
Some information-

This information is based primarily on the study of Fossils and other archaeological remains. Some of this information is also based on scientific research.
Millions of years before Generation I:
The earliest known prehistoric Pokémon existed at this time. It is possible that the Mew population may have decreased at this stage, as they slowly gave way to other Pokémon species. The first humans also appeared around this time.
300 million years ago: An ancient version of Genesect was born. Kabuto and Kabutops lived in the sea.
Team Plasma later used its DNA to create an infinitely stronger version of Genesect with cybernetic technology.
100 million years ago: Species such as Aerodactyl, Lileep, Cranidos, Shieldon, Tirtouga, and Archen were supposedly widespread. They all ultimately went extinct, but not before Archen gave rise to bird Pokémon and Tirtouga gave rise to turtle Pokémon.
A fossil is found millions of years later in Hoenn and revived scientifically by the Devon Corporation.
10,000 years ago: During the last Ice Age, the Mamoswine population flourished, then decreased when the Ice Age ended.
At some point in the distant past, the Magikarp population mutates due to unknown reasons and becomes weaker.
At some point after the Iron Age, Regigigas is sealed away in the Snowpoint Temple, and Regirock, Regice and Registeel are sealed away at various points across the Hoenn region after years of human worship.
3000 years before Generation V:
The Relic items are created.
2500 years before Generation V:
The Relic Castle is built.
1500 years before Generation I:
The earliest known system for writing is developed and used at the Ruins of Alph, Tanoby Ruins, and Solaceon Ruins.
Archaeologists are currently unsure whether the nearly identical Unown preceded or succeeded the writing system, but it is assumed they share some sort of connection.
The Farfetch'd population decreases due to overhunting.