> due to some quirk of evolution, Omega-3, -6, and -9 are the ones biological life uses most. As far as I can tell, there's no specific reason they're all multiples of 3. Probably just a coincidence.

This curio bothered me as well. I didn't yet get a fully satisfying explanation for this either.

There's this diagram, showing for example the full pathway of how linoleic acid catabolizes: https://commons.wikimedia.org/wiki/File:Linoleic_acid_beta_o...

It shows dependencies of the process onto several very specific molecular machines we call enzymes.

(main pathway, handling saturated fatty acids)

   ° Acyl CoA dehydrogenase -- removes 2 hydrogens from carbons immediately after the carboxylic head, forming a π-bond (double-bond) between the α-β carbons;
   ° Enoyl CoA hydratase -- adds water as H-OH to that α-β carbons π-bond;
   ° 3-hydroxyacyl CoA dehydrogenase -- converts the added -OH hydroxyl group to =O keto group;
   ° β-ketothiolase -- grabs the two keto groups, and snips off 2 carbons from the chain, carrying them off in bound form as a molecule of acyl-CoA;

(unsaturated side-branch)

   ° Dienoyl CoA reductase -- collapses two neighboring π-bonds into one;
   ° Enoyl CoA isomerase -- converts cis- to trans- variants, making them compatible with Enoyl CoA hydratase. Pathway continues from there.

These, when viewed as a set of combinators, seem perfectly sufficient to metabolize any fatty acid chain. Their chemistry reads pretty straightforward — and it must deal with cis/trans isomerism shenanigans, with neighboring π-bonds, with odd/even parity of the carbon chain. But it apparently handles all that!

Besides catalysis (combustion of the acids for energy), the two other paths for consumed fatty acids are excretion, and laying them into cell walls and membranes. These two paths aren't selective; they mostly don't care about the length of the chain, and where which π-bonds occur in it, if any.

So this must imply, that the "quirk of evolution" lives somewhere on the anabolic (synthesis/production) side of fatty acids; definitely not on the catabolic side.