Soon a revolutionary era of primordial galaxy observations will be ushered in by the next generation of telescopes, such as the James Webb Space Telescope (JWST) and Extremely Large Telescopes (ELTs). The first stars and galaxies were responsible for initiating the epoch of reionization (EoR) and provided the seeds from which all galaxy evolution grew. Knowledge of their properties will provide the crucial missing link needed to weave a coherent picture of galaxy evolution. To do so, the community must build the needed framework to interpret the spectra of galaxies from z~0‒10, without which we will be woefully unprepared to answer the biggest open questions in galaxy evolution: What properties characterized the first galaxies? How did they reionize the universe? What key physics govern galaxy evolution and are responsible for the diversity of objects seen today?

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We can solve these questions by bringing together detailed expertise of extragalactic rest-frame UV-IR spectroscopy and photometry that are key to deciphering galaxy evolution processes:

• Disentangling spectral signatures that characterize the interplay of massive stars and their surroundings.

• Interpreting the diagnostic imprint of chemical abundance patterns.

• Constraining evolution from stellar populations and star formation histories.

• Observing and modeling metal-poor massive star populations. 

• Understanding the outflow processes that shaped the present-day interstellar medium.

• Constraining the physical processes regulating the dynamic evolution at high redshift.

• Robustly constraining the escape of ionizing radiation from resonant lines.

A bridge of these topics will provide the essential key to deciphering the complexly intertwined stellar+nebular properties of star-forming galaxies, setting the stage to understand the cosmic origins and evolution of galaxies in a unified framework for the first time.