Deep in the Australian outback, several thousand palm trees grow in a 60-square-kilometer region known as Palm Valley. Local lore says the plants are a remnant of the continent's rainier past, the only palm species to survive in the interior when Australia began drying out 15 million years ago. However, tour guides may have to change their tale; a new genetic analysis finds that these red cabbage palms (Livistona mariae) arrived only 15,000 years ago, possibly among the foods carried by indigenous people as they settled the area.
The red cabbage palm was thought to be an ancient remnant mostly because "it didn't seem reasonable that the palms got there any other way," says David Bowman, a biologist at the University of Tasmania in Australia. The red cabbage palm's closest relative, the Mataranka palm (L. rigida), grows in two areas 800 to 1000 kilometers to the north on either side of the Gulf of Carpentaria—too far away, it would seem, for these species to be anything but distant relations.
But a 2010 study led by Australian biologists, including Bowman, and colleagues at Kyoto University in Japan found that L. mariae was genetically identical to L. rigida. Determining just when L. mariae diverged from its relative, however, would require a more detailed genetic analysis. So the Kyoto researchers developed a set of microsatellite markers, short bits of repeated sequences of DNA that tend to evolve very quickly. Slight changes in the sequences can indicate mixing and movement within a population.
For the new study, which is published today in the Proceedings of the Royal Society B, Bowman and his colleagues collected samples from 38 L. mariae palms in Palm Valley and 100 L. rigida palms in the Roper, Nicholson, and Gregory rivers and compared the microsatellite markers. They found that the red cabbage palm and Mataranka palm were so closely related as to be the same species and that the Palm Valley palms were more closely related to the Roper River population. The researchers then calculated the amount of time that had passed since the populations separated, finding that L. mariae had diverged from the Roper River population of L. rigida about 15,000 to 30,000 years ago.
The question of how the palms got to the continent's interior remains, especially because no waterway connects the Roper River to Palm Valley. Fruit-eating birds or bats can distribute the palm's seeds, but the researchers thought it unlikely that the animals could have traveled 1000 kilometers without defecating out the seeds—and sowing palms—along the way.
But a coincidence in timing led the researchers to another hypothesis. The ancestors of indigenous Australians arrived on the continent some 40,000 to 45,000 years ago and intermittently migrated into the interior 20,000 to 30,000 years ago. Livistona species are among the few edible and cultivable plants on the continent—people in Australia today sometimes eat immature palm fronds, called "cabbages." The estimate for the establishment of the Palm Valley L. mariae population "is well within the time window for archaeological sites from indigenous Australians," Bowman says. He and his colleagues speculate that humans brought the seeds with them as they moved into the area.
"The evidence for recent genetic connection among the three populations, across this vast distance, is solid and convincing," says Craig Moritz, a University of California, Berkeley, biologist not affiliated with the study. Pinning down the history is more difficult, though, and Moritz says he's not yet convinced.
Kevin Thiele, a curator at the Western Australian Herbarium in Perth, also cautions that it's probably too early to credit indigenous Australians for transporting the seeds. "Why is it that L. mariae doesn't occur at other suitable sites where there's enough water? People tend to be much more consistent in their behavior than a fruit bat."
However the red cabbage palm arrived in Palm Valley, it appears that the local tale of it being an ancient remnant of a wetter past will have to change. That doesn't necessarily diminish Palm Valley's draw as a tourist attraction, though. "I think we've added to the mystery of the place," Bowman says.