The evolution chamber is the result of the Overmind's view that complacency is the road to defeat, causing it to spur zerg evolution constantly. The evolution chamber provides a testing ground for the genetic code of lesser breeds, helping to increase the offensive and defensive capabilities of the zerg ground-based breeds. It is also required in order for a creep colony to mutate into a spore colony.
The base of the evolution chamber forms from the genetic template of an unknown creature. During development, the "chamber" grows around a full skeleton with a skull, spinal column and ribs, that falls apart when the pit is destroyed. The biological roots of this creature is still unknown to terran scientists.
The evolution chamber contains a hardened skin that grants it near immunity to small arms fire, though can still be destroyed through mass C-14 rifle fire. In addition, it possesses genetic transmission appendages that allow it to transmit genetic information to other structures of the hive cluster.