The largest genetic study ever
performed to learn when land plants and fungi first appeared on the Earth
has revealed a plausible biological cause for two major climate events:
the Snowball Earth eras, when ice periodically covered the globe, and
the era called the Cambrian Explosion, which produced the first fossils
of almost all major categories of animals living today.
According to the authors of the study, which will be published in the
10 August 2001 issue of the journal Science, plants paved the way
for the evolution of land animals by simultaneously increasing the percentage
of oxygen in the Earth's atmosphere and decreasing the percentage of carbon
dioxide, a powerful greenhouse gas.
"Our research shows that land plants and fungi evolved much earlier
than previously thought--before the Snowball Earth and Cambrian Explosion
events--suggesting their presence could have had a profound effect on
the climate and the evolution of life on Earth," says Blair Hedges,
an evolutionary biologist and leader of the Penn State research team that
performed the study.
The researchers found that land plants had evolved on Earth by about
700 million years ago and land fungi by about 1,300 million years ago--much
earlier than previous estimates of around 480 million years ago, which
were based on the earliest fossils of those organisms. Prior to this study,
it was believed that Earth's landscape at that time was covered with barren
rocks harboring nothing more than some bacteria and possibly some algae.
No undisputed fossils of the earliest land plants and fungi have been
found in rocks formed during the Precambrian period, says Hedges, possibly
because their primitive bodies were too soft to turn into fossils.
The early appearance on the land of fungi and plants suggests their plausible
role in both the mysterious lowering of the Earth's surface temperature
during the series of Snowball Earth events roughly 750 million to 580
million years ago and the sudden appearance of many new species of fossil
animals during the Cambrian Explosion era roughly 530 million years ago.
"Both the lowering of the Earth's surface temperature and the evolution
of many new types of animals could result from a decrease in atmospheric
carbon dioxide and a rise in oxygen caused by the presence on land of
lichen fungi and plants at this time, which our research suggests,"
"An increase in land plant abundance may have occurred at the time
just before the period known as the Cambrian Explosion, when the next
Snowball Earth period failed to occur because temperatures did not get
quite cold enough," Hedges says. "The plants conceivably boosted
oxygen levels in the atmosphere high enough for animals to develop skeletons,
grow larger, and diversify."
Lichens are believed to have been the first fungi to team up with photosynthesizing
organisms like cyanobacteria and green algae. Lichens can live without
rain for months, providing protection for photosynthesizing organisms,
which produce oxygen and release it into the atmosphere. The researchers
suggest that the pioneer lichen fungi, which produce acids strong enough
to dissolve rocks, also could have helped to reduce carbon dioxide. When
washed away by rainwater, the calcium released from the lichen-encrusted
rocks eventually forms calcium carbonate limestone in the ocean, preventing
the carbon atoms from forming the greenhouse gas, carbon dioxide, in the
Land plants also can lower levels of carbon dioxide in the atmosphere.
They have molecules called lignins, which contain carbon but do not readily
decompose. After the plant dies, some of its carbon remains locked up
in the lignins and can become buried in the Earth through geologic processes,
preventing those carbon atoms from returning to the atmosphere and effectively
lowering atmospheric carbon dioxide.
"The Earth cools when you take away carbon dioxide," Hedges
says. "Other factors such as the location of the continents may have
had some effect in cooling the atmosphere and creating periods of Snowball
Earth, but I suspect the biggest cooling effect came from the reduction
of carbon dioxide in the atmosphere by fungi and plants, which we have
shown were living on the land at that time."
Fossil fuels like coal and oil are made from plant material, containing
carbon that was taken out of the atmosphere and buried in swamps millions
of years ago. Releasing those same carbon atoms back into the atmosphere
by burning fossil fuels appears to be causing the Earth to get warmer
again, according to many studies.
Hedges and his research team made their surprising discoveries about
the early appearance on Earth of the first land plants and fungi by studying
as many of the genes as possible of their descendants--the species of
plants and fungi living today. They began by sifting through their molecular
fingerprints--the unique sequences of amino-acid building blocks--in many
thousands of genes from hundreds of species archived in the public gene-sequence
Eventually, they found 119 genes common to living species of fungi, plants,
and animals that met the researchers' stringent criteria for use as "molecular
clocks." Previous studies had used a single gene. By detailed comparisons
of the amino-acid sequences of individual genes among numbers of species,
the scientists identified those genes that had accumulated mutations at
a fairly constant rate relative to one another during their evolution.
"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
time of origin," Hedges explains.
The scientists calibrated each of their gene clocks with evolutionary
events well established by fossil studies, primarily those in the history
of animals. Using these known dates as secure calibration points, 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
Hedges says his research might help in the search for life on other planets
by providing a link between the different stages of life's evolution on
Earth and the timing of events in the chemical evolution of Earth's atmosphere,
such as the rise in oxygen. "Possibly the early history of life on
Earth can give us clues for predicting the kinds of lifeforms that are
likely to exist on planets in other solar systems from the chemical content
of their atmospheres," Hedges says.
In addition to Hedges, the Penn State research team includes Daniel
S. Heckman, an undergraduate student whose senior honors thesis formed
part of this research; David M. Geiser, assistant professor of
plant pathology; and undergraduate students Brooke R. Eidell; Rebecca
L. Stauffer; and Natalie L. Kardos. This research was supported,
in part, by the National Aeronautics and
Space Administration through the Penn State Astrobiology