Abstract The widely accepted paradigm that cancer initiation is a monoclonal event has not been definitively resolved. This is largely due to the limitations of experimental systems to study cancer initiation, a definitively transient step in tumorigenesis. Individuals with familial adenomatous polyposis (FAP) possess a germline APC mutation, leading to hundreds of colonic polyps in adolescence and an eventual 100% lifetime risk of colorectal cancer. As such, total colectomies are performed once polyp burden becomes unmanageable and provide the resected colons from FAP donors provide a source of histologically normal mucosa, benign and dysplastic polyps, and, on rare occasions, adenocarcinomas, and serve as an exemplary model disease to understand human tumorigenesis. Through whole genome sequencing of 123 independent samples from six patients, we reveal the polyclonal origins of tumorigenesis in FAP. We characterize and contrast the genomic landscape and relative timing of driver mutation acquisition across normal to dysplastic and invasive colon tissue and compare this with multi-region sequencing data from sporadic colorectal cancers (CRC). We find that only early sporadic CRC drivers are common across the more heterogeneous premalignant stages. Selection, as assessed based on the ratio of nonsynonymous to synonymous mutations (dN/dS), is higher in benign than dysplastic FAP lesions and in sporadic adenomas relative to carcinomas. Phylogenetic reconstruction and pairwise sample comparisons reveal extensive heterogeneity and subclonal sharing within a patient’s benign and dysplastic lesions. Using a mechanistic mathematical model, we infer polyclonality for 90-100% of benign and 53-83% of dysplastic samples. Based on these findings, we propose a conceptual framework for the polyclonal origin of intestinal malignancies where inconsequential mutations, arising during colon formation in utero, are present across the colon and are valuable markers to uncover polyclonal ancestry. Citation Format: Ryan O. Schenck, Aziz Khan, Aaron M. Horning, Edward D. Esplin, HTAN Consortia, William J. Greenleaf, James M. Ford, Michael P. Snyder, Christina Curtis. The polyclonal path towards malignant transformation in familial adenomatous polyposis [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Translating Cancer Evolution and Data Science: The Next Frontier; 2023 Dec 3-6; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_2):Abstract nr B008.