It’s a jolt to the established narrative: stone tools unearthed in Sulawesi’s Walanae Formation are now dated to at least 1.04 million years ago, dramatically pushing back the earliest known hominin presence on this Wallacean island by more than 800,000 years. This discovery not only redefines the region’s archaeological record, but also compels a re-examination of the technological and migratory capabilities of early hominins in Southeast Asia.
The tectonic complexity of Sulawesi, which was developed from the collision of the Eurasian, Indo-Australian, and Pacific plates, produced the Walanae Depression a basin whose infill, the Walanae Formation, preserves a full stratigraphic record of deltaic and fluvial deposits. The uppermost Beru Member, and specifically its Sub-Unit B, has now become the fulcrum of this paradigm-shifting study. These coarse-grained sandstones were long mistaken for comparatively recent river terraces, but new stratigraphic and structural investigations have revealed them to be uplifted relicts of much older basin-fill series whose exposures date back to the Early Pleistocene. Systematic excavations at Calio, initiated following the fortuitous discovery of a flake of chert embedded in cemented conglomerate, exposed seven in situ flaked stone artefacts of variable depths and sedimentary environments.
All were constructed of chert, abundant in local riverbeds but statistically larger than unaltered pebbles from the same deposits. An unpaired t-test indicated that artefacts’ largest dimension averaged 41.3 ± 16.0 mm, compared with 18.5 ± 9.1 mm for natural pebbles, an extremely significant difference. This size difference, coupled with the technological character of the artefacts, emphasizes their anthropic origin and indicates they were manufactured elsewhere and imported to the site, as opposed to being a product of natural fluvial activity. Technological examination indicates that the flakes were created through hard-hammer freehand percussion, a method of hitting a core stone with a hammerstone to remove sharp-edged flakes. The flakes, measuring between 21.9 and 60.1 mm in length, exhibit unidirectional and cross-axis reduction, sharp platform angles, and evidence of core rotation under reduction. Of special interest is the observation that one of the artefacts a Kombewa flake was unfoliated from the ventral face of a bigger flake and retouched along its edge, with evidence for a two-step reduction operation and sophisticated fracture mechanics expertise. These features are characteristic of the early Pleistocene hominins’ technological repertoire in other parts of Eurasia and reflect a “least effort” strategy of tool production, but with technical skill. The firm chronological foundation for the deposits was provided by the amalgamation of palaeomagnetic and US–ESR dating.
Palaeomagnetic analysis of oriented block samples throughout the Calio sequence consistently recorded reverse magnetic polarities, positioning artefact-containing horizons beneath the Brunhes–Matuyama boundary. In agreement, US–ESR dating of two Celebochoerus fossil teeth from the same stratigraphic context yielded a modelled mean age of 1.26 ± 0.22 million years. The intensity of method in US–ESR, which mimics uranium incorporation by a diffusion coefficient and involves laser ablation ICP-MS for high-resolution isotopic mapping, surpassed fallacies of older linear or initial uptake models and is now a routine technique in Pleistocene geochronology to directly date fossil biominerals. The intersection of these self-standing dating methods positions the artefact-bearing layers to the interval between the Olduvai and Jaramillo Subchrons, or around 1.787 to 1.070 million years ago.
Through the fact that the dated fossil is stratigraphically positioned above the bottommost artefact, the lower boundary for hominin activity at Calio stands solid at 1.04 million years, with a longer possibility up to 1.48 million years. This falls ahead of the previously oldest Sulawesi artefacts from Talepu and equals or betters the oldest records from Luzon and Flores, the other two major islands east of Huxley’s Line with early hominin occupation and endemic “hobbit”-sized human relatives. These findings have important ramifications for the investigation of hominin dispersal in Wallacea.
The region’s islands, never a part of the Asian or Australian continental shelves, presented formidable seaborne barriers. The discovery on Sulawesi of archaic hominins at this early date indicates that sea crossings, by accident or intent, had already occurred far earlier than ever documented. “These are simple, sharp-edged flakes of stone that would have been useful as general-purpose cutting and scraping implements,” said Griffith University archaeology professor Adam Brumm. But the absence of cut marks on faunal bones leaves their exact function to further investigation. While the taxonomic status of the toolmakers remains controversial, the time overlap on Java with Homo erectus and the morphological similarities of tools from the same age as Homo floresiensis allow for the possibility of an as-yet-unwitnessed or recent hominin lineage on Sulawesi. As Brumm cautioned, “Until we have found fossils of archaic hominins on Sulawesi, it would be premature to assign a hominin species to the tool-makers.”
The technological and chronological information presented by Calio not only rewrote Sulawesi prehistory but also compels a reappraisal on a wider canvas of early hominin adaptability, technological innovation, and Pleistocene world island colonization dynamics.
