The move from the mostly frugivorous or omnivorous diet of hominin "Australopithecus" to the carnivorous scavenging lifestyle of early "Homo" has been explained by the climate changes in East Africa associated with the Quaternary glaciation. Decreasing oceanic evaporation produced a drier climate and the expansion of the savannah at the expense of forests. Reduced availability of fruits stimulated some proto-australopithecines to search out new food sources found in the drier savannah ecology. Derek Bickerton (2009) has designated to this period the move from simple animal communication systems found in all great apes to the earliest form of symbolic communication systems capable of displacement (referring to items not currently within sensory perception) and motivated by the need to "recruit" group members for scavenging large carcasses.