At various points in the book, especially in Chapters 4 and 7, Buck dreams of a man from the distant past. Buck especially tends to do this when his life is going well and he is in a dominant position. In these dreams, he is being taken back to a time when (according to London, at least) dogs and humans were first living together.
The narrator paints a picture of a "man" who is barely human. He is clearly not simply an animal in that he can make weapons (he has a club with a stone head) and clothes. But he is also clearly not a modern human. He is described in Chapter 4 as having shorter legs and longer arms and bigger muscles -- more like an ape than humans are today. He is very hairy. In Chapter 7 he can even brachiate -- swing from branch to branch in trees.
In other words, the narrator paints a picture of a very primitive human being who is much more like an ape than modern humans are.