Why does Buck dream of the hairy man again

The call of the wild puts Buck in mind of his instincts..... the "ancient man" represents the days before civilization. With little work to do, Buck spends his days by the fire, dreaming of the ancient man. In his mind he wanders in this early, undefined world. He remembers fear most of all, fear and constant vigilence. Along with this vision of the man, Buck nightly hears an ancient call deep in the forest. Sometimes he searches for it. Sometimes he hears it, springs from his place and runs wild through the forest. One night he hears it more clearly than ever before. He goes into the forest and he sees a timber wolf.

The hairy man is representing Buck's wild brother (a wolf). It's because he belongs to the wild and the wild is calling Buck.