'''Firstborn''' , also known as '''Principia'''{{Fn|11}} is the [[Imperial]] term for original -template , pre-[[Primaris Space MarineMarines|Primaris]] [[Space MarineMarines]]s.{{Fn|1}}

For ten thousand years from the [[Great Crusade]] until the end of the [[13th Black Crusade|Thirteenth Black Crusade]], the Firstborn were simply the [[Adeptus Astartes]]. However following the development of [[Primaris Space Marine]]s by [[Belisarius Cawl]] the "Firstborn" as they became known faced the prospect of obsolescence at these larger, physically stronger, and mass produced warriors.{{Fn|4}} Originally, in the early stages of the [[Indomitus Crusade]], most Primaris were produced by Cawl himself and, though developed by the [[Genegene-Seedseed]] of the [[Primarch]]s, lacked a distinct identity, instead being gathered in armies known as the [[Unnumbered Sons]]; however, as the Crusade progressed, the Unnumbered Sons were distributed to either new [[Chapter]]s of the [[Ultima Founding]] or into existing Space Marine Chapters.{{Fn|2a2b}}

===Reaction to Primaris Space Marines===Even though 94% of [[Chapter]]s have accepted Primaris Marines into their ranks{{Fn|22a}} Firstborn-Primaris relations have had issues. To many, the Primaris were new arrivals often recruited away from a Chapter's [[Adeptus Astartes Homeworld|homeworld]] and new to the culture, traditions, and ways of the Chapter. Many arrivals from the [[Unnumbered Sons]] into existing Chapters felt like strangers.{{Fn|2b2c}} When some Firstborn first saw the Primaris, they regarded them as mutants.{{Fn|4}} This tension was exemplified in the [[Space Wolves]], where many [[Wolf Lord]]s resisted accepting Primaris Space Marines.{{Fn|3}} While the likes of [[Krom Dragongaze]] embraced these new warriors, [[Logan Grimnar]] himself was distrustful and say these warriors not of Fenris as means to control the Wolves by Guilliman.{{Fn|11a}} However after experiencing the trials of [[Gaius (Unnumbered Sons)|Gaius]], Grimnar had a change of heart.{{Fn|11b}} In the [[White Scars]], many Firstborn saw that these warriors were not of [[Chogoris]] nor knew the ways of their Chapter's culture, but at the same time understood the necessity of Primaris Marines in defending the [[Imperium]].{{Fn|4}} Notable Space Marine officers who have openly expressed distrust of Primaris Space Marines are [[Flesh Tearers]] [[Chapter Master]] [[Gabriel Seth]]{{Fn|9}} and [[Raven Guard]] 9th Company Captain [[Vos Delorn]].{{Fn|8}} In the [[Unforgiven]], there was much hesitancy over allowing Primaris Marines into the [[Inner Circle]].{{Fn|10}}

Notable Space Marine officers who have openly expressed distrust of Primaris Space Marines are [[Flesh Tearers]] [[Chapter Master]] [[Gabriel Seth]]{{Fn|9}} and [[Raven Guard]] 9th Company Captain [[Vos Delorn]].{{Fn|8}} In the [[Unforgiven]], there was much hesitancy over allowing Primaris Marines into the [[Inner Circle]].{{Fn|10}} In the [[Black Templars]], [[High Marshal (Black Templars)|High Marshal]] [[Helbrecht]] approved of Primaris Space Marines and most of his forces accepted them, but the issue still turned disastrous for the [[Angevin Crusade (Black Templars)|Angevin Crusade]].{{Fn|12}} In an attempt and promote unity between Firstborn and Primaris Marines, [[Roboute Guilliman]] often used them as political equerries and symbols of his authority. For their part, most Primaris Marines looked to their Firstborn brethren for sage advice, drawing from experiences from their many centuries of conflict. Primaris Marines learned this lesson in the early days of the Indomitus Crusade, where many had fallen in battle due to their lack of experience.{{Fn|2b2c}}

*{{Endn|2}}: [[Dark Imperium (Novel)]] - :**{{Endn|2a}}: Chapter 12**{{Endn|2a2b}}: Chapter 13**{{Endn|2b2c}}: Chapter 15*{{Endn|3}}: [[Codex: Space Wolves (8th Edition)]] - ''Annals of the Space Wolves''*{{Endn|4}}: [[Apocalypse (Novel)]] - , Chapter 8*{{Endn|5}}: [[Imperium Nihilus: Vigilus Defiant]], pg. 77 - ''Forces of War: The Rubicon Primaris''

*{{Endn|7}}: [[Codex: Space Marines (8th Edition, 2nd Codex)]]:**{{Endn|7a}}: ''Battleline Squads''**{{Endn|7b}}: ''Close Support Squads''**{{Endn|7c}}: ''Fire Support Squads''**{{Endn|7d}}: ''Veteran Squads''*{{Endn|8}}: [[Codex Supplement: Raven Guard (8th Edition)]] , pg.20*{{Endn|9}}: [[The Devastation of Baal (Novel)]] , Chapter 30*{{Endn|10}}: [[ White Dwarf September 2019]] pg, pgs.29-30*{{Endn|11}}: [[Darkness in the Blood (Novel)]], Chapter 15*11: [[The Wolftime (Novel)]]:**{{Endn|11a}}: Chapters 10-13**{{Endn|11b}}: Chapters 23-25*{{Endn|12}}: [[Throne of Light (Novel)]], Chapter 41