Not just drifting by

Whether ancient humans made sea crossings by choice or by chance is a common topic of debate among historians.

In the case of the well-documented migration of people from what is now Taiwan to the Ryukyu Islands of southwestern Japan some 30,000 years ago, choice seems increasingly likely because new research suggests chance wasn’t.

And that’s despite the fact that the travellers initially would not have been able to see where they would eventually land, and the voyage would have taken them across the Kuroshio Current, one of the strongest in the world.

“Our study looks specifically at the migration to the Ryukyu Islands, because it is not just historically significant, but is also very difficult to get there,” says Yosuke Kaifu from Japan’s University of Tokyo. “The destination can be seen from the top of a coastal mountain in Taiwan, but not from the coast.”

201204 drifting buoy
Credit: Lagrangian Drifter Laboratory/University of California, San Diego

To investigate the likelihood of the journey occurring by chance, the effect of the Kuroshio on drifting craft needed to be measured.

To do this, Kaifu and his team studied the trajectories of 138 satellite-tracked buoys, which drifted past Taiwan or northeast Luzon in the Philippines between 1989 and 2017.

Of the 122 that past Taiwan, 114 were carried northward by the Kuroshio; the only three that came anywhere neart the Ryukyu Islands did so under adverse weather conditions. Of the 16 that past Luzon, 13 drifted with the Kuroshio but only one moved towards the islands due to a typhoon.

As geological records suggest the flow of the Kuroshio has remained unchanged for the past 100,000 years, the study indicates, the researchers say, that humans in drifting boats were unlikely to reach the islands via accidental drift.

“The results were clearer than I would have expected,” says Kaifu. “Only four of the buoys came within 20 kilometres of any of the Ryukyu Islands, and all of these were due to adverse weather conditions.

“If you were an ancient mariner, it’s very unlikely you would have set out on any kind of journey with such a storm on the horizon.”

The Kuroshio directs drifters away from, rather than towards, the Ryukyu Islands, Kaifu says, so that region “must have been actively navigated”.

“If they crossed this sea deliberately, it must have been a bold act of exploration.”

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