Frank Mir was 22 years old when he made his Octagon debut at UFC 34.

It was Mir’s third pro MMA fight, and in it the fresh-faced Brazilian jiu-jitsu purple belt took just a minute, five seconds to tap out (and injure, from the look of it) two-time BJJ world champion and former Abu Dhabi open weight titlist Roberto Traven with an armbar.

A sign of things to come? Yeah, you could say that.

One glance at the UFC 34 card and it’s impossible not to feel the passage of time.

In the evening's dueling main events, Randy Couture defeated Pedro Rizzo to retain the heavyweight title, and Carlos Newton lost his welterweight crown to Matt Hughes. Counting Mir, five future UFC champions fought that night, including BJ Penn, Josh Barnett, Ricco Rodriguez and Evan Tanner. Also on hand were Matt Lindland, Phil Baroni and -- just in case you want to feel really old -- Pete Williams.

Frank Mir, right, was outgunned and out of his league against Junior dos Santos. Rod Mar for ESPN.com

The show took place at the MGM Grand in Las Vegas, which lends an undeniably circular vibe to any discussion of what happened to Mir on Saturday night at UFC 146.

What happened, of course, was that we likely witnessed the end of Mir’s career as a top contender to the UFC heavyweight title.

Obviously, nothing is certain. In the shallow heavyweight division, fortunes can turn on a dime, but on the heels of his fifth fight for some version of UFC gold, it’s tough to imagine the now 33-year-old Mir putting together the kind of prolonged run it would take to earn him a sixth.

Not after he was so thoroughly outgunned by Junior dos Santos this weekend, at least. Not after the champ so effortlessly shook off his takedown attempts and repeatedly punished him to the body and head before ultimately leaving him sprawled out on the canvas midway through the second, just to one side of the Octagon’s shiny new MetroPCS logo.

Like we said, the passage of time.

Mir will no doubt fight another day. Not doing so would show a sort of self-awareness and good sense we seldom see in professional athletes, so we must assume he'll go on to have at least a handful of future fights in the UFC. Early this week, the Internet is atwitter with rumors that his most recent and most bitter nemesis, Brock Lesnar, might return; and right now, there could probably be no better development for Mir’s continued relevance.

Even if Mir never fights again, though, he’ll leave a near unparalleled legacy in the 265-pound division. His 16 wins in the Octagon are the most all time by a heavyweight. He twice held the UFC title (though once it was an interim belt) and his highlight wins over Tim Sylvia, Lesnar and Antonio Rodrigo Nogueira rank among the most memorable submissions in the company’s history.

Perhaps most impressively, Mir battled his way back from a 2004 motorcycle accident that short-circuited his first title reign and threatened to end his fighting career entirely. After shattering his femur and tearing all of the ligaments in one knee, Mir returned to go 8-5 during the next six years, arguably besting anything he’d done in the cage prior to the accident.

If it is not the most remarkable comeback story in UFC history, I’m not sure what is.

Now, Mir faces the unenviable task that eventually befalls all athletes: transitioning to the twilight of his career. How he does it will be entirely up to him. Will he serve as a measuring stick for the UFC’s suddenly healthy crop of up-and-comers; fighters like Stefan Struve, Stipe Miocic and Shane del Rosario? Or will he go a route similar to the one taken by Hughes and Couture, crafting his own conclusion by picking his spots in high-profile, big-money bouts?

A Brock Lesnar return would be good news for old nemesis Frank Mir. Josh Hedges/Zuffa LLC/Getty Images

Aside from Lesnar, the perfect next foe for Mir is probably Barnett, considering their comparable ages, styles and positions in the sport. To make that happen, however, either Barnett would have to be granted his long-awaited return to the UFC or Mir would have to unexpectedly show up in Strikeforce. Not sure either seems overwhelmingly likely.

In any case, Mir spent years in the rarefied air of the heavyweight division’s elite. When the time comes, he should be a shoo-in for the UFC Hall of Fame. Now though, he’ll have to pass the torch -- unceremoniously, as is typical in this sport -- to new standard-bearers like dos Santos, Cain Velasquez and (soon, we hope) Daniel Cormier.

Never was that more apparent than between rounds on Saturday night.

During the final minute of the first, dos Santos had hurt Mir badly, forcing him to stumble sideways, throw a sloppy counterpunch that missed by a mile and shoot for a doomed takedown attempt. In all likelihood, he was saved by the bell. As he sat on his stool receiving orders from his cornermen, the ringside doctor leaned in and asked one of those questions no fighter (no person, really) ever wants to hear.

“Do you know where you are, sir?”

Mir fired off an immediate response: “Mandalay Bay, Vegas, May 26.”

He didn’t sound at all confused. In fact, he sounded completely sure of himself; this declaration issued with the same swagger and unwavering certainty we’ve come to expect from him over the years. The only problem was -- as was instantly pointed out by the UFC broadcast team -- he wasn’t at Mandalay Bay at all.

Mir was back at the MGM Grand.

He was back where it all started, and time was passing.