Microbes reshuffle Earths early history
Some ancient microbes are reshuffling the chronology of lifes early evolution
on Earth. Not only are the organisms apparently hundreds of millions of
years older than previously thought, but they also are challenging current
understanding of climatic conditions of the time having possibly
developed before oxygen was present in the atmosphere and even survived
a global deep freeze.
The first eukaryotes organisms with complex nucleus-containing
cells and ancient cyanobacteria bacteria that produce oxygen
through photosynthesis are ancestors of modern-day life. Previously,
scientists used microfossil evidence to date the earliest eukaryotes to
about 1.8 billion years old, and the earliest cyanobacteria to about 2.1
billion years old. Now, Adriana Dutkiewicz, a geologist at the University
of Sydney in Australia, and colleagues present new evidence suggesting
both types of organisms existed as early as 2.45 billion years ago.
The finding implies that early life forms lived prior to a dramatic rise
in oxygen in Earths atmosphere, the team reported in the June issue
of Geology and could even be responsible for the change.
The apparent survival of eukaryotes through this period also challenges
the snowball Earth theory, which suggests that the planet
was covered from pole to pole in a sheet of glacial ice more than 800
meters (one-half a mile) deep.
Dutkiewiczs team discovered the ancient life by looking at miniscule
droplets of oil contained in tiny vacuoles within the quartz grains of
2.45-billion-year-old rocks at Elliot Lake in Ontario, Canada. The oil
is a product of organic matter that decayed before becoming trapped as
the rocks were cemented. The researchers extracted the oil and analyzed
it for biomarkers, molecules that are produced by specific
life forms and are therefore indicative of their presence. In this case,
the researchers found biomarkers for both eukaryotes and cyanobacteria.
Studies in 1999 and 2003, in which researchers looked for biomarkers
in solid organic matter taken from between the grains of sedimentary rocks,
also found evidence of the earlier existence of eukaryotes and cyanobacteria.
Those studies, however, were unable to rule out the possibility the biomarkers
were contaminants from a more contemporary source, says John Ridley of
Colorado State University in Fort Collins and co-author on the Geology
Dutkiewiczs team could rule out contamination, Ridley says, because
its source of biomarkers the oil was sealed tight within
the quartz grains. This seal effectively isolated it from any fluid
that moves along grain boundaries at any time in the later history of
the rocks, he says.
The teams finding pushes a clear pin in our chronology,
Ridley says, by placing the evolution of eukaryotes and cyanobacteria
about 150 million years before both the oxidation of Earths atmosphere
and the snowball Earth period. It demonstrates these life forms
were there, and the glaciations came afterwards, he says.
Previously, some scientists have suggested the melting glaciers after
the snowball Earth period created an environment that gave life a kick
start. The new finding, however, raises questions about whether cyanobacteria
may have instead helped bring about the glaciation by contributing oxygen
to the atmosphere, Ridley says. The oxygen would have destroyed an insulating
layer of methane in the atmosphere, leading to a colder global climate.
The chain of events that led to the glaciation, however, remains in contention.
Frank Corsetti of the University of Southern California in Los Angeles
says the study indicates the organisms far predate the glaciation,
too much to be linked to the cause as directly. Still, he
says, the finding is significant in that it confirms that such creatures
lived before the oxygen event and that it may end up being one of
the oldest, if not the oldest, report of eukaryotic biomarkers on Earth.
The finding also raises questions about how eukaryotic life survived
the global freeze. One possibility, says Brent Christner of Louisiana
State University in Baton Rouge, is that eukaryotes are hardier than suspected.
Another scenario scientists are considering is that Earth was actually
more like a slushball than a snowball, with parts of the ocean
near the equator probably never freezing over completely (see Geotimes,
December 2005). In that case, Christner says, eukaryotes may
have found refuge in shallow marine environments or freshwater ponds where
both light and liquid water were available.