Students’ engagement with biological evolution remains challenging due to conceptual demands and sociocultural influences. Research suggests that engagement is shaped not only by scientific knowledge but also by epistemic trust, evaluation of evidence, scientific authority, and learners’ positioning toward scientific explanations. However, little qualitative work has examined how these factors interact prior to instruction. This study investigates how upper secondary students reason about evolutionary explanations before formal instruction. A qualitative descriptive approach was used, based on semistructured interviews with 25 Grade 12 biology students in western India. Thematic analysis identified dominant reasoning patterns and epistemic positions. Findings reveal fragmented conceptual understanding, scale-dependent acceptance of evolutionary claims, and varied reliance on authority and evidence. Many students expressed conditional acceptance grounded in epistemic trust, while others negotiated tensions between scientific explanations and personal belief frameworks through reconciliation or resistance. Students also articulated preferences for instructional approaches emphasising evidence, dialogue, and conceptual clarity. These findings show how conceptual reasoning, epistemic evaluation, and epistemic positioning interact in shaping students’ reasoning prior to formal instruction, with implications for evolution education. The study provides a process-oriented account of how these dimensions interact prior to instruction, a phase underexplored in evolution education research.