In my personal, anecdotal experience, every vegan I know has serious health problems. Literally every single one. Never anything that can be traced directly to their diet (e.g. one of those I'm thinking of mostly has severe eczema), but it's such a sharp correlation that I can't help thinking there's something to it.
While I haven't looked at that particular study, the result you're claiming seems implausible; one would expect humans to be optimized by evolution for living on the ancestral diet. While there are specific reasons why certain parts of our traditional diet are unhealthy (e.g. the ancestral lifestyle burned far more calories than current), in the absence of a specific mechanism like that, "I like me therefore it's good for me" is a perfectly reasonable inference.
And if you're really only interested in health, it seems implausible that that would be 100% correlated with animal vs. non-animal. There are so many different possible food molecules and no reliable common factor that differentiates where they came from - even more so when it comes to the animal byproducts that make vegans different from vegetarians, such as honey.
Again, the plural of anecdote is not data, but I've felt much better since going vegan. I'm willing to grant that may be a placebo effect, but my acne seems to also have improved a bit as well.
Hence room for the correct answer: we are built to consume both plants and animals, but different people respond differently to particular combinations thereof and thus have individual reasons to tailor their diets differently.
Apart from what afterburner said (vegetarians going vegan because of health problems), was there really a single ancestral diet? I have read that foraging was actually more efficient than hunting, so maybe it wasn't all meat after all?
When we look at human prehistory we're talking about a small number of individuals living in a small area, so a single ancestral diet is a reasonable approximation. Foraging was indeed more efficient than hunting in terms of calories, and yes the majority of food consumed would not be meat - I certainly wouldn't advocate an all-meat diet. But obviously meat was a valuable or even vital source of some nutrients (the very fact that humans continued to hunt when foraging is more calorie-efficient suggests that meat was necessary in some way), and I find it hard to believe that a diet that eliminates meat entirely could be healthy.
While I haven't looked at that particular study, the result you're claiming seems implausible; one would expect humans to be optimized by evolution for living on the ancestral diet. While there are specific reasons why certain parts of our traditional diet are unhealthy (e.g. the ancestral lifestyle burned far more calories than current), in the absence of a specific mechanism like that, "I like me therefore it's good for me" is a perfectly reasonable inference.
And if you're really only interested in health, it seems implausible that that would be 100% correlated with animal vs. non-animal. There are so many different possible food molecules and no reliable common factor that differentiates where they came from - even more so when it comes to the animal byproducts that make vegans different from vegetarians, such as honey.