If the creature dies, it is returned to its lowest form unless it has successfully evolved into its 'origin' stage, in which case the creature regresses to that instead. After death, any parts gained by evolution are kept, but any gathered food is lost.
If a creature dies and hasn't achieved its 'origin' stage through evolution, it returns to its initial state. Conversely, if it has reached the 'origin' stage, it regresses back to that form instead. After death, any evolutionary enhancements are retained, but all collected food is lost.