***made this shirt ;-o
At around 2.5 million years ago, a feedback loop began to develop between diet and brain size. The Earliest representatives of the genus homo --Homo habilis, had begun a shift in diet that started to incorporate meat. Preceding the evolution of the Homo habilis was Australopithicus whose diet was vegetarian(*). A large gut is required to break down plant materials and it is difficult to find sufficient protein sources needed to maintain a big brain without eating animals. Vegetarianism resulted in a smaller cranial capacity due to the ammount of energy expenditure neccessary for digestion. The consumption of meat facilitated brain growth and subsequent gut shrinage. By the time Homo erectus emerged on the scene around 1.8 million year ago, fossil remains had already begun to evidence larger cranial capacity.
With the increase in brain size due to diet, certain structures which regulate linguistic ability become augmented. One specific instance is the indentations on the fossilized remains of the Nariokotome boy found at lake Turkana. Skull castings of this nearly complete specimen show an enlargement of Brocas area which regulates linguistic comprehesion(*). Of course, morphological features alone do not tell the whole story and it's only when examining the cultural remains of H. Erectus does the case for his ability to communicate become compelling.