The Molecular Clock

For this article, Steve Bunk interviewed S. Blair Hedges, evolutionary biologist, Pennsylvania State University. Data from the Web of Science (ISI, Philadelphia) show that Hot Papers are cited 50 to 100 times more often than the average paper of the same type and age. S. Kumar, S. Blair Hedges, "A molecular timescale for vertebrate evolution," Nature, 392:917-20, April 30, 1998. (Cited in more than 170 papers since publication)

Thirty-five years ago, researchers proffered the remarkable hypothesis that substitution of amino acids in proteins, and of nucleotides in genes, occurs in a more or less clocklike fashion throughout the evolution of organisms.¹ But the use of these molecular evolutionary clocks often has yielded differing estimates of when any two given lineages diverged from a common ancestor. A major reason for such inconsistency is that divergence times have been based on sequence differences in a single protein or gene common...