Welcome to the last post in the series. Time to summarize and wrap it up!
I respect each person's right to choose the diet they prefer. This includes vegetarians and vegans, particularly because most of them make daily sacrifices to try to make the world a better place for all of us. I'm an omnivore, but I sympathize with some of the philosophy and I often eat beans or lentils instead of meat*.
Our history with meat
Our ancestors have probably been eating some form of meat continuously for at least two hundred million years. However, the quantity has waxed and waned. The first mammals were probably largely carnivorous (insectivores). Yet our primate ancestors went through a 60-million-year arboreal phase, during which we probably ate fruit, leaves, seeds, insects, and perhaps a little bit of vertebrate meat. We only outgrew this phase in the last few million years, when we developed the tools and the brains to pursue prey more effectively.
During our 2.6 million-year stint as hominin hunter-gatherers, we ate an omnivorous diet, although we really have very little idea how much meat it contained (it probably varied by time and place). Historical and contemporary hunter-gatherer cultures are all omnivorous, and typically eat significant to substantial quantities of meat, suggesting that our ancestors may have done the same. Non-industrial agricultural populations eat as much meat as they can get, although they usually can't get as much as hunter-gatherers.
If there is such thing as a natural human diet, it is clearly omnivorous.
Meat, obesity, and chronic disease
Vegetarians and vegans do tend to be leaner and have a lower risk of some chronic diseases than omnivores, although it's difficult to separate meat avoidance from the other trappings of the vegetarian and vegan diet and lifestyle.
Regularly replacing meat with plant protein such as beans may reduce cardiovascular risk, and for people at very high cardiovascular risk, a very-low-fat, high-unrefined-carbohydrate vegan (or near-vegan) diet may be something to consider.
Most of the chronic disease risk associated with meat consumption comes along with red and processed meats-- particularly the latter. There are good reasons to believe that processed meat increases the risk of digestive cancer, obesity, diabetes, and perhaps cardiovascular disease. As much as I dislike this conclusion, unprocessed red meat probably does increase the risk of digestive cancer, and possibly also diabetes in people with elevated iron stores. There are hints that it might also contribute to cardiovascular disease, although the overall evidence is not very strong in my opinion. That doesn't mean we need to completely eliminate these meats from our diets to be healthy, but moderation is probably in order.
Seafood is one category of meat that tends to be associated with positive health outcomes. Again, it's difficult to completely disentangle seafood eating from socioeconomic factors, but the evidence overall does suggest a protective effect. Poultry tends to be neutral.
Meat and mortality
Vegetarians, vegans, and omnivores all have a similar total mortality risk. Overall, there is little association between total meat consumption and mortality in the general population. The only type of meat that is consistently associated with higher mortality is processed meat. This suggests that avoiding meat, even red meat, probably isn't an effective way to live a longer life (even if it does impact the risk of specific diseases).
Meat, development, and physical function
If we take a broader view of health that includes more than just disease resistance, meat seems to play a positive role. Meat supplies nutrients that are complementary to those contained in plant foods. This is expected, since our nutritional requirements have presumably been shaped by hundreds of millions of years of omnivory.
A whole food-based omnivorous diet is the easiest and most effective way to meet the nutritional needs of the human body, and the further a person departs from this pattern, the harder she has to work to be well nourished. For example, vegan diets require a lot more thought and planning than omnivorous diets to meet nutritional needs, and because of this, vitamin and mineral deficiencies are more common among vegans.
As one would expect, these nutritional limitations sometimes restrain the body from achieving its full potential. When we look at the most physiologically demanding scenarios, such as reproduction, growth/development, and athletic performance, omnivores tend to have an advantage.
This may explain why many people don't feel well on vegan diets, and often develop strong cravings for meat over time**. Those who stick with the diet for many years are those who are able to tolerate it the best and have the most resolve.
Many well-studied traditional cultures have demonstrated that an omnivorous dietary pattern including modest quantities of meat, dairy, and eggs, and also including plant protein such as beans, can support excellent health and longevity over multiple generations. We have limited evidence that the same can be true of a vegetarian diet containing dairy and eggs, however we do not have such evidence for a vegan diet containing no animal foods. While I have no problem with adults eating a vegan diet in general, I remain uncomfortable with vegan pregnancy and feeding children a vegan diet. Adults can choose but children can't.
The bottom line
The overall evidence leads me to believe that meat is a valuable part of the diet, but we don't necessarily need to eat much of it to reap the lion's share of the benefits. From a chronic disease perspective, it probably is preferable to focus meat intake primarily on seafood and poultry, and limit processed meat.
* I don't think I would ever become completely vegetarian or vegan again, even if I was convinced meat is unhealthy. The all-or-nothing phase of my life is over, and I have little desire to completely eliminate anything from my diet. I occasionally eat unhealthy foods such as pizza, ice cream, and potato chips-- and I don't feel guilty about it. I keep 90+ percent of my diet in the healthy range and don't worry too much about the rest.
**Although personally, I felt fine during my six months of eating vegan. I missed eating meat, but I don't recall my desire for it intensifying over time.