The "Firstborn" is the term used within the Adeptus Astartes and the wider Imperium since the start of the Indomitus Crusade for the original Space Marine genetic template created by the Emperor of Mankind in the late 30th Millennium.
Firstborn Astartes possess only the original 19 gene-seed organs crafted by the Emperor. As a result, though still transhuman warriors, they lack the greater strength, height, resilience and reflexes of their Primaris Marine brethren.
In the immediate wake of the Ultima Founding, it was not possible for Firstborn Marines to undergo the procedure created by Archmagos Dominus Belisarius Cawl to become Primaris Marines. The survival rate was considered too low for the attempt to be made.
This changed just before the War of Beasts began on the world of Vigilus when the Ultramarines Chapter Master Marneus Calgar volunteered to undergo the surgical procedure known as the Rubicon Primaris.
Archamagos Cawl succeeded in implanting the three missing Primaris gene-seed organs into Calgar's body. After the procedure, Calgar was reborn as a Primaris Marine, proving the conversion was possible. Since then, many other Firstborn from Chapters across the galaxy have crossed the Rubicon Primaris to become even more effective defenders of Mankind.
For most of Imperial history, from the start of the Great Crusade in ca. 798.M30 until the end of the 13th Black Crusade and the fall of Cadia in ca. 999.M41, the Firstborn were the only type of Space Marines that existed.
When Belisarius Cawl finally completed the creation of the Primaris Space Marines in his laboratories deep beneath the surface of Mars after the end of the Terran Crusade, the "Firstborn" Astartes now faced the prospect of their own obsolescence.
Most of the early Primaris Marines produced in what became known as the Ultima Founding had been kept in stasis for 10,000 standard years, since the end of the Horus Heresy. These young men had hibernated the long millennia away while Cawl worked to enhance the Emperor's original work and improve the Space Marine template.
The resulting Primaris Marines were physically superior and more resilient warriors than the Firstborn, but were initially deployed by the resurrected Primarch Roboute Guilliman as part of the great army known as the Unnumbered Sons. The Unnumbered Sons were organised into mixed units, where Primaris Marines bearing the gene-seed of different Primarchs all fought together.
In this way, Guillliman hoped to break down many of the divisions that had grown up among the original Space Marine Legions because of the personality differences that had manifested among the early Space Marines.
As the Indomitus Crusade progressed and its fleets spread across the galaxy, the forces of the Unnumbered Sons were slowly whittled down. Many were provided as reinforcements to the existing, depleted Chapters of the Firstborn. Others were transferred into new, Primaris-only Chapters that were made up solely of Primaris Marines drawn from a single Primarch's genetic line.
In the course of the Indomitus Crusade, 94% of the existing Firstborn Chapters accepted Primaris Marines into their ranks. Most were grateful for the new blood to face the terrible foes that threatened Mankind in the wake of the Great Rift's birth.
But some Firstborn resented the arrival of their Primaris brethren. For many, the Primaris Marines were new arrivals who had not been recruited from their Chapter's homeworld and were unfamiliar with the Chapter's culture and traditions.
Those who came from the Unnumbered Sons and had fought for solar decades with Battle-Brothers of other lineages often felt like strangers to the existing Firstborn of a Chapter, even if they shared the same Primarch.
Some Firstborn even believed the Primaris Marines to be mutants when they first saw them, as some remembered the tales of how certain Space Marine Legions had once tried to improve their own Astartes during the Horus Heresy to disastrous results.
Such feelings were embodied within the close-knit Space Wolves Chapter, where many of the Great Companies ruling Wolf Lords refused at first to accept Primaris Marines, fearing that they were not true sons of Fenris or of Leman Russ.
The Firstborn of another independent-minded Chapter, the White Scars, also had trouble accepting their Primaris brethren at first, for they had not been born of Chogoris and thus were not familiar with the culture and traditions of the steppe tribe warriors who comprised the Chapter's brethren.
But the scions of Jaghatai Khan also understood that the Primaris reinforcements were vital if they were to continue to defend the Imperium. Such pragmatism ultimately prevailed among the Firstborn of most of the existing Adeptus Astartes Chapters.
To help the Firstborn better accept their Primaris brethren, Roboute Guilliman often used the Emissaries Imperatus of the Adeptus Custodes as his heralds in introducing Primaris reinforcements to the existing Space Marine Chapters.
As the Custodes were viewed with awe even by the Astartes as the warriors who spoke for the Emperor, their acceptance of the Primaris Marines made it easier for many Firstborn to accept that the creation of the Primaris represented the will of the Emperor Himself.
The presence of the Adeptus Custodes also ensured that even the most traditional Chapters accepted the Primaris warriors into their ranks. One does not decline a gift from the Emperor's own hand, after all.
The Primaris Marines themselves harboured no such reservations about their Firstborn brothers, who they looked to for advice and counsel. They understood that the Firstborn's many centuries of combat experience more than made up for their less advanced physical capabilities.
The Primaris Marines learned this lesson the hard way in the early years of the Indomitus Crusade, when their casualties proved high from their lack of experience despite their greater physical gifts.
Even after the crossing of the Rubicon Primaris, Firstborn Marines remained highly important to the existing Space Marine Chapters.
They continued to comprise important combat units in Adeptus Astartes battleline, close support, fire support and Veteran squads, including in the following roles.
- Scout Marines
- Tactical Squads
- Assault Squads
- Devastator Squads
- Vanguard Veterans
- Sternguard Veterans
- Apocalypse (Novel) by Josh Reynolds, Ch. 8
- Codex: Adeptus Custodes (8th Edition), pp. 27, 32, 48
- Codex: Space Marines (8th Edition) (Revised Codex), pp. 69, 72, 76, 77-79, 82
- Codex: Space Wolves (8th Edition) - Annals of the Space Wolves
- Dark Imperium (Novel) by Guy Haley, Chs. 12, 13 15
- Imperium Nihilus: Vigilus Defiant (8th Edition), pg. 77
- Spear of the Emperor (Novel) by Aaron Dembski-Bowden, Ch. 5
- Warhammer 40,000: Rulebook (7th Edition) (Digital Edition), "Space Marines" (Image)