Humanity developed the Warp-Drive and spread at an explosive pace across the galaxy, settling thousands of new colony worlds and making first contact with the other intelligent races of the galaxy, including the Eldar and the Orks.
This period ended in violence and horror as the Men of Iron revolted against their human masters and the first widespread appearance of human psykers caused the deaths of hundreds of millions of people when these men and women, unprepared to wield their new powers, unleashed the horrors of the Warp upon their unsuspecting fellow citizens.
So great was their suffering that a morbid fear of Space Marines became a permanent fixture of Terran culture ever afterwards.
But the Emperor and his Loyalist servants ultimately prevailed, and slew Horus at the height of his power on the bridge of his own flagship, the terrible Gloriana-class Battleship, Vengeful Spirit, in high Terran orbit.
Terra is perhaps the most massive Hive World within the Imperium, with an uncounted population that numbers several hundred billion human beings.
The bulk of Terra's massive population is divided between the upper class that includes the Imperial nobility and Adepts of the Adeptus Terra and the lower classes who serve as basic labourers.
And so for the last 10,000 Terran years stagnation, technologically, culturally and even morally has come to define the Imperium of Man.