The felids evolved in Asia and spread across continents by crossing land bridges. DNA sequence analysis shows that ancient cats evolved into eight main lines. There were at least 10 migrations (in both directions) from continent to continent via the Bering land bridge and Isthmus of Panama. The "Panthera" genus is the oldest and the "Felis" genus is the youngest. 60 percent of the modern species of cats developed within the last million years.