The yrast level structure of ²¹⁵Rn has been studied by means of in-beam spectroscopy α-γ-γ coincidence techniques through the ²⁰⁷Pb(¹⁸O,2α2n) reaction at 93 MeV bombarding energy, using the 8π GASP-ISIS spectrometer at Legnaro. New spectroscopic information has been obtained. The deduced low-lying level scheme of ²¹⁵Rn does not exhibit the alternating parity structure observed in the heavier known isotones ²¹⁶Fr, ²¹⁷Ra, ²¹⁸Ac, and ²¹⁹Th. From this result, the lightest nucleus showing evidence for octupole collectivity is ²¹⁶Fr, defining the lowest-mass corner for this kind of phenomenon as N ≥ 129 and Z ≥ 87.