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.