People have been around in Avatar: The Last Airbender for at least ten millennia, as shown when Korra experiences the life of the first Avatar. Regardless of whether humans existed before this timeline, however, the narrative implies that Spirits may have called the world their home for thousands of years before Wan.
Unfortunately, most major global events that take place after Raava and Wan combine have been lost to history, or perhaps are collecting dust in some ancient library. As far as the stories of ATLA & TLOK go, there have been a few significant instances that have been important in shaping the world.
The Birth Of The Lion Turtle Civilization
It's not clear when exactly humans started living and building their civilizations on the backs of Lion Turtles, but the symbiosis had been well-established long before Avatar Wan.
This is because the Ancient Ones offered humanity protection from the Wild Spirits. The Lion Turtles were compassionately just, as they freely offered the power of bending to anyone who wanted to use it — although they expected humans to return their gifts when the task was complete.
The Creation Of The Avatar
Wan accidentally saves the wrong spirit when his Firebending separates Vaatu from Raava; as atonement, he travels with the Spirit of Light to learn all four styles of bending from different Lion Turtles.
With Raava maintaining the balance between the elements, Wan becomes capable of manifesting a magnificent combination of bending that almost keeps Vaatu at bay. In the end, the Avatar is born when Wan draws from the fountain of energy created by Harmonic Convergence.
The Manifestation Of Wan Shi Tong's Library
Wan Shi Tong, a Spirit in the shape of an owl, is interested in nothing more or less than collecting, storing, and preserving knowledge, whether it be scientific, technical, educational, artistic, or a simple recipe.
The library takes physical form in the Si Wong Desert many millennia prior to the events of the series. Wan Shi Tong opens it to all of humanity, hoping that his books might sway them from their violent ways. His optimism turns out to be misguided folly, and the Owl Spirit hides his library within the burning sand.
The Formation Of The Four Nations
Around the same time as the first Avatar, humans begin leaving the safety of their Lion Turtle cities to scour the wild. They request their benefactors to provide them with the respective bending discipline, which is inevitably granted.
Humans roam the world, slowly establishing villages and towns on all the continents. Over time, and obviously based on an affinity for one of the four elements, the Earth Kingdom, Fire Nation, Water Tribes, and Air Nomads came into being.
The Hegemony Of Ba Sing Se
Wan Shi Tong's library contains a map of Ba Sing Se that's said to be over five thousand years old, which means that the initial settlements in the region are much older. Recorded information suggests that the first dwellers mined a certain shiny crystal, using its value to develop a complex economic system.
As the official capital of the Earth Kingdom, Ba Sing Se is one of the most powerful political entities in the world — it doesn't hesitate to display its strength, either. As such, it's a bit strange that the Earth King allows Omashu to be effectively independent.
The Life Of Avatar Kyoshi
Avatar Kyoshi lived for more than two centuries, several times longer than any known Avatar. During her time in the world, she was responsible for changing not only the Earth Kingdom, internally as well as externally improving its relationship with the other three nations.
Kyoshi is as central in death as she was in life, given that both Aang and Korra learn from her experience, wisdom, and, most of all, her mistakes.
The Hundred Years' Invasion
The era of peace brokered by Avatar Kyoshi is rudely shattered by Fire Lord Sozin's desire: to bring the world under his sovereign thumb (while pretending to invade other territories for the sake of "sharing prosperity.")
Roku initially manages to quell his old friend's ambitions with a show of overwhelming force, but the Avatar's death allows Sozin free rein to do whatever he wants. The first thing that the Fire Lord does, however, is seek out Roku's Air Nomad successor.
The Air Nomad Massacre
When Sozin's Comet arrives, the Fire Lord for whom it is named employs its Firebending augmentation to massacre the Air Nation. His reasoning is entirely dependent on the existence of Aang; thankfully, Sozin fails to find the new Avatar, as does his son, Azulon.
Regardless, the Air Nomads are completely wiped out in the process, destroying centuries worth of culture, history, and, more importantly, thousands of innocent lives.
The Birth Of Republic City
After Ozai's fall in the Hundred Year War, the future of the Fire Nation colonies is left to the new Fire Lord and the Avatar, who are initially unable to come up with a compromise. Zuko wants "his citizens" out of the occupied territory, but they argue that their lives now incorporate both Earth Kingdom and Fire Nation traditions.
Finally, Aang and Zuko enter into a treaty between the two countries. Known as the Harmony Restoration Movement, it results in the founding of the United Republic of Nations, headquartered in Republic City.
The Third Known Harmonic Convergence
The Harmonic Convergence takes place when all the planets in the Avatar solar system enter into perfect alignment, which opens and combines the Northern and Southern Spirit Portals. It ends up in a struggle between Light and Darkness, between Raava and Vaatu, every single time.
As of now, there have been three known Harmonic Convergences, all won by Raava, but the most recent one, occurring during Korra's lifetime, is the most impactful. Raava is destroyed, leaving the Avatar to take extreme measures to restore their powers. Unfortunately, she can no longer connect with her previous incarnations, making Korra the second first Avatar.