Imagine the cave man on the road, traveling to lush fields of vegetation where fruits and grains and tubers could be located with ease. The troop kills a deer or elephant, and feasts on nothing but meat and blood for days, consuming the entire kill before it can spoil. They do this repeatedly while on the road. When they arrive at their destination, they find they no longer need to take the physical risks that hunting invariably presents - flailing hooves and charging frightened beasts. They become vegetarians.
Modern man misunderstands what the cave man ate while lolling about during their vegetarian periods.
They did not live strictly on vegetables and fruits and grains. They ate any and everything that was handy, and this included numerous insects and slow moving life forms such as mollusks and possums. They ate less meat, but the diet was highly varied and included occasional small bites from sources other than plants. Thus, those modern humans who try to live what they interpret to be a strictly vegetarian life suffer from malnutrition - poor immunity, anemia, lack of strength, and inability to deal with stress. Man was not designed to live by vegetables alone, and must accommodate their body with protein sources from living creatures other than plants, or suffer the consequences.