Life expectancy in the late paleolithic was 33 years; today it is 67. So we would expect (ceteris paribus) that "natural" things are unhealthy. Instead, the tag line "All-Natural" is almost always a claim that the "All-Natural" product is healthy.
This argument tends to misunderstand a basic evolutionary principle:
- Evolution selects for gene propagation, not health
Everyone is probably familiar with extreme examples, like black widow spiders who practice sexual cannibalism. A canonical example of evolution's imperfection in humans is the appendix, whose function may be largely vestigial. The appendix shows the dangers of basing our actions on "evolution": even if we evolved an appendix so that our primate ancestors could better digest leaves, that doesn't necessarily imply that eating those leaves is the healthiest thing to do.
At the other end of the spectrum are the numerous fad diets based around the proposition that humans evolved to eat some certain amount or type of meat. But why should the evolutionary benefits of eating meat be related to health?
This was brought up by Jared Diamond in his book Why Is Sex Fun?, based largely on the paper "Why do Men Hunt?" by Hawkes, et al. They note that, as far as anyone can tell, hunter-gatherer societies would be better off (more and healthier kids) if they were just gatherer societies; that is, if men didn't hunt.1 So why do men hunt even though it hurts not just themselves but their children? Because if you bring down a big animal, you can trade that for extramarital sex, and it's better to have lots of extramarital kids, some of whom might starve to death, than to devote all your time to making sure a few intramarital kids live.
That's probably not something you'll find in the average diet book.
Voltaire parodied the idea that there is a compassionate god ensuring that natural events are good. Were Voltaire alive today, Candide might instead have poked fun at pseudo-Darwinians who claim that "Mother Nature" looks out for us.
- Note that "gathering" in this case can mean "hunting" small animals and fish, so it's not that the nutritional value of meat is greater. It might be better phrased as "Why do men hunt large animals instead of small ones?" There are several other possible objections that Diamond addresses in that book, which I won't repeat here.