One of the most remarkable events in cellular evolution is the endosymbiosis of α-proteobacteria with a single archaean host cell, a rare evolutionary process, which eventually led to the transformation of symbionts into fully functional mitochondrial organelles in eukaryotes. Evolutionary events related to plants occurred almost 1.6 billion years ago, when eukaryotic heterotrophs acquired a β-cyanobacterium (containing 1B RUBISCO) in what is termed as primary endosymbiosis. Further, this composite cell lineage evolved into three photosynthetic lineages: green algae (plants), red algae and the glaucophytes. Thereafter, a secondary, and tertiary endosymbiosis event occurred giving rise to distinct kinds of green and red-derived photosynthetic plastids, which can be observed in a few haptophytes and dinoflagellates respectively. Eventually, these endosymbionts acquired characteristic cellular properties such as two/multiple envelope membranes and reduction of their genomes through either loss or concerted endosymbiotic gene transfer (EGT) into the nucleus, which ultimately led to the decline of more than three quarters of coding capacity and complete loss of several metabolic pathways. This loss, however, is partly compensated by import of nuclearencoded proteins as well as proteins acquired by horizontal gene transfer (HGT). For most proteins, specific transport mechanisms from nucleus/cytoplasm to organelle exist. The proteins are typically translated as a preprotein with specific signal sequences targeted to the organelle membrane. These membranes harbour receptors, in some cases soluble receptors, for recognition of these signal sequences. Proteins are then internalised using a set of translocation machineries (Gould <i>et al.</i> 2006).

novel phenotypes to their hosts permitting them to leap between adaptive landscapes with new trait axes and peaks. Except for the establishment of mitochondria during eukaryogenesis, all other endosymbioses are built on the more ancient ones that preceded them. Arrows track the evolutionary progression of these interactions. Peaks and labels illustrate some of the major endosymbiotic events. Over evolutionary time, endosymbioses have become ecologically pervasive, playing integral roles in shaping—and reshaping again and again—Earth's biological diversity. One could summarize life on our planet as comprising just the prokaryotes and a broad union of organisms derived from endosymbiotic interactions (i.e. anything with mitochondria, plastids, and beyond; Yutin et al. 2008 ; Martin et al. 2015 ; Archibald 2015a ; McCutcheon et al. 2019 ). As such, there is an intrinsic and even urgent need to understand the biology and ecology of these interactions. This knowledge is key to discerning the main origins and drivers of biological diversity, as well as to clarifying even our most basic biological and evolutionary theories. Pursuing such grand research goals requires the application of evolutionary principles. This framework elevates questions of “ how endosymbioses function” to “ why they function,” “ where they came from,” and “ why they even exist and persist in nature.” It further provides a predictive framework to project our understanding well past our contemporary moment in the evolution of life on Earth. Building an evolutionary framework into endosymbiosis research necessitates developing a baseline understanding of the diversity, origins, and evolutionary processes underlying these interactions. Over the past 40 years, researchers publishing in the journals, Molecular Biology and Evolution ( MBE ), and later Genome Biology and Evolution ( GBE ), have tackled these questions and greatly expanded our knowledge of endosymbioses. What we have learned is profound and