Dust attenuation evolution in $z \sim 2$-$12$ JWST galaxies

A sizable fraction of the heavy elements synthesized by stars in galaxies condenses into sub-micron-sized solid-state particles, known as dust grains. Dust produces a wavelength-dependent attenuation, $A_\lambda$, of the galaxy emission, thereby significantly altering its observed properties. Locally, $A_\lambda$ is in general the sum of a power-law and a UV feature ('bump') produced by small, carbon-based grains. However, scant information exists regarding its evolution across cosmic time. Here, leveraging data from 173 galaxies observed by the James Webb Space Telescope in the redshift range z = 2 - 12, we report the most distant detection of the UV bump in a z ~ 7.55 galaxy (when the Universe was only ~ 700 Myr old), and show for the first time that the power-law slope and the bump strength decrease towards high redshifts. We propose that the flat $A_\lambda$ shape at early epochs is produced by large grains newly formed in supernova ejecta, which act as the main dust factories at such early epochs. Importantly, these grains have undergone minimal reprocessing in the interstellar medium due to the limited available cosmic time. This discovery opens new perspectives in the study of cosmic dust origin and evolution.