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The zerg emerged from the first spawning pool. The zerg fed on each other's essence, gaining strength, a practice that was carried out as they emerged beyond the pool and assimilated Zerus's other lifeforms.
As the xel'naga uplifted the zerg, Amon intended to use the zerg for his own purposes. However, the independent nature of the zerg necessitated the creation of a hive mind to direct the species in order to make them more controllable. As a result, the Overmind was created, with many zerg falling under its control. This event was witnessed by the creature known as Zurvan.
After turning on the xel'naga, the zerg under the Overmind departed Zerus. At this point, the zerg went down two different paths—those of the Overmind were known as the Swarm. However, a number of zerg had burrowed deep below the planet's surface, escaping the Overmind's control. They remained on Zerus, becoming known as the primal zerg. Here, they continued to prosper, hunt, and evolve, the Swarm unaware of the primals' existence.
Second Great WarEdit
By the Second Great War, the primal zerg had existed for at least millennia. During the conflict, the primal zerg encountered their Swarm counterparts. Sarah Kerrigan, after being de-infested by a xel'naga device, sought to find a new means of increasing her power. Whilst aboard her leviathan, she met with the Dark Templar Zeratul who revealed to her the existence of Zerus and of the primal zerg who were needed to transform her once more.
Upon arriving at Zerus, she awakened Zurvan and battled the primals, who intended to kill the "corrupted" zerg. Kerrigan achieved her primal form , defeated the primal leaders and their packs joined the Swarm.  She departed Zerus with Dehaka, his pack and the other primal zerg who joined the Swarm; they who followed in the hopes of acquiring new essences. While the packs were brought into the Swarm, Dominion field reports suggest that the primal zerg have retained a degree of autonomy.
During Kerrigan's attack on Korhal, the Terran Dominion activated the Psi Destroyer, a superweapon which killed zerg through their hive mind. Because the primal zerg were all sentient individuals who had no hive mind, they were immune to its effect. Kerrigan dispatched Dehaka and his pack to weaken the enemy. Finally, the zerg destroyed the Destroyer.
Shortly after this, the primal zerg participated in Kerrigan's final assault on Augustgrad. Kerrigan's forces destroyed the Dominion entrenchments barring their way, and allowed Dehaka's pack into the city. After Kerrigan killed Mengsk, the primal zerg, along with the rest of the Swarm, departed Korhal to fight Amon.
"The toolmakers might create something stronger than your armor and claws."
"Their tools stay the same. I collect. I change."
The primal zerg bear the genome of the zerg race before the Overmind established control over their species.While the strains of the Swarm often bear an insect-like appearance, primal zerg bear more resemblance to reptiles, and in some regards, mammals.
Primal zerg have the ability to take essence from their foes, consuming their flesh and through it, their DNA. They are in a constant state of evolution, incorporating beneficial traits from their victims. Such traits can become manifest in a short amount of time, ranging from seconds to hours, or a few days. Through this, the primal zerg were able to mimic numerous Swarm strains, while retaining their own physiological markers.
Primal zerg are individuals. Many despise the Swarm zerg for being "corrupted" by the latter's inter-entity psionic connection and hivemind. Zurvan described subjugation to the hivemind as a "terrible fate". Without the hivemind, the primal zerg are immune to psionic measures that would otherwise cripple Swarm zerg. They were immune to control by the Queen of Blades. They remain vulnerable to sigma radiation.
Primal zerg may form into packs, led by pack leaders. Their philosophy/culture is Darwinian on the principle of survival of the fittest. Loyalty is commanded by the ability to survive challenges from underlings; ascension is through consuming weaker "superiors" when the opportunity arises. For primal zerg, form and function are one—with each change comes greater strength, and thus survival.
Pack leaders were powerful primal zerg. Save for Dehaka, they were all killed by Kerrigan, and their pack members integrated into the Swarm.
- Sarah Kerrigan (by virtue of position, biologically a terran/primal zerg hybrid)
- Slivan, the Eternal Mother
- Zurvan, the Ancient One
- Samwise Didier did concept art for the primal zerg while Phill Gonzales did unit modeling. Most of the primal zerg variations were based on dinosaur species he enjoyed as a child. The idea was to explore how beastly the zerg were before the Swarm reduced their physical forms to a skeletal physique featuring only components essential for combat.
- Gonzales has conceded that primal zerg dialogue was heavy handed. As the developers didn't have time to show primal zerg evolving through stealing essence, the point had to be stressed through dialogue.
- The concept of the primal zerg bears resemblance to the development idea of the "metamorph", a zerg strain developed by Kerrigan and Abathur that could direct its own evolution.
- Primal zerg units does not gain any movement speed bonus from standing on creep, unlike their Swarm counterparts. They also do not receive any benefits from the mutation and evolution traits in the Evolution pit.