While crystallization is a ubiquitous and an important process, the microscopic picture of crystal nucleation is yet to be established. Recent studies suggest that the nucleation process can be more complex than the view offered by the classical nucleation theory. Here, we implement Single Crystal Nucleation Spectroscopy (SCNS) by combining Raman microspectroscopy and optical trapping induced crystallization to spectroscopically investigate one crystal nucleation at a time. Raman spectral evolution during a single glycine crystal nucleation from water, measured by SCNS and analyzed by a non-supervised spectral decomposition technique, uncovered the Raman spectrum of pre-nucleation aggregates and their critical role as an intermediate species in the dynamics. The agreement between the spectral feature of pre-nucleation aggregates and our simulation suggests that their structural order emerges through the dynamic formation of hydrogen-bonded networks in a linear fashion. The present work provides a strong impetus for accelerating the investigation of crystal nucleation by optical spectroscopy.