Thanks to decades of relentless lobbying, advertising and promotion, most people think that animal flesh has always been the major source of protein in our diet. The truth is that for most people during most of history, meat has been at most a "once in a while" food, and that lentils, beans, nuts and other plant foods have been far more important sources of protein.
Now, as the animal agriculture industries are running roughshod over the planet, a great many people are starting to rediscover the abundant health and flavor that comes with eating only plants. Our future depends on us making the same decisions.