It always depends on the particular circumstances. Beef is a bad polluter in a large-scale, industrialised setting with their fodder being produced as cheap as possible on larger areas than food is produced for human consumption, many of them being deforested, and antibiotics being fed standardly. And that is the reality for most beef production on the globe. Then we find out that it's bad for our health, too, surprise surprise. All these things change if the cattle is held free-range and managed well, by which the carbon that is sequestered in soil outweighs pollutions by far, stimulate the growth of the pasture and the meat is rich in omega-3s instead of omega-6s etc (so yes, sustainability is so much more complex than Co2; you don't restore ecosystems just by being carbon neutral). Prices are much higher, but only because so many costs of the industrial production are externalised. That being said, if we would just let all the cows go onto grasslands, we'd see many ecosystems collapse, which happens everywhere we try large-scale monocultural food production, also with fishes etc. The same even applies to some large-scale industrialised biodynamic farms...-Almost all 'meat is bad' comparisons rely heavily on comparing vs beef, which is indeed by far the worst polluter, and massively stacks the deck. When you start looking at fish, eggs & chicken the picture changes drastically. Insects would likely be even better but I think no one wants to go there... But the difference between vegetarian and vegan is actually quite minimal as a result.
-I have yet to see a comparison that includes 'organic' vs 'industrial / bioindustry' produced meat. Probably because they prefer to hide the uncomfortable truth that animal-friendlyness and environmental-friendlyness are often at odds with eachother. Large scale, industialized produciton with fast-growing breeds is usually significantly more efficient when it comes to resources consumption / CO2. But the fact that these aren't seperated / specified properly is on its own indicative that the numbers are deliberately 'vague', considering there's a significant difference in their footprint.
-There's more to sustainability than just Co²...there's also things like water consumption. And just as an example, a lot of nuts are actually quite horrible at their water consumption. A kilo almonds costs about 16k liters of water. A kilo of cashews 14k. A kilo of chicken? Only 4k.
What recent research has shown is that we really can't generalise these things. A diet that works for someone could be the cause of all sorts of diseases for somebody else, and what is good for you at one point in your life might not be good at another. Humanity has seen cultures with all kinds of diets, and among those cultures that didn't deplete their lands within a few centuries, there's the whole range from diets that are mostly based on meats or fish to mostly (but not exclusively) vegan cultures. So as @Jovidah said, we're omnivores. What all of them have in common is that their diet was strictly local and seasonal, fresh, varied across the years both in variety and in amount, and that all these people spent a lot of time with food-related activities. Oh, and none of them were trying to extract as much profit as possible in as short as possible out of the grounds.I’m pretty convinced through my self experimentation humans are meant to be primarily carnivores, with supplementation for micronutrients from vegetables and mushrooms. How that works in a world with too many people I have no idea, but I know it’s the best way for me. Living with a significant other who prefers variety though, and crazy rising food costs have kept me on a more mixed diet.