First living organisms were
Initial life faced anoxic environment without free oxygen or ozone, making aerobic metabolism impossible. Simplest cells would be prokaryote-like, lacking nucleus and organelles, obtaining energy by fermenting pre-existing organics or inorganic reactions. Anaerobic prokaryotes such as methanogens and fermenters fit this description and have ancient lineages. Eukaryotes and oxygenic phototrophs appeared later after atmospheric oxygenation. Fossil stromatolites and molecular phylogeny support Anaerobic prokaryotes as earliest cellular forms. 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.