  The spectral features observed in kilonovae (KNe) reveal the elemental
composition and the velocity structures of matter ejected from neutron star
mergers. In the spectra of the kilonova AT2017gfo, a P Cygni line at about
1$\mu$m has been linked to Sr II, providing the first direct evidence of
freshly synthesised $r$-process material. An alternative explanation to Sr II
was proposed - He I $\lambda 1083.3$nm under certain
non-local-thermodynamic-equilibrium (NLTE) conditions. A key way to robustly
discriminate between these identifications, and indeed other proposed
identifications, is to analyse the temporal emergence and evolution of the
feature. In this analysis we trace the earliest appearance of the observed
feature and detail its spectro-temporal evolution, which we compare with a
collisional-radiative model of helium. We show that the 1$\mu$m P Cygni line is
inconsistent with a He I interpretation both in emergence time and in
subsequent spectral evolution. Self-consistent helium masses cannot reproduce
the observed feature, due to the diminishing strength of radiative pathways
leaving triplet helium.
