There are two theories regarding the evolution of paleognathes. According to the Gondwana vicariance hypothesis, the paleognathes evolved once, from one ancestor, on Gondwana during the Cretaceous, and then rode on the daughter landmasses that became today's southern continents. This hypothesis is supported most strongly by molecular clock studies, but it is weakened by the lack of any Cretaceous or southern fossil paleognaths, as well as the early radiation of paleognaths in Laurasian landmasses. According to the Tertiary radiation hypothesis, they evolved after the Cretaceous–Paleogene extinction event from multiple flying ancestors on multiple continents around the world. This hypothesis is supported by molecular phylogeny studies and matches the fossil record, but it is weakened by morphological phylogenetic studies. Both hypotheses have been supported and challenged by many studies by many authors.