> You'd expect that with big brains being so costly, evolution has only equipped us with brains big enough to store a lifetime of memories

It hasn’t, and rightfully so. I don’t think there’s any evolutionary advantage in remembering the first steps you did or the first time you drank milk, for example, and indeed, nobody remembers those.

A brain that remembers where the berries grew last year, where that cosy cave is, or where that angry tribe lives has evolutionary advantages. Most of the rest such as remembering specifics about events we visited, I think, is collateral non-damage, plus, oftentimes, reinforcement learning (many childhood memories get reinforced by your (grand)parents telling you repeatedly what they remember happened)