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Figure 1. The evolution of relative brain size in groups of vertebrates over the past 200 million years (adapted and updated from Jerison, 1976, p. 96; Jerison, 1991, fig. 17). This plot seems to show an evolutionary trend towards increasing relative brain size (=E.Q. = Encephalization Quotient) and is probably the most definitive evidence for such a trend. Average living mammal E.Q. is defined as 1. The broken lines indicate gaps in the fossil record. Variation within groups is not shown. The lineage that led to humans is drawn thicker than the other lineages.

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