All life started out desperately trying to ingest enough nutrients to live, grow and often mutate. The diversity of climates and organic material available determined what was eaten, how it was processed and how from this diversity so many diverse life forms developed and how they in turn used the world to provide for themselves.
A brilliant book on the subject by the physicist, Gino Segre, is called A Matter of Degrees--the degrees referring to temperature. It is clear how long--billions of years--it took for life to evolve into the forms that we know today, including humans. Much of that time was a struggle for food and water. Only recently have we developed tools, agriculture, the use of fire and the eating of meat.
In earlier times, being fat was a symbol of wealth. A man who could afford a harem of plump women was rich. The thin man was mistrusted like Shakespeare's "lean and hungry" Cassius.
Today, things have turned upside down; the upper classes wish to be lean and obesity is overwhelmingly a problem of the poor.
Partly, this is understandable. Many years ago, I was a counselor a a camp for underprivileged children. Many of them would stuff their mouths at a meal and chew on the food throughout the day. They could not believe that a steady supply of food would be coming their way.
More normally, it is a simple case of not valuing dieting or realizing that cooking need not be onerous and that it is cheaper than ready-prepared food and snacks. Food is often used as a reward and recently a study with ice indicated that fattening foods can awaken the pleasure centers in the brain causing addiction.
Also, in many cases, traditional diets that were healthy for their populations have been undermined by the easy presence of fattening foods and even well-meant and horrific advice such as that given to Italian immigrants around the turn of the last century by social workers that they should give their children fewer vegetables and more meat.
It is ironic that the animals of the world--particularly humans--have gone from struggling against deprivation to a societal struggle--important to the health of people and the economy--against overweight and obesity.