Over 118,000 years ago, on the island of Sulawesi in Indonesia, a group of humans settled down and made a home. The only evidence of their existence is a large collection of stone tools, carefully crafted, preserved in the sediment at the edge of a river. A group of archaeologists recently spent several years excavating in the area and dating what remains they found. Astonishingly, their work suggests that humans may have arrived on this island as early as 195,000 years ago. And it's extremely unlikely they were Homo sapiens.

Sulawesi is part of the Indonesian island chain that forms a gentle curve in the waters between Southeast Asia, the Philippines, and Australia. But at the time these early humans were arriving during the Pleistocene, Australia and New Guinea were one continent, called Sahul, and many of the Indonesian islands were connected by land. Sea levels were often much lower than today due to glaciation, which locks water up into polar ice. Previous research has shown that early human groups crossed over to the islands during this time, before Homo sapiens evolved. Indeed, Sulawesi's neighboring island Flores was home to the recently discovered Homo floresiensis, or Hobbit people, a group of unusually small hominins who arrived on the island roughly 1 million years ago.

The researchers published their findings this week in Nature, detailing the stone tools they found and explaining how they determined their age. The tools were typical of hominins during the Pleistocene, which is to say they were simple stone slivers called flakes made from banging one rock against another to produce small, sharp-edged pieces that could be used as knives, scrapers, weapons, and more. "There is patterning in the flaking techniques," write the researchers, but "there is little evidence that the stoneworkers were creating tools of specific form." So these were general purpose tools.

Embedded in the ancient silt along with the flakes, researchers also found a few tooth fossils, including from an anoa, or miniature water buffalo, a Celebochoerus, or Pleistocene-era boar, a Stegodon, or dwarf elephant, and an alligator. These fragments offer a hint of the environment where these ancient humans lived on Sulawesi, surrounded by boar, elephants, and buffalo. The researchers found no human remains.

But the animal teeth did allow the researchers to fix probable dates on Sulawesi's human occupation by using uranium-series dating on the fossils. They also dated the silts surrounding the tool finds by measuring their magnetic polarity. Combining these dating techniques, the researchers write they are very confident the tools were left behind between 195 and 118 thousand years ago.

This places the ancient humans on Sulawesi at least 68 thousand years earlier than had been previously thought, based on 45-thousand-year-old Homo sapiens skeletons found in the area.

So who were these early humans? The researchers detail a few ideas in their paper. One possibility is that they're just modern humans who spread to the region from Africa roughly 120 thousand years ago, but that seems unlikely. We know that Homo floresiensis, the Hobbit group, was on Flores at least a million years ago, so it stands to reason that another human group might have made it to Sulawesi. Possibly, say the researchers, it was a group of Homo floresiensis who somehow got to Sulawesi, too. Other possibilities include Homo erectus, our hominin ancestor, who might have come to the island from Java. It might even have been Denisovans, who lived in Southeast Asia over 200 thousand years ago and whose genetic traces can still be found in the DNA of modern people in the region.

"Considering the predominantly southerly flowing currents of the Indonesian through-flow, we speculate that the most likely points of origin for the Sulawesi colonizers are Borneo to the west (part of mainland Asia during periods of low sea level) and the Philippines to the north (the northern extremity of Wallacea)," conclude the researchers. They believe other islands in the area probably have archaic hominin remains and suggest more excavations should be done to solve this new mystery in the increasingly tangled tale of human origins.

Nature, 2016. DOI: 10.1038/nature16448