Food needs to be broken down in the mouth before it can be swallowed and digested further. How this is being done depends on many factors, such as the mechanical properties of the foods and the morphology of the masticatory apparatus. Palaeoanthropologists spend a great deal of their time reconstructing the diets of our ancestors, as diet holds the key to understanding our evolutionary history. For example, a high-quality diet (and meat-eating) likely facilitated the evolution of our large brains, whilst the lack of a nutrient-rich diet probably underlies the extinction of some other species (e.g., P. boisei). The diet of South African hominins has remained particularly controversial however.