Main sequence A-stars provide a critical cosmic clock with powerful application to the first galaxies. As relatively simple stars whose light dominates stellar populations with ages 100 Myr to 1.5 Gyr, their lifetimes align perfectly with the billion-year-old universe where galaxies formed prior to reionization and grew rapidly to redshift z~5. Following methodology used in the discovery of "late-bloomer" galaxies, my maximum-likelihood code SEDz* analyzes SEDs to calculate -- not just estimate -- stellar mass buildup in the first galaxies. I will present first results of my program to characterize and quantify the nature of SFHs --- burst or episodic, steady or continuous -- and explore these behaviors with respect to epoch, environment, and the rapid physical changes during the reionization epoch.

First Star Formation Histories Galaxies from JWST-NIRCam