Argon and luminescence dating of fossil shell infills from Trinil in Java, where Homo erectus lived, reveals that the hominin-bearing deposits are younger than previously thought; perforated shells, a shell tool and an engraved shell indicate that Homo erectus ate freshwater mussels, used their shells as tools and was able to create abstract engravings. Homo erectus made tools from shells, and even decorated some of them with what look like intentional incisions. The fossils of the hominid that came to be known as Homo erectus were discovered at Trinil in central Java by Eugene Dubois in 1891. Josephine Joordens and colleagues have been looking over the historic Dubois collections, now in Leiden in the Netherlands, concentrating on the freshwater shells. They find evidence for shellfish consumption by hominins, a shell tool and other shells showing signs of intentional modification. Age determination on the sediment directly associated with the shells show that they were used sometime between 380,000 and 640,000 years ago, well within the time during which Homo erectus lived in Java, and pre-dating the oldest geometric engravings described previously by more than 300,000 years. The manufacture of geometric engravings is generally interpreted as indicative of modern cognition and behaviour1. Key questions in the debate on the origin of such behaviour are whether this innovation is restricted to Homo sapiens, and whether it has a uniquely African origin1. Here we report on a fossil freshwater shell assemblage from the Hauptknochenschicht (‘main bone layer’) of Trinil (Java, Indonesia), the type locality of Homo erectus discovered by Eugène Dubois in 1891 (refs 2 and 3). In the Dubois collection (in the Naturalis museum, Leiden, The Netherlands) we found evidence for freshwater shellfish consumption by hominins, one unambiguous shell tool, and a shell with a geometric engraving. We dated sediment contained in the shells with 40Ar/39Ar and luminescence dating methods, obtaining a maximum age of 0.54 ± 0.10 million years and a minimum age of 0.43 ± 0.05 million years. This implies that the Trinil Hauptknochenschicht is younger than previously estimated. Together, our data indicate that the engraving was made by Homo erectus, and that it is considerably older than the oldest geometric engravings described so far4,5. Although it is at present not possible to assess the function or meaning of the engraved shell, this discovery suggests that engraving abstract patterns was in the realm of Asian Homo erectus cognition and neuromotor control.