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.