Keyboard Navigation to Prior Commit

I would like to navigate from a selected commit to its prior commit in one keystroke.  Attached is an illustration.  I think the keystroke might be option-arrowdown, and likewise the other direction would be option-arrowup.

This will help especially if the prior commit is far offscreen, when the number of branches are many, or when enough merges criss cross – basically where visual line tracing is not  helpful.