The NFL’s 100th season will kick off Thursday night in Chicago, where 88 seasons ago the league played perhaps the most significant game in its history on a 60-yard, tanbark-covered indoor field that reeked of elephant manure so badly that some of the Bears players threw up on the field.

Yeah, the league sure has come a long way in reaching a $15 billion behemoth that will open 2019 with the Packers and Bears at Soldier Field.

One giant step forward took place on Dec. 18, 1932.

Unable to declare a champion based on the regular-season standings, the NFL scheduled the 6-1-4 Portsmouth Spartans to play the 6-1-6 Bears at Wrigley Field in the league’s first postseason game. Deep snow and forecasts of brutally cold weather forced the game into Chicago Stadium, where the circus — and those smelly elephants — had just visited.

The smaller field and a key play in the game set in motion the league’s first major rule changes, a break from the rules governing college football and the establishment of the NFL’s own rules committee. The latter was the predecessor of the competition committee, which continues to make annual changes and set points of emphasis.

This year, the notable point of emphasis is calling more offensive holding, an interesting and rare pushback in the evolution of this pass-happy league. And, of course, the biggest rule change is the one-year trial allowing pass interference calls and no-calls to be reviewed by instant replay.

Portsmouth coach George Clark would have loved instant replay back in 1932. With the title game tied 0-0 in the fourth quarter, Bears fullback Bronko Nagurski faked a dive play, stepped back, jumped and completed a short pass to Red Grange for a touchdown in a game the Bears won 9-0.

The Spartans argued that Nagurski’s pass was illegal. In their opinion, and many others, Nagurski wasn’t at least five yards behind the line of scrimmage when he threw the ball, which was the rule at the time.

The game also was the first played with hash marks, since the sidelines butted up against the stands. Until then, the ball was spotted one yard from the sideline following plays that went out of bounds.

After that game, the NFL decided to keep the hash marks, among other rules for that game. The league also decided that passes could be made from anywhere behind the line of scrimmage.

From 1922 to ’33, NFL teams scored fewer than 10 points a game 10 times in 12 seasons. In 1934, they hit a record 10.8 and never dipped below 10 again.

Passing and scoring has continued to evolve ever since. Rules have changed. Players have become more skilled. Coaches are more creative.

In 1948, teams averaged 23.2 points per game, a record that stood until 2013 (23.4). Through 13 weeks a year ago, the NFL was threatening to obliterate that record thanks in part to points of emphasis governing pass interference, roughing the passer and helmet-to-helmet contact.

In Week 3, journeyman Ryan Fitzpatrick became the first NFL player with three consecutive 400-yard passing games. In Week 8, Vikings receiver Adam Thielen became the first player to start a season with eight 100-yard receiving games in a row. In Week 11, the Chiefs became the first team to score 51 points and lose.

Heck, it got so wide open that the league actually listened to defensive coaches as they vehemently complained, week after week, about widespread offensive holding that was going uncalled. The resulting one-week point of emphasis in Week 14 resulted in 94 holding penalties which, according to ESPN, was 42% more than any week since at least 2012.

Scoring leveled off the final four weeks, as it often does as the weather turns and injuries mount. Teams averaged 23.3 points per game, second-highest ever, and the season was capped by the lowest-scoring Super Bowl ever (Patriots 13, Rams 3).

Where do we go from there?

“Looking at the preseason, it’s … ‘There have been so many holding calls,’ ” said Mike Pereira, former vice president of NFL officiating and current Fox Sports rules analyst. “Usually, there’s three holding calls a game. But it was seven, eight, nine holds a game in the preseason.

“So that’s the one thing I think is going to maybe have a greater effect — maybe a far greater effect — than the new pass interference rule.”

In other words, buckle up, unholster those thumbs, fire up Twitter and turn your attention to Chicago. The smell of elephant manure is long gone, but there’s still change in the air as the NFL’s 100th season kicks off.