In a previous entry, I discussed the problems with appealing to "humanity's natural state", and I briefly touched on what I want to get into in a little more detail here - the fact that tools are part of humanity's evolutionary package.
One common variant of the Caveman Fallacy is to point to elements of the human body and claim that they are proof that humans should not be engaging in certain activities (usually involving the eating of certain items) because, lacking tools, we are lacking in the biology necessary to do the task or digest the food.
Claims such as this should not be dismissed out of hand. Humans are in the habit of doing all manner of things that are physically hard on us and that we would likely be better off not doing. However, these claims should not be seen as pursuasive without a good deal of thought either.
The basic problem is that the claim that humans should not engage in an activity because the human body is not built for that activity is based on a basic misconception about the role of technology in human evolution. As stated int he previous entry, our ancestors had tools well before we were anatomically modern humans. Indeed, many animals, especially some of our fellow primates, can be seen using tools in their environments. As out ancestor's brains became more complex, so did their tools, and so did their ability to adapt to the environment. By the time anatomically modenr humans arrived on the scene, we had evolved to a creature for whom tools were necessary extensions of the body - indeed, the ability to make, use, and rely on tools is physically ingrained into our brains.
The tools used for food production - grinding implements, cutting tools, and fire - are especially important. For the human brain to develop requires a huge amount of calories, and while most of us living in North America and Europe can take a steady supply of food for granted, our ancestors weren't so lucky. Over the evolutionary path that led to humans, the development of these food production tools allowed our ancestors to not only make use of a wide variety of plant and animal foods, but to process these foods in such a way as to improve the ability of each ounce to yield nutrition in order provide out infants and adolesecents with the necessary fuel to feed their growing brains and provide adults with the calories that they needed to store up for lean times. By the time the Holocene dawned, some 10,000 years ago, eating an omnivorous diet was as much a part of being Homo sapien as walking upright and using language (another tool, when you think about it).
When someone tells you that humans should not be doing something because we are not physically built for it, you should stop and consider what they are saying. They may have a point. However, if our ancestors developed tools for it, then it is part of our evolutionary kit, and our ability to fashion the tool means that we are, in fact, built for the activity.