Look at schoolchildren today. There isn't a skinny one amongst them. The girls have huge thighs and hardly any waist. The mind boggles just thinking about what their body weight will be when they reach their mid thirties. Only the very old today resemble people seen in the old black and white movies. Before the Second World War, people were very skinny. Mind you they did do a lot of manual labor, many working 12 hour days. If you burn a high amount of calories you can eat what you want and not gain weight. However, most today are sedentary.
Remains of early humans show that they did eat a gruel of wild seeds. This evidence only proves that our ancestors ate seeds in the last ten thousand years. There is strong evidence of a meat, berry and tuber diet before this time. There was variation in lifestyle of premodern humans. None of them farmed cereals though. They did keep domesticated animals.
The story of Man has been never ending periods of starvation and misery, with disease being prominent due to ignorance of sanitation. Food has only been plentiful in recent times. It is notable that obesity and malnutrition is spreading through developing countries as they adopt the Western diet. This on its own is a condemnation of the modern diet. We can be sure our ancestors ate a wide range of food, but they did not gorge themselves on white flour.
Science by Ty Buchanan