"Most artifacts occur in an interval bracketed by dates of 85,000 to 118,000 years, but three meters below the dated level of 118,000 years we still find stone artifacts," he said, adding that the animal fossils from the site that were dated at a minimum 200,000 years old were just 20 centimeters below these lowest artifacts.
"That tells us [the undated lower stone artifacts] are probably older than 200,000 years.
"It means it must have been archaic humans that made these stone tools."
Cave art on Sulawesi shows modern humans (Homo sapiens) lived in Sulawesi around 40,000 years ago, Dr van den Bergh said.
"These modern humans must have encountered these archaic humans and what happened we don't know yet," he said.
He said the dense vegetation of the region made it difficult to find human fossils.
In Flores they were now using 100 local laborers for two months each year to dig up earth in the hope of finding more human fossils.
Dr van den Bergh said it was likely they would take the same approach in Sulawesi.
He said the team's "working hypothesis" was that based on the sea current patterns in the area the ancestors of the hobbit (Homo floresiensis) came from Sulawesi.
"We have now proven these pre-modern humans were on Sulawesi and the search for their fossil remains is now open."
Colin Groves, Emeritus Professor from the School of Archaeology and Anthropology at the Australian National University, said the finding was "quite significant".
Professor Groves agreed with the researchers' hypothesis that an older human species had been on the island.
"Compared to both Java and Flores, the date of a little over 100,000 is quite recent - nonetheless, it does extend human occupation of Sulawesi well back, making it highly probable that we have here a human species that is not Homo sapiens," he said.
"Whether it is a remnant Homo erectus, a dwarf-sized species in the mold of Homo floresiensis, or a fresh, undiscovered species endemic to Sulawesi, is just a matter for speculation at the moment, but certainly it is a very exciting prospect."