Well many Indians have been vegetarian for many generations.
Documented with a sufficient sample size, and studied?
About a third of India is vegetarian; the majority are not. I wonder what the percentage is of people who had strictly vegetarian parents for five generations or more. And is the health of this sample of people very good overall, in terms of both physical and mental ability?
Despite having a climate that is optimized for growing plants, with plenty of rainfall, India currently has one of the higher rates of malnutrition in the world while eating among the lowest amount of meat in the world. There are a lot of factors involved but I'd be hesitant to use anecdotal reports from India as an example of optimal nutrition.
There's a pretty solid amount of evidence that dietary sources of DHA omega 3 fatty acid in the hunter gatherer diet played a key role in developing the over-sized brain seen in our species. DHA is a big component of the dry weight of our brain, and virtually all human consumption of it comes from fish, bivalves, grass-fed meats (especially the brain of the animal, for hunter-gatherers), and chicken eggs from chickens that are fed flax seed. The only semi-realistic lacto ovo vegetarian source of it is a huge amount of algae concentrated down into supplement form.
Women pass on DHA to their babies, helping to grow their brains. The body can also convert ALA omega 3 fatty acid (found in some plant sources, like flax seed) to DHA omega 3 fatty acids, but only a small fraction is converted, and with a high degree of variability. Hence why dietary sources of it are so helpful, and considered an important part of our evolutionary history. When a person goes vegetarian in their life, they at least received a lot of DHA from their mother, usually. But when you have several generations in a row of zero dietary source of DHA, then the body is 100% reliant on the body's ALA to DHA conversion process, generation after generation. I'd like to see more evidence that almost all people can do that with no health consequences; absolutely no impairment to intelligence or mood.
Personally, my mood and strength increased a lot when I switched from a vegetarian diet to one that includes moderate amounts grass-fed beef, organic chicken, free range eggs, Alaskan wild-caught fish, and mussels along with my vegetables and fruit, and a reduction in grains.
And I think that'll be more than compensated for by the massive decrease in burden on the land if 99% of people stopped eating meat (trophic levels and all).
Among the significant problems to solve with growing plants are 1) how to fertilize them and 2) how to keep pests away.
And big problems to solve with raising animals are: 1) how to feed them and 2) what to do with the waste.
Nature, and farms that mimic nature, largely avoid those problems. Plants feed the animals, and animals fertilize the plants and eat the pests that attack them.
So let's take an example of a permaculture orchard. There's a place out in California I have in mind that grows trees for fruits and olive oil, maintains grasslands, and also raises chickens, goats, and other animals. They routinely let in chickens and goats into their orchid, where the chickens peck away at the ground to get rid of bugs that would otherwise harm the trees, and goats eat the grass around the trees that would otherwise have to be mowed. The farm then sells all types of plant and animal products throughout the seasons of the year, and is highly diverse.
A lacto ovo vegetarian version of that would be a lot less economically sustainable and perhaps less environmentally sustainable. If they did everything they do in that example, but sell none of the animal products, and don't harvest any animal meat or eggs or milk, then that same amount of land only offers calories from the trees, and only derives income from the trees, and that entire set of animals is just strictly a cost to the farmer, and those animals will eventually die off over time anyway. The prices of the fruits and olive oil would have to go way, way up, and they'd need to use far more land in order to produce enough calories to feed the same number of people. And if they didn't use animals at all, even for pest control, fertilization, and grass mowing, and therefore in this case likely not being a permaculture, then they'd have to do all of those services for the trees in other ways, with any environmental and economic consequences from that.
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Another example would be cultivation of mussels, which spend their adult lives immobile and have no actual brains. People put ropes into the water, mussels grow on them, and then they harvest and sell them. It actually improves water quality unlike horrible fish farms, is considered sustainable, and doesn't need to use up any land. Mussels are a great source of EPA and DHA omega 3 fatty acids, and an absolutely phenomenal source of vitamin b12. In contrast, studies routinely show that strict lacto ovo vegetarians tend to have vitamin b12 deficiencies.
Now, we could eat those brainless sustainable mussels for all the lean protein, EPA and DHA, and vitamin b12 they contain, without using any land for it. Or we could not, and get those calories from a plant farm instead. And if we do that, it's not likely that we're really killing fewer animals. Countless bugs die from natural and synthetic pesticides used to keep plants safe on farms, and certain types of plants result in a deaths of mice and snakes when harvested, like one estimate of up to 100 mice per hectare of wheat. Fertilizers often drain into rivers and oceans and kill fish. So in that scenario we still kill small animals, but have to supplement vitamin b12, use more land, and have possible health issues from no dietary DHA.
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Another example would be semiarid grasslands. They're not the best place for growing most fruits and vegetables, but they're ideal for raising cows on natural grasses, with modern rotational grazing. A variety of omnivorous permacultures are more adaptable to environmental conditions than purely vegetarian farms.
