2025NatAs...9..458M

A sizable fraction of the heavy elements synthesized by stars in galaxies condenses into submicrometre-sized solid-state particles known as dust grains. Dust produces a wavelength-dependent attenuation, A _ λ _ , of the galaxy emission, thereby substantially altering its observed properties. Locally, A _ λ _ is in general the sum of a power law and a ultraviolet 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 detection of the ultraviolet bump in a z ≈ 7.55 galaxy (when the Universe was only ∼700 Myr old) and show that the power-law slope and the bump strength decrease towards high redshifts. We propose that the flat A _ λ _ 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 offers crucial insights into the redshift-dependent evolution of dust attenuation properties, shedding light on the role of supernovae-driven dust production, grain size distribution at early cosmic times and the processes driving dust evolution at later epochs.