The idea of humans not eating meat is highly inaccurate and had to be made by people long ago ignorant to reality. You can't just magic humans into meat eating, from our DNA up, everything would change drastically, we would literallly become a different species over night.
Our teeth, our mouths, our stomach, our intestines, our liver, our pancreas, our saliva all have functions specific to eating meat alone. When you eat something you break it down over several stages to a tiny chemical molecule. Every stage of breaking down requires chemicals and proteins to make it happen, these chemicals and proteins all need making and distributing in the body. By adding meat to the diet we are changing the human from the ground up! A 'human' that did not consume meat would be very different, have longer intestines, different teeth, jaws and faces, would have a different pancreas and liver and a wildly different lifestyle. Animals that feed on vegetation spend their days grazing, some human ancestors spent up to 90% of their time eating as it was so energy inefficient to consume vegetation.
Only when we evolved enzymes to break down meat and developed fire to aid that process could we have 'spare' time to develop cultures and societies, over hundreds of thousands of years.
This is not guess work, this is all evidence based. As for modern day vegetarians, they have to supplement their diet with essential amino acids and take advantage of farmed and processed foods with high calories and sugar. Adam of the bible had wild greens and fruit trees....... even if they were easy to find his whole life would have been consumed by eating.
We have plants with energy from the sun, tiny animals and insects eat the plants, then slightly larger animals eat the smaller animals and plants in greater quantity, then larger animals again eat the animals that eat the animals and the plants .......at every stage the energy being carried in an animal increases drastically, so that the energy and raw material available increases every level. The higher up the chain you eat, the mire energy you can consume in less time. The animal and plant kingdom work on this basis of energy transfer, the idea of animals not eating animals is ridiculous, the energy from the plants to the larger animals can only get there by consumption of other animals OR by spending all day eating vegetation non stop and by having long intestines, consantly metabolising what you consume. Such as with cows etc.
When I learned all this, I gave myself a big facepalm. While we are at it, try finding a source that lions can or ever have lived off straw lol..... doesn't exist. The urban myth about the london zoo feeding the lions straw during the war as repeated in WT literature has no basis.