Understanding how galaxies form and evolve is a fundamental goal of modern astrophysics. It touches on critical problems at both small scales, e.g., star-formation, and large scales, e.g., the hierarchical growth of structure as expected from the cosmological paradigm. Ironically, the very nature which makes galaxy evolution so important to understand is the same property that makes it so difficult to understand. The vast range in physical scales that need to be understood and non-linear evolutionary behavior make ab initio models describing galaxy evolution impossible. In this thesis, I adopt the ``archaeological'' approach to unraveling the evolutionary histories of galaxies that exist at the extreme ends of the observed galaxy population.
I first describe the empirical stellar spectral library that forms the foundation of the stellar population synthesis models that underpin this work. The expansion in stellar parameter and wavelength coverage of this library enables the broad applicability of the models to objects that collectively populate various ends of the range of galaxy parameter space.
The first such application demonstrates that the primary correlate of initial mass function variability is not metallicity, like suggested by previous work. Then I show that the internal properties of the ancient star clusters that are used as primary fossil record to infer the histories of the most massive galaxies in the Universe are not as well-understood as previously considered. I discuss how this result both alters the interpretations of galaxy evolution based on these fossils and the implications this has on the understanding of star cluster formation and, thus, star formation. I then, with the new models for the individual clusters and a novel statistical framework for the systems of these clusters, provide new constraints on the assembly processes in a massive galaxy. Finally, I apply the new models to the spatially-resolved galaxy light of a contentious new galaxy type to show that it is indeed a galaxy with hitherto unprecedented stellar population properties.
