The ancestors of major groups of animal species began populating Earth more than 600 million years earlier than indicated by their fossil remains, according to the largest study on the subject using gene sequences, recently completed at Penn State. The research suggests that animals have been evolving steadily into different species for at least 1200 million years, which challenges a popular theory known as the Cambrian Explosion that proposes the sudden appearance of most major animal groups, known as phyla, 530 million years ago.
A paper describing the research will be published in the January 22, 1999, issue of the Proceedings of the Royal Society of London (Series B) by Penn State Undergraduate Student Daniel Y.-C. Wang, Postdoctoral Fellow Sudhir Kumar, and Associate Professor of Biology S. Blair Hedges.
To gauge the pace of evolution, the research team tested hundreds of gene sequences to find those that developed mutations at a constant rate over time. "Because mutations start occurring at regular intervals in these genes as soon as a new species evolves--like the ticking of a clock--we can use them to trace the evolutionary history of a species back to its actual time of origin," Hedges explains.
By comparing individual genes in pairs of species, the researchers identified 75 nuclear genes that had accumulated mutations at a fairly constant rate relative to one another during their evolution. The genes were from species representing three major taxonomic groups, or phyla, of animals (arthropods, chordates, and nematodes), plus plants and fungi.
The scientists then calibrated these molecular clocks to an evolutionary event
well established by fossil studies--the divergence of birds and mammals about
310 million years ago. "A clock isn't any good unless it is calibrated to a
time that everyone else agrees on," Hedges explains, "and just about everyone
agrees on the date when reptilian ancestors of birds and mammals appeared
Contact: Barbara K. Kennedy