Legumes and actinorhizal plants are capable of forming root nodules symbiosis with rhizobia and Frankia bacteria. All these nodulating species belong to the nitrogen fixation clade. Most likely, nodulation evolved once in the last common ancestor of this clade. NIN (NODULE INCEPTION) is a transcription factor that is essential for nodulation in all studied species. Therefore, it seems probable that it was recruited at the start when nodulation evolved. NIN is the founding member of the NIN-like protein (NLP) family. It arose by duplication, and this occurred before nodulation evolved. Therefore, several plant species outside the nitrogen fixation clade have NLP(s), which is orthologous to NIN. In this review, we discuss how NIN has diverged from the ancestral NLP, what minimal changes would have been essential for it to become a key transcription controlling nodulation, and which adaptations might have evolved later.