In comparison with other mammals, humans grow up slowly. This is integral to the way we learn the complexities of our language, social organisation and material culture. It is part of what defines us as human. Slow development is also one of the distinctive features of primates as an order of mammals and there is a relationship with size. The larger species within any mammalian order tend to develop more slowly than the smaller species. Humans are among the largest of primates but, even taking that into account, we are still by far the slowest primates to reach full maturity. In any development graph, we are an outlier, in a different category to the rest. One crucial question for archaeologists and palaeoanthropologists is therefore to determine when our slow rate of development evolved. If the earliest stone tools date to 2.6 million years ago, were they made and used by hominins (the subfamily of living and fossil primates to which we belong) for which the pace of development was as slow as ours, or was the pace relatively faster, as in other living primates? This is surely central to understanding the meaning of these tools, the social and behavioural context within which they fitted and the cognitive abilities of their makers.