Practice question

Photosynthesis likely evolved from chemoautotrophic electron transport chains using bacteriochlorophyll absorbing infrared light without splitting water, thus not producing oxygen. Anoxygenic photosynthesis in purple and green sulfur bacteria uses H2S as electron donor, matching early anoxic, sulfide-rich environment. Oxygenic photosynthesis requiring manganese cluster to oxidize water appeared later in cyanobacteria, triggering great oxidation event. This sequence explains why earliest phototrophs were Anoxygenic, avoiding oxygen toxicity before protective enzymes evolved. This concept integrates genetics, ecology and molecular evidence, frequently tested in NEET, GATE and CSIR-NET, highlighting links between genotype, phenotype and environment.