Tangent: Playing Dragonborn in Pathfinder

Dragonborn have an interesting (and very recent) past, in terms of their attachment to the Dungeons & Dragons game. Their first appearance in 3.5 was as a transitive race made up of those who willingly underwent a process by which they were reborn into new, draconic bodies out of dedication to their god. It wasn't until 4th Edition that they saw an incarnation as a "true" race. They lack a clear place in Golarion. I will try to offer a few ways for you to establish dragonborn history within your Pathfinder campaign.

As draconic demihumans, it's clear that however they are incorporated they ought to have some connection to true dragons. It needn't be as direct as the result of a breeding process, but the clear influence of dragons in their physiology needs to be explained somehow. They also need a place of origin, and a place of refuge in the modern world. Here are a few suggestions on ways to insert dragonborn into Golarion:

• The gold dragon Mengkare, ever in determined pursuit of societal perfection, experimented briefly with instructing a handful of his people to breed with him (or with other dragons he recruited for the purpose) in order to study the offsprings' worth. Though capable humanoids, he eventually determined that this new race would not move his society closer to perfection. Abandoning the project, he sent the dragonborn away from Hermea, scattering them to the nations of the inner sea. If you use this option, dragonborn have an extremely short history, and no personal culture save a shared bitterness at Mengkare's treatment of them (or their parents, if they are descended from those exiled by Mengkare).
• During the height of Thassilon when the Runelords' magic might was at it's strongest, Karzoug the Claimer hatched a twisted plan. Though he numbered many dragons among his followers, their conniving minds prevented him from placing much trust in them. On the other hand, while he could easily cow and subjugate the human population, they lacked the might and ferocity of his dragon allies. In a moment of terrible insight, he realized that if he could combine the two he might create a race with the ferocity and cunning of a dragon, but without the force of will that made them impossible to utterly control. Using his powerful transmutation magic he was able to fuse dragon and humanoid together, creating a new race: the dragonborn. They served their master well, proving stronger than his human troops. But after the fall of Thassilon and the loss of their master, the dragonborn scattered. Some joined those humans who would eventually become the Shoanti tribes, others devoted themselves to the dragons, and more still traveled as far away from Thassilon as they could, making their way to other countries. (Credit to Eldim for this suggestion)
• In the forming of the world, as Apsu and Tiamat created the gods themselves, they instilled a tiny portion of themselves in creation. During the Age of Darkness, hidden far away in the unexplored regions of the Mwangi Expanse, the dragonborn carved out a place for themselves in the endless jungle. Infused with the barest spark of divinity, dragonborn shaman foretold a rise to greatness in the Age of Glory to come. In preparation, a number of dragonborn were sent into the rest of the world to take stock of what they were certain would eventually become their domain. When word of the sundering of the magics of prophecy reached the ears of dragonborn civilization, however, scores of disillusioned tribesmen abandoned the Mwangi Expanse to try and bring about what they considered their destiny the old-fashioned way: by earning it.

Dragonborn in your Rise of the Runelords campaign might come from a rich tribal history, or enter play with a complete lack of personal heritage or identity. Here are some ways to give dragonborn a special relevance to the events of the adventure path:

• As the progeny of a race that once belonged to the personal army of a Runelord, Thassilonian lore has seeped into dragonborn culture as legend and fable. Runes might have developed special significance for the dragonborn people, and the PC might find disturbing similarities between familiar dragonborn motifs and his increasing exposure to the ruins and remains of ancient Thassilon.
• The PC took up a life of adventuring in order to embrace an imagined personal or shared racial destiny unmade at the advent of the Age of Lost Omens. As his adventures earn him more prestige and glory, he might become increasingly hopeful that the key to that destiny lies within reach. The prospect of uncovering the mysteries of ancient Thassilon only serve to fuel this drive.