 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.The closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Chaco tortoise ("Chelonoidis chilensis"), a much smaller species from South America.