They were formerly assigned to Geochelone, but a recent comparative genetic analysis has indicated that they are actually most closely related to African hingeback tortoises. Their ancestors apparently floated across the Atlantic in the Oligocene. This crossing was made possible by their ability to float with their heads up and to survive up to six months without food or water.
The members of this genus on the Galápagos Islands are among the largest extant terrestrial chelonians. Giant members of the genus were also present in mainland South America during the Pleistocene.