Comment by adrian_b

Flax was included in the first set of domesticated plants, together with the cereals barley, emmer wheat and einkorn wheat, and together with a few legumes, including lentils and peas.

This set of domesticated plants does not appear random, because any more restricted set would have made impossible the substitution of the animal food used previously with plant seeds.

The seeds of cereals and of legumes together could provide an acceptable protein source, while the flax seeds could add the alpha-linolenic acid, which can be transformed by humans, with modest efficiency, into the needed DHA and EPA.

We know that the first generations of people who had become dependent of agriculture had serious health problems in comparison with their ancestors, which have become less severe after many generations, presumably after they have learned to better balance their diet and when those who have survived might have been better adapted to eating such food.

Nevertheless, regardless where you are located you do not know the properties of your genes, unless you do some very expensive study, by using various diets and monitoring how they influence the content in your blood of various substances, e.g. of DHA and EPA when eating various sources of omega-3 fatty acids, of vegetable or of animal origin.

In the absence of such a study, the safer hypothesis is that you belong to the people who are not efficient at the elongation of ALA into DHA and EPA, so it is safer to eat food with enough DHA and EPA, instead of eating food with ALA, like flax seeds or oil, and hoping that you belong to the people for whom this is good enough.

This is similar for a few other conditionally-essential nutrients, which can be produced by humans, but in most cases only in too small quantities compared to necessities, so it is safer to ensure that they are present in food, e.g. vitamin K2, choline, taurine, creatine.