Humans mostly evolved as carnivores and have used grains and starches as sustenance only when more nutrient dense food was not available. One of the reasons there are not many megafauna left in the world is because we ate them all and had to invent farming to survive. A carnivore diet of ruminant animals would be more sustainable and help slow climate change.
Similar to a paleo diet, it relies on biology and anthropology as a starting point. The carnivore diet has lots of science behind it. Human nutrition requirements and biology point to a requirement for animal food. They do not indicated plant foods are necessary.
Human stomach acid resembles a scavenger. More acidic than a carnivore, and even more than an herbivore. This would allow us to digest food found after a kill and not be harmed by parasites that may be in them.
Meat allowed our brains to evolve our large brains. It would have been impossible for an ancient human to feed their large energy demands of their brain with plant food.
Hunter gatherers tens of thousands of years ago had abundant access to megaherbivores. It is possible that megafauna like Wooly Mammoth and Glyptodon died out because humans hunted them could be ample evidence for our carnivore history.
Ruminant animals are part of the natural carbon cycle of the earth and are necessary to maintain soil health. Eating grass fed beef can even reduce atmospheric carbon.
Most people on a carnivore diet eat about 2lbs of meat per day. Mostly beef, with some eggs, and occasionally others like lamb, chicken, pork, and fish.