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Abstract

Humans are distinguished from other great apes by their unusually high metabolic rates, which supported the evolution of several energetically costly traits, including larger brains and exceptional endurance capacity. Sustaining these demands requires the cardiovascular system to deliver substantially greater amounts of oxygenated blood to metabolically active tissues. These selective pressures likely shaped the evolution of the human left ventricle, favoring structural and functional traits that enhanced stroke volume and consequently, cardiac output. The objective of this dissertation, therefore, was to comprehensively assess left ventricular variation across primates to gain insight into potential evolutionary divergence of the human heart. Study 1 first considered left ventricular trabeculation, a structural characteristic of the myocardium defined by protrusions of the ventricular wall, and its relevance for ventricular mechanics across extant great apes using two-dimensional echocardiography. All non-human great apes exhibited prominent trabeculation, particularly at the left ventricular apex, which was markedly reduced in humans. Functional analyses using speckle-tracking echocardiography revealed a negative association across hominids between apical trabeculation and left ventricular mechanics, whereby a greater extent of trabeculation was associated with lower apical rotation and twist. Study 2 then characterized the left ventricular phenotype of humans, chimpanzees (Pan troglodytes) and an evolutionary outgroup of rhesus macaques (Macaca mulatta), to determine whether the derived human phenotype directly supports a greater stroke volume. Principal component analysis revealed distinct ventricular phenotypes across the three species. While both non-human primates exhibited relatively spherical, trabeculated left ventricles, rhesus macaques had rapid rates of myocardial deformation, whereas chimpanzees exhibited relatively thicker ventricular walls. Humans alone, however, had a uniquely elongated ventricle with proportionately greater compact myocardium, alongside enhanced apical rotation and myocardial deformation. Importantly, this human phenotype was associated with higher stroke volumes. Collectively, the findings of this dissertation provide novel evidence for adaptive evolution of the human left ventricle. The divergence in both structure and function likely enhanced humans’ stroke volume, representing a key adaptation required to support high energetic demands.