An Alternate History

This one I think I can get several posts out of. So, on with the first one!

Point of Divergence: c. 3000 BC.
As the Sahara desert creeps northward, a small tribe make a lucky discovery. They stumble on the proper combination of plants, in the proper setting, that can hold the dry earth in place and leave it relatively fertile. With this agricultural package, they are able to maintain an existence further into the arid areas of North Africa then was otherwise possible in our timeline.
Not very far, mind. Probably only about 50 miles further, at least for most of their history. The tribe that made the discovery did so out of necessity, as it had always been hindmost on the long retreat from the desert. While their agricultural techniques and water management skills gave them an ability to farm more arid regions than their neighbors, it never gave them enough surplus to make any significant attempts at conquest. The tribe ended up in what is now Libya.
For the next two thousand years, the tribe's existence changed little of history. While they did add slightly more arable land to the desert's edge, no other tribe or nation managed to take advantage of it; the complete knowledge of the plant species and water management techniques needed to maintain the land was only the property of a select priesthood, who worshiped the tribe's god of rain and water.
The tribe developed a simple defensive tactic: they built fortified buildings on the dry hills and mountains of their land, fortifications that always contained at least a year's worth of food, and, more importantly, a water supply - usually a spring, often augmented with cisterns. If invaders came, the people retreated to the hill forts, and waited. The invaders inevitably found that the land they took quickly began to dry up and become useless; the plants that preserved the soil required careful, knowledgable tending to properly flourish. Invaders usually left after their first failed harvest, and the people of the tribe would come down from their fortresses, replant the important plants, and start growing their crops again.
The people actively avoided domesticated animals larger than the dog. Cows, sheep, even goats and pigs, were too hard on land already barely able to support the humans. Thus, the tribe never gave up hunting as a major source of protein. Of course, since game was scarce on the edge of the desert, this only contributed to the tribe's relative poverty.

2000 years after the tribe's effective beginning, this would all change. The beginning, of the end for the tribe, but of something much larger for the world, came in the form of an ambitious young priest. He was the younger son of a (relatively) rich family. His father was a trader, trading what little surplus the tribe could produce to other, richer tribes futher towards the coast. Thus, the young man was exposed to wealth. Unfortunately, this only awakened his greed. He knew that other people had far more than his people ever could, and this always chaffed. His father eventually gave him to the priesthood, hoping they could curb his avaricious spirit. A significant donation to the local shrine to the water god helped them accept him as an excellent canidate.
Fortunately, the young man was a good canidate, at least in one respect. He was a very quick learner, and was easily able to absorb all the knowledge the priests could impart about the plants and practices that helped the soil remain fertile. The young man entered the priesthood at 14, and was made a full priest at 18, two years ahead of schedule.
His first 5 years as a priest were not only not distinguised, they were actively notorious. He was moved to three different small settlements in that period, as at each village his greed got the better of him, leading him to various improprieities.
Finally, in desperation, the priesthood sent him to a place they though he could do no possible harm: a tiny settlement, with only a single family, right on the very edge of the desert, the poorest place possible. This turned out to be the priests' fatal mistake.
The young man had, by this point, learned patience. His schemes had failed him, almost always because he rushed in. Now he had learned the value of thinking and planning ahead. And there, on the edge of the desert, his greatest plan was born. The priest realized that the priesthood was not likely to move him anywhere else again, lest he again cause trouble. So, the only way to gain wealth and status was by gaining them where he was. But how? He thought and thought, until one day, he realized that the most signficant barrier to wealth in this tiny village was its lack of arrable land. He thought to himself "what if I could somehow increase the land these people could farm?"
To any of the tribe, this was a revolutionary idea. Always, they had maintained a holding action, preserving what they had, but not expanding. Expansion was difficult: it used water that could water fields instead, leaving the possibility open of starving both old field and new. The priesthood had formed an almost taboo attitude toward it.
But the young priest was never one to be overly concerned with social mores when his own well-being was at stake. He began casting about his immediate surroundings for something to help him with his plan. Almost at once, he found an ideal test bed: a small oasis, 5 miles out into the desert. He soon explored it, and found out all he could. The oasis was supported by a small spring, which seemed quite regular, not significantly affected by the dry season or the rainy season. The young priest began to work out how much water the spring produced, and how much water a given quantity of land needed. He practically had to invent new math himself to do it, but he got a good approximation by the end. He determined that with careful management, the spring could support almost four times as much land as it now did, although none of the land would be terribly lush. Nonetheless, a significant step. But to do it, he needed support: the task of water management was nothing to do on one's own.
His next step was to enlist the aid of the clan that he was assigned to. He went to the clan head, and explained that he could add a large new area of land to the clan's holdings, if they followed his lead. The clan chief was willing, both due to respect for the dictates of a priest, and because he himself had often desired a little bit better social status. Soon, the priest had all the able-bodied folk of the clan working for him, planting the seedlings of the plants needed to hold the water and the soil, building cisterns and resivors to hold excess water, and creating irrigation ditches to spread the water around. Now, the priest had to wait.