Hippocampus
The brain's map-maker and memory filer. Helped by walking and learning, smaller in people under long stress.
A curved structure on each side of the brain that turns today's experiences into memories you can find tomorrow, and that keeps your sense of where things are. Cut it out and a man can hold a conversation but won't remember it in an hour. Evidence: solid
It's one of the brain areas that visibly changes size in adults. In people over 55, a year of regular walking grew it a little instead of the usual slow shrink; people under long stress tend to have a smaller one, though which came first is less clear. Taxi drivers who learned every street in London had a bigger back part of it than bus drivers on fixed routes, and it grew while they trained. Evidence: decent
Why it matters
Walking, sleeping, learning routes without the satnav and keeping stress from running for months are all, in the end, hippocampus maintenance. Evidence: worth trying