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The Galapagos Islands are one of the few regions of the world where the predominant land animals are not mammals, but reptiles. This is thought to be a result of their isolation and the difficulty for animals of reaching them. It seems that the Galapagos have always been as isolated as they today so making the journey to them for animals long and difficult, prevailing oceanic currents are also only favourable from the South American mainland for a part of the year.

Reptiles with their greater capacity for surviving without food or water are better equipped to make this journey than mammals and the first arrivals on the Galapagos are thought to have drifted probably from South America on driftwood, or small "islands" of matted vegetation and branches that are sometimes seen being swept down rivers in flood.

When the first tortoises arrived on the Galapagos Islands, they were unchallenged by mammals and so evolved to better take over the niche of large herbivore that is taken elsewhere by sheep, goats, deer or the like. There are also a number of advantages to being large which in the absence of any natural predators on the adults, the Galapagos Tortoises could exploit in these unusual conditions unlike other tortoises almost anywhere else in the world.

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