The original reason for numbering the years from this point is because it was believed to be the year of the birth of the ancient religious leader Jesus Christ, which has long been forgotten by Humanity. Mankind now possesses only one religion -- a deep faith in the God-Emperor of Mankind and His Imperial Cult. During the long years of the Age of Terra, Humanity came to dominate the Earth culturally and technologically. Myriad human civilisations came and went during this long span of now mostly forgotten human history. The first three Chaos Gods -- Khorne, Tzeentch and Nurgle -- were birthed in the Immaterium during this period, all coming into existence at various points during Old Earth's European medieval period. Their emergence at this time was the result of the constant warfare, the growth of sophisticated early modern nation-states and continued outbreaks of global plagues, respectively, which marked that era and fed the final formation of the Dark Gods' consciousnesses.
The Sol System was ultimately colonised once Humanity developed the necessary technology to leave the Earth's orbit in the late 2nd Millennium and the early 3rd Millennium. Mankind first colonised Mars and lived on the moons of Jupiter, Saturn and Neptune. Mars became the first planet to be terraformed by Humans when it was settled by various industrial cartels. The Red Planet soon became synonymous with technical expertise and scientific advancement, a characteristic that would mark it for many millennia to come. Advanced nuclear fusion and anti-matter propulsion technology was developed that eventually allowed Humanity to construct interstellar starships that could colonise other star systems, though the slow travel times of these vessels meant these Human colonies had to be self-sufficient to survive, as no aid could arrive from Earth in time. At the end of the period and stretching into the next era in Human history, the Age of Technology, Humanity began to colonise the stars using these sub-light spacecraft.
At first only nearby star systems could be reached and the Human colonies established on them were required to survive as independent states since they were separated from Old Earth by up to ten Human generations' time of travel. Almost nothing of this period's events is known to current Imperial historians save as myth and legend, though it is clear that the Emperor was alive and on Earth throughout this period. He is believed to have been born in the 8th Millennium B.C. in Anatolia, then disguised His true abilities and immortal Perpetual nature as He moved from identity to identity throughout Old Earth's ancient history, always trying to better the lot of Humanity, physically, intellectually and spiritually.
209-299.M2 Birth of the Ragged Knight - The Bloodthirster daemon known as the Ragged Knight is born in the Warp.
066.M2 Battle of Hastings - The famous Battle of Hastings is fought in northwestern Europa on Terra, beginning a long series of events culminating in the onset of the Industrial Revolution near the end of the millennium.
ca. 500.M2 Awakening of the Chaos Gods - The three elder Chaos Gods -- Khorne, Tzeentch and Nurgle, respectively, have awakened to full consciousness by the end of the Terran European Middle Ages.
ca. 500.M2 The Visconti di Modrone Deck of Cards Printed - The ancient Visconti di Modrone deck of Visconti-Sforza cards that eventually comes into the possession of Chief Librarian Ahzek Ahriman is printed around this time.
ca. 750-900.M2 Industrial Revolution - The Industrial Revolution begins in the western regions of Old Earth and eventually spreads across the planet by the first solar decades of the 3rd Millennium. The economies of the nation-states of Terra begin to rapidly expand with the onset of industrial production and the rapid scientific discoveries that mark this period. However, the Industrial Revolution also breeds new problems for Mankind, including unequal distribution of resources, extreme environmental degradation, and rapid shifts in cultural values that lead to societal strife.
914-918.M2 World War I - First industrialised global war known as "World War I" begins in Europa and spreads across the planet.
939-945.M2 World War II - Second industrialised global war, called "World War II," a conflict even more devastating than the first, engulfs Terra.
945.M2 The Nuclear Age Begins - Detonation of the first human-developed atomic weapon. Fission-based nuclear power soon becomes widespread on Terra as do stockpiles of nuclear weapons.
945.M2 Recruitment of Damon Prytanis - The mysterious group comprised of various alien races known as the Cabal recruits the Terran Perpetual who will one day come to be known as Damon Prytanis. Having lived many untold lifetimes, he is an ideal choice to become an agent of the Cabal, to battle the forces of the Primordial Annihilator. Throughout many ages of human history he works as an assassin, and shows utter loyalty to the Cabal and their cause.
946.M2 First Computer Created - The first digital computer, called UNIVAC, invented by humans on Terra in the long-lost city of Philadelphia.
961.M2 First Human in Space - First human being, Yuri Gargarin, crosses Terra's atmosphere and orbits the planet.
969.M2 The Moon Landing - First human landing on Luna.
971.M2 First Space Station Built - First human space station, called Salyut, is placed in orbit of Terra.
001.M3 Human Genome Unlocked - Mankind's scientists complete the sequencing of the human genome, allowing for significant advances in genetic research.
100.M3 Mars Terraformed - By the start of the 22nd Century, Mars becomes the first human colony world to be terraformed. It is given an atmosphere and its deserts are transformed into fertile soil. Mars is then settled by industrial cartels and their workforces, quickly growing into the first human Hive World in subsequent centuries.
Discovery of the Immaterium (Unknown Date.M3) - Scientific advances of the third and subsequent millennia bring some basic knowledge of the Warp to humans.
Colonisation of the Solar System (Unknown Date.M3) - Humanity begins to colonise the Sol System using conventional sub-light spacecraft. Colonies are established on Mars, and on the moons of Jupiter, Saturn, and Neptune.
Humanity Colonises the Galaxy (Unknown Date.M3) - Humanity uses Ohnyl Cylinders (O'Neill Space Colonies) to spread beyond its own home solar system to local habitable planets in other nearby star systems. However, such early interstellar colonies would not reestablish contact with Terra until millennia later.
M4 - The events of this time period have been lost to Imperial records.
Unknown Date.M5 Alpha Centauri System Colonised - The 5th Millennium saw much of Mankind's early sub-light interstellar expansion outside of the Sol System. It was during this millennium that the Alpha Centauri System, the closest star system to Earth's own, was colonised and terraformed by the earliest human settlers.
M6 - The events of this time period have been lost to Imperial records.
Unknown Date.M7 - First contact made between Mankind and an intelligent xenos species.
M8 - The events of this time period have been lost to Imperial records.
M9 - The events of this time period have been lost to Imperial records.
M10 - The events of this time period have been lost to Imperial records.
M11 - The events of this time period have been lost to Imperial records.
M12 - The events of this time period have been lost to Imperial records.
M13 - The events of this time period have been lost to Imperial records.
M14 - The events of this time period have been lost to Imperial records.
-Galaxy in Flames (Novel) by Ben Counter, pg. 280
-Realm of Chaos: The Lost and the Damned (1st Edition), pp. 174, 176
-Warhammer 40,000: Rulebook (8th Edition), pg. 40
-Warhammer 40,000: Rulebook (5th Edition), pg. 122
-Realm of Chaos: The Lost and the Damned (2nd Edition)
-Warhammer 40,000: Rulebook (7th Edition) (Digital Edition), pg. 293
-A Thousand Sons (Novel) by Graham McNeill, Ch. 17
-The Unremembered Empire (Novel) by Dan Abnett, Ch. 7
-Abaddon: Talon of Horus (Novel) by Aaron Dembski-Bowden, Part I, Ch. IV