The paleo paradigm has been proposed, all fine and dandy. And it is quite convincing. Scientists looked at markings on bones, studied nucleic acids from early Homo’s poo samples (coprolites) and it is immediately obvious that eating grains was out of the question till the dawn of agriculture, between 4 and 10 thousand years ago. And the argument is that these few thousand years were not long enough for any evolutionary adaptation to new diet to take place. Clearly, there are some reports (I have not seen any raw data though) that soon after people started to eat grains and legumes (lentils were the first) their started to decay and that the jaws did not grow sufficiently to accommodate the teeth, which was a problem particularly with wisdom teeth. Then there was maloclusion (misalignment of teeth of the jaw and the mandible), the skull got smaller as did the stature. Apparently, the life span also shortened. And finally, there was narrowing of the pelvis in women, which led to complications at birth (Not as in the Garden of Eden anymore). Clearly dentists and midwives were in high demand. The records of these ancient times are scarce, but there are also observations, most notably by Weston Price, of the contemporary hunters-gatherers, who appeared to have none of these. This is being explained mostly by the phytic acids in grains which inhibit absorption of vital minerals from food thus affecting skeletal and dental health. Only birds have phytase which digests grain without problems.
So, where is the controversy? The Paleo stand is that humans could not have adapted their digestion and metabolism over few hundred generations. If indeed, the skeletal problems resulted from grain consumption, they would have been manifest at post-reproductive age, so neither natural nor sexual selection was likely to play a role. Besides, time was to short to make considereble changes to the mechanism fine-tuned in the course of eons. We share something like 99% of the DNA with chimps and they do not eat grains either (though they do eat some meat).
There may be another take on that if you forget Charles Darwin and think Jean-Baptise Lamark. Inheritance of acquired traits had been generally discredited as heresy, but some neo-lamarkists survive and even argue that such miracles are possible:
In any case it is possible that some adaptations were passed in children at the level of regulation of DNA or RNA expression, rather than modification of sequence of nucleic acids.
If you look around, you will discover people who appear not to be bothered with grains or high carbohydrate diet. They do not do endurance sports and yet they are slim. They never develop diabetes and may not even have heart disease (the association between high carbs and atherosclerosis is probably more convincing that the cholesterol story). I noticed, however, that they tend to have smaller mandibles as well, incidentally the trait of some British aristocrats. I doubt that these people have phytase in their pancreatic juice, so they likely have mineral deficiencies, but they may have better insulin control. Possibly, the descendants of the first farmers from the Fertile Crescent of 10,000 years ago would be better adapted to grains, while the people from the parts of the world where hunters-gatherers prevailed even during the Roman Empire (e.g. northern and eastern Europe), may struggle. Or, to the contrary, those having been exposed to grains for more generations, would have inherited the consequences of grain consumption and now have not only more tooth decay but also smaller brains? Unless of course, Lamark is pure nonsense.