I think the most conscientious choice is informed omnivory. Permaculture and organic gardening and farming techniques, combined with a in-depth knowledge of the ecosystem and a focus on local food strikes me as far more admirable than vegetarianism that relies on supermarket produce.

Properly raised, naturally grown livestock need not be harmful to the environment--it can be considerably less harmful than growing food crops. Remember--hoofed animals can be grazed on native grasslands, and if proper attention and rotational grazing are employed, they will improve rather than degrade the habitat. Raising food crops in a conventional manner always destroys the native habitat.

It's not enough to know what labels were put on the food (such as 'organic' or 'vegetarian'). You have to know where the food came from and how it was actually produced. Venison steak and rotationally grazed, grass-fed beef is infinitely greener (AND more humane) than factory farm wheat crops soaked in pesticides which ran off into the rivers, and contributed to the oceanic dead zone at the mouth of the Mississippi. The sheer amount of death and suffering caused by fertilizer runoff is staggering to contemplate. Obviously any diet that includes tropical fruits if you do not live in a tropical area is not one that is ecologically sound.

Is it necessary to be ecologically optimal in all areas? I would say, no--it's not always practical, either. But it is important to consider your reasons for various actions that impact your health, and make sure that your logic is sound, and truly fits in with all of the facts.

But then, this was more about what humans are adapted to eat, more than what humans should eat to minimize their negative impact on ecosystems and the environment.