Lucy was similar to Australopithecus, and was physiologically nowhere near Cro-Magnon Man (us). She wasn't "early man." She was early man's ancestor.

That being said, you are right, grains were used by early man, as were fruit and vegatation, but meat was their main source of sustanance, and vital to survival. And when we say grains here, we are talking about smashed-up grains, mixed with water and cooked on hot stones, highly unprocessed. The time-intensive process of gathering grain only allowed for limited consumption of grain products anyway (until the advent of farming).

So, when people like Ray Audit talk about eating like early (Cro-Magnon) man, he is talking limited grain, fruits, vegetables and organic meat (with meat being the main source of nutrition). The steep rise in Diabetes, heart disease and obesity didn't really start until the early 20th century with the coming of processed foods, flour and refined sugar.