It's physics.
About 60% of the dry weight of the human brain is lipid, and one of its most abundant fatty acids is DHA (docosahexaenoic acid). For decades, scientists wondered why evolution seemed to favor this unusually long, highly unsaturated molecule.
The answer lies in flexibility.
DHA contains six double bonds, giving it an extraordinary ability to bend and twist. Instead of behaving like a rigid rod, it acts more like a molecular spring. When incorporated into cell membranes, especially those of neurons and retinal cells, DHA helps keep the membrane flexible and dynamic.
Global Chemical Database