Short Answer:
Apples.
Long Answer:
I will tell you the tale of how the Great Tradeway came to be. Many aspects of culture will be explained in the process - the Water Festival, the Bargain, the tribute paid every winter solstice at the bronze statue of the Mad Wizard, and the enduring symbol of the Great City - Copperopolis of Gondwana.
Legend has it the Mad Wizard was originally from the city across the sand. He crossed the great desert to our city of Copperopolis to study magic. Everyone knows Copperopolis is the center where fabulous things are built and wondrous crafted items are easy to come by.
(Incidentally, do you know how hard it is to get a shadow demon to carry a box of precision-crafted glass beakers GENTLY across the desert? After receiving one’s fourth or fifth crate of broken glass, serious students of wizardry give up and relocate to Copperopolis. Sure, it’s in the middle of nowhere, but there’s something about that sand that makes the best glass.)
Time went by and the Mad Wizard’s power grew, but he was unhappy. He missed his homeland. He had grown up in the lush vales on the other side of the desert and was used to eating fresh apples at all times of the year. In Copperopolis, there were no apples to be had.
He frequently sent his shadow demons across the desert to the apple orchards, and they brought him crates of the treasured fruit. But his dissatisfaction grew with each trip. Surely this sort of menial work was beneath him! With all his power and knowledge, surely he could come up with a better solution than to waste his precious magic on breakfast!
Other wizards, less canny wizards, have used their magic to rule as tyrants over the villagers. Such cruel wizards solve their problems through force and fear. The Mad Wizard was not a kind man, but he was wise. He didn’t want to waste his time ruling a kingdom any more than he wanted to waste his magic using shadow demons as fruit vendors. He chose to cooperate with the non-wizard inhabitants of the city.
He called together the greatest merchants, the best craftsmen, the most reknowned cartographers, and together they formed a plan. The route was planned, the Bargain struck.
That fall, once the harvests were in, teams of builders set out across the sand. They built a waystation a mere four hours’ walk into the desert. They dug great pits deep in the sand, down where the sun could not reach. Water carriers brought them sustenance from the town, crossing to the work site at night and returning to their homes before the sun rose. Meanwhile, the local potters were busy crafting vast pottery cisterns. Once the pits were dug, a wooden framework was built around each cistern with carrying poles. It took twelve people working together to lift each cistern. Teams of workers hauled each cistern out to the work site and placed them in the pits. The builders then crafted a simple stone shelter over the pit, protecting the cisterns from dust and sun.
That night was the very first Water Festival. Everyone in Copperopolis gathered together. Each able-bodied person brought some dried provisions and the biggest flask of water they could carry. They walked out to the waystation, gave the provisions to the laborers, poured their flasks into the cisterns, and returned to the city before dawn. Meanwhile, those unable to make the journey remained in the city and cooked a great feast for the water carriers. Upon their return at dawn, the whole city held a marvelous celebration.
From the first waystation, the builders moved another four hours’ walk into the desert and dug a second pit. Each night, water carriers moved between the new work site and the newly-complete waystation, bringing food and water to the workers. Each night, water carriers would bring water from the city to the first waystation to replenish the cisterns. The pits were completed and new cisterns were brought to the second waystation. These were slowly filled via flasks carried from the first waystation.
The builders advanced another four hours’ walk and began their work again. In this fashion, the first oasis was reached. Then the second, the third, and so on. The path became well marked with stone markers at regular intervals, and stamped flat by thousands of feet. Soft sections of sand were reinforced with timber and gravel. Work on the Great Tradeway halted during the planting and harvesting season, then began again every Fall Equinox.
It took years to complete.
Finally, when the last chasm was bridged and the last waystation complete, the Great Tradeway was ready for use. Merchants at both cities gathered together every wheeled conveyance at their disposal, loaded them high with goods, and pulled them across the desert. They carried little water with them, as the cisterns were kept well stocked by the water carriers stationed at each city and each oasis.
The first carts arrived at Copperopolis, stuffed to the brim with apples. The Mad Wizard selected the juciest crate for himself and gave the rest to the city. Every inhabitant received at least a few. Now, this may not seem like much of a reward nowadays, but back then only the wealthiest merchants ever saw fresh fruit from the vale across the sand.
Every fall, after the harvest, is the Water Festival. It has grown considerably since the first one. Now, teams of runners sprint across the sand in a great race to stock the cisterns with cartloads of water. Long-distance runners compete to be first to stock the most distant waystations. There is great honor in winning the race. The Dawn Feast has grown in size and reputation, eventually becoming a week-long festival. The Mad Wizard has long since passed on, but they still honor his memory with a tribute of apples at the base of his statue at the start of each trading season. Every inhabitant of Copperopolis still receives a share of the first cart of apples across the desert.
And this is why the symbol of endurance in the face of great challenge - the symbol of Copperopolis overcoming any obstacle - is an apple.
Footnotes:
Eventually this road could be replaced by a series elevated canals built out of cement. They pump water through it by use of a series of water towers and Archimedes’ Screws. They float barges through the flat parts, build a road through the mountain parts. Once the Great Tradeway became well established, the engineers and merchants would try to improve the system, and this might be a natural evolution of the route.
Bibliography:
Bedouin using underground concrete cisterns to store water in the desert, sometime between 6500 BC and 700 BC:
International Association of Certified Home Inspectors: The History of Concrete by Nick Gromicko and Kenton Shepard https://www.nachi.org/history-of-concrete.htm
A real-world example of a civilization using the power of teamwork to build 25,000 miles’ worth of highway through the Andes Mountains (the Capac Ñan) without iron tools or draft animals:
Smithsonian Journeys Quarterly: The Inca Road, How the Inca Empire Engineered a Road Across Some of the World’s Most Extreme Terrain by Hannah Bloch
http://www.smithsonianmag.com/smithsonian-institution/how-inca-empire-engineered-road-would-endure-centuries-180955709/?utm_source=smithsonianhistandarch&utm_medium=email&utm_campaign=201507-hist&spMailingID=22963197&spUserID=NzU3NjY0OTQzOTMS1&spJobID=600087191&spReportId=NjAwMDg3MTkxS0
The United Nations Educational, Scientific, and Cultural Organization World Heritage Center’s entry for the Capac Ñan (Qhapaq Ñan):
http://whc.unesco.org/en/list/1459
Wikipedia article for Archimedes’ Screw, which was actually in use 350 years before Archimedes lived. This screw was originally cast in bronze and was used to pump water uphill, theoretically used to water the Hanging Gardens of Babylon:
https://en.wikipedia.org/wiki/Archimedes%27_screw
