Just to add a little more context to that quote:
When Buck earned sixteen hundred dollars in five minutes for John Thornton, he made it possible for his master to pay off certain debts and to journey with his partners into the East after a fabled lost mine, the history of which was as old as the history of the country. Many men had sought it; few had found it; and more than a few there were who had never returned from the quest. This lost mine was steeped in tragedy and shrouded in mystery. No one knew of the first man. The oldest tradition stopped before it got back to him. From the beginning there had been an ancient and ramshackle cabin. Dying men had sworn to it, and to the mine the site of which it marked, clinching their testimony with nuggets that were unlike any known grade of gold in the Northland.
This paragraph is talking about the long-running hunt for a lost mine. Given the preceding sentence about "Many men had sought it ...", probably the part about "the first man" refers to the first man who went looking for this mine among the many who have tried.
There are stories and traditions surrounding this mine and the frequent searches for it. A sort of folk history has sprung up around it, "as old as the history of the country". But among all those stories, not one goes back as far as to tell about the man who first started the quest: "the oldest tradition" concerning the hunt for the mine is still not old enough to tell us about the origins of that hunt. The passage doesn't tell us what the traditions are, but we may imagine that some stories relate to specific quests and some traditions might have originated with specific questors, but even the oldest one doesn't "go back" as far as the very first questor.
The origin story is so old that it's been lost in the mists of time, and only newer stories and traditions still remain now. To quote a related theme from a completely unrelated work of literature:
Ages come and pass, leaving memories that become legend. Legend fades to myth, and even myth is long forgotten when the Age that gave it birth comes again.