IN DECEMBER 2016, , Germany, was doing fieldwork in Colombia when something incredible crossed his path. While chugging across a vast expanse of wetland, he passed an . “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.”
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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 theory 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 Hydrographic Office in Washington DC received . Described as “a piece of forest covering ¼ acre, topmost branches reaching at least 30 feet above sea level”, it was sighted again later the same year, 1850 kilometres further north-east. What became of it after that isn’t known.
Then along came plate tectonics, which holed the rafting hypothesis below the waterline. If continents moved around, and sea levels fluctuated to expose transient land bridges, there was no need to invoke implausible oceanic voyages. Animals simply walked to their distant homes and were then cut off.
But the rafting hypothesis didn’t sink entirely. It began to resurface in the 1990s when new molecular dating techniques revealed that the reptiles and amphibians of the Greater Antilles – a group including the islands of Cuba, Hispaniola, Puerto Rico and Jamaica – had .
Favourable currents
Biogeographers then took a second look at other animals in far-flung places and surmised that they, too, had some questions to answer. Again, Madagascar loomed large. The new techniques showed that the ancestors of hippos, tenrecs, lemurs and fossas all colonised it long after it rifted from Africa 157 million years ago and that they all arrived at different times. There were no known land bridges. This has helped to reinstate long-distance over-water dispersal as the dominant theory, says .
We have a clear idea of how this “sweepstake colonisation” might happen, says . Very occasionally, animals living in or near rivers get stranded on rafts of soil and vegetation ripped from the riverbank by flash floods. The rafts can be huge, and stocked with enough food and water for even quite large animals to survive for weeks. Some wash out to sea and get carried away on currents. The vast majority drift aimlessly and break up, and their passengers end up starving, dying of thirst or exposure, drowning or getting eaten by sharks. But very occasionally, a raft will catch a favourable current and make landfall on some distant shore. If enough members of a single species make it – especially if their number includes a pregnant female – then they may establish a new colony. Each of these possibilities is remote, but over geological time they stack up to become quite probable.
“Obviously it’s improbable, but you only need one crossing event”
One leading proponent of this scenario is Ali. He points out that the flora and fauna of distant islands generally consist of a fairly random selection of descendants of those found on the nearest bit of continent, which is . He also points out that the colonists tend to be small, hardy creatures such as lizards, shrews and, at a push, primates, which are better adapted to surviving a long voyage. There is a reason nobody ever claimed that lions or giraffes crossed the ocean on a mat of vegetation, he says.
But Ali and his fellow travellers do claim something that seems to stretch credibility: that 35 million years ago, monkeys rafted across the Atlantic Ocean. We know that the ancestors of the New World monkeys evolved in Africa around 40 million years ago, but then they suddenly pop up in South America 5 million years later – a seemingly instantaneous teleportation, in geological terms, across the then 1500 kilometres of ocean.
“Obviously it’s very improbable, but you only need one crossing event,” says Ali. There is good evidence that rodents also made the trip separately, he adds, and in any case it is hard to see any other way monkeys could have got there. “We see what we see. It is what exists,” says Ali. In unpublished work, he has estimated that with a decent current, a raft could have crossed the Atlantic in about 14 weeks – just about long enough for a small primate to survive.
This doesn’t float everyone’s boat. “It’s a crazy idea,” says Mazza. “How can animals like monkeys cross an ocean without food and water, with exposure to salt, with overheating? It’s virtually impossible. It’s the same as saying that Martians transported them.”
The sheer implausibility also renders the geological time argument bogus, he argued in a . “We’re told that what is impossible is made possible by millions of years,” says Mazza. “This is like saying that if we throw ourselves from windows for millions of years, maybe somebody can fly. Piling up impossibilities and figuring that we end up with a possibility statistically makes no sense.” Invoking rafting is grasping at straws, he says.
That isn’t to say sweepstake colonisation never happens. Invertebrates, plant seeds and small vertebrates have more of a shot as they can probably survive for longer on smaller flotsam. After the 2011 Tohoku earthquake and tsunami in Japan, around 300 Japanese marine species were found on the shoreline of British Columbia having been carried on artificial debris. Some larger vertebrates, such as tortoises, crocodilians and possibly even hippos, may be able to float or swim.
Of course, sceptics can’t just pour cold water on the idea. The presence of particular animals in certain faraway places still requires an explanation, so what have they got? “There must be some other more logical and more reasonable explanation,” says Mazza. “Maybe we don’t know enough about the geology of the sea floor.”
For Madagascar at least, that could be the answer. According to a , Masters and others, previously unknown land bridges between Madagascar and the African mainland rose and fell three times in the past 65 million years. The timing coincides with those difficult-to-explain colonisations.
A related possibility is stepping-stone colonisation, where animals make short hops along an island chain. If tectonics then erases the intermediate islands, this creates the illusion of a long-range oceanic dispersal, says Mazza. Another option, at least for more recent animal crossings, is that small creatures such as lizards were accidentally or deliberately transported by prehistoric humans. We know that our Stone Age ancestors were skilled seafarers, navigating across hundreds of kilometres of ocean to reach Japan 35,000 years ago and perhaps even sailing from South-East Asia to Australia 65,000 years ago.
What about the New World monkeys? Mazza says people are investigating that.
What would really help to settle the matter is a direct, unambiguous observation of a raft out to sea, preferably with animals on it. Floating islands are commonplace in the inland swamps of Colombia, says Fritz, and during the rainy season they are flushed into the Magdalena river, which flows into the Caribbean. Nobody has ever seen one go the distance, but Fritz says he believes it is possible. The one he saw looked like it would have been seaworthy for months.
To that end, Ali says he wants to tag some floating islands with GPS trackers to see where they end up. But with rivers around the world increasingly dammed and otherwise clogged up, the possibility of a raft ever making it out seems remote. There are two dams across the Magdalena. , but it now drains into the Panama Canal and there is no chance of a raft successfully reaching the sea. So if those cheeky monkeys are planning another epic crossing, they may have to wait a few more million years.
