The evolution of football from its earliest days has resulted in a game that would be barely recognizable to any of the founding teams of the NFL back in the 1920s.
Much like the March of Progress, football has evolved from a primitive grind on the ground to a fully-upright and modern species with the ball in the hands of powerful grinders less and less by the year.
Every rule change or development in the modern game seems to have only furthered football's ascent to the skies, meaning today's game is very much a pass-first entity.
The game used to be played by the old adage "three things can happen when you throw the ball, and two of them are bad." Consequently, the Hall of Fame is stacked full of players who were the best at battering their way through powerful defenses for yardage and touchdowns.
Running backs were once 230-lb bruisers who would take carry after endless carry into the line, churning their legs for yardage and running over defenders to move the chains. Winning in the NFL meant hard-nosed football involving three yards and a cloud of dust on more plays than not.
Today, the game is more about finesse, speed and precision. There are effective players checking in at well under 200 lbs, with guys like Dexter McCluster tipping the scales at 170. Players of that size had no chance in the era of smash-mouth football, but today's NFL is a passing battle, and now they are dangerous weapons that can exploit the new-found space on the field.
When teams only need to concentrate on shutting down the run, the field becomes concentrated on a smaller area in the middle by the line of scrimmage. When you know the ball is coming towards the line, you only need to shut things down at the line of scrimmage and seal off the edge to prevent it from going to the sidelines.
That is relatively easy to do, and you can see it happen even today when teams have no viable passing threat. Adrian Peterson has seen his fair share of this type of front with the Vikings, as they only had a brief period of passing success under Brett Favre since AP arrived.
Teams would stack the box with eight- or nine-man fronts, cluttering up the running lanes and banking on the offense not being able or willing to pass to exploit the space. Running forces the space out of the game, making football a battle of power and wills.
Everybody knows where everybody else will be lining up, and the only question is who has better strength, leverage and technique. That's what dictates whether or not the play will be successful.
What passing does is open up the whole field, taking defenders away from one concentrated spot at the line of scrimmage and forcing them to try and defend huge areas of real estate, any of which can be targeted by an offense at any moment.
Generally speaking, playing defense is an exercise in reactive work rather than the proactive nature of playing offense. Being reactive in space is far harder than doing so in the confined nature of run plays at the line of scrimmage.
In those plays, there is only so much scope for being in completely the wrong place at the wrong time with a false step. Get things wrong when covering in space, and you could find yourself 10 yards out of position in a heartbeat with a receiver running free to the end zone on your watch. The job is tougher, the stakes much higher.
The way the rules have progressed over the years, passing has become easier and safer.
The rate of interceptions has plummeted, as quarterbacks' completion percentage has steadily risen. It wasn't long ago that a 60 percent completion rate was considered the benchmark for an elite season, but in 2011, 22 passers eclipsed that mark, and another few were within a couple of throws of doing so.
Joe Namath finished playing in 1977, and he was elected into the Hall of Fame with a career completion percentage of just 50.1 over 3,762 attempts. He threw almost 50 more interceptions than touchdowns over his career in the Jets' high-risk offense. At that time, passing was a dangerous way to play football, and those Jets teams were pioneering a high-risk/high-reward style of play.
Today, Peyton Manning has a career interception rate of just 2.7 percent compared to the 4.6 Namath ended his career with. Manning's career completion percentage is a hair under 65 percent, and he has thrown 201 more touchdowns than interceptions over his career. Those numbers would look farcical in Namath's era.
With a Peyton Manning playing for you, why would you ever run the ball?
Running started off as the only viable way to move the chains when playing football. Over the years, it developed into a way to set up the big pass play. You could sucker a defense to the line of scrimmage and then attack them deep over the top against relatively few defenders.
Today, running has become the reverse, as it's primarily used just to keep a defense honest or to exploit the space the passing game has created by forcing defenders into coverage.
This is why today's running backs look more like Mercury Morris than Larry Csonka. They need to be able to exploit space quickly, so speed is the greatest asset they can have.
Their ability to play within the passing game also becomes vital. In most NFL offenses, the running back is the primary check-down receiver. When a quarterback can't find a downfield target, he knows he has the running back as a safety valve to which he can dump the ball without worrying about throwing it into danger.
Running backs also need to be able to pick up the blitz and block for the passing game. To be an effective runner in today's league, you need to be a jack of many trades, but you don't need to be a master of any.
Teams will still spend high draft picks on truly elite prospects like Peterson or Trent Richardson, but if they don't get a shot at those guys, they will often wait until late in the draft, knowing they can get a player who can do most of what they want without breaking the bank.
Running backs have never had a tougher time getting big-money contracts, because teams think they can replace that production for a fraction of the cost.
When running backs were the driving force behind a team's offense, they could command big-money deals and high draft slots because everybody knew those guys were hard to find. It wasn't easy to stumble into a complete and durable back you could rely on for every carry in every season.
Now, few teams are built around a running back, and most have a stable of backs they use to get the job done rather than one workhorse. They can afford to let one of them walk rather than paying him, and they'll replace that part of the stable with a low-risk, cheap, young prospect.
In essence, running backs no longer have any leverage against teams in a league where running now gets much of its work done for it by the passing game. Running backs used to have to make their own yardage, but today they have yardage made for them by the way the passing game can open up space and creative play-calling can catch a defense off-guard and out of position.
The league has been steadily and inexorably moving away from running as the foundation of offense for decades, and things are only intensifying in that area. The running back has fallen victim to that march away from three yards and a cloud of dust as football took to the skies.
It isn't so much that running has become unimportant to an offense, but the changes have meant that running is now seen as an easier pursuit with the way the pass game sets up the run.
Teams no longer place a premium value on the ability it takes to run within the confines of a modern NFL offense, and so they place a far lower worth on the players expected to get the job done.
With running often being done by committee rather than by a single player, that worth gets further diluted by being shared amongst a stable of backs. Therefore, running back has devolved into one of the most worthless positions the league has.
Until running the ball becomes more than a way to help with the passing game, running backs will only continue to see their value plummet.