The family Felidae is believed to have originated in Asia about 11 million years ago. Taxonomic research on felids remains partial, and much of what is known about their evolutionary history is based on mitochondrial DNA analysis. Significant confidence intervals exist with suggested dates. In the latest genomic study of the Felidae, the common ancestor of today's Leopardus, Lynx, Puma, Prionailurus, and Felis lineages migrated across the Bering land bridge into the Americas 8. 0 to 8. 5 million years ago (Mya). The lineages subsequently diverged in that order. North American felids then invaded South America 2–4 Mya as part of the Great American Interchange, following formation of the Isthmus of Panama. but the relationship is unresolved. The cheetah lineage is suggested by some studies to have diverged from the Puma lineage in the Americas (see American cheetah) and migrated back to Asia and Africa, while other research suggests the cheetah diverged in the Old World itself. A high level of genetic similarity has been found among North American cougar populations, suggesting they are all fairly recent descendants of a small ancestral group. Culver et al. propose the original North American cougar population was extirpated during the Pleistocene extinctions some 10,000 years ago, when other large mammals, such as Smilodon, also disappeared. North America was then repopulated by South American cougars.