I think vegans here chose their dieting habits for individual reasons.
A good post by bwb (Brian) about becoming vegan is here:
A good reason to go vegan
Brian tried going vegan for himself -- not for the ulterior motive of undermining the "evil" ways of meat eaters. Quite simply, he experienced positive results so he stuck with the diet. What more is there to say?
The only crtique about meat eaters is their "out of sight, out of mind" thinking, where as long as they are unaware of the details behind how their meat was prepared, they have no mental obligation to worry about it.
I don't think anyone has criticized a meat eater for making the conscious decision to eat meat, so there's no reason a meat eater should feel out of place in these forums.
Now, onto the "that's how our ancestors lived" arguments.
Our ancestors also had a life expectancy of around 16 years. Antibiotics and other modern medicine were not part of their diets, thus brain development. Nor were processed foods, trans fats, refined sugar or refined grain. Preservatives. Multivitamins. Ground meat. Microwaved foods...
I could go on, but the point is that modern health habits are far removed from those of our ancestors. Yet life expectancy has skyrocketed. Through the advent of modern medicine, science has essentially found a way to exploit the imperfections of the human body.
With modern, large scale health studies (i.e. the China study), science again has found a way to objectively observe all the nuances of diet -- how saturated fat affects heart disease, how omega-3s affect brain development, etc.
Yes, we can look to our ancestors and try to mimick their health habits, noting the various elements that contributed to this or that evolutionary path. But modern health and nutritional science give us a much stronger, better, more objective tool at finding out what's good for our bodies and what's not.
And health studies, so far, have told us that vegetarians and meat eaters alike are entirely capable of functional, healthy lifestyles.