The Zookeeper's Dilemma
Imagine you are a zookeeper. A clear and pressing question about each animal is: What do they eat?
To maintain healthy animals, the first priority is to try as well as possible to replicate what they eat in the wild. Feeding carnivorous lions only rice cakes and herbivorous zebras only fish cakes will lead to sick and eventually dead animals.
One of biggest-picture signs that something is not quite right today with human diets was expressed by Dr. Barry Groves (his lecture is featured below). He pointed out that although we observe a great deal of chronic and degenerative illness among modern humans, this is largely unheard of among wild animals. However, it is seen among captive and domesticated animals, specifically, animals that are being fed the wrong food.
So what do humans eat? Well, a great many things. So what is the next question?
More ideal information would be an answer to: What do they eat in the wild?
This, too, is quite a challenge because with very few exceptions, humans today no longer live "in the wild" in any helpful sense. However, it is possible to investigate what modern human ancestors ate when they much more nearly did so during evolutionary formative periods, say, 50,000–100,000 or more years ago.
Answers to another question would also help, and this one can be applied to moderns: What kinds of foods do they do best on?
Humans are able to eat a wide range of food and survive doing it, but what would be ideal? This shifts the emphasis to what foods humans thrive on indefinitely versus merely what they can manage to stay alive on for some years. This is a subject of extensive medical research, sadly, much of it deeply flawed, due mainly to over-reliance on study designs that are inherently incapable of demonstrating causation. Such "studies," however, are far cheaper to fund and then use as the basis for getting another paper published. The mainly evolutionarily relevant idea being "publish or perish."
Balancing a number of different lines of evidence, I have arrived at the view that humans are basically carnivores that can also survive on plant foods as a fall-back, even for very long periods of time, though not without suffering gradual degenerative harm in the process. Feeding humans primarily—and especially only—plant foods causes them to become gradually malnourished, to sicken in a variety of ways, and to "fail to thrive."
This is commonly obscured for two reasons. First, the process of degeneration can take years and decades to progress. Second, moderns who move toward vegan diets often report feeling better, so it must be good, right? On examination, however, they are quite often feeling better after moving away from from something rather specific—modern diets of processed foods. They are not moving away from an ancestral diet rich in fresh fatty meat. With some exceptions, many find their health deteriorating noticeably after a few years of veganism and are forced to quit.
Just because something is better than something else by some measures such as feeling better or losing weight in the short term, this does not necessarily mean it is also ideal or even good.
An excellent introduction to the argument in favor of the foregoing view is available in the lecture below. Have a look. It's about 50 minutes. Many additional papers and lectures are linked in the resources page.
If this is true, how could large scale meat eating possibly work for a modern society? Tiny populations of paleo hunters could do it, but they were working with massive roaming herds, and many of those species still went extinct! And isn't meat production already bad for animals and the environment already, even without being expanded still further?
Although this belief has become quite popular, the balance of evidence I have seen indicates that it is severely mistaken. To explain this view, we must turn to some still different perspectives and sources: