In the last 25 years, no piece of gear has been more influential to the sport of climbing than artificial gym/training holds. A close second might be the cordless drill, and then way down the list you'll find such items as sticky-rubber shoes and spring-loaded cams, critical items that, for sure, pushed climbing standards and made the sport easier and better.
But cams and sticky rubber were not responsible for introducing millions of people to climbing. They did not make the sport automatically seem fun. They did not grow the sport or the industry. They were not responsible for the development of competition climbing, which has produced the world's best rock climbers. They did not provide the training apparatuses that are now allowing even teenagers to climb 5.15.
Plastic holds did.
The evolution of artificial climbing holds is murky. Perhaps because plastic/indoor climbing is often regarded as something not real or legitimate, no one has really bothered to write its history. François Savigny and the French company Entre Prises (EP) are credited with being the first to commercially manufacture artificial holds as early as 1984, 25 years ago. The first holds from EP were hexagonally shaped holds made of resinous concrete. They were painful to grab, heavy and came in a very limited number of shapes.
However, though we think of indoor climbing as not real, the desire to pull down on manmade problems is a natural instinct in climbers, as evinced by the fact that climbers had been making their own holds since the early 1970s. Brooke Sandahl of Metolius says that he screwed wooden blocks onto rafters as early as 1976. People bolted rocks to the undersides of tunnels. And, with no climbing gyms around, climbers used buildings to fill the void.
What followed next, and continues to this day, is a crucible of art and engineering, where creativity and vision meet technology.
The earliest holds just weren't that fun, says Mark Jackson, a designer of 18 years at Entre Prises USA. They were sharp. There was a trend in the early 90s toward more rounded shapes. Now it has come full circle, back toward more natural, sculpted shapes, while still keeping in mind ergonomics.
Holds and walls that are fun to climb not only introduce people nowadays to the sport, but get them hooked. The sheer numbers of different shapes are approaching the near-infinite permutations of natural rock grips, allowing climbers to gain comfort, familiarity and strength in various hand positions more quickly and easily than ever. Atmospheric ratings, and the rate of improvement for most climbers, not to mention their attitudes, can be attributed to the plastic hold.