Funny

This is one argument @darganon. We need nutrients from a variety of things. We can live without some of them but that can come at the cost of health later in life e.g Vegetarian women more likely to fracture hips in later life.

Furthermore:

animal-source foods (ASFs) are dense in bioavailable vitamins and minerals. ASFs are the only intrinsic food source of vitamin B12 [7] and contain more bioavailable forms of vitamins A and D, iron, and zinc than plant source foods (PSFs)

Source

Then there's land-use:

• 86% of the global livestock feed intake in dry matter consists of feed materials that are not currently edible for humans

• Contrary to commonly cited figures, 1 kg of meat requires 2.8 kg of human-edible feed for ruminants and 3.2 for monogastrics

• Livestock consume one third of global cereal production and uses about 40% of global arable land

Source

It wouldn't surprise me if we evolved to have balanced diet from multiple sources because they have the nutrients we require. We most likely don't need all the meat we're eating and we do a terrible job in developed countries with reducing waste. But just like a purely meat based diet, a purely plant based diet is just one of the extremes. To each their own though.