Convinced that the evolutionary history of man lay in East Asia, physician Eugène Dubois enlisted as an army surgeon in the Royal Dutch East Indies Army in Sumatra. He had little success in finding fossils in Sumatra.

In 1890, he moved to Java, where his excavations brought him to the Solo River. In 1891, Dubois discovered a heavily mineralized cranium belonging to an early human. Many of the features were worn flat, but the shape of the cranium was distinctively long and the forehead was flat. A heavy browridge was evident, along with a distinct sagittal keel, visible in frontal and three-quarters view as a bony ridge passing lengthwise along the skull.

It should be noted that this sagittal keel is distinct from the sagittal crest of the Paranthropus species (e.g. KNM WT 17000: Paranthropus aethiopicus, and OH 5: Paranthropus boisei). In three-quarters view, the temporal line, where the chewing muscles attach to the skull, is visible to the side of the skull, and distinct from the keel.