Bipedalism, fire, cooked meat, language, the number 0, Bitcoin.
Discussion
Electricity
Also, I'd put language before fire in the enumeration.
I thought that one through. Cooked meat enabled rapid brain growth, which enabled language. They happened gradually in tandem, but IMO you don’t get language without cooked meat, hence the sequencing.
I've changed my mind over the years about the theory of evolution. I think Darwin got it backwards. Therefore, my shortlist is:
1. Word (In the beginning was the Word)
2. Love
3. Bitcoin
I thought about that one too. I see it more as an incremental stepping stone to Bitcoin rather than a 0-1 step. Some people trim it all the way down to just Fire, Bitcoin, and (perhaps eventually) teleportation.