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MILLIONS of years before the Galapagos Islands existed, there was another archipelago in the same stretch of water off the west coast of South America. And it seems those long-vanished lands probably shaped the evolution of some of the unusual Galapagos wildlife that later inspired Charles Darwin鈥檚 theory of evolution.

Today鈥檚 archipelago probably owes its existence to a nearby geological phenomenon, a plume of unusually hot rock rising from deep in Earth鈥檚 interior. When the plume meets the crust beneath the Pacific Ocean, it triggers intense volcanic activity, which forms underwater mountains that can grow tall enough to rise above sea level and become islands.

The parts of the Galapagos that lie near the plume today are about 3 million years old. But geologists think the plume is much older, and has been forming volcanic islands for much of the past 20 million years. These have since been dragged to the east and northeast of the Galapagos by drifting tectonic plates. During this process, almost all of the island-volcanoes became inactive and sank below the water, where their remains can be found.

For the first time, researchers have worked out how big these lost lands were. Felipe Orellana-Rovirosa and Mark Richards at the University of California, Berkeley, used data on the rate at which they sink to work out how much land was above the waves at various points over the past 20 million years. They say that there is likely to have been a Galapagos-like archipelago off the west coast of South America for most of this time.

Their work suggests that 16.5 million years ago, the land area of the archipelago was 22,500 square kilometres, more than twice that of the current chain. Some of the lost islands had peaks that may have risen as much as 500 metres above sea level. The findings were presented at a conference of the American Geophysical Union in Washington DC.

This tallies with earlier work. Kenneth Petren at the University of Cincinnati in Ohio and his colleagues previously highlighted that some Galapagos species, including beetles and marine iguanas, seem to have evolved in isolation for millions of years before the formation of the current islands. Orellana-Rovirosa and Richards say these species must have lived on the earlier islands, migrating to the modern archipelago when it formed.

Given that the earlier archipelago was substantial, and its potential role as an ancestral home for some of the wildlife now living in the Galapagos, Richards and Orellana-Rovirosa suggest naming it Darwinia, to honour Darwin, whose time in the current islands informed his theory of evolution by natural selection.

But Peter Grant of Princeton University, who studies the unique Galapagos finches, suggests a different approach to naming the lost lands. 鈥淒arwin never set foot on them, so [Darwinia] wouldn鈥檛 be my choice,鈥� he says.

Instead, he suggests taking inspiration from the animal that is most likely to have occupied the ancient chain 鈥� the marine iguana. 鈥淏y this logic, I would suggest Iguania is more apt.鈥�

This article appeared in print under the headline 鈥淭he lost islands of the Galapagos鈥�