A new climate record suggests that Homo floresiensis — pint-size human relatives nicknamed “hobbits” — endured thousands of years of intensifying drought before disappearing from their Indonesian island home of Flores. The prolonged drying may have stressed both the hobbits and the miniature elephant-like animals they hunted for food, researchers report December 8 in Communications Earth & Environment.
“This is the first good, quality climate record [for Flores],” says Nick Scroxton, a paleoclimatologist at Maynooth University in Ireland. While multiple factors may have driven the hobbits to extinction, he says, “the climate almost certainly played a big role.”
Nicknamed after the diminutive people of J.R.R. Tolkien’s fantasy novels, the hobbits of Flores stood barely a meter tall. Skeletal remains and stone tools from Liang Bua cave in the island’s highlands indicate they lived there as recently as 50,000 years ago.
But after that, there are no further signs of them.
To learn why, Scroxton and colleagues reconstructed Flores’ rainfall history using a stalagmite in another cave. Because stalagmites grow from water trickling through cave ceilings, their chemistry preserves past climates.
The researchers looked at two of those climate records. First, shifts in magnesium and calcium levels reflect mean annual rainfall. Since calcium precipitates out of water before magnesium, rainfall reaching the cave during dry periods loses much of its calcium. Second, oxygen isotopes — variants with different atomic weights — reflect summer rainfall. Oxygen-18 tends to rain out of clouds first. During the summer monsoon season, this happens over the sea, resulting in rain on the island that contains little of this isotope.
The stalagmite record shows that mean annual rainfall decreased from about 76,000 to 61,000 years ago. Summers then got especially parched between 61,000 and 55,000 years ago, with the area receiving just about 450 millimeters of rain during the season, roughly half the rainfall it receives today.
Such aridity may have caused highland rivers to dry out in summer, the researchers suggest, causing problems for Stegodon, a major food source for H. floresiensis. The animals “are not going to survive with a seasonal river,” Scroxton says. “They’re either going to move or die.”
Oxygen isotopes in Stegodon fossils from Liang Bua show the same trends, which allowed the team to date the fossils. The animals vanished from the site around 57,000 years ago — “when summer rainfall dropped to record lows,” says coauthor Michael Gagan, a paleoclimatologist at the University of Wollongong in Australia.
Paleontologist Julien Louys says the study “provides a very strong mechanism to explain the extinction of the larger-bodied mammals from Flores towards the end of the Pleistocene,” starting roughly 50,000 years ago.
“The fact that the hydrological changes are being reflected in the teeth of the Stegodon provides good direct evidence that these changes were being felt by at least one of the organisms in question,” says Louys, of Griffith University in Brisbane, Australia.
It’s not clear what happened next, Scroxton says. Stegodon may have moved to the coast, where water could still be found, with hobbits following. This relocation may have put them in the path of modern humans (Homo sapiens) moving through Southeast Asia. There was also a volcanic eruption in the vicinity 50,000 years ago. All these factors could have been important. “It’s all about stresses,” he says.
Finding evidence of H. floresiensis at other sites will be crucial for tracking their population. While they disappeared from Liang Bua 50,000 years ago, Scroxton says, they could have survived elsewhere — perhaps for millennia.
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