It is often repeated in the literature that humans have been eating grains, dairy and other Neolithic products for only 10,000 years, which is why we are not well adapted to this kond of diet. 10,000 years is a long time but seems much shorter when compared with one million on human existence or even with 100-200,000 years of Homo sapiens.
But 10,000 years ago was when agriculture was first adopted in the "fertile crescent" of the Middle East. While humans populated Europe about 45,000 years ago (Neanderthal had been there), they were hunters-gatherers and remained so until the agrarians came. And they came to Europe from the south-east, along the Mediterranean coast and then northwards to present France. So in southern Europe agriculture was adopted probably around 6,500 BC, a little later more to the North reaching Germany and southern Britain about 5,500 BC and the Baltic, Northern Britain and Scandinavia 3,500 BC.
Eastern Europe was probably inhabited by hunters-gatherers for longer, though they fell under the influence of migrants from Asian steppes, who were not farmers, but bred horses for food, transportation and warfare.
Suddenly Paleolithic story becomes much closer to home. While it can be argued that descendants of the citizens of ancient Mesopotamia could have acquired greater tolerance for grains and dairy, possibly via selection due to influence of Neolithic diet on fertility, this may not be the case with the descendants of the people who lived in what is now Scotland, Northern Ireland, Finland, Poland or Ukraine. It is interesting to observe that incidence of numerous chronic diseases is relatively high in those countries. This could possibly be explained by "bad diet", but can also be explained by genes which make adaptation to "bad diet" more difficult or impossible.
Highly recommended reading:
Lutz W: The Colonisation of Europe and Our Western Diseases. Medical Hypotheses, Volume 45, Issue 2, Pages 115-120
There have also been some simplistic attempts to link dietary factors with blood types. I disregarded the D'Amato story when the book appeared long time ago. But now it is making me wonder, as the association may have something to do with our heritage. To put is simply: the European hunters are mostly type 0, the farmers type A, and the horsemen type B. It would be very instructive to look at other genes, as already attempted by Dr. Lutz.
Also, we have to be cautious in associating present inhabitants of any region in Europe with the origin of their ancestors. For example, most of the gene stock in Britain appears to have originated from the ancient European hunters-gatherers, such as proto-Basques, who migrated North more than 15,000 years ago. Later Celtic (farmers) influence was more cultural than genetic and the Viking invaders we not avid farmers. Influence of Romans and Normans was also mostly cultural and limited to the elites. Another example: in ancient Grece and Rome there were many African (and possibly northern/eastern European slaves, whose genes might have contributed later to local populations, while Spain was occupied by Arabs who had been eating grains and dairy for much longer.
So, if you are type 0 and your ancestors came from Scotland or eastern Europe, you should be more serious about Paleo lifestyle than if they came from Southern France.