With football now dominated by rocket-armed quarterbacks and fleet-footed receivers, it’s hard to imagine the sport without the forward pass. The play, however, was illegal for nearly four decades after the sport’s inception. When passing was finally permitted in 1906, to improve player safety, critics predicted it would dilute the sport's  rugged essence and drive away fans. But it had the opposite effect.

In football's early years, yardage was tough to acquire; points were even scarcer. Wearing little padding and protective equipment, players who used their bodies as battering rams suffered not just kicks, bites and eye gouges but wrenched spines and cracked skulls.

But football wasn’t just extremely violent. It was deadly.

The Chicago Tribune reported 18 football-related fatalities in 1904, mostly among prep school players. After another 19 died the following year, universities such as Stanford, Northwestern and Duke dropped football. Others threatened to do the same unless changes were made.

Prodded by President Theodore Roosevelt, an avid football fan who worried that the game could be outlawed if not made safer, more than 60 schools met after the 1905 season and approved rulebook revisions. Among them were the abolition of dangerous mass formations, the creation of a neutral zone between offenses and defenses, the doubling of the first-down distance to 10 yards and legalization of the forward pass.

Although any player behind the line of scrimmage was permitted to pass, the rules committee imposed severe restraints that hampered offenses. Passes couldn’t be thrown or caught within five yards of each side of the line of scrimmage, and only the two ends on the line of scrimmage were eligible to make catches.

Additionally, passes that crossed the goal line resulted in touchbacks to defenses, and out-of-bounds throws were given to defenses at the spots where they left the field. Passes that hit the ground without being touched by any player resulted in turnovers.

“The forward pass has been so well hedged about with restrictions as to make it a play that must be thoroughly practiced and well executed to be of use,” wrote rules committee member Walter Camp, a staunch foe of the play.

Pass proponents such as Georgia Tech coach John Heisman believed the forward pass would inject speed and skill into football and open up the game by compelling defenders to spread out in coverage. But opponents such as Camp believed it emasculated the sport’s brute nature.

“Many predict the ruination of the game through the drastic reformation,” reported the New York Times of the sport’s rule changes heading into the 1906 season.

Those who feared the forward pass would immediately ruin football needn’t have worried because old-school coaches of the East’s top colleges viewed it as a risky gimmick. Yale tried only three passes in its season opener. All failed.

“Well executed they are undoubtedly highly spectacular, but the risk of dropping the ball is so great as to make the practice extremely hazardous and its desirability doubtful,” the New York Times editorialized.

Unlike the Eastern elites, Saint Louis University coach Eddie Cochems gave the new rule the old college try. Before the start of the 1906 season, he cloistered his team in a Jesuit retreat in Wisconsin, as he later wrote, for “the sole purpose of studying and developing the pass.”

In the opener for Saint Louis against Carroll College on September 5, 1906, Bradbury Robinson threw football’s first legal forward pass. The toss hit the ground untouched, resulting in a turnover. But Robinson later connected on a 20-yard touchdown pass. Thanks in part to the forward pass, undefeated Saint Louis outscored its 1906 opponents, 407-11.

Glenn “Pop” Warner also embraced the forward pass as a way for his 1907 Carlisle Indian Industrial School squad to compete against collegiate powers with stronger, deeper rosters. Warner designed the “Carlisle formation,” forerunner of the single-wing offense, which gave players options to run, pass or kick.

Carlisle showcased its aerial game in front of 20,000 fans in Philadelphia against the University of Pennsylvania in a battle of unbeatens. Playing in his first college game, Jim Thorpe was among the Carlisle players who completed a pass in Carlisle's decisive victory.

Carlisle's shutout loss the following week at Princeton, however, demonstrated the limitations of the forward pass. Without pass interference penalties, Princeton’s defenders continually grabbed Carlisle receivers to prevent them from catching the ball.

Because the rulebook discouraged passing, football continued to be a ground-and-pound game—and a lethal one. The Chicago Tribune reported 31 football-related deaths between September 1908 and the summer of 1909, and Army and Navy cancelled their 1909 seasons after each team had a player die from football injuries.

The continued fatalities brought additional tweaks to passing rules, such as no longer making untouched throws turnovers. The aerial game, however, remained a passing fancy until a relatively unknown Catholic school used it to score one of college football's greatest upsets.

Before the 1913 season, Notre Dame’s Gus Dorais and Knute Rockne practiced the forward pass while lifeguarding in Ohio. The training paid off when the Irish unleashed a high-flying assault at West Point that overwhelmed Army on November 1, 1913.

Notre Dame opened the scoring when Dorais threw a 40-yard touchdown pass that Rockne caught in stride, and the throws kept coming as the Irish scored five passing touchdowns. Dorais completed 14 of 17 passes for 243 yards in the victory that put Notre Dame on the football map.

“Everybody seemed astonished,” Rockne later wrote. “There had been no hurdling, no tackling, no plunging, no crushing of fiber and sinew. Just a long-distance touchdown by rapid transit.”

The passing techniques used in the early 1900s differed from what football fans are familiar with today. Some players left their feet to make jump passes, while others tossed footballs underhanded or end over end.

As the forward pass became more common, the football itself evolved from a watermelon-shaped orb that could be shot-putted to a slimmer oval that was easier to grip and could be thrown in a spiral.

In the 1930s, the spirals were flying out of the hands of Texas Christian University quarterbacks Sammy Baugh and Davey O’Brien, winner of the 1938 Heisman Trophy. Employing a spread offense with two wide receivers and two slot receivers, TCU threw the ball as many as 40 times in a game, and their star quarterbacks eventually took their aerial skills to the fledgling National Football League.

After being drafted by the Philadelphia Eagles, O’Brien set an NFL record in his 1939 rookie season with 1,324 passing yards, only to be surpassed by Baugh the following year. “Slingin’ Sammy” set another record in 1947 when he threw for nearly 3,000 yards, a mark that wouldn’t be surpassed until Johnny Unitas did so in 1960.

The running game, however, remained an offensive mainstay.

When Cincinnati Bengals quarterback Ken Anderson led the NFL with 2,667 passing yards in 1974, it was more than 1,000 fewer than Washington’s Sonny Jurgensen tossed in 1967. The NFL responded in the 1970s by allowing offensive linemen to block with their hands and tightening restrictions on the contact defenders could make with receivers.

Since the turn of the century, the NFL enacted further rule changes to promote passing, such as barring helmet shots and low hits on the quarterback. The pass-happy game has fueled the growth of fantasy football leagues and television ratings. Defying the prediction that it would cause the sport’s demise, the forward pass instead made football America’s most popular sport.