You have to boil the wort for hours otherwise it's not beer, it's porridge. It's that process that kills any pathogens, not the fermentation (obviously, since fermentation requires the presence of pathogens).
In old times, a metal vessel large enough to boil an entire batch of wort may not have been feasible. Other techniques were used in these cases.
Say you have a wooden barrel or tub:
* Take off a portion of the wort, boil it, and add it back. This is decoction.
* Heat rocks and add them into the wort.
Define 'old times'?