Legumes arrive and start fixing nitrogen

Legumes appear in the Paleocene and become one of the most consequential plant families on Earth by hosting rhizobial bacteria in root nodules, where the bacteria convert atmospheric nitrogen into a usable form in exchange for sugars. Nitrogen is the limiting nutrient in most ecosystems, so the arrangement lets legumes colonise poor soils and enrich them. They are abundant in the Cerrejon rainforest almost from the start. Much later, agriculture would be built on this partnership through crop rotation, long before anyone knew why it worked.