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Most organisms possess time-keeping devices called circadian clocks. At the molecular level, circadian clocks consist of transcription–translation feedback loops (TTFLs). Although some components of the negative TTFL are conserved across the animals, important differences exist between typical models, such as mouse and the fruit fly. In Drosophila , the key components are PERIOD (PER) and TIMELESS (TIM-d) proteins, whereas the mammalian clock relies on PER and CRYPTOCHROME (CRY-m). Importantly, how the clock has maintained functionality during evolutionary transitions between different states remains elusive. Therefore, we systematically described the circadian clock gene setup in major bilaterian lineages and identified marked lineage-specific differences in their clock constitution. Then we performed a thorough functional analysis of the linden bug Pyrrhocoris apterus , an insect species comprising features characteristic of both the Drosophila and the mammalian clocks. Unexpectedly, the knockout of timeless-d , a gene essential for the clock ticking in Drosophila , did not compromise rhythmicity in P. apterus , it only accelerated its pace. Furthermore, silencing timeless-m , the ancestral timeless type ubiquitously present across animals, resulted in a mild gradual loss of rhythmicity, supporting its possible participation in the linden bug clock, which is consistent with timeless-m role suggested by research on mammalian models. The dispensability of timeless-d in P. apterus allows drawing a scenario in which the clock has remained functional at each step of transition from an ancestral state to the TIM-d-independent PER + CRY-m system operating in extant vertebrates, including humans. [ABSTRACT FROM AUTHOR]