<p>The δ¹³C and δ¹⁵N values of human collagen from ancient human bones can be used to learn about sources of protein in their diet. Isotopic analysis was performed on collagen from the skeletal remains of people living along the Pacific Coast between Point Conception and western Los Angeles, as well as offshore islands and sites deeper inland beyond coastal mountain ranges. Graphs of the δ¹³C and δ¹⁵N values of these data produced line segments with little dispersion along these lines (average variance r² = 0.79 ± 0.14). This indicates that the diet of the people at each site consisted of mixtures of two principal components: marine mammals and fish; and terrestrial foods, mainly C₃ plant foods. There is no evidence of significant consumption of invertebrate marine foods (e.g., shellfish). As was previously shown by Walker and DeNiro (<CitationRef CitationID="CR67">1986</CitationRef>), people living on the islands favored consumption of marine foods. Because only two food components contributed to the diet, we could treat the data as one-dimensional arrays and determine the fraction of terrestrial and marine foods. At coastal sites most people consumed about 40% terrestrial foods while at island sites they consumed ~ 20% terrestrial foods. At sites deep inland (isolated from the sea by low mountain ranges) people consumed on average about 30–40% marine protein. This may represent either frequent migration to coastal regions or transport of fish to inland sites.</p>