Legumes.

Whatever continent you are on (besides Antarctica), whatever type of vegetation you are in, and however that vegetation has been disturbed and modified by humans, there will very likely be a legume growing nearby. Leguminosae or Fabaceae, commonly known as legumes, with ∼22,500 species is the third largest family of flowering plants, after the daisies (Asteraceae) and orchids (Orchidaceae). A central question in legume biology is understanding why the family is so diverse, geographically widespread and abundant, and how legumes came to form significant components of almost all terrestrial ecosystems across the globe. Economically, legumes are also important as major world food crops, and have been so since the dawn of agriculture. The ability to fix atmospheric nitrogen through root nodule symbiosis with bacteria - the hallmark of many legumes - is important in both ecosystem functioning and agriculture, and current research even aims to engineer nodulation in non-legume crops. This combined eco-evolutionary and societal importance means that legumes have occupied a central position in botanical and wider biological research ever since the late 19 century, when Gregor Mendel used the garden pea in his experiments, which famously provided early insights into genetics. In this Primer, we present an overview of the diversity, evolution and ecological and economic importance of legumes across the globe, and discuss the evolution of nodulation, one of the key traits of the family.