The Pleistocene Imprint: Evolutionary Asymmetry of Thermal Risk, Atmospheric CO2 Starvation, and the Existential Threat of Glaciation

Abstract Contemporary environmental discourse operates under an acute "hyperthermic bias," conceptualizing anthropogenic planetary warming as the singular existential climate catastrophe. However, human evolutionary biology, comparative epidemiology, and Pleistocene paleoclimatology demonstrate a profound, neglected asymmetry: cold and glaciation represent an incomparably more lethal, irreversible, and existential threat to the biological hardware of Homo sapiens. This strategic monograph deconstructs planetary thermal risk across three empirical pillars: Epidemiological Asymmetry: Empirical evidence from the largest global temperature-mortality dataset (Gasparrini et al., The Lancet, 2015), analyzing 74 million deaths across 384 locations, proves that cold-related mortality (7.29% of total deaths) exceeds heat-related mortality (0.42%) by a factor greater than 17 to 1. As an anatomically tropical, hairless, eccrine-sweating primate, the human body is bioenergetically ill-equipped for chronic cold. Biospheric Collapse: During the Last Glacial Maximum (LGM; ~20,000 BP), atmospheric carbon dioxide plunged to ~180 ppm—critically close to the ~150 ppm physiological starvation threshold where C3 plant photosynthesis collapses entirely. Continental glaciation locked vast freshwater reserves into continental ice sheets, triggering planetary megadroughts and rendering agriculture thermodynamically impossible. The Paleolithic Genomic Bottleneck: The human genome carries the indelible imprint of Pleistocene glacial contractions, marked by metabolic adaptations (thrifty genotypes, brown adipose tissue thermogenesis, lipolytic-ketogenic substrate dependence) engineered strictly to endure severe caloric deprivation and thermal collapse. While global warming expands high-latitude growing seasons and stimulates biomass via carbon fertilization, planetary cooling destroys primary net productivity and obliterates global carrying capacity. Genuine civilizational security requires recognizing the profound thermodynamic asymmetry between warming and glaciation.