What we do know is that by three and a half billion years ago the highly complicated molecule DNA had emerged. DNA is the basis for all life on Earth. It has a double-helix structure, like a spiral staircase, which was discovered by Francis Crick and James Watson in the Cavendish lab at Cambridge in 1953. The two strands of the double helix are linked by pairs of nitrogenous bases like the treads in a spiral staircase. There are four kinds of nitrogenous bases: cytosine, guanine, adenine and thymine. The order in which the different nitrogenous bases occur along the spiral staircase carries
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