Every diet "requires a lot of knowledge and effort."

People learn their culture's version of a healthy diet as children, which is why learning other sorts of diets as adults seems difficult.

Some cultures have healthier diets than others, but no culture has a traditional diet that is so unhealthy it can't sustain the population.

Human diets evolve. In human history people have created healthy diets by trial and error. They may not have had specific scientific knowledge of nutrition, they just knew which combinations of foods they needed to eat to stay healthy.

In some animals this dietary knowledge is largely instinct, in other animals such as humans, bears, sea otters, etc., specific diets are learned behaviors.

There are seven and a half billion people on this planet now. We can't all eat large quantities of meat every day without doing very significant damage to whatever is left of the earth's natural environment.

About half the people in my extended family are vegetarian, many of them nearly vegan. The reason they have chosen these diets are varied. I've seen children thrive on such a diet and grow up to be very healthy adults.

Times change. Three of my grandparents were the children of ranchers and dairy farmers. All of my great grandparents were Wild West, the meat they ate they killed themselves. As a little kid I used to watch my great grandmas cut up fish, chickens, and small mammals for dinner with awe. Their hands moved faster than I could follow.

My dad doesn't like hunting much, but he's an avid fisherman. When my siblings and I were children most of the animal protein we ate was fish he caught, followed by cheap powdered milk, bulk cheese, and ground up dairy cows.