Calibrating And Constraining Molecular Clocks

Abstract For well over 200 years, natural scientists have used fossils with varying degrees of confidence to date the evolution of life. The field has advanced dramatically in the last few years, and it would be useful now to review some of the key issues and to suggest an outline of a modus operandi for the future. In explaining the role of fossils in establishing timescales, it is useful to review the historical sequence in which key observations were made. Much of this early history predates the 1960s concept of the molecular clock; but the way in which fossils should be used today depends crucially on those earlier geological and paleontological observations. In presenting these observations in a logical sequence, we highlight what can and cannot be done with fossils, and link this to the relative strengths and weaknesses of molecular data. The value of carrying out this survey now is that it is not framed in the old and rather worn narrative of a “conflict” between fossils and molecules (e.g., 1, 2). It is no longer a question of which is better than the other, or how far one can go with one or the other source of data. It is evident that both fossils and molecules have great strengths, but in recognizing the weaknesses of each source of data, a realistic plan for collaboration between paleontologists and molecular biologists can be proposed (3, 4).