Mercury isotopes show vascular plants had colonized land extensively by the early Silurian.

The colonization and expansion of plants on land is considered one of the most profound ecological revolutions, yet the precise timing remains controversial. Because land vegetation can enhance weathering intensity and affect terrigenous input to the ocean, changes in terrestrial plant biomass with distinct negative Δ 199 Hg and Δ 200 Hg signatures may overwrite the positive Hg isotope signatures commonly found in marine sediments. By investigating secular Hg isotopic variations in the Paleozoic marine sediments from South China and peripheral paleocontinents, we highlight distinct negative excursions in both Δ 199 Hg and Δ 200 Hg at Stage level starting in the early Silurian and again in the Carboniferous. These geochemical signatures were driven by increased terrestrial contribution of Hg due to the rapid expansion of vascular plants. These excursions broadly coincide with rising atmospheric oxygen concentrations and global cooling. Therefore, vascular plants were widely distributed on land during the Ordovician-Silurian transition (~444 million years), long before the earliest reported vascular plant fossil, Cooksonia (~430 million years).