People who lived in areas without fish also lived in areas without algae.
>The 2 essential fatty acids are
It is worth noting that there is basically no evidence that those 2 fatty acids are actually essential. A single study done in the 1930s is the basis for that belief. A study in which rats fed no fat got symptoms we now would consider indicative of vitamin B6 deficiency. This study was done prior to the discovery of vitamin B6. Although adding "essential" omega 6 fatty acids alleviated the rat's symptoms, later studies were able to alleviate the symptoms using vitamin B6 while the rats continued to receive absolutely no fats.
>Linseed oil (flaxseed oil) is a good provider of essential fatty acids
And linseed oil consumption is correlated with an increase in all cause mortality.
>Experts recommend a vegan diet against heart disease and cancer.
Experts recommend all sorts of things that contradict evidence. Like taking statins even though they increase mortality. Or switching from perfectly safe saturated fats to "heart healthy" trans fats even though it increases mortality. Or once trans fats were too obviously bad they simply changed their recommendation to "polyunsaturated fats" which are also correlated with increased mortality.