Primitive Earth atmosphere was considered to be
Geochemical evidence indicates early Earth atmosphere lacked free oxygen and ozone. Instead it contained reducing gases such as methane, ammonia, hydrogen and water vapor from volcanic outgassing. Such reducing conditions prevented oxidative breakdown of newly formed organics, allowing accumulation of amino acids and nucleotides. Laboratory simulations like Miller-Urey replicate this setting. Without oxygen, ultraviolet radiation reached surface driving reactions. Understanding atmospheric chemistry explains why Reducing state is crucial for chemogeny preceding biological evolution. This concept integrates genetics, ecology and molecular evidence, frequently tested in NEET, GATE and CSIR-NET, highlighting links between genotype, phenotype and environment.
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 1: Origin of Life.