 All subspecies of Galápagos tortoises evolved from common ancestors that arrived from mainland South America by overwater dispersal . Genetic studies have shown that the Chaco tortoise of Argentina and Paraguay is their closest living relative . The minimal founding population was a pregnant female or a breeding pair . Survival on the 1000 @-@ km oceanic journey is accounted for because the tortoises are buoyant , can breathe by extending their necks above the water , and are able to survive months without food or fresh water . As they are poor swimmers , the journey was probably a passive one facilitated by the Humboldt Current , which diverts westwards towards the Galápagos Islands from the mainland . The ancestors of the genus Chelonoidis are believed to have similarly dispersed from Africa to South America during the Oligocene . 
