Geoclimatic events driving South American aridization have generated biota differentiation due to barriers and new environment formation. New environments allow species climatic niche evolution, or the geographical expansion of an existing one. Understanding the role these processes play may clarify the evolution of South American biota. Gomphrena L. ranges across almost all the continent's arid environments. We tested whether South American drylands are biogeographically connected through the Gran Chaco but, due to different aridity levels, lineage diversification could have also been associated with the evolution of climatic niches and morphological or physiological traits. With available data, we generated a dated phylogeny, estimated ancestral ranges, performed diversification analyses, reconstructed ancestral states of two characters, and examined if niches have changed between lineages. Results showed that Gomphrena diversified throughout the easternmost South American drylands ~15.4 Ma, and subsequently three independent clades colonized the western arid regions during the last Andean pulse, and after the marine transgressions (~4.8–0.4 Ma) via the Gran Chaco. The colonization implied an increase in the diversification rate of annuals over perennials and the progressive east–west differentiation of the occupied climatic niche. This diversification was influenced by C₄ photosynthesis, which could have acted as a niche opener to conquer new environments after the Paranaean Sea withdrew. Spatiotemporal patterns found in Gomphrena suggest that geographical expansion and evolution of climatic niches played a common but decoupled role in promoting diversification. These results show that the Gran Chaco may have acted as a historical connection linking South American drylands.