New geological findings suggest that volcanoes existed over the Galápagos hotspot long before today's islands emerged less than 5 million years ago. The evolution of some of Galápagos' biota might have taken place on these former islands. This study investigates the evolutionary history of two of the archipelagos' older vertebrate taxa, the endemic Galápagos marine and land iguana (genera Amblyrhynchus and Conolophus).

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.

Nobody is absolutely certain just how the tortoise arrived or how it spread to most of the islands in the group. Perhaps we will never know.

However, it does seem quite likely that the tortoise arrived by sea, floating on a natural raft of driftwood. We know that the tortoise can survive for long periods without food or water, so the theory is a reasonable one.

The tortoise played an important role in the Theory of Evolution. When Charles Darwin visited the Galapagos Islands, the vice-governor of the Islands told him that he could identify what island the tortoise was from simply by looking at him.

Naturalists believe tortoises arrived in the islands clinging to a piece of driftwood from a river mouth along the Pacific Coast. A relatively large tortoise, related to the Galapagos tortoise lives on the South American mainland. The tortoises arrived in San Cristobal then spread throughout the archipelago. Those on individual islands or in isolated parts of the larger islands developed into its own sub-species.