Urbanization and Careers
As years passed, techniques for growing food improved, the range of land under cultivation spread, the number crops increased, allowing previously "poor farmland" to be cultivated with newly recognized crops or simply providing variety in the diet of early settlers which led to better health. The great river valleys provided the best farmland, often permitting multiple growing seasons, and so it was in these locations that experienced the first transformative consequences of the agricultural revolution. It became possible for a human being to survive without growing their own food, building their own shelter, or defending themselves; you could have other people do it for you. Farmers had learned to grow enough food for their families and still have a surplus left over.
At first, this surplus merely went to sustaining a ruling elite. But gradually the first professions developed: solider, artisan, builder, priest. These people were not capable of supporting themselves, they required the support of the local populace either willingly in exchange for their services or more frequently by coercion of force from rulers who wanted professionals to serve their courts. These developments reached a critical mass in 4000 BC when Uruk became the first city in world history. It boasted a population of 10,000 and was heavily walled to defend those inside from anyone who might wish to take what they had. The city was critical; it was one thing for people to specialize in a trade or profession, it was another to be able to find a location where they could purchase everything else they needed. Cities were highly focused centers of population that allowed specialization, a relatively high volume production, and the easy ability to sell what you made and purchase what you didn't.
The larger the city, the more specialized workers could become. Cities benefited tremendously from professionals who specialized and produced goods of higher quality and in greater quantities. This was one factor in the rise of cities and careers, and one reason why careers emerged in tandem with the trend toward urbanization. The flip side was that careers were highly desirable; people could focus on doing one thing they did well and / or enjoyed while purchasing goods they didn't want to make for themselves. Urbanization made it possible to find buyers and sellers in large enough volumes for specialization to work for the first time, so they exploded together. On the strength of this economic reality, Uruk ruled unchallenged for 1000 years.