ragir and Savage-Rumbaugh 2009 and references therein). Taken together, this means that social play provides the evolutionary context within which meanings can be generated and shared by convention, hence the emergence of symbols and with them of creativity. This will be explored further below in relation to fantasy play.

Play can also have a significant effect on learning in general. Phenomenological and neurobiological evidence suggests that during play individuals feel positive and that play is pleasurable and rewarding (Konner 2010:511). Studies show that individuals learn better and in a greater variety of ways when they are feeling playful (Konner 2010:512). Furthermore, researchers have studied neurotransmitter systems involved in play and it appears that opioid systems are implicated. As Konner (2010:510) notes, “during social play activity increases in the nucleus accumbens and other reward mediating brain areas.”

Finally, play, as we have seen is key to cognitive development, problem solving and innovation (Bateson 2005). For example, play affords opportunities for the generation of new and possibly adaptive responses to novel environments (Pellegrini et al. 2007; Smith 2010). It reproduces culture but can also change culture over time (Smith 2010). It gives individuals an opportunity to probe particular behaviors or explore potential solutions (Bateson 2005:18). Play rearranges previously unrelated thoughts and ideas into new combinations and while most are discovered to be fruitless, this is seen as a “powerful means of gaining insights and opening up possibilities that had not been previously recognized” (Bateson 2005:18–19). Thus, play allows for the breaking away from established patterns (Bateson 2005). Bateson (2005:22) argues that “aspects of play can increase the total sum of spontaneously developing behavioral structures that serve to solve complex problems” and it is these characteristics of play that many researchers argue were selected for evolutionarily. In fact, some researchers assert that this is the most important aspect of play. They argue that many tasks could be practiced and learned through direct observation without engaging in play but because play can lead to innovation and these innovations can be derived and transmitted within the social group, play is especially important evolutionarily (Pellegrni et al. 2007).

Why Stop Playing?

The overwhelming benefits of play beg the question of why the frequency of play behaviors declines later in life around sexual maturity. Adults of all species seem to engage in play far less often than younger individuals. Most bouts of play in adults are associated with infants and this interaction likely contributes to the overall healthy development of their offspring (Lewis 2010; Smith 2010). But there appears to be a significant shift in the benefit to cost ratio against play as animals reach maturity (Smith 2010). It is seems clear that costs of play might increase as play fighting becomes rougher, for example, but do the concomitant benefits of play decrease with age as well (Smith 2010)? I believe that there are two things happening. First, the ways in which play shapes the developing brain may be less effective or actually impossible in an adult brain. Second, there is something special about the kind of learning and information transfer that takes place during play. Some researchers have argued that the innovative outcomes of play discussed above could result in changes in gene frequencies through the process of organic selection (Bateson 2005). Organic selection is a