Not every Super Bowl represents a line of demarcation for the legacy of pro football, yet Super Bowl LI was a special circumstance. Featuring the largest comeback in Super Sunday's history, fans all over the world were either introduced to the sport in its most stunning form, or reconnected with what was always a hallmark of the old NFL: unpredictability. Down 28-3 in the third quarter, Tom Brady engineered drive after to drive to ensure that people will be glued to sports' biggest game for years to come.

William "Pudge" Heffelfinger is considered to be the first professional football player. For a game against the Pittsburgh Athletic Club, the Allegheny Athletic Association football team paid Heffelfinger $500. While it's entirely possible athletes were paid to play football before this historic day, an accounting ledger provides documented proof that Heffelfinger was paid to play football. Heffelfinger was worth the expense, as he recovered a fumble and returned it for a touchdown for the game's only score in a 4-0 Alleghany Athletic Club triumph.

In the midst of moves to abolish the game of football (18 deaths had been reported during the 1905 season), Theodore Roosevelt personally encouraged reform in the sport. The result of Roosevelt's efforts to eliminate the violent nature of football in 1905 led to two milestone events in the game's history: the formation of the Intercollegiate Athletic Association of the United States -- which is now known as the National Collegiate Athletic Association -- and the forward pass. Dangerous mass formations such as the wedge were also banned, and the distance necessary to attain a first down was extended from 5 to 10 yards.

A group of men, including the most famous of the time -- Jim Thorpe -- and a man who would be involved in the game for more than six decades -- George Halas -- gathered at the Hupmobile showroom in Canton, Ohio to create what was first known as the American Professional Football Association (APFA), later renamed the National Football League in 1922. Thorpe was named the league's first president.

When the NFL was in its infancy, the league was in desperate need of a major star to attract fans to games. On Thanksgiving Day 1925, the Chicago Bears signed "The Galloping Ghost" and the back made his debut against the rival Chicago Cardinals in front of a standing-room only crowd of 36,000 at Wrigley Field. Over the next few months, Grange and the Bears traveled the country on a barnstorming tour, promoting the pro game to large collections of spectators. Grange's signing ultimately helped the NFL grow to a new level in the late 1920s and early 1930s.

The 1932 NFL championship tilt was historic for a number of reasons: 1) It was the first time a one-game playoff was held to determine a champion; 2) it was held indoors; 3) it forced rules changes, most notably the use of hashmarks and forward passes becoming legal from anywhere behind the line of scrimmage. The Portsmouth Spartans and Chicago Bears finished the season tied for first. A title game was scheduled to be played at Wrigley Field, but a blizzard forced the game indoors at Chicago Stadium. The Bears won 9-0, with the game's lone touchdown coming on a Bronko Nagurski pass to Red Grange, which the Spartans argued was not thrown from at least 5 yards behind the line of scrimmage, as the rules of the time dictated. The score stood, and the new passing rule set in 1933 and enjoyed to this day would forever alter the game. Official league championships started being held in 1933, and those games would evolve into what football fans now know as the Super Bowl.

The 500 or so New Yorkers who actually owned televisions were the fortunate few who were able to witness a defining moment in pro football's relationship with mass media. NBC was the first network to televise a pro football game when the Philadelphia Eagles and Brooklyn Dodgers faced off in front of 13,050 spectators at Brooklyn's legendary Ebbets Field. The game featured three future members of the Pro Football Hall of Fame: quarterback Ace Parker and tackle Bruiser Kinard of the Brooklyn Football Dodgers, and Eagles end Bill Hewitt.

The second play of the 1940 NFL Championship went 68 yards for the Chicago Bears. Chicago wouldn't stop at 68 yards, much less 68 points. The 73-0 thrashing of the Washington Redskins was a pushpin on the evolution of football. Basically, Bears head coach slash owner George Halas and innovator Clark Shaughnessy showed that scheme -- in this case the "T" formation -- could greatly impact football games. The modern pro set would dominate the landscape of pro football, with its in-your-face introduction coming on the Bill Osmanski touchdown scamper pictured above.

What became known as "The Greatest Game Ever Played" was a climactic event in professional football's rapid rise in popularity during the 1950s at a time when baseball dominated the sporting scene. In a game that was broadcast nationally on television, the Baltimore Colts and New York Giants engaged in a thriller that was the first championship game decided in sudden-death overtime. The Colts ultimately triumphed, 23-17, to earn the league title in a game that featured 17 future members of the Pro Football Hall of Fame.

The First AFL-NFL World Championship Game -- later to be known as Super Bowl I -- was the first major step in the AFL-NFL merger and laid the groundwork for the NFL landscape as fans know it today. The Green Bay Packers were the ideal representative for the NFL in its initial on-field showdown with the upstart AFL, whose champion in 1966 was the Kansas City Chiefs. Vince Lombardi added to his legacy by leading the Packers to victory in the first two Super Bowls. In all, the Packers won five championships in seven years during the 1960s. That run of success provided the impetus to name the shiny prize given to Super Bowl winners the Vince Lombardi Trophy.

One finger, one league, one huge statement. that pro football would be better because competitive balance was at a premium -- and in the sole possession of the NFL. The league had merged with the shockingly successful American Football League two years prior, but joint games as one big "happy" family were two years away. Most NFL teams felt the AFL was a weaker league and that the two sides didn't belong together. That was until Joe Namath and the New York Jets beat the Baltimore Colts, 16-7, in Super Bowl III, driving home the point that AFL teams -- what became the AFC -- could hold their own.

The New York Jets fell to the Cleveland Browns in Cleveland Municipal Stadium back in Week 1 of the 1970 season. Big deal. Ahh, but what did the NFL gain? Prime-time football. ABC executive Roone Arledge and Commissioner Pete Rozelle joined forces to create Monday Night Football, from which television's longest-running series this side of 60 Minutes has flourished. The success of Sunday Night Football, Thursday Night Football and the NFL as a media darling owes a big thank you to Arledge and Rozelle's collaboration.

If you had to pick one play from the 120-year history of professional football, you could hang your hat on "The Catch" -- you just couldn't hang your Dallas Cowboy hat. The most famous photo of NFL history changed NFL history. The San Francisco 49ers' 28-27 win over the Cowboys in the 1981 NFC Championship launched Bill Walsh's short passing game on pro football, as well as the 49ers' dynasty.

The Minnesota Vikings were over the moon about getting Heisman Trophy winner Herschel Walker via trade in 1989. The rest of the NFL wasn't, not because of what he did for the Vikings, but what the trade built for the Dallas Cowboys. Cowboys head coach Jimmy Johnson ultimately received six Vikings draft picks. Two of those were used on the NFL's all-time leading rusher in Emmitt Smith and a three-time All-Pro safety in Darren Woodson. The other picks were used in trades, one of which resulted in the acquisition of Russell Maryland. The Dallas dynasty was born.

The "Tuck Rule" as it came to be known, stated that if a quarterback loses the football during his throwing motion, including bringing the ball back after changing his mind, it was an incomplete pass. Enter Tom Brady and the upstart 2001 New England Patriots, who were the fortunate recipients of this obscure ruling. Brady's apparent fumble in the 2001 Divisional Playoffs would have led to the Oakland Raiders sealing the win. Instead, it kickstarted the league's latest dynasty, as New England won three of the next four Super Bowls. The play also heightened the power of instant replay.

A small turning point -- and major moment -- in the history of the NFL: David Tyree's catch to help the Giants upend the undefeated Patriots in Super Bowl XLII. The one-handed, pin-it-against-your-helmet grab showed that, despite all of the breathless analysis and analytics that lead up to every Super Bowl, nobody really knows what's going to happen until the game kicks off. Tyree was special teamer -- a backup virtually his entire career -- yet this clutch play extended a drive to beat a team many were calling the best ever.