But, based on the data currently at hand—evidence that is vastly improved over that available only a scant two decades ago—what can be said about the Proterozoic evolution of primitive prokaryotes? How did prokaryotes evolve over the nearly two billion years of Proterozoic time?
The answers: A lot can be said; even at present, a great deal is known. Surprisingly, however—in fact, absolutely remarkably—cyanobacteria in particular, and perhaps all prokaryotes in general, seem to have evolved hardly at all between early in the Proterozoic and the present day! That, indeed, is strange. The history of life, at least that of familiar Phanerozoic life, is one of change: Evolution, an unending progression of new forms of life . . . survival of only the fittest . . . the old supplanted by the new. But prokaryotic cyanobacteria seem to have played the evolutionary game by a different set of rules: What once succeeded, continued to succeed . . . survival of the already fit . . . if it’s not broken, don’t fix it!
Schopf, J. William, The Oldest Fossils and What They Mean, “Major Events in the History of Life”, Jones and Bartlett Pub., Sudbury, MA, p.22.