All Dragonborn are aware of these things (if you’re not a Dragonborn and playing in the game, don’t be reading this):
In ancient times, this land was verdant and full of life. This included a native species of men and women. Physically they most closely resembled half-elves, and they were powerful wielders of powerful psionic and magical energies. They were kind and fair, and quite peaceful.
Inevitably disaster struck. A creature from the farthest places, the outer realms found its way to their world and infected many of the native animals. The local species were warped into monsters, and then formed into armies to attack the peaceful villagers. They tried to use their vast magics to protect themselves, but the monsters were given protections by the vile thing which infected them, and the spells were nearly powerless to protect the populace. Many were killed, a few were enslaved, and warped to better serve their masters. These mutated, twisted slaves were the beginnings of the Tieflar race.
The war ravaged the locals, and they seemed powerless to protect themselves. As their last strongholds began to fall, one group of refugees tried to escape the slaughter and enslavement by hiding out in areas that had already been conquered. Little by little they were picked off as the aberrant magics were used against them to detect their location.
However, what they discovered during this time of fear and hiding was that they had some small degree of protection. Many of refugees kept small, dragon-like creatures as pets and companions. Those who kept them were less likely to be found. It appeared as though their dragon companions provided a small amount of protection from the scrying effects of the aberrations.
Having learned of this, the remaining members conceived of a dangerous gamble. Using the last of their waning mystic powers, they conceived of a spell that would merge themselves with their dragons, in hopes that the protection might be amplified enough to keep them hidden indefinitely. The spells were successful, although with a few odd side-effects.
-The really important bits-
All Dragonborn are hermaphroditic. In their ancestors, dragon and man paired up in opposite-gender groups. When they merged, both genders merged into one. Many Dragonborn adopt a gender so as to appear more normal to outsiders, but physically they have both genitalia.
The fortress where the original merging occurred still exists, and is heavily guarded by current dragonborn. It’s called Pyazesh Draefor (“Egg Heart” in the ancient language) and contains the most vulnerable aspect of the Dragonborn race. At the heart of the fortress is a small egg, barely six inches across. It is the last egg of the dragon race that became one-half of what are now the Dragonborn. The egg is held in permanent magical stasis, unable to hatch or change. It also exists as a physical object focus for the merging spell. Should anything happen to the egg, should it be destroyed the spell that holds the dragons and men in hybrid for would unravel. For the original participants, this would have simply meant separation. For their descendants however, this would mean death. Their bodies and spirits are too intertwined to be separated in such a harsh manner. Should the egg break, the entire Dragonborn race would cease. Thus the fortress is both the most sacred of Dragonborn places, as well as their most guarded secret.
Further, all Dragonborn share a racial memory of certain key pieces of information. This is considered one of the side-effects of the merging spell. All are aware of their ancient history as mentioned above, as well as the existence, location and importance of Pyazesh Draefor. Other pieces of ancestral memory differ from individual to individual.
Some have retained the ability to speak the Ancient language. Draconic is not the native tongue of the Dragonborn people, it is the language of the larger dragons and kobolds. However, it has since been adopted as their own in further effort to mask their true nature and to protect their great vulnerability. Some Dragonborn do have the ability to speak and read the Ancient tongue in addition to (or in place of) Draconic.
For rules purposes, this means a Dragonborn can choose to take Ancient in place of Draconic, or can take it as an additional when choosing the Linguist feat. Keep in mind that very few other people can speak this language, and it’s generally only going to be useful when trying to translate old texts and such.