There is no fossil record of the evolution that preceded cellular life, but the process can be elucidated by molecular investigation of modern organisms. One issue is the differentiation between genotype and phenotype, mediated by the mechanisms of transcription and translation that express genes as proteins. In Chapter 1, W. Ford Doolittle and James R. Brown explore the question whether the last common ancestor of all life was a "progenote" endowed with a genetic information transfer system that was much more rudimentary than at present.
Nancy Maizels and Alan M. Weiner, in Chapter 2, conclude that transfer RNA-like molecules predate the progenote. They evolved, before the advent of messenger RNA and templated protein synthesis, as regulatory elements of replication. These molecules were later hijacked for their present function in amino acid transfer, which required the evolution of the bottom half of modern tRNA molecules, which includes the anticodon.
In 1944, when Simpson's Tempo and Mode was published, the only fossils known were animals and plants that had lived since the Cambrian Period, some 550 million years ago. Since the 1960s, numerous fossil remains have been discovered of prokaryotic microbes, some of which are more than 3 billion years old. These simple asexual organisms were ecological generalists that evolved at astonishingly slow rates. As J. William Schopf, in Chapter 3, puts it: "In both tempo and mode of evolution, much of the Precambrian history of life … appears to have differed markedly from the more recent
Phanerozoic evolution of megascopic, horotelic, adaptationally specialized eukaryotes."
Eukaryotic microfossils appeared in the late Paleoproterozoic, some 1700 million years ago. Their evolution was at first slow, but diversity and turnover rates greatly increased around 1000 million years ago. Evolutionary rates accelerated again during the Cambrian, as Andrew H. Knoll shows in Chapter 4: protistan diversity increases by a factor of two and turnover rates by a factor of ten.