Vegetarianism and especially veganism can get pretty ideological sometimes. People who have strong beliefs like to think that their belief system is best for all aspects of their lives and the world, not just bits and pieces. Many vegetarians believe their way of eating is healthier than omnivory or carnivory. It's easy to believe, since mainstream nutrition research has a distinctly pro-vegetarian slant. One of the classic arguments for vegetarianism goes something like this: our closest living relatives, chimpanzees and bonobos, are mostly vegetarian, therefore that's the diet to which we're adapted as well. Here's the problem with that argument:
Where are chimps (Pan troglodytes) on this chart? They aren't on it, for two related reasons: they aren't in the genus Homo, and they diverged from us about 5 million years ago. Homo erectus diverged from our lineage about 1.5 million years ago. I don't know if you've ever seen a Homo erectus skull, but 1.5 million years is clearly enough time to do some evolving. Homo erectus hunted and ate animals as a significant portion of its diet.
If you look at the chart above, Homo rhodesiensis (typically considered a variant of Homo heidelbergensis) is our closest ancestor, and our point of divergence with neanderthals (Homo neanderthalensis). Some archaeologists believe H. heidelbergensis was the same species as modern Homo sapiens. I haven't been able to find any direct evidence of the diet of H. heidelbergensis from bone isotope ratios, but the indirect evidence indicates that they were capable hunters who probably got a large proportion of their calories from meat. In Europe, they hunted now-extinct megafauna such as wooly rhinos. These things make modern cows look like chicken nuggets, and you can bet their fat was highly saturated.
H. heidelbergensis was a skilled hunter and very athletic. They were top predators in their ecosystems, judged by the fact that they took their time with carcasses, butchering them thoroughly and extracting marrow from bones. No predator or scavenger was capable of driving them away from a kill.
Our closest recent relative was Homo neanderthalensis, the neanderthal. They died out around 30,000 years ago. There have been several good studies on the isotope ratios of neanderthal bones, all indicating that neanderthals were basically carnivores. They relied both on land and marine animals, depending on what was available. Needless to say, neanderthals are much more closely related to humans than chimpanzees, having diverged from us less than 500,000 years ago. That's less than one-tenth the time between humans and chimpanzees.
I don't think this necessarily means humans are built to be carnivores, but it certainly blows away the argument that we're built to be vegetarians. It also argues against the idea that we're poorly adapted to eating animal fat. Historical human hunter-gatherers had very diverse diets, but on average were meat-heavy omnivores. This fits well with the apparent diet of our ancestor H. heidelbergensis, except that we've killed most of the megafauna so modern hunter-gatherers have to eat frogs, bugs and seeds.