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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 hypoth

By | November 13, 2000

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 to the two lineages being studied. This paper helped to break that pattern, by analyzing 658 nuclear genes representing 207 vertebrate species. It showed that the ancestors of most contemporary mammals arose more than 80-110 million years ago, well before the extinction of dinosaurs.

This finding did more than merely challenge the hypothesis that mammals filled ecological niches left by the departed dinosaurs 65 million years ago. It also demonstrated an average difference of 30 percent between divergence times estimated by the molecular clock method and those determined by the traditional, fossil-based method. The conclusion followed that large gaps exist in the fossil record for the Mesozoic era, 65-250 million years ago.

Blair Hedges, an evolutionary biologist at Pennsylvania State University, has little doubt about why the paper by he and coauthor Sudhir Kumar (now at Arizona State University) has been cited in so many other papers: "Biologists in general really seek out this type of information. They want to know why and when organisms originated, evolved, and differentiated from each other."

A key forerunner of this study, he notes, was their 1996 paper providing a similar result with only 50 genes and implicating continental breakup in the splitting of bird and mammal lineages.2 Last year, his team published a study showing that at least six animal phyla are Precambrian in origin, arising more than 400 million years before their first appearance in the fossil record.3 It could be that the so-called "Cambrian explosion" of fossils about 530 million years ago was preceded by ancestors that left a scant fossil record because they were soft-bodied and either very small or microscopic.

Researchers have not determined why the clocklike substitution of sequences occurs. According to the prevailing neutral theory, most mutations that eventually spread throughout an entire population are not under strong natural selection. They are selectively neutral, random occurrences that yield a constant relationship to time, like the degradation of isotopes in radiometric dating of rocks or minerals. "That's only a theory and, to be honest, you don't need a theory for a molecular clock," Hedges points out. "It's an empirical clock."

One of his team's current projects, in which a large number of genes is being examined, is a joint endeavor with Pennsylvania State University anthropologist Alan Walker. "We are looking at the timing of the great ape divergences, and how it constrains the evolution of the apes and the fossil record," Hedges says. Another study is an extension of this papar, involving analysis of several- fold more genes than the original effort.

"When thousands of genes are used, the amount of data is staggering," he notes. "So people should realize that molecular clocks can only get better."

Steve Bunk (sbunk@uswest.net) is a contributing editor for The Scientist.

References

1. E. Zuckerkandl, L. Pauling, "Evolutionary divergence and convergence in proteins," in V. Bryson and H.J. Vogel, eds., Evolving Genes and Proteins, New York, Academic Press, 1965, pages 97-165.

2. S.B. Hedges et al., "Continental breakup and the ordinal diversification of birds and mammals," Nature, 381:226-9, 1996.

3. D. Y.-C. Wang et al., "Divergence time estimates for the early history of animal phyla and the origin of plants, animals and fungi," Proceedings of the Royal Society of London Series B - Biological Sciences, 266:163-71, 1999.

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