Fox, Ronald F. “Origin of life and energy.” Encyclopedia of Energy 4 (2004): 781-792.
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The phrase origin of life refers to those natural geophysical processes that may have occurred on the primitive Earth some 3.5-4.0 billion years ago that gave rise to life. Presumably there are no extant representatives of the earliest forms of life since they were surely driven to extinction by more advanced forms very early in the evolution of life. Fossils do not preserve macromolecular structures inside cells so that no direct fossil evidence exists. Even putative cellular fossils from 3.5 billion years ago, the stromatolites, are not absolutely proven to be what they appear to be. On the other hand, their similarity to contemporary formations deposited by bacterial mats topped with cyanobacteria, that may not be too different from the primitive organisms of 3.5 billion years ago, suggests that the metabolic and protein biosynthesis macromolecular machinery of contemporary cells was already developed that long ago. How these macromolecular structures came into being is the problem of the origin of life.