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This paper extends the Mammalia candidus pan-mammalian co-evolution hypothesis (Craddock, 2026b) by proposing a specific environmental mechanism: cyclical lake salinity variation in the East African Rift Valley during the Plio-Pleistocene as the driver that activated and deepened the symbiosis between Candida species and hominid hosts. Drawing on paleoclimatological evidence of alternating humid and arid periods producing dramatic lake-level and salinity oscillations (Maslin et al., 2014; Trauth et al., 2005), paleoanthropological evidence of concurrent hominid speciation and encephalization events (Shultz and Maslin, 2013), and established literature on the endocannabinoid system (ECS) as a conserved master regulatory system across mammals (Elphick, 2012), we propose that periodic exposure to increased electrolyte concentrations in drinking water followed by freshwater periods producing electrolyte disruption analogous to the syndrome of inappropriate antidiuretic hormone secretion (SIADH) provided the environmental conditions under which a fungal symbiont capable of managing host perfusion and electrolyte balance gained decisive selective advantage. The symbiont’s capacity to fill this role is not limited to the ECS. We present a synthesis of peer-reviewed evidence demonstrating that Candida albicans occupies a unique position in the mammalian internal ecology: it is the only organism in the host microbiome that simultaneously signals across kingdoms (to bacteria, competing fungi, and the mammalian host), possesses physical tissue mobility through hyphal morphological transition, and accesses the host’s endogenous receptor infrastructure. Confirmed molecular targets of C. albicans metabolites include nuclear transcription factors (FXR, PPARs), voltage-gated calcium channels, GABA-A neurotransmitter receptors, the GLP-1 incretin system, cholinergic receptors, and multiple arms of both innate and adaptive immunity. The endocannabinoid system, while the primary and most ancient interface, represents the trunk of a signaling architecture whose canopy extends across the broader GPCR superfamily and beyond. We reinterpret farnesol, the first quorum-sensing molecule identified in a eukaryote (Hornby et al., 2001), not as a self-regulatory signal but as a multi-target effector molecule deployed to manage the host environment, consistent with the twenty-five-year absence of any identified farnesol receptor in C. albicans itself. The organism possesses confirmed receptors or binding proteins for at least six classes of host hormone, including estrogen, luteinizing hormone, corticosteroids, and androgens, while governing additional endocrine axes through upstream management of pituitary perfusion and ECS-mediated signaling — a two-tier architecture in which the organism senses hormones that provide inbound information and modulates hormones it controls through the producing gland We further propose that the social component of the co-evolutionary architecture was initiated before the salinity oscillations through the discovery and communal use of exogenous phytocannabinoids, which promoted peaceful social bonding, group cohesion, and cooperative behavior. This pre-linguistic social flywheel, reinforced epigenetically through transgenerational cannabinoid-induced methylation changes, established cooperative social structure before the emergence of language. Language did not create the co-evolutionary trinity of symbiont, host physiology, and social structure. It completed it, and allowed it to accelerate. Single-cell transcriptomic evidence (Dumeaux et al., 2023) demonstrating pre-positioned bet hedging, distributed survival strategies, and controlled genome destabilization in C. albicans populations is reinterpreted within this co-evolutionary framework as architectural rather than merely pathogen-adaptive, consistent with an organism refined across approximately 200 million years of mammalian co-evolution. It is the ultimate survivor: a biochemical computer continuously recalculating what moves might be required next. The C. albicans genome (14.3 Mb, approximately 6,400 genes) encodes over 1,300 genes with no orthologs in other yeast species, the majority of which remain functionally uncharacterized. We designate the symbiont-active hominid phenotype Homo candidus and argue that a subsequent genetic shift in cardiac architecture from suction-dominant to pump-dominant circulation disrupted the co-evolutionary trinity, producing the modern human condition in which the symbiont persists commensally but can no longer execute its full physiological program. Sixteen testable predictions are presented, including proposed experiments in simulated gastric environments, comparative mycobiome analysis of Rift Valley populations, computational genomic analysis of uncharacterized C. albicans genes using biological foundation models, and molecular dating of the C. albicans / C. dubliniensis divergence.