This occurs in chapter 4, "Who has Won to Mastership," where Buck is dreaming and remembering before the campfire. The image he sees in the fire is evidently that of primitive man, who is barely clothed, carries a club, does not stand fully erect, and lives "in perpetual fear of things seen and unseen."
Buck sees this image because it is part of his own ancestral memory. We're told that in the Yukon he sometimes remembers Judge Miller's house in the warm Southland and the comfortable life there, but what is stirring more within him is a sense of his own primitive ancestry being awakened. These are the memories of "his heredity that gave things he had never seen before a seeming familiarity." Buck's new and dangerous life in the Northland has granted him an entry into his own past, into the lives of his wolf ancestors who lived in a similar environment. These are instincts that have been aroused within him, and the primitive man he re-imagines in the flames is part of that primordial existence, before civilization, when man and wolf/dog were first coming together as partners.
Eventually Buck does rejoin the wild, as a leader himself, but not until after his own partnership, an idyllic one, with John Thornton. It is the tragedy of Buck's story that he loses two Edens, two states of innocence: the first in the Southland, and the second in his stay with Thornton. The novel is thus a parable of the Fall of Man, but one in which the loss of innocence leads Buck, like mankind, into a new world of challenge and fulfillment he could not have otherwise had.