The evolution of the Deru and Slithis was entirely natural and unaffected by any conscious or unconscious meddling by the divine forces around them... at least until the Slithis started trying to tap the power of the Nameless. Their fictional role was to grandly represent the ordinary living things that were affected by the struggle between the divine powers, but allied with neither.
I saw them as concurrent evolution away from whatever their original form was.
Did the Deru and Slithis evolve on Dereth around the same time, or did one pre-date the other? Was their evolution natural , or were they guided by the divine forces?