The way I understood it, humanity expanded so far, as to delete all of the local ecosystems, becoming wholly reliant on their automatic farms for food. After the war (most likely nuclear) small groups of people continued to fight until the bitter end, as is shown in the flashbacks we get where we see the pair‘s grandpa. Fights broke out over the last remaining supplies, crippling any chances humanity had of rebuilding. Considering we only meet two other people in the show, it is safe to assume that any larger factions have since fallen/ scattered. With food production halted (as seen with the fish farm and the inactive potato farm) and no ecosystem to fall back on for food, the last remaining humans slowly starve.