For most of Arabia’s history (and prehistory), the Eastern region was oriented toward Persia, the Southern region was oriented toward Africa and Egypt, and the central desert was culturally different from both. But during the years before Muhammad’s birth, a great natural disaster sent many people of the South into other regions as refugees, settlers and traders. This was a major step in the creation of an “Arabian” identity.
Keeping up a civilization based on large-scale water technology requires a lot of central planning and power. We know that ancient Saba (Sheba) used its military and economic muscle to harness the thousands of workers required to build and maintain irrigation canals and dams. But all kingdoms weaken eventually, and Saba’s rule came to an end. None of its successor kingdoms, Ma’in, Qataban and Himyar, were as powerful, and gradually, the water engineering grew weaker too. The Jewish kingdom of Himyar (ca. 450) and the later Ethiopian (Aksumian) occupiers (ca. 540) both tried to repair the dams, especially the great Marib Dam. They were also up against forces of nature, since the soil of Yemen was basically sandy and didn’t do well at sealing in water.
Medieval Arabic historians give different accounts of when the Marib Dam finally failed. It was before Muhammad’s birth, but it may have been three centuries before, or two decades. It sounds like it was probably at least a century before, because of the developments that came after and were already in place by Muhammad’s time. Additionally, the dam’s health must have wavered long enough to have breaches that each seemed like “the big one.” In any case, the event’s timing matters less than its result: a mass emigration of South Arabians whose way of life had been dependent on irrigation.
Legends tell that a local king’s wife, a seer, foretold the collapse of the Marib Dam. Tarifah said that the dam was being eaten by rats from within, and that the people should all leave the area. Her people, the tribe of Ghassan or Jafnan, obeyed. They emigrated en masse northwards, out of Yemen and into modern Jordan. There they set up a capital at Jabiyah and became a client and buffer state for Rome.
The tribe of Lakhm also migrated northward, staying to the east of the Ghassanids. They resettled along the eastern coast of Arabia and into Iraq, where they set up a kingdom with a capital city of al-Hirah. They were allied with, and a buffer state for, Persia. The two Arabic immigrant kingdoms faced each other, and the empires counted on them to stay even in their arms race. Both of these Arab-origin buffer states come into the early story of Islam.
The tribe of Azd migrated north and settled in the area of modern United Arab Emirates. This fact is important in our time because in the 1980s, Sheikh Zayed, president of the UAE, funded a new Marib Dam in Yemen. He probably had many reasons to do this, but among them, he said that his tribe of Azd had come from Yemen, so he was just doing something for the Old Country.
The tribe of Kindah moved northward toward Bahrain, but they were not allowed to settle. They established a caravan town at Qaryat al-Faw in central Arabia, and some of them became neo-nomads. Some of the tribe went back to the south and re-established a power base. The Kindah tribe in central Arabia is part of Muhammad’s story because they were the local rivals to Mecca’s power. Other southern tribes, such as the Madh’hijj, also settled in the central area but did not become Mecca’s rivals. Rather, they were part of the pilgrim wave that swelled Mecca’s business each year.
This massive northward migration was somewhat balanced by a tendency among the nomads to move closer to the cities. Nomadic tribes originally from the north and central areas drifted southward, setting up camp close enough to cities like Mecca that they could interface with other caravan businesses. They brought their families near to the cities who were hiring mercenaries and security guards. By the time of Muhammad, there had been two centuries of ethnic and linguistic mixing, moving toward a national identity.