Did you know that Bruce Jenner was once the most famous person -- man, woman or child -- on the planet? (If you are under the age of 40, probably not.)

Did you know he starred on "CHiPS?" (Under the age of 30, probably not, and also wondering what a “CHiPS” is.)

Did you know he starred in commercials, skated on reality shows, was a combatant on “Battle of the Network Stars,” had a golf tournament, was a race car driver, starred in some dreadful movies, and some of the worst reality shows? (Under 25, ummm ... no.)

Bruce Jenner: From the most famous person in the world, to the not-quite-so-famous, to famous-for-being-famous status, to accessory in "Keeping up with the Kardashians.”

And finally, Friday night, to this: Jenner will be interviewed by Diane Sawyer on ABC in what will almost certainly be the most-viewed news (or more specifically) "informational” program on television this year. He will be back to where he started: The most famous person in the world.

It's a remarkable moment in history of American celebritydom -- that monster that consumes our insatiable interest and sometimes the lives that it's built upon. Jenner is back. But it is also fair to ask: Where has he been?

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The answer's not easy to pin down because Jenner is not. He's undergone continual transformation -- personal, professional, even physical. Indeed, Jenner has spent a lifetime transforming himself, from a failed college football star (bad knee) to Olympic gold medal decathlete.

From then ... till now. Friday night, the wheel turns once again.

"The celebrity," cultural historian Daniel J. Boorstin once observed, almost redundantly, "is a person who is known for his well-knownness." But the modern celebrity -- unattached to movie stardom or TV stardom or pop music stardom -- is really someone known for his adaptability, or the ability to morph according to the dictates of the news cycle that is driven by the dictates of the ever-shifting interests, tastes and demands of ... you.

And over the decades, just about five of them now, Jenner has been the very personification of well-knownness yoked to adaptability. He pursued well-knownness with the ferocity and determination that he pursued his gold medal. Well-knownness became the goal, the end, the whole idea. He succeeded, then failed, then succeeded all over again. He was the cat with nine lives, then 10 lives. Tomorrow night is No. 11.

He used every available means in pursuit of fame, but as it always does in these pursuits, television became the vehicle. After his Olympics triumph in 1976, Jenner became the Wheaties pitchman, following a relatively new template (first the commercial endorsement deal, then the TV series or movie ... )

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Bill Cosby had consecrated the whole idea with Jello, while Suzy Chaffee (Chapstick) had established the notion that Gold Medal winners can pitch stuff too.

But TV stardom was to become at first a fickle and then a brutal mistress. No one asked -- including apparently Jenner himself -- whether he could actually act. "Grambling's White Tiger" (1981) and then "CHiPs" (he played Officer Steve McLeish) emphatically answered that question. He could not.

Jenner didn't give up. Jenner never gave up. There were other series: "The Love Boat," "Murder, She Wrote," something called "Me and Mom." Even “Silver Spoons.” They were all mercifully quick appearances, almost over before they began. He even appeared as himself on "Donald Duck's 50th Birthday." Then on to infomercials. No luck there either.

Still, Mr. Adaptable forged ahead. In 2000, reality television became a network obsession. Jenner got himself a spot on the hot new import from Britain, "The Weakest Link." But if commercials and TV series were, fickle, reality was the brutal part of this leg of the trip. He was cast on "I'm A Celebrity -- Get Me out of Here," an entire series predicated on the joke of the celebrity has-been.

Shows like "Pet Star," "The Contender, " "The Apprentice," came and instantly went. The nadir was a skating celebrity show. Observed The Washington Post critic: "There are times when Bruce Jenner, the 1976 Olympic decathlon champ, looks downright scared while skating with U.S. pairs champion Tai Babilonia."

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He couldn't act, couldn't skate. What could Jenner actually DO? Little did he realize through all these trials -- nor did anyone else -- the answer to that question was right by his side -- his wife, Kris.

They were married in 1991; "Keeping up with the Kardashians" would not arrive until 2007. It subsequently redefined an entirely new way of developing, then exploiting, fame -- the TV series starring people who were famous for not much of anything at all, but then who became enormously famous simply by virtue of starring in a show about themselves.

Jenner -- once the most famous person in the world -- was largely reduced to an accessory role, as the eccentric mopheaded spouse who shuffled between compounds. If "Kardashians" was -- say -- "Gilligan's Island," with Ginger as Kim, Jenner was either the Professor ... or Gilligan.

The show of course was his crowning anti-achievement, and as far from Olympic fame (or for that matter, "Skating with the Stars" ignominy) as he could have possibly wandered. Yet it was a show that made him a household name among the most media-Internet-saturated "cohort" in world history: Young adults, between the ages of 18 and 30. It even made him a reasonable selection for a bit part in "The Hunger Games."

Jenner had become world-famous once again for doing nothing more than being Kris Jenner's consort.

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And so as you watch tomorrow night -- which you will -- and wonder why this is even "news," know this: Jenner has spent almost 50 years, since Newtown High School, in Connecticut, transforming himself. He's both a product of the Age of Celebrity and a cause of the Age of Celerity. He was a great athlete, then spent the next 40 years parlaying that fame so relentlessly that few would even remember what he was actually once famous for.

His trajectory went this way: Gold Medal winner ... former athlete ... a would-be star ... a "cautionary tale” ... a flailing has-been ... and finally ... a bona fide reality TV star. Tomorrow night, the wheel turns again, when he'll tell Diane Sawyer that he has decided to become a woman.

To write a TV post -- a personal ambition of mine, by the way -- that includes a reference to both Bruce Jenner and Philip Roth in the same sentence, I end with this:

Some years ago, the famous novelist said of fame, "The American writer in the middle of the 20th century has his hands full in trying to understand, and then describe and make credible much of American reality ... The actuality is continually outdoing our talents, and the culture tosses up figures almost daily that are the envy of every novelist."

Take a bow, Bruce. You've stumped Philip Roth.