In 1976, Fran Tarkenton threw for his 40,240th yard, breaking Johnny Unitas’ NFL record for career passing yards.

In 1995, Dan Marino launched his 47,004th yard, breaking Tarkenton’s mark.

In 2007, Brett Favre slung his 61,362nd yard to move ahead of Marino.

And last week, Peyton Manning reached his 71,839th yard and passed Favre in what, unfortunately, might be remembered as one of the saddest days for an NFL icon in league history.

“Football is hard,” Tarkenton said this week. “Peyton breaks a record of Brett Favre’s, a record I once held for [19] years. A big record. And the next thing you know, he’s on the bench and people are booing him and yelling for the other guy.

“But, hey, that’s football.”

Indeed.

Manning was benched as the Broncos lost their second straight game, because he also was the reason they lost their second straight game. He had thrown four interceptions, extending his league-leading number to 17 during a season in which he’s completing only 59.9 percent of his passes, with a career-low passer rating of 67.6.

Numbers like that will get even living legends booed by the fans and benched in favor of a guy like Brock Osweiler, who trails Manning in regular-season touchdowns, 539-2.

Sunday, Manning will be back in Denver nursing injuries to his feet and throwing shoulder while the Broncos play in Chicago. Coach Gary Kubiak says the job still belongs to Manning when he’s healthy, but we all know a win this week will earn Osweiler a start at home against the Patriots. And a win there would essentially be the end of Manning’s career, barring an injury to Osweiler.

Talk about the NFL’s circle of life.

“Hey, I heard fans boo John Unitas,” Tarkenton said. “I heard them boo Bart Starr. I heard them boo Sonny Jurgensen.”

And?

“And I heard them boo Fran Tarkenton because when you get to the end of your career they want to see the new guy,” Tarkenton said. “They want to see the new pretty face even after all that Peyton has done. That’s because football is a different world. It’s an emotional world. It’s week to week.

“It’s up and down. Look at how the Vikings did in San Francisco, losing [20-3] in Week 1. You say to yourself, ‘How can this happen?’ But it does. How do you think Cincinnati feels after losing to Houston? How about Green Bay losing to Detroit at home last week?”

When Tarkenton retired from the Vikings in 1978, he left the game with the NFL career records for attempts, completions, yards, touchdowns and wins. He holds none of them anymore, but his ability to play the position wasn’t defined by statistics alone anyway.

“It’s the toughest position in all of sport,” Tarkenton said. “It’s the least understood position in all of sport.”

The Vikings’ current quarterback, Teddy Bridgewater, isn’t turning heads with his numbers this season. But he is turning Tarkenton’s head with how he’s handling himself, both within the framework of the team and when called upon in higher-pressure situations such as the closing moments of the Bears game.

“I like Teddy because I think he’s comfortable in his own skin,” Tarkenton said. “Quarterbacks, sometimes the moment is too big for them. For most of the people who play quarterback, they don’t get it. And they’re not dumb guys. These are good guys who have talent. But it’s the mental package. Understanding all the nuances and are you comfortable or is it too big for you?

“Christian Ponder probably had more physical ability [than Bridgewater]. But he wasn’t comfortable in his own skin. I looked at him and he just wasn’t comfortable. Never was. And you cannot overcome that. I don’t care what your talent is. Teddy, from the moment he stepped on the field, it doesn’t look like the moment, the stage is too big for him.”