The M series is a continuation of the A series development that's been going since 2008 when they bought PA Semi, maybe earlier. The M1 could have been branded

The M series is a continuation of the A series development that's been going since 2008 when they bought PA Semi, maybe earlier. The M1 could have been branded the A14X but for whatever reason they chose not to, probably to make better marketing buzz. I would guess they've been booting macOS on ARM in the lab since the A7 and after that just a long grind to get to a good enough user experience with sufficient app compatibility to switch. You can see stuff like TSO in the CPU showed up in earlier chips at least a couple years before the M1 launched.