In 2003, scientists made a startling find in a remote cave on the Indonesian island of Flores: The skull and skeleton of an adult female hominin, a group consisting of modern humans and extinct human species, who stood only about a meter tall. That discovery sparked a fierce debate about whether the hominin—officially dubbed Homo floresiensis but often called the “hobbit”—was a separate species or a diseased modern human. Now, many of the same scientists who made the discovery have radically revised their estimate of the fossils' age, based on an exhaustive new analysis of the cave’s geology. Instead of living 18,000 years ago, as they originally reported, the hobbit lived between 60,000 and 100,000 years ago—some 10,000 years before H. sapiens arrived in the region.
That new, much older date range for H. floresiensis makes it “impossible to argue that it is a pathologically-dwarfed modern human,” says Russell Ciochon, a paleoanthropologist at the University of Iowa in Iowa City who was not involved in the study. “In my opinion, this paper drives the final nail in the coffin” of that hypothesis.
A chief argument underpinning the diseased Homo sapiens hypothesis was the original 18,000-year age of the fossils—long after H. sapiens arrived in southeast Asia and Australia. However, that 18,000-year-old date was based on only a geological analysis of the fossils' surroundings and not on direct analysis of the bones themselves. And the complexity of the cave's geology initially misled the scientists, says Matthew Tocheri, a paleoanthropologist at Lakehead University in Thunder Bay, Ontario, Canada, and a member of the discovery team.
Liang Bua is a cathedral-like cave with a high domed ceiling. The original excavations, conducted from 2001 to 2004, dug into four regions of the cave, including a couple of sites along its eastern wall, where some fossils were found. Rather than damaging the fossils by dating them directly, the team looked to the sediments in which they were found. They discovered pieces of charcoal in sediments at similar depths, and considered those to be proxies for the ages of the fossils themselves. The charcoal bits were dated to around 19,000 and 13,000 to 11,000 years before present.
But Liang Bau boasts a devilishly complicated geological history, as layers of silt and clay interleave with layers of weathered limestone, loose gravel, and volcanic ash. In many places, these layers have been scoured by erosion, altered by seeping water, and jumbled by tectonic activity. About a year after the discovery, paleoanthropologist Thomas Sutikna, the new study’s lead author and a Ph.D. candidate at the Centre for Archaeological Research in Jakarta, pushed the team to take a closer look at the series of sedimentary layers, or stratigraphy, along the eastern wall. “It became clear that there were two very different deposits that just happened to be superimposed against each other on a 45-degree angle,” Tocheri says. Still, he says, it “took several years to convince members of our team that there was something we were missing in the stratigraphic layers.”
Sutikna also noted that a distinctive layer of volcanic fragments called a tuff appeared in several places around the cave. The tuff provided a kind of universal time stamp for the entire site: By tracing its appearance in various excavated regions of the cave, the team was able to “connect the dots,” Tocheri says. From 2007 to 2014, the team dug into a number of new areas near the center of the cave, he says. Then, they found a powerful key with which to fully reconstruct the cave’s history: a buried “hill” consisting of layer upon layer of sediment, unaffected by erosion.
The layers included a handful of volcanic tuff layers, sediments bearing additional artifacts linked to H. floresiensis—and, beneath them all, a sediment layer that matched the one in which the original fossils were found. The team then used numerous techniques to date the various sediments. Dating on the gravelly sediment layer containing the fossils suggested it was deposited between 100,000 and 60,000 years ago; just above it is a layer of volcanic ash that was dated to about 60,000 years ago. This suggests that the fossils themselves couldn’t possibly be younger than 60,000 years old, the team reports online today in Nature.
To cap it off, the team has now directly dated the fossils themselves. Originally, researchers shied away from that analysis for fear of damaging them, Tocheri says. But this time, they felt they needed to do it. “It wasn’t until [we] were reasonably convinced that [the fossils] were probably older than 50,000 years old that we decided we needed to date them directly to be absolutely certain,” he says.
The team dated three arm bones found at different locations within the cave during previous excavations. Using uranium-thorium dating—which is based on the radioactive decay of uranium and thorium isotopes in a material—they found dates ranging from about 66,000 to 87,000 years old.
H. sapiens is generally thought to have spread throughout Southeast Asia’s islands and into Australia by about 50,000 years ago. The team’s report of the much older dates of the sediments, as well as of the fossils themselves, suggest that the fossils could not have belonged to H. sapiens, Ciochon emphasizes. There is evidence of H. sapiens in the cave around 11,000 years ago, he notes, and they may have reached Flores long before that date. Whether H. floresiensis survived long enough to interact with modern humans remains an open question.
Meanwhile, Tocheri says the work is far from over in Liang Bua. “We excavate pretty much every year,” he says. “We suspect we’ll be working there for the rest of our lives.”