It was originally proposed that the radiation occurred around 3.5 million years ago, when an ancestral form, perhaps similar to the modern-day black-headed siskin, was able to adaptively radiate into South America. In this version of events, the dispersal into South America happened before the isthmus of Panama was complete (around 2.7 million years ago), but when mesothermal montane plant species were able to disperse across the narrowing strait. The novel plant communities in South America would have created many new niches into which the ancestral species adaptively radiated.