A team of researchers, led by Professor Gerhard W. Weber from the Department of Anthropology at the University of Vienna in Austria, thinks that our early ancestors (specifically the australopithecines) probably started eating nuts and seeds because they were hungry and that was the only food available.
These adaptations are important in an environment where food is scarce. While being able to adapt to new food source is important, it probably would not be possible if they can't process these foods mechanically (chewing and processing). Adaptations such as large molars, robust teeth or thick enamel (morphological adaptation) are also important. Read more here: European scientists crack the mystery of why we eat nuts.
Also read, The feeding biomechanics and dietary ecology of Australopithecus africanus by Strait et al. (2010)