As a student of Gustav Heinrich Ralph von Koenigswald in the 1950s, Jacob claimed to have discovered and studied numerous specimens of Homo erectus.
After the end of the Japanese occupation of the Dutch East Indies during World War II, Jacob served in the Indonesian armed forces.
[2] Jacob came to international prominence when he expressed his disagreement with scientists who claimed that remains found on the island of Flores constituted a new human species, labeled Homo floresiensis.
Whoever was responsible misaligned the pieces and put them at an incorrect angle"; and, "The pelvis was smashed, destroying details that reveal body shape, gait and evolutionary history".
He stated that such damage occurred during transport from Yogyakarta back to Jakarta[8] despite the physical evidence to the contrary that the jawbone had been broken while making a mold of bones.