Constraint relaxation and repair flexibility under long-term environmental stress: a lineage-specific epigenetic framework for rare cross-lineage SNP emergence on the human Y chromosome

Rare cross-lineage single-nucleotide polymorphism (SNP) states observed in high-coverage Y-chromosome datasets present a persistent challenge to conventional interpretations of lineage specificity, homoplasy, and phylogenetic exclusivity (Jobling & Tyler-Smith (Nat Rev Genet 18:485–97, 2017); Poznik (Nat Genet 48:593–99, 2016)). In particular, a focal downstream lineage within haplogroup E-CTS1454 (E-Y250637) exhibits a reproducible set of isolated SNP states canonically annotated within other haplogroups, while early-diverging sister branches within the same tripartite topology do not. These observations are difficult to reconcile with stochastic sequencing error, recombination, or classical mutagenesis models, yet do not entail collapse of higher-order haplotypic structure. Here, we propose a constraint-relaxation model in which prolonged environmental stress transiently reduces repair canalization in lineage-specific genomic architectures, expanding the space of permissible repair outcomes without invoking genetic transfer, recombination, or directed adaptation. Integrating Y-chromosome phylogenetic topology, temporal ordering of variant emergence, and archaeological–environmental context, we identify the Levantine Iron Age Anomaly (LIAA) as a historically bounded stress envelope during which E-Y250637, but not its early-diverging sister branches, remained resident in the southern Levant. We argue that extended exposure to environmental instability during this interval plausibly modulated germline repair dynamics through epigenetic and biophysical boundary conditions, enabling rare convergent repair outcomes that were subsequently stabilized and observed only after later demographic relocation. No claim is made that environmental stress specifies nucleotide identity; rather, any influence would be indirect and mediated through generalized modulation of repair pathway weighting. This framework reframes rare cross-lineage SNP observations as context-dependent convergent repair outcomes permitted by transient constraint relaxation rather than evidence of mutagenesis, horizontal transfer, or classical homoplasy. By situating repair flexibility within a temporally bounded environmental stress model, the approach provides a conservative, falsifiable explanation for delayed SNP emergence in narrowly constrained genomic systems (Siebert (Clin Epigenet 15:145, 2023); Ben-Yosef (PNAS 114:2425–30, 2017)). More broadly, the approach motivates formal investigation of how long-term environmental stress may modulate repair permissiveness in lineage-specific contexts, with implications for population genetics, ancient DNA interpretation, and epigenetic inheritance. The present framework builds on Waddington’s concept of canalization, originally formulated to describe the buffering of developmental trajectories against perturbation (Waddington (Nature 150:563–65, 1942), (London: Allen & Unwin, 1957)), and extends this logic to the regulation of DNA repair outcomes and genome stability under long-term environmental stress.