In A Dictionary to Basic Japanese Grammar, it says under the conditional ば that this is the origin of the topic particle は but I haven't been able to find where this claim comes from. Does anyone know how the conditional came to be the topic particle?
It's not that one is the origin of the other. Adverbial particle は, conjunction ば and sentence ender わ (as in 出るわ出るわ) share the same origin.
The reason why adverbial particle は came to denote the topic is probably because one needed something to tell boundary when it had been originally unmarked.