"Ability" isn't a single variable either. There's a combination between natural talent and hard, determined work. For instance, many people thought Ricardo Quaresma and Cristiano Ronaldo had similar amounts of natural talent as young players, but Quaresma wasn't as determined as Ronaldo to become the greatest player in the world. Conversely, Michael Jordan's contemporary Charles Barkley didn't really have Jordan's natural talent, but made up for it by working exceptionally hard. So you have at least two (and possibly more) variables, all of which have to reach extreme outliers together, to produce an athlete like Messi, Jordan, or Gretzky.