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 exactly what you would expect if they
arrived by chance. 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 In unpublished work, he has estimated that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 2019 paper. “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 waterNIC
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Graham Lawton
dreams of escaping
on a floating islandon 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 2020 paper by Mazza,
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. Rafts have also been seen (and
filmed) on the Chagres river in Panama, 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. ❚The ancestors of
lemurs somehow
crossed from
mainland Africa
to Madagascar“ Obviously it’s
improbable,
but you only
need one
crossing
event”
52 | New Scientist | 18/25 December 2021