Discover, January 31, 1993
The Galápagos Islands changed the way we look at the world. After Charles Darwin visited this archipelago off the coast of Ecuador in 1835, he noted how the different islands kept groups of closely related animals apart from one another, allowing them to develop into separate species. Those ideas would eventually become part of our shared intellectual heritage with the publication, in 1859, of the Origin of Species.
In this century geologists learned that the Galápagos are no more than 3 million years old, a finding that made the evolutionary pace of these animals remarkably fast. Then, in the 1970s, biochemists discovered this pace was truly astounding: some of the Galápagos animals, they found, have been evolving for at least 10 million years–that’s 7 million years of evolution that took place on islands that didn’t exist! Not until this past year did geologists finally figure out how animals could be older than the land beneath their feat: there were older Galápagos Islands, but they were drowned.
Darwin, in his day, had no way of measuring the rate of evolution directly. In the 1970s, however, biochemists Vincent Sarich and the late Allan Wilson of the University of California at Berkeley discovered that every organism, in every one of its cells, contains an evolutionary clock: the steady rate of mutation of proteins and DNA over the course of millions of years. Measure the difference in these molecules from two related organisms, they said, and you can determine how long ago the organisms’ last common ancestor lived. In one test of this technique, Sarich and Jeffrey Wyles, also of Berkeley, measured blood proteins in two different kinds of Galápagos iguana, one of which lives on land while the other swims. All Galápagos animals descend from South American colonizers, but there is no sign of two stocks of iguanas on the mainland. Therefore the iguanas probably evolved from a common ancestor that rafted over to the islands. But according to the proteins, Wyles and Sarich discovered, that ancestral iguana lived at least 10 million years ago, long before the islands were there.
The answer to the puzzle lies in geology. Island chains can form when a hot plume of molten rock rises through Earth’s mantle. When it hits the outer crust, it forms a volcano, which eventually builds an island. But Earth’s crust doesn’t sit still. It is divided into moving plates, and as one of these plates drifts over the hot plume, new islands are formed one by one, in a chain. Gradually, as the islands age, they begin to erode, and the crust below them sages; ultimately they disappear underwater.
A team led by Oregon State University geologist David Christie believed that this was happening at the Galápagos Islands, which sit on a plate moving toward South America. The researchers took sonar measurements of the seafloor to the east of the islands and found undersea mountains with suspiciously beachlike slopes. Dropping a probe to their surfaces, the geologists collected smooth, rounded pebbles. Such pebbles could have been formed only in the surf on a beach, and so the team concluded that the mounts were indeed once islands. Geologic dating techniques revealed that the rocks–and the islands they came from–were up to 9 million years old, and Christie has reason to believe there are older ones farther east.
Now it’s clear that ancestral iguanas from South American could have come to the islands about 10 million years ago. They made their home there, and their descendants branched into different species. When the older islands vanished, the animals made the short swim over to a new island, and during all this time their molecular clocks kept ticking.
The discovery reminds biologists to take into consideration the changing Earth when they study the evolution of life on islands. “Some biologists used to talk about an ‘Atlantis hypothesis’ as if it were a bit too imaginative to be real,” says Christie, “but it’s really not that unusual.”
Copyright 1993 Discover Magazine. Reprinted with permission.