I'm wondering how much of our eating habits is based on culture rather than biological necessity. Arguably, humans (or the genus Homo) may not have developed that way, if those prehistoric homonids hadn't eaten meat. Today, however, I think it's more of a cultural thing. I think it would be very much possible to change our eating habits, but as culture develops only slowly this might take a very long time. And who knows, maybe this concious decision might give us an evolutional disadvantage over millions of years.