The early human colonisation of islands might not have been plain sailing. Instead of using boats to deliberately settle on Indonesian islands, hominins may have arrived as castaways, carried on floating debris after floods.

David Wilkinson of Liverpool John Moores University and Graeme Ruxton of the University of St Andrews, both in the UK, used population estimates from the early settlement of Polynesia to model the likely success of island settlement attempts in human prehistory.

They found that five young couples had a 40 per cent chance of giving rise to a population of 500 or founding a population that survived for 500 years. Ten random castaways had only a 20 per cent chance of similar success. But throwing in between one and four additional castaways every 50 years raised the chances of an accidental settlement succeeding to 47 per cent.

Stone tools show that hominins possibly Homo erectus reached Flores 1 million years ago. The famous Homo floresiensis may have descended from this population of H. erectus. Meanwhile, a study earlier in the year concluded that Neanderthals had access to boats 100,000 years ago, which they used to reach the Greek islands of Lefkada, Kefallonia and Zakynthos, where their stone tools have been found.

The new finding suggests that in both cases the hominins could have reached the islands without boats. We already know that other mammals managed the feat: rats and Stegodon, an extinct relative of the elephant, crossed the deep-water channel between the Indonesian islands of Java and Flores. Elephants are strong swimmers, and rats could have travelled on storm debris.

Accidental colonisation of Flores by hominins would have been difficult, but not impossible, says Mike Morwood of the University of Wollongong in New South Wales, Australia. However, he adds, "the rapid colonisation of Greater Australia and major islands of western Melanesia 45,000 to 50,000 years ago indicates deliberate colonisation voyages by people using directed craft".

Journal of Human Evolution, DOI: 10.1016/j.jhevol.2012.05.013

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Hominins did not need boats to settle islands