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 because it is based on well-accepted studies of fossils." Using this date as a secure calibration point--and the mutation rate for each of the constant-rate genes as a timing device--the researchers were able to determine how long ago each of the species originated.
"Not only are all these genes telling us that a wealth of animal species in at least three phyla were already on Earth millions of years before their fossils start appearing," Hedges says, "but they also are telling us when three of the major kingdoms of living things--animals, plants, and fungi-- first diverged from a common ancestor and began evolving down separate evolutionary paths." That date--about 1.6 billion years ago--is the earliest yet obtained by gene studies for this evolutionary event, according to Hedges.
The Penn State team used more than twice as many genes to date the origin of the three major animal phyla as had been used in any other study since gene sequences first became available in the Genbank public databases maintained by the National Institutes of Health (NIH) during the 1970s. "We wanted to have so much data that the conclusions from our study of this controversial issue could be very robust," Hedges comments. Earlier studies using many fewer genes were disturbing to some researchers because they yielded a wide range of dates for the origin of animal species, although all the gene studies agreed that the event occurred well before the Cambrian period. "Our methodology and our larger data set should have had a stabilizing effect; and in fact, our study resulted in a date intermediate between the earlier estimates," Hedges says.
If the results of his team's genetic study are correct, Hedges says the scientific question must change from "How did all these species evolve so suddenly early in the Cambrian period?" to "Why don't we see any fossils of these species long before the Cambrian period?" Among the suggested answers are that changes in the Earth's atmosphere led to the development of hard external skeletons in animals that had only soft external skeletons before the Cambrian period. "Hard body parts like external skeletons are most likely to become fossils," Hedges explains. Species not likely to fossilize, like earthworms, typically live and die without leaving a trace of their existence--except in the genes of their descendants.
Another hypothesis is that many species of animals with skeletons were living on Earth before the Cambrian period, but they were so small that their fossils have not yet been found. "The further back in time you want to look in the fossil record, the fewer places there are on Earth to look," Hedges explains. Fossils have to be safely encased in sedimentary rock, which, over time, melts or becomes deformed by the movement of the Earth's crust. Sedimentary rocks over 3 billion years old are very rare. "If we can find very-old and very-fine-grained phosphate sediments, which can preserve even soft bodies, we might have the potential of finding fossils of these early animals, even if they were only microscopic in size," Hedges says. "We seem to be missing the fossils of a lot of species."
Hedges says his research might be useful for finding life on other planets. "If we can learn when different stages of life evolved on Earth, we can compare those dates to events in the chemical evolution of Earth's atmosphere and ocean, such as when oxygen and other important gases increased," Hedges explains. Research with this goal is an important focus in Penn State's Astrobiology Research Center. "Our goal is to see if the early history of life on Earth can give us clues for how to predict life on other planets and in other solar systems," Hedges says. "We hope to be able to predict the kinds of lifeforms that are likely to exist on other planets, based on those that existed during Earth's history, just by measuring the chemical content of the planet's atmosphere."
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