Need to quickly move to the start (or end) of a layer? Press I to jump to the In point of a layer, or O to jump to the Out point. Remember though, if a layer starts or ends outside of your composition's time range, you won't see the playhead (content) in After Effects.
If running AE 6.5, there's additional shortcuts between in and out points in your comp.
Press Cmd+Shift+Option+Right Arrow (Ctrl+Shift+Alt+Right Arrow) to move to the next in or out point
Press Cmd+Shift+Option+Left Arrow (Ctrl+Shift+Alt+Left Arrow) to move to the previous in or out point