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Pancreas cancer (PC), a highly aggressive tumour type with uniformly poor prognosis, is an exemplar of the classical view of cancer development based on stepwise progression1. The current progression model, based on analyses of precursor lesions termed pancreatic intraepithelial neoplasm (PanINs) lesions, makes two predictions: 1) PC develops through a particular sequence of genetic alterations2–5 (KRAS > CDKN2A > TP53/SMAD4); and 2) the evolutionary trajectory of PC progression is gradual because each alteration is acquired independently. One shortcoming of this nearly two decade old contention is that clonally expanded precursor lesions have been identified that do not always belong to the tumour lineage2,5–9, indicating that the evolutionary trajectory of the tumour lineage and precursor lesions can be divergent. This prevailing view of tumourigenesis has contributed to the clinical notion that PC evolves slowly and presents at a late stage10. However, the propensity for this disease to rapidly metastasize and the inability to improve patient outcomes despite efforts aimed at early detection11, argue that PC progression is anything but gradual. By tracking DNA copy number changes and their associated rearrangements from tumour-enriched genomes using novel informatics tools, we found that PC tumourigenesis neither is gradual nor follows the accepted mutation order. Two-thirds of tumours harbour complex rearrangement patterns associated with mitotic errors, consistent with punctuated equilibrium as the principal evolutionary trajectory12. In a subset of cases, the consequence of such errors was the simultaneous, rather than sequential, knockout of canonical preneoplastic genetic drivers that likely set-off invasive cancer growth. These findings challenge the current model of PC tumourigenesis and provide novel insights into the mutational processes giving rise to these aggressive tumours.