The advent of landscape design isn't a recent event it has been in use since the beginning of civilization.
When people first developed the sensibility to appreciate the natural beauty that surrounded them, in nature, they have ever since been trying to duplicate that scene in their home surroundings.
I imagine those early people had gone about the process of landscape design by trial and error, and those with the eye for detail and arrangement were probably the first ones to set down the earliest rules of landscape design.
Some people I've known have questioned the practicality of landscaping, and instead of arguing that the aesthetics of landscaping is more than it's "just pretty" - because they'd never fathom that - I tell them that garden design has developed over millennia and if there wasn't a need for it, it would have been discarded long ago.
Societies all over the world have garden designs that are unique to their cultures, some are very tranquil and calm, some are very structured and symmetrical while others are in stark contrast to the surrounding environment.
All of these designs where developed to fill a niche in their respective societies that reflect their social norms.
Asian countries that revered tranquility and harmony have this as a major component in their gardens, Western Europe has very symmetrical gardens showing order and grandeur.
Design continues to evolve as peoples tastes change - so fickle.
Who is responsible for shouldering all this landscaping history? none other than your local landscape gardener the one who does all the tedious work of weeding, watering, grading, trimming, mowing, building, plumbing, maintenance of all their tools and vehicles as well as designing your landscape.
Pretty good for someone who digs in the dirt for a living.