The Zerg are a completely different race from humans and starlings

Their population was formed by infecting other races. In order to achieve zerg dominance, these creatures, once infected, selectively evolve rapidly into effective and deadly killing machines. The Zerg don't need technology to make weapons, armor, or ships. Instead, these functions are efficiently implemented through adaptations and planned mutations. Even zerg structures are specialized organs attached to growing organisms near their nests. When the Zerg first arrived in Koprulu, they were ruled by the absolute dominance of a zerg collective consciousness. The overlord fundamentally directs the actions of every zerg in the swarm through some lower level of consciousness. The first and most important link is the brain worm, each of which is entrusted with the direct control of a sizeable portion of the swarm. The host does the brain worm's bidding, directly directing and controlling most of the zerg in the nest and on the battlefield. Although the overmind's primary purpose was to destroy the highly evolved protoss and absorb their genes, it found something very useful and untapped in humans. After infecting a powerful human, Sarah Kerrigan, the Overlord evolved a new and unique creature: The Queen of blades. When the Overmind was destroyed by the Starspirit during the invasion of El, Queen Blade manipulated the Dark Templars to destroy the remaining living brain worms and eventually rule the Zerg.