From what i can see, only fish contain the available form of omega 3 in decent amounts, and then flax and chia seeds that contain ALA which has a ridiculously low conversion rate to EPA and DHA which is what our body actually uses. So PUFA omega 3 are essential for brain function and other organ functions but the only real source for omega 3 is oily fish. Because 90% of our history was spent in Africa, where did our primal ancestors get their omega 3 from?
I’m guessing a lot of early humans lived near water sources like rivers, lakes, ponds, streams if not near the coast so many would probably eat fish occasionally if not regularly. Fish can be preserved (like drying them) which would make them tradeable.
And of course, you do get some omega 3 from other animals too.
There are bodies of water most places, and usually smaller tribes would locate near them. I believe trade was important for our ancestors as well, so people inland could trade whatever resources they had with local fishing tribes. And as others have said, animals eating a more natural diet tend to have a bit more omega 3s in their fat.
Vegetables, especially leafy greens, contain a lot of omega three fatty acids relative to their overall fatty acid content. Just because omega 3s are essential doesn’t mean you need a lot of them to survive/thrive.