Around 470 million years ago, in the middle Ordovician, a lineage of green algae left the water and stepped onto land. This seemingly modest transition was the opening of a profound transformation in Earth's biosphere and climate.

The earliest land plants were simple, non-vascular organisms resembling today's mosses and liverworts: they used rhizoids instead of roots, reproduced by spores, and could not stray far from water. Yet adapting to terrestrial conditions posed formidable challenges — desiccation, gravity, and competition for light. These pressures eventually drove the evolution of vascular tissue (xylem and phloem). Cooksonia, known from fossils around 425 million years old, is among the earliest plants with such tissue: small, centimetre-scale stems, unbranched, bearing spore capsules at their tips. Inside those tiny stems, the conducting channels were already sketching the blueprint for every tree and flowering plant that would follow.

Plant colonisation of land directly altered the atmosphere. Oxygen produced through photosynthesis rose measurably through the Devonian. Carbon dioxide fell simultaneously — cooling the climate and accelerating chemical weathering of rocks as plant roots broke mineral surfaces; the weathering products washed into the oceans, providing nutrients to marine life as well.

Plant roots began to hold soil together, slow erosion, and accumulate organic matter — what we now call soil formed, in a meaningful sense, for the first time. Without that foundation, the later evolution of insects, amphibians, reptiles, and mammals on land would not have been possible. Over the next hundred million years plants grew into towering forests, the Carboniferous coal swamps rose and fell, and atmospheric oxygen reached a peak. That first small step onto dry land was the starting point of every terrestrial ecosystem Earth has ever known.