In the beginning, before Angels, Demons, Immortals, and Reapers came into public light, Earth was just like it was for you. Cities, Continents, Government... Ignorance. Time has no meaning anymore. Some Reapers can manipulate time, and now our world is a parallel of yours. This is how it happened.
0: Life begins everything by creating Cain and Abel. These two are the First of the Demons and Angels, respectively. Abel creates the Overworld and Cain creates the underworld. They both mutually create the middleworld. Both create mates, and begin "populating"
200: Both species establish a mutual contract. Angels in the Overworld only. Demons in the Underworld only. Middleworld is free ground.
225: The Demons have free reign of the Earth. The angels want no part of the barbarism the Demons act with. The Angels have the Sky. Mortals and Reapers don't exist. Underworld, Middleworld, and Overworld hierarchies are established.
500: Mortals are created. Angels realize that Demons and Mortals can't exist together. They banish the Demons to the Underworld.
1000: A war breaks loose in the Gates of the Underworld and Overworld, with Demons and Angels attacking each other. This war shatters the world. Many die, and a large flood envelops the world.
1250: The demons are allowed free reign of earth again.
1500: Life creates mortals once more, and the demons are once again Banished. They come together under one leader, Akuma-Oni, to think up a battle plan.
1775: The mortals overpopulate the planet. Life himself creates Reapers in direct contrast to Mortals, to make sure they stay balanced.
1800: The Reapers do their job too well. They are given time off to let the Mortals repopulate. Life Chords are established.
1922: The Seal on the Gates of the Underworld begins to weaken. A few demons break loose. They set up a colony and refuge on Earth.
1935-1943: One Demon, Adolph Hitler, gets too Excited and slaughters an entire subsection of Humans. The angels are alerted of his presence, and slaughter the entire Camp of demons. This enrages Akuma-Oni . The Angels come together under a leader, Uriel, to better protect themselves as well.
1970: The Reapers feel lonely, and decide to join under a leader as well. Their leader, Thanatos, has big plans for the Harbingers of Life and Death.
2000: Another epic war begins. This war brings the world to it's knees, and the Mortals have no idea what is going on. Thanatos finds their so called "Leader" and explains everything to them. How their very lives were a lie, an experiment. They understand and take refuge on their new continent. The war has shifted their entire planet into one large continent and one large super ocean.
2010: Present Time - The continent's have all formed an alliance under a single mortal leader. No government or currency system exists. Everyone helps out everyone in order to merely survive the wars the angels, demons, and reapers have. Angels, Demons, and Reapers have discovered they can take an apprentice, and create "Immortals." A new age has begun. The Age of Immortals. Thanatos has been removed from the game (Stripped of his powers and memories, mortal in the middleworld) so his apprentice, Fenrir, has taken over as leader of the Reapers.