The First Great Ocean Voyagers? Hint: It Wasn’t the Polynesians
Posted by Richard Conniff on February 25, 2014
An old theory held that one of the earliest domesticated crops—bottle gourds, widely used for drinking vessels, food storage containers, musical instruments, and even medicine—came to the New World from Asia, either drifting across the Pacific or being carried by humans via the Bering Land Bridge. But new evidence argues that gourds made the ocean voyage on their own, not from Asia but from the coast of West Africa.
The new study, published in the journal Proceedings of the National Academy of Sciences, is interesting partly for what it says about the hazards of putting too much faith in narrow genetic analysis. It also fits into a much larger scientific movement arguing that plants and animals—including amphibians, monkeys, and perhaps even flightless birds—didn’t need continental drift, land bridges, or human migrations to reach their current locations. They did it, instead, more or less on their own, through a series of chance long-distance ocean voyages.
For the study, Logan Kistler of Pennsylvania State University and his coauthors looked at 36 modern samples of bottle gourds and nine ancient ones from archaeological sites around the Americas. Their results contradict a 2005 study—published in the same journal and including one of the same coauthors—that tracked the original gourds back to Asia.
Why the difference? The earlier study examined only three narrow sites on the gourd genome. Studying a larger sample of the genome revealed that “Africa is the clear source region of the bottle gourds that populated the Americas,” the study concludes, adding that the results “highlight the risk of basing conclusions on very small genetic datasets.”
Using computer models of prevailing currents, Kistler and his coauthors also calculated that sea-going gourds could have hitchhiked from West Africa to Brazil in as little as 100 days, though the average crossing would have lasted nine months. Either number fits existing evidence that bottle gourd seeds can remain viable after almost a year afloat in seawater.
According to Kistler, the first bottle gourds in the New World show up in the archaeological record about 10,000 years ago, in Florida and Mexico. Their seeds have also turned up in mastodon dung at a site in northern Florida, suggesting the critical role of now-extinct large mammals in spreading gourds around the New World.
The work on gourds fits a shift among evolutionary thinkers away from the old idea that plants and animals arrived at their present locations as passengers, carried along in comfort as shifting tectonic plates pushed continent-size land masses across the face of the planet. That idea became conventional wisdom in the 1960s and 1970s, when the scientific community finally accepted the theory of continental drift. But science is constantly overturning its own orthodoxies, and the development of increasingly precise molecular clocks—timelines based on changes in DNA—has forced scientists to seek other explanations for long-distance migrations.
For instance, monkeys also originally came to South America from Africa. But we now know it couldn’t have happened when the two continents pulled apart 100 million years ago. Molecular clock studies say Old and New World monkeys diverged from a common ancestor less than 50 million years ago, and monkeys don’t even show up in the New World fossil record until 26 million years ago.
In his new book The Monkey’s Voyage: How Improbable Voyages Shaped the History of Life, evolutionary biologist Alan de Queiroz makes the case for an alternative explanation: Even now, large land rafts periodically calve off from river banks and drift out to sea. A few hapless monkeys and other species trapped aboard could have been carried along on favorable currents—and 40 million years ago, the Atlantic Ocean may have been just 900 miles across at its narrowest point. With the most favorable winds, the voyage for the first pioneering monkeys in the New World could have taken as little as a week.
Other studies suggest that lizards and amphibians can also sometimes survive long-distance voyages. As recently as 1995, for instance, a hurricane carried green iguanas from Guadeloupe 175 miles across the Caribbean to Anguilla, which they promptly colonized. Even the massive flightless birds known as moas, now extinct, may once have been ocean travelers. The idea that their ancestors may have flown to New Zealand recently became more plausible when the tinamou, a large-bodied South American bird that can fly, turned out to be a member, along with the moa, in the otherwise flightless ratite evolutionary group.
All this suggests several useful conclusions: Science is an endless search for the truth but always subject to modification as better facts come along. Improbable chance plays a greater role in the history of life than we generally like to admit. Species of all kinds—but especially those first pioneering monkeys staring out desperately across an endless blue horizon—are far more enduring and heroic than we can even imagine.