The Thunder Warriors

The Space Marines can trace their origins back to the Unification Wars on Terra in the late 30th Millennium.

Origins
When the Emperor of Mankind first revealed his existence and lead regiments of deadly genetically-engineered soldiers known as Thunder Warriors in a great campaign to unite all of the myriad techno-barbarian tribes and nation-states of Terra under his rule. From the outset of his retaking of Terra, the Emperor employed genetically modified warriors within his forces and in these early enhanced troops lay the origins of what would later become the Space Marine Legions. During the Age of Strife, known as "Old Night" on Terra, the cradle of Mankind had seen more than its fair share of augments and "super" soldiers created both from bio-alchemy and cybernetic augmentation. But it was the Emperor's own Thunder Warriors, named for the early thunderbolt and raptor's head heraldry used by their master in the Imperium's earliest days, that were to prove superior to all of them. These superhuman warriors were a gestalt mix of unprecedented superhuman physical power, gene-programmed resistance to environmental and even psychic attack, a warlike spirit and the Emperor's own strategic genius. The Thunder Regiments were an army unlike any that had come before them, and the forces of the powerful tyrants of Old Earth had nothing to match them.

This Unification of humanity's homeworld marked the beginning of the Imperium of Man and the Emperor's quest to reunite all of humanity under a single interstellar government. This quest was intended to prevent his race's extinction from the growing threats which confronted the human-settled galaxy in the wake of the Age of Strife. But despite their many early victories in the Unification Wars, the Thunder Warriors were far from perfect. Some were mentally unstable, others suffered catastrophic biological failure after an unprecedented span of years as their own superhuman physiques turning against them in the end. It seems obvious in retrospect that the Emperor knew early on that a more permanent and stable force of enhanced warriors was needed, so even while the Thunder Warriors waged war in their early days the Emperor gathered about him a team of savants and gene-wrights, some willing and others as captives taken from his foes, and constructed new genetics laboratories deep in the vast dungeons of his Terran fortress beneath the Himalazian (Himalayan) Mountains. Labour there went on for decades in absolute secrecy and resulted in the creation of the Primarchs and other wonders of gene-craft known and unknown. Foremost amongst these were the Space Marine Legions, the Legiones Astartes. Into their creation went all the secret history and genetic lore of the Age of Strife, hard wisdom gained through the success and failure of the Thunder Warriors and the Emperor's own inimitable genius.