Looming over the northern edge of the Amazon rain forest are some of the most remarkable mountains on earth. Known as tepuis,or tabletop mountains,they are typically ringed by sheer cliffs that rise thousands of feet from the surrounding lowland jungles. Instead of peaks,tepuis have enormous flat expanses at their tops. To reach the tops of many tepuis,the only choices are scaling the cliffs or flying in a helicopter.
For all their isolation,the tops of tepuis are not barren. They are like islands in the sky,covered with low forests and shrublands that support a diversity of animals likes frogs and lizards. Many of the species that live on top of the tepuis are found nowhere else on the planet.
In a paper to be published in the journal Evolution ,a team of scientists report the first DNA-based study to address an age-old question about the tepuis: How did animals and plants end up in such an inaccessible place?
The researchers focused on tiny tree frogs that live only atop the tepuis. They came to the surprising conclusion: Over the past few million years,the frogs climbed up the colossal cliffs.
We may see those walls as being impossible for something to climb because just theyre so hard for us, said the lead author,Patricia Salerno,an evolutionary biologist at the University of Texas. But it is possible for these frogs to get up there. Theyve gotten up there several times.
Some intrepid scientists scaled the tepuis to learn how they formed. Their history is very different from that of the nearby Andes,which formed like many mountain ranges do,as two continental plates collided 25 million years ago and thrust rock layers upward. The tepuis are far older. The highest layers of rock were formed by sand settling on the bottom of oceans more than two billion years ago. The sandstone was later lifted and became dry land. It began to erode and fragment about 300 million years ago,and by about 70 million years ago,the tepuis towered above the surrounding lowlands.
Even more intriguing than the tepuis long isolation is that many species living atop them are found nowhere else. To many biologists,the only explanation that made sense was that the ancestors of those unique animals and plant species have lived on the tepuis for more than 70 million years. In honour of Arthur Conan Doyle,author of The Lost World,they called this notion the lost world hypothesis.
Salerno realised that genetics offered a straightforward way to test this hypothesis. Over time,species accumulate new mutations in their DNA at a roughly regular rate. When two species diverge from a common ancestor,their DNA becomes steadily more different. Biologists can use this so-called molecular clock to estimate how long ago their common ancestor lived. Salerno decided to compare species from different tepuis to see how different their DNA had become. If the species had been on their tepuis ever since they formed,she reasoned,their common ancestor would have to be at least 70 million years old. Tree frogs would be a good group of species to study,she decided,because they move around very little during their lives. One species of tree frog found on the tepuis hides out inside one species of carnivorous plant during the day,for example,coming out only at night to hunt for insects. Its therefore not surprising that 90 per cent of frog species that live on the summits of tepuis are found nowhere else.
Working with US and South American collaborators,Salerno has collected frog DNA from museum collections and trips to tepuis. For their study,the researchers compared the DNA from four closely related species of tree frogs,each found on a different tepui. They also analysed the DNA of their closest lowland relatives.
Salerno and her colleagues found that the frogs evolved long after the dinosaurs. The common ancestor of all four tepui frog species lived about 5.3 million years ago,not 70 million years ago. Some of the species diverged only a few hundred thousand years ago. This is the first effort to get a feeling for the evolutionary aspects of this thing,and in that sense its a step forward, said Roy McDiarmid,a herpetologist at the Smithsonian. He finds it plausible that the tree frogs scaled the tepuis only in the past few million years. But he doesnt think a study on one group of frogs rules out the lost world hypothesis for other species.
There may not be much time left to gather data. Global warming has been driving mountain-dwelling species to higher altitudes and will drive them up even farther. But the frogs and other species that are found only on top of tepuis have already gone as high as they can go. Theyre just going to go extinct, Salerno said.