Thanks, Bobbit! 
I came onto the thread in response to the discussion about leather - and then segued into talking about diet.
And thanks to you, OP, I was just about to start a new thread, but if you're happy to let the conversation run, then fine.
I agree that we have it in us to be omnivore, but our bodies have not developed to eat meat:
"You’ve got a long, winding digestive tract, weak stomach acid and no fangs. Carnivores have short digestive systems, SUPER powerful stomach acid, and razor sharp teeth (claws, too). Is it possible you were born for fruit of tree - NOT flesh of beast? I’d say so. But then I’m a ‘sucker for straight logic." JSekerak.com
We are in fact, opportunists - historically, we've taken our food where we could find it. And I wouldn't argue against the theory that our big brains are the result of eating meat.
However, there is much evidence that a plant-based diet is healthier for us than one which includes meat and meat products.
Here's a post from an earlier MN thread, by AbFabT:
"I truly believe going vegan is the best thing we can do for our health. We have cancer in my family so I did loads of research into prevention being better than cure, and am convinced cutting out meat and dairy is beneficial. One book I'd highly recommend is The China Study, by Dr.T.Colin Campbell (www.amazon.co.uk/China-Study-Comprehensive-Nutrition-Implications/dp/1932100660/ref=sr_1_1?s=books&ie=UTF8&qid=1280963998&sr=1-1&tag=mumsnetforum-21) - very convincing about switching to a plant-based diet for health reasons."
And there's also the Adventist Health Study.
And, for those who really want an in depth examination of the effects of eating a plant-based diet, there's this.
None of this addresses the animal welfare issue or the effects on global warming of eating meat. 2 other good reasons to consider eating less meat.