So boiling isn't strictly required, though it has nice benefits on the wort (in a phase called the "hot break"). However, it is necessary to reach certain temperature (140-160 F, though broader may work) to convert starch into sugar.

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.

Eventually the most alcohol tolerant bacteria, the ones primarily fermenting the sugars, get killed by the alcohol itself (by denaturation, the alcohol makes the membranes soluble in water). At that point there are very few more alcohol tolerant bacterium.