There is a set of molecules common to all forms of life on Earth that are often described as ‘the molecules of life’. This should not be taken to imply that life can only be made from just these components, or that life on Earth always was. These molecules are parts of a machine, the essential design of which is also common to all life on Earth, but which is so complex and well organized that it looks like a product of evolution through natural selection, an evolution that was effectively frozen at some stage before a ‘last common ancestor’. There are hints in the structure of our biochemical pathways as to which of the present components of this machine came first, and intimations of an earlier altogether different genetic system in our ancestry.
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