Life, origin of

The appearance of maggots on meat or of intestinal tapeworms supported an ancient belief in the spontaneous generation of life. This idea was challenged in the seventeenth century but not abandoned before Pasteur’s experiments. Scientists now agree that terrestrial life had a single origin, but differ in explanations. Some believe that life began with the onset of protein-based metabolism, supported by evidence of spontaneous abiotic amino acid synthesis and theoretical models of self-sustaining and evolving systems of enzymes. Others believe life began with the appearance of nucleic acid-based molecular replicators and have organized their research efforts around the vision of a primordial ‘RNA world’.