Some ancient microbes are reshuffling the chronology of life’s 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 Earth’s 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.

Dutkiewicz’s 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 paper.

Dutkiewicz’s 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 team’s 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 Earth’s 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.”

Jennifer Yauck

Links:
"Slushball life," Geotimes, December 2005

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