July 9, 2012
The 2003 discovery of the diminutive Homo floresiensis, better known as the Hobbit, on the Indonesian island of Flores was a shock. Anthropologists never expected to find a 3-foot, 6-inch-tall hominid living in Southeast Asia at the same time as modern humans, as recently as 17,000 years ago. Aside from the controversy over the hominid’s true identity—a diseased Homo sapiens or a member of its own species—another intriguing question was how the ancestors of the Hobbits got to Flores.
One possibility is that the Hobbits’ forefathers sailed over on a raft. Or their arrival might have been an act of nature: A powerful storm or tsunami could have washed a small group of hominids out to sea, and then floating vegetation carried them to Flores. That idea sounds implausible, but it’s also an explanation for how monkeys reached South America.
Scientists will probably never know for certain what the Hobbit’s ancestors went through to get to Flores. Such ancient wooden boats are unlikely to be preserved and there’s no way to prove it was a freak accident.
But recently a pair of researchers offered a novel way of assessing the issue. Ecologist Graeme Ruxton of the University of St. Andrews in Scotland and biologist David Wilkinson of Liverpool John Moores University in England simulated population growth over time of planned colonizations versus accidental castaways to see which scenario could lead to successful inhabitations of an island. They reported their results in the Journal of Human Evolution.
The premise of the model is that a group of hominids reach an island. The hominids mate monogamously and each year there is a set probability that a female of a certain age will give birth. There’s also a given probability that individuals in the population will die, based on age and sex.
For the scenario of a planned trip at sea aboard a raft, Ruxton and Wilkinson assumed colonists were sailing as groups of families. So the founding populations in this model had an equal number of adult males and adult females. Ruxton and Wilkinson ran their simulations using different group sizes for a founding population. After running each scenario a thousand different times, they concluded such populations could be successful—defined as lasting 500 years or reaching 500 individuals. The likelihood of success increased with founding population group size, reaching a success plateau at groups of just 20.
To simulate an accidental island arrival due to a storm or tsunami, the pair changed one of their starting assumptions. Instead having an equal number of adult males and adult females at the onset, they assumed the sex ratio was random. No one plans to be washed out to sea, after all. Under this scenario, colonizations were 50 percent less likely to succeed compared to the planned trips aboard a boat. But with slight modifications, that number went up. By adding a 2 percent chance that one to four additional castaways might reach the island each year for the first 400 years, Ruxton and Wilkinson found that unintentional colonizations were as likely to succeed as planned ones. These newcomers increased a stranded population’s chance of long-term viability by introducing new genes to the island and/or balancing out skewed sex or age ratios.
Although the chance of different storms washing different groups of hominids to the same island sounds as likely as lightning striking twice, it may not be that far-fetched. Ruxton and Wilkinson point out ocean currents and wind patterns can lead floating objects to the same place over and over again.
So what does all of this computation really tell us? On the one hand, the models are only as useful as the assumptions Ruxton and Wilkinson used to build them. If the hominids didn’t mate monogamously, for example, then the pair’s conclusions may not be valid. But putting such concerns aside, the results indicate that both rafting and accidental ocean dispersals are possible explanations for the Hobbits’ inhabitation of Flores. Therefore, the researchers warn, a hominid’s presence on an island isn’t necessarily evidence of some kind of sailing technology.
Today, humans live on tens of thousands of islands—even if they didn’t necessarily mean to.
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