To interrogate genes essential for cell growth, proliferation and survival in human cells, we carried out a genome-wide CRISPR/Cas9 screen in a B-cell lymphoma line using a custom extended knockout (EKO) library of 278,754 sgRNAs that targeted 19,084 RefSeq genes, 20,852 alternatively-spliced exons and 3,872 hypothetical genes. A new statistical analysis tool called RANKS identified 2,280 essential genes, 234 of which were unique. Individual essential genes were validated experimentally and linked to ribosome biogenesis and stress responses. Essential genes exhibited a bimodal distribution across 10 different cell lines, consistent with a continuous variation in essentiality as a function of cell type. Genes essential in more lines had more severe fitness defects and encoded the evolutionarily conserved structural cores of protein complexes, whereas genes essential in fewer lines formed context-specific modules and encoded subunits at the periphery of essential complexes. The essentiality of individual protein residues across the proteome correlated with evolutionary conservation, structural burial, modular domains, and protein interaction interfaces. Many alternatively-spliced exons in essential genes were dispensable and were enriched for disordered regions. Fitness defects were observed for 44 newly evolved hypothetical reading frames. These results illuminate the contextual nature and evolution of essential gene functions in human cells.