Life's building blocks may have formed for free in Earth's early, oxygen-free air
Working independently in the 1920s, biochemist Alexander Oparin and geneticist J.B.S. Haldane each proposed that Earth's early atmosphere — rich in methane, ammonia and hydrogen, with no free oxygen to break organic molecules apart — would let simple carbon compounds accumulate into a 'primordial soup,' setting the stage for the first self-replicating molecules. The hypothesis gave Stanley Miller and Harold Urey a concrete atmosphere to simulate in their famous 1952 spark-discharge experiment.