Did monkeys really sail the oceans on floating rafts of vegetation?

The mystery of how some species colonised new continents is as old as the theory of evolution itself. Now, with fresh clues surfacing, the rafting hypothesis might finally sink or swim


15 December 2021

New Scientist Default Image

Brett Ryder

IN DECEMBER 2016, Uwe Fritz at the Museum of Zoology in Dresden, Germany, was doing fieldwork in Colombia when something incredible crossed his path. While chugging across a vast expanse of wetland, he passed an enormous floating island complete with tall trees and a resident colony of howler monkeys. “Have you ever seen a howler monkey?” says Fritz. “They’re huge! But the trees were large enough so the monkeys can permanently live in them. They do not swim.” All told, the island covered an area about the size of two Olympic swimming pools.

Fritz later told a collaborator, Jason Ali at the University of Hong Kong. Ali’s jaw hit the floor. “For me, it was just a random observation,” says Fritz. “But he is the floating island guy. He has worked on them for years, but never seen one.”

Ali is one of the leading advocates of one of the most controversial ideas in evolutionary biology: that the presence of certain species in certain places can only be explained by long-distance maritime voyages. The hypothesis, essentially, is that animals were carried across the ocean on rafts of vegetation and started afresh on the other side.

The sheer unlikeliness – some would say preposterousness – of this idea has always been an obstacle to its acceptance, and the arguments for and against the rafting hypothesis have sloshed back and forth for 160-odd years. But now, with floating islands in Colombia and fresh clues from the sea floor, both sides are claiming to have evidence that could finally see the idea sink or swim.

The rafting hypothesis is as old as the …