*NOTE- Stainless Steel: Steven Stone's Adventures in Hoenn is intended to be able to fit rather snugly into any particular universe. This is simply the version of the universe as it would apply to the video game series.
Approximately 500,000,000 years ago (Cambrian through Carboniferous periods)- Life begins to creep from the ocean as highly adaptable creatures known as Mew. Three species of reptilian creatures evolve from the Mew species: one living deep underground, one deep beneath the sea, and one high in the atmosphere, each becoming more and more long-lived with each passing generation. Eventually, these species slowly become extinct due to climate shifting. The last of each become Groudon, Kyogre, and Rayquaza, respectively. Groudon survives by consuming minerals deep in the Earth, Kyogre by eating microscopic creatures, and Rayquaza by consuming airborne minerals and water vapor. Massive insectoid creatures evolve toward the end of this period.
Approximately 300,000,000 years ago- The Permian Period begins. Life on Earth diversifies incredibly. However, Groudon and Kyogre meet for the first of several times, causing a cataclysmic battle that wipes out approximately 97% of all life on Earth. As a result of this battle, Pangaea, Groudon's creation by primal instinct, begins to split apart.
Approximately 250,000,000 years ago to 65,000,000 years ago- Saurian Pokémon begin to appear, flourish, and then mostly die out, with a few exceptions. Groudon and Kyogre do not appear.
Approximately 10,000 years ago- Human civilization begins to develop, and their deep bond with Pokémon forms. Jirachi is born.
Approximately 5,000 years ago- Ancient human civilizations flourish, go to war, die out, and are born again. Humans discover that Jirachi is able to grant three wishes when found. However, after these wishes are granted, Jirachi enters a deep slumber for one year, and teleports to a different location on Earth. Wars over the possession of Jirachi are waged.
18 BCE (Before the Common Era)- During one such war, Regigigas, under the command of its master, arouses Groudon and Kyogre. The two rivals meet and their clash threatens to split apart the Earth itself. To prevent the Earth's destruction, Regigigas locates Jirachi, and this year's wishes are used to create the Red Orb and the Blue Orb and their original guardians (Latias and Latios), to create Regirock, Regice, and Registeel, which are used to lock Regigigas (willingly) in a temple in the far north, and to restore the balance of nature. Rayquaza arrives to end the battle, and the Orbs are used to send the titans into a deep slumber. The Hoenn region has been shaped by the battle, and Kyogre retreats to the Seafloor Cavern, and Groudon to the depths of Mt. Chimney. It is agreed upon by all sides that Jirachi is much too dangerous for mankind, and Jirachi agrees to sleep for one thousand years, rather than just one. Regirock, Regice, and Registeel retreat to their respective temples around the Hoenn region.
982 CE (Common Era)- Jirachi awakens. The wishes are used to turn the world into something of a Utopia. Mankind and Pokémon alike flourish. The third wish is used to create a Green Orb to summon Rayquaza should Groudon and Kyogre ever reawaken. However, the requirements of its utilization are lost to the years, rendering the sphere practically useless. A monument is built to honor Rayquaza in southern Hoenn, known as the Sky Pillar. It endures the ages better than its sister monument, which was built in southwestern Johto, the Embedded Tower, which was created in reverence to the balance of nature represented by the trio.
1966 CE- Steven Stone is born.
1970s CE- In a joint venture, Devon Corporation and Silph Company invent and begin the production of the Poké Ball. At first, only three varieties are available- the regular Poké Ball, the higher-performance Great Ball, and the most powerful Ultra Ball. Devon begins work on specialized Poké Balls, and Silph begins work on an ultimate Poké Ball.
1982 CE- The events of Stainless Steel occur.
Late 1980s/Early 1990s CE- Wallace follows Steven's steps through the Hoenn Pokémon League, and eventually replaces him as Champion.
1996-circa 2004 CE- The events of Pokémon FireRed, LeafGreen, Emerald, HeartGold, SoulSilver, Platinum, Black, and White occur.