No one knows what came before the Reign of Giants. Thousands of years of slavery and cultural repression will do that. For as long as anyone can remember (and the elves and dwarves have very long memories), the various sorts of giants kept the smaller races subjugated.
Until one day, humans showed up. Humans breed like orcs and fight like dwarves and carve empires out of the wildnerness like nothing the world had ever seen, and one by one the giants fell to human conquest. Now the only giants that remain are the brutish, particularly stupid breeds (hill giants, ogres, trolls, etc.) and scholars are debating whether this is the Age of Humans. After seven hundred years or so of human empires on the rise, it certainly seems so.
The god Moradin gave the dwarves the skills of metal casting and ironworking, Teach those skills to the humans, and they come back with machinery. The god Corellon Larethian taught the elves music and magic. Teach that to humans, and they come back with acoustics and architecture and thaumaturgy. The god Yondalla taught the Halflings agriculture and irrigation and animal husbandry. Teach that to the humans, and they build an entire socioeconomic system around civil service and the taxation of crops to feed their ever-growing cities. Each human is creative and adaptable in their own way, pushing their societies to ever-greater heights, all without the guidance of a god. The other races find this (at the very least) deeply unsettling.
But while the dwarves have returned to their halls to rebuild their clans, and the elves returned to their forests, and the halflings to their fields and rivers, the humans have spent the last few hundred years carving out kingdoms and fiefs wherever they can spread. One of those is the free city of St. Merridell, in the Berremont mountains, the gateway into a wide expanse of untamed land that begs to be explored. The locals simply call it the Frontier…