Relative brain size has long been considered a reflection of cognitive capacities and has played a fundamental role in developing core theories in the life sciences. Yet, the notion that relative brain size validly represents selection on brain size relies on the untested assumptions that brain-body allometry is restrained to a stable scaling relationship across species and that any deviation from this slope is due to selection on brain size. Using the largest fossil and extant dataset yet assembled, we find that shifts in allometric slope underpin major transitions in mammalian evolution and are often primarily characterized by marked changes in body size. Our results reveal that the largest-brained mammals achieved large relative brain sizes by highly divergent paths. These findings prompt a reevaluation of the traditional paradigm of relative brain size and open new opportunities to improve our understanding of the genetic and developmental mechanisms that influence brain size.

The growing fetus compresses maternal abdominal organs against the diaphragm, constraining diaphragmatic respiration and increasing dependence on intercostal muscles during pregnancy. In conjunction with females’ smaller thoracic cavities, this fetus‐induced abdominal displacement and its theorized impact on respiration has been cited as the explanation behind females’ relatively longer and more angled ribs and transverse thoracic processes. Arguably, this morphology should enable females’ intercostal muscles to produce greater ribcage elevation and a more powerful inhale than the more horizontal intercostal muscles and ribs of males. This “abdominal displacement” hypothesis is often used to explain human thorax dimorphism. However, intraspecific variation in ribcage dimorphism due to ecogeographic rules (e.g., Allen’s rule) demonstrates that human populations adapted to low latitudes have narrower trunks, which in turn may display different patterns or degrees of dimorphism than populations adapted to temperate climates with wider trunks. This study’s objective is to explore how the human thorax has responded to obstetric and thermoregulatory demands to assess the underlying premise of the “abdominal displacement” hypothesis that ribcage dimorphism is uniform across populations. Focusing on thorax volume and rib morphology, the following predictions were tested: 1) the pattern of sexual dimorphism (i.e., female‐biased) remains the same across human populations and conforms with that of the “abdominal displacement” hypothesis, yet 2) the degree of sexual dimorphism (i.e., the extent of differences between the sexes) varies with geographic ancestry due to ecogeographic rules. Using 3D scans of articulated torsos and isolated ribs, size‐adjusted thorax volume, size‐adjusted rib arc and chord lengths, and rib angles were calculated for low‐latitude adapted (represented here by 23 African American individuals) and moderate climate adapted (represented by 22 European American individuals) adults. Factors were compared using ANOVA (rib angle) and ANCOVA (all other variables) using height as the covariate. Despite admixture, African Americans have 11% smaller size‐adjusted total thoracic volumes than European Americans, as expected given their descent from low‐latitude adapted individuals. Given females’ 12% smaller size‐adjusted thoracic volumes, the “abdominal displacement” hypothesis would predict female‐biased dimorphism in rib morphology to ameliorate shifting organs into this smaller space during pregnancy. This prediction is not met as the size‐adjusted arc and chord lengths show 4 to 10% significant male‐biased dimorphism across ribs. Dimorphism in rib angles is variably female‐ or male‐ biased. The magnitude of sexual dimorphism in these features does vary among the populations, following predictions. Cumulatively, these results suggest that the angles and lengths of isolated ribs do not follow expectations of the “abdominal displacement” hypothesis. It is possible that the transverse processes of thoracic vert...