They managed a balanced diet by eating what was available-imagine that-apparently without interference from a Neanderthal Michelle, too...
Ancient man ate what was available as you said.
Fruits, root vegetables and any animal he could kill.
Our genes are descended from that type of diet over a million years or more. Our genes do not tolerate a diet made up largely of pasta, breads & rice and hormone fed meat.
But the real reason diabetes and obesity is exploding is lack of exercise. Ancient man walked miles every day in search of food. Modern American drives to get his food.