The two ancestral sources of polyunsaturated fats that I can think of are olive oil and hazelnuts.
In Britain at least, hazlenuts are a traditional Christmas food, which makes sense, hazlenut trees bear nuts in the autumn, and people gather them and eat them. They taste lovely. But I don't think they were ever a principal source of calories for the British, more a seasonal food.
In Southern Europe, olive oil was spread all around the Mediterranean basin by the Greeks and Romans, and extensively cultivated and traded. It's a fair bet that Southern Europeans since ancient times have eaten olive oil in large quantities, especially in regions where it's extensively cultivated. Why would you not eat the local food? Much like nuts, olives taste very good, and have lots of calories in them.
So if polyunsaturated fats were bad news and a cause of obesity, then we should probably expect that Southern Europeans run to fat.
And that was very much the stereotype of my youth. "Don't marry an Italian girl! They are lovely but the minute there is a ring on their finger they turn into their mothers." (And their mothers are always very large!)
These days, it's the British and Americans who are fat! America leads the charge, but Britain is not far behind. Southern Europe has resisted the obesity epidemic for a long time. (Remember the book "French Women Don't Get Fat"? They didn't use to.
I can think of two possible reasons.
One is just that the southern european cultures, already having a culture of eating olive oil, have stuck with it rather than adding seed oils to everything.
And another is that two thousand years is enough to cause evolution. Maybe Southern Europeans have actually evolved a little towards being able to use polyunsaturated fats as a major source of energy?
If that's true then we should see the hallmark of recent speedy evolution, homozygote deficiency diseases in the affected areas.
So, does anyone know of specifically Southern European genetic diseases of fat metabolism?
And we should also see that the descendants of Southern Europeans in America are more resistant to America's current obesity epidemic.
Again, I have no idea, I just register that this is a straightforward prediction of the idea of olive oil being bad for you.