Wild primates take most of the daily diet from plant sources, eating moderate to small amounts of animal source foods (ASF). Plant materials make up from 87% to >99% of the annual diet of great apes, the closest living relatives of modern humans (Homo sapiens sapiens). Reflecting their close genetic relationship, gut form and nutrient requirements of apes and humans (Hominoidea) are very similar, as is their pattern of digestive kinetics—one predicated on a relatively slow turnover of ingesta. In plant-eating mammals, in contrast to carnivorous mammals, greater body size is associated with lower dietary quality. Turning to ASF as a routine rather than occasional dietary component would have permitted the evolving human lineage to evade the nutritional constraints placed on body size increases in apes. Without routine access to ASF, it is highly unlikely that evolving humans could have achieved their unusually large and complex brain while simultaneously continuing their evolutionary trajectory as large, active and highly social primates. As human evolution progressed, young children in particular, with their rapidly expanding large brain and high metabolic and nutritional demands relative to adults would have benefited from volumetrically concentrated, high quality foods such as meat. Today, many humans, particularly those in high income nations, have a variety of high quality, non-ASF dietary alternatives, but such foods were not generally available to paleolithic human ancestors nor to many people today in low income nations.
It's worth noting before anyone gets too carried away with the idea of Man the Hunter
that the increased consumption of animal source foods occurred at around the same time that humans started making greater use of roots and tubers as described in a previous thread - (Link