The Yuan-Ti were the first mortals to have cities.
With the discovery of and obsession with collecting gemstones, many dragons discovered that humanoids, especially Dwarves, were quite useful for extracting gemstones. Red Dragons and Blue Dragons especially made extensive use of humanoid slaves to mine rubies and sapphires and all kinds of precious gemstones.
Tired of being forced to mine gemstones, an alliance of dwarves, high elves, yuan-ti and other enslaved people rebelled, using the magic they'd learnt to fight their overlords. Many dragons were killed by their minions, others gave up on holding their slaves.
After overthrowing the dragons, the Dwarves, Yuan-Ti and High elves almost immediately began to war with each other for supremacy over the land.
Areainra/Pahice and Scainnios/Nozohr decided to start stripping Yuan-Ti of their snake elements to create Humans.
A group of Yuan-Ti sorcerers made deals with a Green Dragon to help them conquer an empire, in the process learning a great deal of magic which then spread to their enemies.
Yuan-Ti methods of counting and recording livestock and slaves evolved into a writing system consisting of wedges pressed into clay tablets.