So what was around back when we were evolving and living off the land? Well animals were plentiful, and we were very active beings and we would have no problem going out and catching them. This mean't that our diet was high in protein very important in building muscle and repairing ourselves from the harms of everyday life. Instead of using carbohydrate as our number one every source we used fats. They are a much more efficient energy source and pack around 2.5 times as many calories as carbohydrate per gram.
A historical note, fats up until recently were the most valuable part of animals. It would give you more energy and thus the ability to survive. It was the fat that was traded at markets and not the fillet steak!
So onwards, what else did we have? Fruit and vegetables. Scavengers would soon be able to identify which fruits were edible or not etc. Most likely through trail and error. This then made fruit which would be abundant in the vast vegetative landscapes to be high on the meal adjenda as it's easier to find than meat.
For information on how to use the paleo diet as an athlete i recommend this link, Paleo diet for athletes.