Rhythm derives from natural body repetitions like heartbeat and walking.

Evidence

Probably an even more important musical structure than melody and harmony is rhythm and its related features. Rhythm is a complex concept which refers to different temporal structures in music. The existence of rhythm is based on natural repetitions in time, such as walking, running, the heartbeat, and breathing. In general, a systematic description of rhythmic structures is harder than one for the structures in melody and harmony. The concepts related to rhythm are also slightly imprecise. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1315

Categorization

• XREF: Bodily grounding of rhythm connects to the notion of keynote/ambient sounds shaping human tempo in The Soundscape and to circadian/biological rhythms.
• QUESTION: Why is systematic description of rhythm harder than melody? Worth chasing the structural reason.
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1315