On a raft
and a prayer
Fresh clues could finally tell us if monkeys and
other animals sailed across oceans on floating
islands of vegetation, finds Graham Lawton
I
N 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.
BR The rafting hypothesis is as old as the theory
ET
T^ R
YD
ER
of evolution itself. In On the Origin of Species,
Charles Darwin pointed out that the flora and
fauna of the Galapagos Islands were clearly
related to those of South America, while Cape
Verde’s were distinctly African (Darwin visited
Cape Verde’s main island Santiago in January
1832). His point was to discredit the belief that
each species was a unique, divine creation, but
he inadvertently launched the idea that the
inhabitants of distant islands must have
somehow blown in from the mainland.
Improbable as they seemed, epic
oceanic voyages by animals clinging to
logs or vegetation seemed the only possible
explanation for the presence of certain
species in certain places. At that time,
scientists thought that the continents’
positions were fixed, so alternative
possibilities didn’t readily present themselves.
As well as the Galapagos Islands and Cape
Verde, there was the mystery of Madagascar.
Despite being separated from Africa by the
deep, fast-flowing Mozambique Channel,
which is some 400 kilometres wide at
its narrowest point, the island’s fauna
nevertheless have a clear African history.
On top of now-extinct Malagasy pygmy
hippos, these include three groups of living
mammals with African ancestry: lemurs,
tenrecs and mongoose-like carnivores such as
fossas. Unlikely as it sounded, they must have
crossed the channel.
Occasional anecdotal observations of ocean-
going floating islands lent some credibility
to the hypothesis. One of the most detailed
comes from 1892, when the US Hydrographic50 | New Scientist | 18/25 December 2021