Plumbing is about systems which convey fluids about the place, which means that they get involved in waste disposal as we all know. But they also become experts at heating and cooling (HVAC) and water supplies.
The root of the word
The name plumber comes from the Latin word for lead which is plumbum. This was the material which original pipes were made from before anyone realized the issues associated with lead pipes.
It all started with the need for public baths
One thing ancient cities heading east from Greece all the way to China had in common was the need for public baths. More people in close proximity needed waste removal services and drinking water too. All developed over time and improved. The Minoan palace at Knossos in Crete had hot and cold running water, and they worked out how to get water to flow uphill too.
It was the Romans who really developed the technologies for serving a large population. At its height, Rome housed over 1 million people. (This volume of population did not happen in Western civilization again until Victorian London passed the number in the 19th century.)
Aqueducts and drains
Dionysius of Halicarnassus comments that the Roman Empire was founded on three things; roads, drains and the aqueduct. All three were so exceptional that there are examples of all three still extant today.
The Roman plumbers were part inventor, part geographer and part engineer. They succeeded in bringing fresh drinking water to Rome from great distances away.
The first of the aqueducts carried water over 16 kilometers, down an average incline of 10 meters and succeeded in bringing 75,000 cubic meters into the city every day. This was achieved 300 years BCE.
By the 3rd century, Rome had 11 aqueducts which ran for a combined 800 kilometers and supplied the city with 1 million cubic meters a day.
After that, we lost it
With the decline of the Empire, the techniques and need for plumbing on such a scale were no longer necessary. The great epidemics of plague in Western Europe mean that it would not be needed again for well over a millennium.
Roman plumbing still functioning
The city of Bath in England still has a functioning Roman Bath that draws over a million visitors a year. They are not safe for bathing though, as the pipes are made of lead.