The brain’s left and right hemispheres differ dramatically in functionality. All these differences could have evolved from any genetic mutation that affected the functioning of only one hemisphere. Evolutionary adaptations occur at the level of individual genes, and thus the environment in which a gene variant “lives or dies” includes the host genome in which that gene sits, as well as the physical “body” constructed by that host genome. And because the survival of every gene within a host genome depends on the survival of its one host organism, every gene in that genome must either live or die together (ignoring random gene transmission during sex). If, in the distant past, a genetic mutation occurred that affected the functioning of only one hemisphere, then it might have caused a spiral of symbiotic co-evolution to emerge between hemisphere-specific gene variants. That is, the genes that coded for left-hemisphere attributes might have exerted selection pressures on genes that coded for right-hemisphere attributes, and vice versa, eventually producing two highly divergent structures, each of which was adapted to function in the environment created by the other.