  Age-dating and weighting stellar populations in galaxies are essential steps
to study galaxy formation through cosmic times. Evolutionary population
synthesis models with different input physics are used towards this aim. In
particular, the contribution from the thermally pulsing asymptotic giant branch
(TP-AGB) stellar phase, which peaks for intermediate-age 0.6-2 Gyr systems, has
been debated upon for decades. Here we report the detection of strong cool-star
signatures in the rest-frame near-infrared spectra of three young (~1 Gyr),
massive (~10^10 Msun) quiescent galaxies at large look-back time, z=1-2, using
JWST/NIRSpec. The co-existence of oxygen- and carbon-type absorption features,
spectral edges and features from rare species such as Vanadium, and possibly
Zirconium, reveal a strong contribution from TP-AGB stars. Population synthesis
models with significant TP-AGB contribution reproduce the observations better
than those with weak TP-AGB, which are commonly used. These findings call for
revisions of published stellar population fitting results, pointing to lower
masses and younger ages, with additional implications on cosmic dust production
and chemical enrichment. New generations of improved models are needed,
informed by these and future observations.
