Colonization of these remote rocky outcrops was made particularly difficult by the fact that the nearest large landmass is over 600 miles away, which made the crossing of this barrier virtually impossible for many groups of plants and animals.
Reptiles are greatly over-represented here because of certain properties that made them particularly suitable to surviving a long oceanic crossing, such as thick scaly skin with no sweat glands and an enormous resistance to hunger and thirst. Approximately fifteen million years ago, a spiny-tailed iguana fell into the water somewhere in Central America, clung on to a vegetation raft for dear life, and was eventually carried all the way out to the Galapagos. This one arrival of what was probably an egg-carrying female eventually gave rise to three completely new species, endemic to this archipelago, by a process called adaptive radiation. Two of these were land animals; the third became the only sea-going lizard in the world.
During low tide, such as we were lucky enough to have for our walk on Santiago Island, these unique animals -- marine iguanas -- entered the intertidal area to graze off several species of algae, their food source.
