This paper introduces the concept of Systemic Environmental Coherence (SEC) as a theoretical framework for understanding how built environments influence adaptive human functioning. The work argues that environmental performance cannot be adequately evaluated through isolated variables—such as lighting, acoustics, thermal conditions, air quality, or spatial configuration—but must instead be analyzed as a dynamically interacting environmental system. The paper proposes a systemic model describing how multiple environmental variables converge to shape cognitive load, physiological stress, behavioral reliability, and human performance in complex built environments. Within this framework, the Space Genome Index (SGI) is introduced as a conceptual diagnostic metric designed to evaluate the systemic environmental performance of architectural and infrastructural spaces. The framework emphasizes that environmental performance depends not only on the quality of individual variables but also on their degree of alignment and coherence within the overall environmental system. When environmental parameters diverge significantly, the resulting systemic incoherence may increase cognitive load and physiological stress, affecting adaptive human functioning. By shifting the analytical focus from individual environmental parameters to systemic environmental coherence, this work establishes a formal theoretical foundation for future empirical research, environmental diagnostics, and performance-oriented design strategies. The framework aims to support new approaches to evaluating built environments in relation to human reliability, public health, and operational performance across high-demand settings such as healthcare facilities, educational institutions, workplaces, and other complex infrastructures.