Today shortstops are considered a premium position and more balls are hit to that player than any other on the field. But, there were no shortstops when baseball began in America; it was the last position to be added to the defense.
Today, even shortstops who can barely make it to .250 at bat are still highly valued, if they can make all the routine plays and that one spectacular play that can save a run and a game.
Prior to 1849 the three basemen stood on, or very near, their assigned bag; that left a rover, who usually roamed between the outfielders and the infielders. Sometimes the rover would station himself just outside the infield, between the bag-hugging “base men.”
"The man we call the “shortstop” today started as a fourth outfielder."
In a very early version of today’s “shift,” the rover would fill the hole either side of the infield, depending on whether it was a Right or Left-handed batter.
With the advent of a livelier ball in 1896, the outfielders had to play much deeper and this created the need for the ball to be relayed back to the infield and gave the rover a new assignment. With the “base me” playing close to the three bags, the rover would race out to take the throw from whichever outfielder was making the run for the ball.
“The short-stop acted as a utility man and would go out in the field and take the ball from the out-fielders and send it to the home-plate or to the in-field.” 
Reporter, Tim Murnane, a former player, mentioned another reason that the outfielders required a relay man:
“Men played but few games and their arms were not in condition to make long throws.” 
In 1859, the “fourth outfielder” was used in a variety of ways with no designated location or position title:
“At each of the bases is stationed one man to watch the runner and the fielders, who are outside, throw the ball to him in order that he may touch the runner with it before he reaches the bag.” 
The hoary lore of baseball makes it difficult to definitively say who was the “first” man to play the position that we call “shortstop” today.
A member of the club that founded most of the rules, Daniel Adams, of the Knickerbockers [N.Y.], lays claim:
“I used to play shortstop and I believe I was the first one to occupy that place, as it had formerly been left uncovered.” 
Although shortstop is the prominent, “star,” position in today’s infield, Peter Morris notes that, it was nearly insignificant in the early days of the game:
“Since all of these functions were to some extent afterthoughts, shortstop was not a very glamorous position during these years. That started to change in 1856 when Dickey Pearce joined the Atlantics of Brooklyn…he helped to helped to transform the position into a key defensive one.” 
Sportswriter Tim Murnane agrees:
“Dickey Pearce…is the first man to play the position as it is played now [in 1888].” 
And, Murnane adds:
“George Wright was the first man to play the position deep and close to second base…and in 1869 Wright and Charley Sweasy were the first players to work the two positions as they are worked today .” 
"Which shortstop turned the first “double play”?"
One of the earliest published reports of the 6-4-3 Double-play appeared in August 17, 1861 edition of the New York Clipper. Although the play was quite a trick then, it became a commonplace defensive maneuver within a few years.
Obsessive baseball maven, Henry Chadwick, observed the evolution of the “shortstop” position from Fourth outfielder:
“In selecting your short-stop, let him be an accurate thrower to begin with, but especially should he be noted for his activity in backing up every player in the infield as occasion may require…always on the move and on the lookout, first behind third base, then running home to help the catcher.” 
Twelve years later , Chadwick updated his job description:
“short-stops are now required to cover the bases so frequently—second base in particular—that they have become almost part and parcel of the basemen-team of a nine.” 
Four years later Sporting Life added a few more chores:
“…[He] backs up the infield throwing, has rambling orders for pop flies in the space between the [in and out] fields, and is especially useful in run-outs that are so frequent in these days  of fast and daring base running.” 
In 1904, Hall of Famer Honus Wagner, one of the first great “modern” shortstops observed:
“[shortstop] has become almost a duplicate of that of second base, except that one man plays to the right of the bag, while the other plays to the left. Ten years ago a short fielder had a position all to himself…The large increase in the number of Left-handed batters has brought about a change, and the shortstop is as much a second baseman as the player so designated.” 
Wagner might be amazed to see how Ozzie Smith, Rey Ordonez, Omar Vizquel and other current shortstops have added new wrinkles and sparkle to defensive plays* and how Cal Ripken established the large-frame players with power at the position today.
[“In the Big Inning” will appear here on BSI every FRIDAY.]
 Cincinnati Enquirer, April 1, 1888.
 New York Herald, October 16, 1859.
 Sporting News, February 29, 1896.
 Peter Morris, A Game of Inches, p. 37.
 The Ball Player’s Chronicle, August 15, 1867.
 New York Clipper, December 20, 1879.
 Sporting Life, December 11, 1883.
 Sporting Life, July 3, 1904.
* Wizardry: Baseball’s All-Time Greatest Fielders Revealed by Michael Humphreys.