Office in Washington DC received reports of
a stand of trees floating in the Atlantic Ocean.
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 arrived in dribs and drabs long after the
islands had drifted away from the mainland.
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 Paul
Mazza at the University of Florence in Italy.
We have a clear idea of how this “sweepstake
colonisation” might happen, says Judith
Masters at the University of Fort Hare in South
Africa. 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.
One leading proponent of this scenario is >
18/25 December 2021 | New Scientist | 51