LONDON (Reuters) - Life on land began more than 1.4 billion years earlier
than scientists had thought, geologists said Wednesday.

Scientists have known that microorganisms have lived in oceans for about
3.8 billion years, but they weren't sure when early life forms made the
transition to land.

The oldest proof of terrestrial life had been found in 1.2
billion-year-old fossils from Arizona, but scientists in South Africa and the United States
have now discovered organic matter in 2.6 billion-year-old South African
rocks.

``This places the development of terrestrial biomass more than 1.4 billion
years earlier than previously reported,'' Yumiko Watanabe, of the
Pennsylvania State University, said in a study in the science journal
Nature.

Knowing when microorganisms made the transition from oceans to land is
important because it gives scientists new information about the presence
of oxygen that is needed to sustain life and the formation of the earth's
protective ozone shield.

Hiroshi Ohmoto, a geochemist at Penn State who contributed to the
research, believes there are even earlier samples of life on land.

He and his colleagues are planning to scour sites in Australia, Canada and
elsewhere to find them.

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Ancient South African Soils Point To Early Terrestrial Life
[http://www.psu.edu/ur/2000/biomats.html]

November 29, 2000

University Park, Pa. -- Remnants of organic matter in ancient soil more
than 2.6 billion years old may be the earliest known evidence for
terrestrial life, according to a team of Penn State astrobiologists.

"Our work shows that the organic matter in this soil very probably
represents remnants of microbial mats that developed on the soil surface
between 2.6 and 2.7 billion years ago," says Dr. Hiroshi Ohmoto, professor
of geochemistry and director of The Penn State Astrobiology Center. "This
places the development of terrestrial biomass more than 1.4 billion years
earlier than previously reported."

Evidence that microorganisms flourished in the oceans since at least 3.8
billion years ago exists, but when these microorganisms colonized on land
is not clear. The oldest undisputed remnants of terrestrial biomass have
been 1.2 billion-year-old microfossils found in Arizona.

Examining samples taken from Mpumalanga Province, South Africa, using a
variety of geochemical methods, the researchers report in this week's
issue of Nature, that a paleosol dating to between 2.6 and 2.7 billion
years ago contains organic carbon that was neither created by high
temperature fluids nor is the remnant of later petroleum migration, but is
in-situ biological in origin.

A paleosol is a layer of ancient soil, in this case buried and preserved
where it formed. Because the 55-foot thick layer of soil found at Schagen
is located between a layer of 2.7 billion-year-old serpentine and a 2.6
billion-year-old quartzite bed, the researchers can date the soil to
between 2.6 and 2.7 billion years ago. Showing that the carbon in the soil
is biological in origin and that it accumulated during soil formation is
much more difficult.

The researchers, who include Ohmoto; Yumiko Watanabe, Ph.D. candidate at
Penn State and at Tohoku University, Sendai, Japan; and Jacques E.J.
Martini, Geological Survey of South Africa, evaluated three possibilities
for the formation of reduced carbon in the soil.

The first of these was that the carbon was graphite crystals created when
the underlying serpentine formed under high temperatures. The graphite
then was concentrated during the soil formation.

"The crystallinity and hydrogen/carbon rations of the organic matter
suggest it is not of igneous or hydrothermal origin," says Ohmoto, a faculty
member in Penn State's College of Earth and Mineral Sciences.

The second possible origin of reduced carbon is liquid hydrocarbons
introduced after the soil formation ended. Materials introduced after
formation should show up along fractures in the rocks.

"The organic matter is almost always concentrated in clay-rich parts of
the rocks paralleling the ancient surface," says Ohmoto. "Organic matter
and clays are so intimately mixed together that the size and morphology of
individual 'grains' of organic mater can only be recognized under electron
microscopes."

The Penn State researchers conclude that the reduced carbon was not
produced by high heat and then incorporated into the soil as it formed, nor was it
deposited after the soil formed by migrating petroleum. The third
possibility is that the organic carbon represents remnants of biomats
developed on the soil surface. The researchers found that the organic-rich
clays in the upper portion of the paleosol appeared as seams between
fine-grained and coarse-grained layers of quartz.

"These features suggest that the organic matter in the uppermost soil zone
is an indigenous remnant of microbial mats that developed on the surface
of clay-rich soil during the rainy season," says Ohmoto. "The mats were
blanketed by aerosol deposits laid down during the dry season."

In the lower portion of the paleosol, things are less clear because the
effects of seeping water and the dissolution and precipitation of
materials suggest some decomposition. While identifying the organism in
the microbial mats is difficult, the researchers are certain that they
were not photosynthetic sulfur bacteria as there is no sulfur present.
Photosynthetic blue-green algae, however, are a likely possibility for the
mat formation because the ancient remnants have nearly identical carbon
isotope ratios as modern blue-green algal mats in fresh water.

The researchers are also certain that the mats formed on land, not in the
oceans, because the carbon isotope values for the carbon in the paleosol
are distinctly different from the organic carbon found in marine
sedimentary rock.

"Although terrestrial bacterial communities were predicted by previous
researchers, this is, to our knowledge, the first study presenting several
lines of evidence for an extensive development of microbial mats on soil
surfaces in the Archaean," says Ohmoto. "Our finding may then imply that
an ozone shield developed before 2.6 billion years ago.

"The ozone shield would have protected land-based biological forms from
the effects of cosmic radiation. Development of the ozone shield requires
an oxygen-rich atmosphere. Our finding of ancient biomats on land is an
important addition to a growing line of evidence suggesting that the rise
of atmopsheric oxygen took place more than 2.6 billion years ago."

The University receives research funding for this and other efforts
through the NASA Astrobiology Institute, a research consortium of academic,
non-profit and NASA centers including Penn State. NASA's Ames Research
Center is the agency's lead center for astrobiology, the study of the
origin, evolution, dissemination and future of life in the universe.

**aem**

POWRÓT