Based on the evolutionary biology of primates and conclusions drawn from the nutritional, ecological and social history of Homo sapiens, we know that modern humans are designed to eat fats, protein, and carbohydrates, probably in that order.
We also know that for most of human pre-history, we ate a combination of foods with these ingredients as and when we could. Few early human societies persisted without a significant source of animal protein, and many, such as the ancestors of the Inuit ate mostly animal fat and protein.
And althoug the early humans were few in number, they were able to migrate and find suitable food almost everywhere on the planet. This alone is remarkable. There are few species with enough dietary range to persists in the dry deserts of modern day Namibia, the rainforests of the Congo and the cold steppe habitats of northern Europe.
A mixed diet gave us the evolutionary edge. Those nutrient bombs from hunting and scavenging—and in modern times from feedlots and battery houses—are a huge advantage. But the more comprehensive plant-based nutrition gives us flexibility and, through that, survivability.
Our biology makes us generalists, and our brains give us the remarkable adaptive capacity to become general almost anywhere on the planet.
Animal protein is energetically expensive
Food ecology tells us that animal fat and protein cost a lot to make—they are energetically expensive. Animals are energy sinks, while plants are energy sources, or perhaps more like energy factories fuelled by the sun. Energy is lost in the conversion of plants to animals, and energy is required to maintain the metabolism of the animals.
If all humans were vegetarians, the energy used to maintain animals and the energy lost in that process would be saved. In real terms, that means less total energy and by extension, less land needed to feed a human vegetarian than a human omnivore.
This is the logic of the argument that eating plants is how we save the planet.
Land requirements to feed everyone
Food ecology, the engine room of food production for subsistence and intensive agriculture, tells us that it is never this simple. However, it is worth thinking about the land requirements of different diets to test the logic.
In a neat study that simulated the land requirements of 42 simulated diets that varied in the amount of meat and fat, researchers at Cornell University established that the average annual land requirements per person for each diet ranged from 0.18 ha to 0.86 ha.
In general, per capita land requirements increased with increasing quantities of meat in the diet, while the influence of the quantity of fat varied depending on the quantity of meat consumed. In addition, the proportion of land in perennial crops was greater in higher meat treatments than in lower meat and vegetarian treatments.Peters, C. J., Wilkins, J. L., & Fick, G. W. (2007). Testing a complete-diet model for estimating the land resource requirements of food consumption and agricultural carrying capacity: The New York State example. Renewable Agriculture and Food Systems, 22(2), 145-153.
Fair enough and consistent with food ecology—meat in the diet needs more land than no meat. But the relationship is not linear and is affected by the amount of fat consumed.
Even on a vegetarian diet, eating fats needs more land than no fat.
Eating meat requires more land than no meat, but the effects compound as the fat consumption increases,. If there is some fat in the diet the range in the amount of land needed for a meat and meatless diet declines.
Here is the graph using carrying capacity as the measure of land needed.
Based on these analyses, the researchers estimated that the land base for agriculture and average yields of crops and livestock from New York State could feed between 6.08 million people (0 g meat, 52 g fat) to 2.04 million people (381 g meat, 52 g fat).
So, in general, lower meat diets supported more people than higher meat diets, but the differences between diets with different meat levels decreased as fat increased—fat makes a big difference.
This study was completed when the population of New York city was 8 million. At that time, the carrying capacity of New York State could feed between three-quarters and a quarter of the residents, depending on what they ate.
This example for New York is consistent with other estimates of land requirements and the basic logic—if humans only ate plants we would need less land to feed them.
But the advantages of eating some meat and especially some fat might outweigh land use gains.
What sustainably FED suggests
Remember that humans can convert fat into energy and protein into carbohydrates. We don’t have to eat plants. And fat is an efficient source of energy for humans.
Our digestive physiology is designed for the nutrient and energy bonus we get from eating animal fat and to a leser extent animal protein. And we have always done this whenever the opportunity presented.
And so to the answer to the question.
Will vegetarianism save the planet?
Not really, for two reasons.
The first is our design, our basic biology. We are fat and meat eaters. Our metabolism handles both these food types efficiently and effectively. We are flexible and can live on plants alone, but our bodies process energy and nutrient-dense foods well.
There was a reason our ancestors lived near the water on the coast and shores of lakes and rivers. Water bodies are a great source of energy-dense food. Ask an Inuit.
The second reason is irony.
Suppose we follow the logic and the evidence that the human carrying capacity of the land is greater when we only eat plants. The 8 billion people would use less land for food.
Except this is unlikely to happen. More likely, there would be more food for people to eat. All the grains fed to livestock and chickens could be made into pasta—a delightful example of Jevon’s paradox.
What happens next is more people.
Peters, C. J., Wilkins, J. L., & Fick, G. W. (2007). Testing a complete-diet model for estimating the land resource requirements of food consumption and agricultural carrying capacity: The New York State example. Renewable Agriculture and Food Systems, 22(2), 145-153.