Lately, I have become addicted to the videos of a certain young lady named Kerry McCarpet. She's witty, eloquent, and I just love that British accent of hers - I could watch this video of hers over and over, just to hear her utter the phrase "betwixt the hand and the gob" again and again.
I also agree with a lot of what she says, even though I'd probably be much more cautious in some respects. Her mind is more speculative than mine. I think this is a good thing - it makes for inspiring and lively videos. At the very least, she does what many vegans sadly don't do - she explains her reasons for believing the way she does.
However, let's focus on the one negative thing I found, shall we?
Here, and at a few other places, she presents an argument for high-carbohydrate/low-fat veganism: Evolution shows you what we should eat; our body runs on sugar, our blood cells seem modeled on chlorophyl to some extent. Therefore, we should eat a high-carb vegan diet.
(Personally, I think the chlorophyl argument has a chemist weeping somewhere, but that's just an aside.)
There is a general problem with arguments from evolution. When you try to argue that one specific trait is good, it is of course tempting to say that this trait has survived for so long, so it has to be good - right?
Well - no, sadly. When you do that, you will always end up going in circles.
See, the theory of evolution was invented to explain how we (and all the other species) ended up where we are. But that doesn't tell you anything about whether one specific trait is "good" (i.e., well-adapted) or not. After all, (the theory of) evolution necessarily has to explain the bad along with the good - or to put it differently: In evolution, there is no such thing as good or bad.
The theory of evolution is descriptive, not prescriptive.
We evolved to love sugar and salt, and this is reflected in our taste buds. And we evolved to have a strong sense of empathy, which we may judge as a good thing. But we also evolved to be primates with a rather nasty habit of dropping bombs on each other, and we evolved to have an overweight brain that lets us to fancy stuff like cooking our food, and making hamburgers. We cannot predict, from a merely evolutionary standpoint, which ones of those traits will still be around in a million years.
If you were in possession of a time machine (or a supernatural gift of foresight), and you were to peek into the far future to tell us how this whole game of natural selection actually turned out, that would be something entirely different. Then you could tell us whether red hair will prevail (it doesn't look like it will, to my utter dismay), or whether we will become more peaceful (again, not the obvious winner... *sigh*). But as things are, evolution cannot be used to argue for or against any one lifestyle. It is entirely possible that only the most territorial and aggressive will survive. There may be lots of other arguments against this, but evolution alone just can't tell you.
I suspect that, at its heart, this is really a vitalist argument. It sees evolution not as a blind process guarded by natural laws, but as an intentional agent. It is extremely tempting to fall into this trap - I think even Dawkins once remarked that it is almost impossible to talk about evolution while avoiding it - so this is not a damning accusation at all. It's totally understandable.
After all, the situation is extremely frustrating. We're still left with a guessing game when it comes to something so fundamental as our choice of food. We just don't know enough about nutrition to give definitive answers to the most rudimentary questions.
Personally, I'm going for a "part-time vegan" style, and while I might change this in he future, I'm fairly certain about one basic principle: I will never submit to one philosophy of nutrition to the absolute exclusion of all others. There will always be some meat in my diet - probably very, very little -, some fish, some oil, some cooked and some raw foods - and probably lots and lots of fruits and veggies. But even this diet, which I consider the most rational choice I am currently capable of, is based on guesswork, word of mouth, and personal experience.