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- 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.
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- 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.
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- 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.
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- 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.
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- "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.
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- 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.
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- 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.
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- 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."
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- 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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- This research was supported by grants
from the National Science Foundation and the National Aeronautics and Space
Administration.
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