So if anyone cared to do the research, I'm sure their findings would show exactly what I'm about to describe. The problem is, there is no money in researching things that don't show an advantage toward one thing or another.
As with all things, balance is the key...
If one were to graph life expectancy based on average daily physical activity over the years, I expect that we are currently in a decline. We have developed technology to allow us more leisure time, which in turn has reduced the amount of physical activity required of the average human living in a developed society. At one time, physical demands kept our bodies working at their peak for most of our lives, but medicine had not advanced to the point where life expectancy was at a maximum, due to disease and other factors.
If one were to graph life expectancy based on average daily caloric intake over the years, again I expect we are in a decline. Most people in developed societies now eat an excess of calories, especially in the U.S. Our meal portion sizes have grown steadily in recent years, due to a percieved increase in value with larger portions.
Dietary needs have changed little over the course of human evolution. A balance of proteins, carbohydrates, and fats are recommended for a healthy diet. The body also requires certain vitamins and minerals in order properly to function. It matters little the source of those nutrients, whether derived from plant or animal sources. Our bodies have evolved to make use of the nutrients from both those sources. Any study which purports to show the benefit of one to the exclusion of the other must have an agenda, and their conclusions will obviously support the outcome they were seeking. Otherwise, the results would not be published, and the research thrown out as flawed.