An Oxford University study published in the journal PLOS ONE concluded that ancient humans in Africa subsisted not on seeds, nuts and red meat, but mainly on corms.
Corms are underground, bulbous tubers that some plants use to store nutrients during the winter or adverse weather conditions such as droughts. Humans in some parts of the world still eat similar tubulars such as "tiger nuts."
The popular Paleo Diet encourages avoiding carbohydrates in favor of grass-fed beef, seafood, fresh fruits and vegetables, eggs, nuts, seeds and certain types of "healthful" oils like olive, walnut, avocado and coconut. It claims that our cavemen ancestors thrived on such foods and that following the same type of eating plan will result in weight loss and overall better health.
Oxford study author Dr. Gabriele Macho examined the diet of Paranthropus boisei, our ancient ancestors in East Africa from 1.4 to 2.4. million years ago. Dr. Macho determined that these early humans had jaw structure akin to modern baboons and likely subsisted on a similarly soft diet. The study results claim that the early humans' teeth and jaws weren't equipped to handle tough foods such as meats and hard nuts.
Their diet probably consisted of mostly corms (tubular plants), flowers, fruits and invertebrates like worms and grasshoppers. However, the author notes that, "Ascertaining the diet of an extinct species is imprecise at best, and the present study does not pretend otherwise."
Dr. Macho bases her conclusions not only on the subjects' dental structure and food that was likely to have been available at that time in history, but also on many other factors. These include how much of certain foods would have been required to sustain the estimated size of ancient humans and how much time they would have had to devote to foraging for their food and eating it.