Well, first off, you seem to be confusing the concept of survival of the fittest and evolution, which are two separate things. Evolution is nothing but the genetic mutations we get and give to our offspring. And it's happening constantly, regardless of whether it meaningfully affected reproduction. It's just that over long periods of time, mutations which are well adapted to helping a species survive their environments and reproduce tend to make it into the next generation. Thus, species tend to evolve/adapt to suit their environments.

So if our diets largely involved meat for millions of years during as our time as hunters and gatherers, it's likely we have evolved to eat and digest meat quite well by now without it negatively impacting our health. We do know we have eaten meat for at least a million of years, as we are omnivores and have some carnivorous teeth. And we likely evolved to be omnivores because it probably helped some of our ancient ancestors survive periods of food (plant) scarcity. Some evolutionary researchers think meat eating may even be apart of how we were able to develop our human intelligence.

But during those millions of years we didn't have the meat diet alternatives we have now, so how can you assume that meat is better (or worse) than those?

Given how incredibly complex human health is, it’s no trivial task to identify precisely how/why some specific behavior is “healthy” or “unhealthy”. Given that predicament, our best guide is what our ancestors did, since it’s passed the test of evolutionary time to at least get us to our present moment. The burden of proof is on any reformer to demonstrate why his reform is better than the tradition.

In the case of something as ancient as meat eating, which is so fundamental to our species as to be reflected in our very physiology (teeth, digestive system), this is an extremely high bar.