Why the dietary ratio shifted from roughly 1:1 to 16:1
Estimates of ancestral diets place the omega-6 to omega-3 intake near 1:1 to 4:1, built from wild plants, fish, game, and foraged nuts. The modern Western pattern is often cited near 15:1 to 20:1, driven mostly by the cheap, widespread use of high-linoleic vegetable oils in processed food and frying.
The ratio is a population-level descriptor, not a target a person eats directly. Two diets with the same ratio can carry very different absolute amounts of each fat, so the ratio is best read as a marker of how the food supply changed, not a precise clinical dial.
Delta-6-desaturase as the shared bottleneck
Both linoleic acid (omega-6) and alpha-linolenic acid (omega-3) compete for the same delta-6-desaturase enzyme on the path to longer-chain derivatives. When omega-6 intake is very high, it can occupy more of this enzyme's activity, which is one mechanism proposed to lower conversion of plant omega-3 into EPA and DHA.
Conversion of alpha-linolenic acid to EPA and DHA is limited in humans regardless of ratio, which is why direct EPA and DHA sources such as oily fish are discussed separately. The eicosanoid-balance argument is mechanistically plausible but human outcome data remain mixed, so this is best treated as a working model rather than settled fact. General educational information, not medical or dietary advice; consult a qualified professional before changing intake.