THE GRAND EXPERIMENT.
    Driven to perfect their science of proto-genetic evolution, the ancient Xel'Naga traveled to the distant fringe world of Aiur. The vast of Aiur had produced the most advanced race that the Xel'Naga had ever seen. Believing that they could steer the race's evolution to the pinnacle of physical perfection, the Xel'Naga began to conduct their proto-genetic experiments. The race that the Xel'Naga would eventually name 'Protoss' advanced rapidly and gained what creators termed 'a distinct purity of form'. Unfortunately, the Xel'Naga pushed their experiment too far. The inherent essence and sentience of the Protoss developed far to rapidly, leading to bitter strife and division between them and their creators. The Xel'Naga deemed that the purity of form they sought to create had been sullied by a conflict of essence and thus decreed that the Protoss were, in fact, a failed creation. The Xel'Naga abandoned their children and launched themselves into the void.

THE BIRTH OF THE ZERG.
    Traveling thousands of light years into the burning core of the galaxy, the Xel'Naga eventually settled upon the volatile ash-world of Zerus. The Xel'Naga planned to continue their Grand Experiments of evolution, only this time they dismissed their tenets of physical form and focused chiefly on the pursuit of a distinct purity of essence. Residing in their massive ships high above the fires of Zerus, the Xel'Naga began once again to challenge the wiles of fate.
    The Xel'Naga were more successful with their second venture than they could have imagined. They labored to advance the evolution of the most insignificant life form on Zerus, a race of miniature  insectoids known as the Zerg. Through Xel'Naga proto-genetic manipulations, the Zerg survived the torrential firestorms of their world and thrived. Although extremely small, worm-like, and possessing no ability to manipulate their physical surroundings, the Zerg adapted to survive. They developed the ability to burrow into the flesh of the less vulnerable species indigenous to Zerus. Feeding off the nutrients contained within the spinal fluids of their hosts, the Zerg learned to parasitically merge with heir host creatures. Once they became capable of controlling the metabolic and anatomical processes of their hosts, the Zerg used their new bodies to manipulate their surroundings.
    As the Zerg incorporated more and more host creatures into their fold, they began to assimilate their various genetic strains and processes. Zerg chemistry began to mutate and adapt accordingly to the volume of new genetic material being processed. However, as diverse as the range of host creatures became, there was always the undeviating drive to consume only the most evolutionary advanced species encountered. The Zerg were innately selective as to which species they consumed, ensuring that at every stage of their development they were at the top of the proverbial food chain. Any race that the Zerg came across that was deemed unworthy of assimilation was eradicated to further purify the strains.
    The Xel'Naga soon made an alarming discovery. The original races assimilated by the Zerg were hardly recognizable after only a few generations of their inception. Somehow the Zerg had developed the ability to supercharge and steer the latent evolutionary processes within their host creatures. The host creatures fell prey to the effects of gradual physical mutations that caused all the various strains to grow armor-piercing spines, razor-sharp limbs, and ultra dense carapaces. Over a surprisingly short amount of time, the strains grew to resemble a terrifying ravenous and unified race.

OVERMIND.
    Xel'Naga, remembering all too well that their failure within the Protoss was a result of pushing the sentience of the fledging species too quickly, decided to follow a different path with the burgeoning Zerg. Attempting to waylay the potential hazards of different egos, the Xel'Naga structured into the collective sentience of the Zerg a unified, amalgamated 'Overmind'. The Overmind coalesced into semi-sentient being that represented the primary drives and instincts of all of the Zerg strains. As time passed, the Overmind developed rudiments of personality and advanced intellect.
    Although the Overmind directed the actions of every creature within the swarm, it did so through the use of secondary agents. The Overmind created a new strain of Zerg that could facilitate the communication of its whims. These Cerebrates were massive versions of the original Zerg insectoids and were engineered to process and carry out the various objectives of the Overmind. Each Cerebrate had a unique objective, such as 'defend the Hive', 'scout for potential strains', produce more warriors', or eradicate all life forms'. The Cerebrates were then empowered to build their own Hive clusters and carry out their intended agendas. Over time they developed personalities that corresponded to their primary drives. However, the Cerebrates were still genetically incapable of ignoring or overriding the commands of the Overmind.
    The Zerg chain of command lengthened as new races were incorporated into the swarm. Cerebrates began to use secondary agents to communicate their orders as well. For the defense and productivity of all the immediate Hive, the Cerebrates turned to Queens to further extend their control over the swarms. The Queens oversaw drone activity throughout the resource lines of the Hives. And kept a vigilant watch over the bugging spore colonies if he collective. In times of battle, Cerebrates called upon Overlords to directly relay their commands to the myriad warriors of the swarm. Overlords not only transported Zerg warriors into the fray, but also directed their attacks against their enemies. Just as the Cerebrates followed the Overmind, the Queens and Overlords were incapable of disobeying the commands of their Cerebrates. This rigid system of command kept the swarms at peak, savage efficiency.
    As the swarms continued to grow and strengthen, the Overmind turned its thoughts to its own future. It realized that within a few short centuries its race had assimilated all the indigenous life on Zerus. It knew that to further evolve the Zerg swarm, the Zerg would need to leave Zerus. The Overmind began to reach out with its senses, looking for something - anything - which would provide them with transport from this world. That opportunity soon arrived. A race of gargantuan, space-faring life forms passed through the Zerus system, and the Overmind called to them. Drawn to the barren world by this beacon, they were quickly assimilated by the swarm. This inclusion of super-dense hides and the ability to exist in a vacuum bolstered the genetic pool of the swarm. Soon the Zerg warriors were conditioned to survive the harshness of space.
    This pivotal moment in the development of the Zerg was not lost to the Xel'Naga. The Zerg, despite having an extreme physical handicap, had succeeded in not only surviving, but in retaining the purity of their terrible overriding essence. The Xel'Naga knew they had achieved their goal.

THE FALL OF THE XEL"NAGA.
    The pride in their achievements proved to be the fatal downfall of the Xel'Naga. The Overmind, while slowly expanding itself into the void of space, became aware of the mighty Xel'Naga world-ships hovering ominously above the skies of Zerus. The Xel'Naga having kept a constant watch over the Overmind, were horrified to find that it had actually severed their psychic link, effectively hiding  itself from their view. With its need to consume driving its minions into a lustful frenzy, the Overmind launched the now space-faring Zerg swarms at the unsuspecting Xel'Naga. The ancient race did what they could to stem the tide of the ever advancing Zerg onslaught , but in the end their efforts were in vain. Wave after wave of Zerg swarms hammered the reinforced hulls of the Xel'Naga ships with no sign of abatement. Within only a few hours the Zerg overran the defenses of their creators and laid waste to the Xel'Naga fleet.
    As the greater whole of the Xel'Naga race was consumed by the raging, genetic whirlwind of the Zerg, the Overmind gained the knowledge and insights of its masters. The Overmind processed thousands of sentient beings into itself, causing it to grow far more powerful than it had ever imagined. It learned the secrets of the sacred Khaydarin Crystals and began to incorporate the energies of those Crystals into into its own. Through the intimate knowledge of evolution and proto-genetic physiology gained from the Xel'Naga, the Overmind was able to increase the level of sentience in many of the higher strains of Zerg strains, while still keeping them fully under its control.
    Through dissecting the memories of the Xel'Naga, the Overmind was made aware of the myriad races that had at one time or another been influenced by the ancient race. The Xel'Naga had kept detailed genetic history of each race , giving the Overmind a clear understanding of their respective strengths and weaknesses. Most importunately the Overmind learned of the exceedingly powerful race that lived near the galaxy's fringe known only as the Protoss. The Overmind knew that the Protoss and the Zerg would eventually be caught in an inevitable, apocalyptic conflict.

THE DETERMINANT.
    The Zerg left the lifeless, burning world of Zerus and laid waste to every planet they found along their path towards the Protoss Homeworld. As they progressed slowly through the trackless dark between the stars, the Zerg assimilated only the strongest of the races that they came across. The swarm continued to build steadily, ever-increasing in size and power. As they progressed, the Overmind sent out numerous deep-space probes that scouted ahead of the swarm, searching for new worlds to plunder.
    Despite innumerable victories, the Overmind was greatly disturbed. The Overmind was aware that the Protoss had become a highly psionic race, able to bend and warp the very fabric of reality to their whims. It sought a way to counter the awsome might of the Protoss, but found no answers among the genetic strains it devoured.
    On the verge of despair, the Overmind made an amazing discovery. One of its deep-space probes had relayed the location and vital statistics of a race that occupied a series of non-descript worlds, right under the shadow of the Protoss.
    The new race, called humanity, was mere generations away from developing into a formidable psionic power. But the Overmind also knew that Humanity was still in its infant stages, hardly capable of defending itself from the ravenous Zerg. Although a short lived and seemingly frail species. The Overmind knew that Humanity would be the final determinant in its victory over the Protoss. If it could assimilate the psionic potential of Humanity, the Overmind would have the ability to combat the Protoss on its own terms.
    Thus, the Zerg swarms made their was towards the burgeoning worlds of Humanity. The journey lasted for sixty years, but eventually the massive, extended Zerg swarm reached the outskirts of the Terran Sector of Koprulu.  Sending in a scouting Brood, the Overmind soon discovered that the Humans dwelt on over a dozen different worlds within the Sector. Seeding the atmosphere of the planet called Chau Sara with rudimentary Hive-Spores, the Overmind began to unfold its master plan to enslave Humanity. The Hive-spores gradually drifted down to the surface of Chau Sara and saturated the ground with their denuding, alien toxins. Although the Human colonists had no idea that the Zerg had subtly infested the topsoil of their world, Zerg minions began to descend to the planets surface and began to construct their bizarre structures and Hive clusters. Once the infestation of the colony was well under way, the Overmind sent its voracious children to the other nearby worlds. Insidious and elusive in their tasks, the agents of the Overmind soon infested the colonial worlds of Chau Sara, Mar Sara, Brontes, and Dylar IV, unnoticed by the denziens of these colonies.
    Yet, from the cold void of space, a mighty fleet of Protoss warships emerged to combat the Zerg invasionary forces. He Overmind, anxious to learn what it could about the enigmatic Protoss, decided to let them hamper the initial infestation process. Holding its ravenous warriors at bay, the Overmind watched as the Protoss razed the colony of Chau Sara. Apparently, the Protoss were aware that the Hive Spore had already despoiled the planet and seeking to prevent further infestation, incinerated the planet.
    Such decisive action pleased the Overmind, who could only marvel at how the Protoss wrought absolute devastation with such grace and power. Knowing that the coming conflict would be the greatest challenge of its existence, the Overmind pulled its forces back to observe how the Protoss, and Humanity, would react next.

Credit must go to BLIZZARD ENTERTAINMENT as well as Davidson & Associates for this COMPLETE extract of the history of the Zerg, as seen above.