Large Gene Study Questions
Cambrian Explosion
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."
This research was supported by grants from the National
Science Foundation and the National Aeronautics and Space
Administration.
Art
http://geology.swau.edu