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Ancient methane-makers

Living on Earth 3.5 billion years ago was tough. The sun was faint and weak, and there were no plants, no animals and not much oxygen. But some hardy methane-breathing microbes did make a go of life back then, and they were living and “breathing” 700 million years earlier than previously thought, according to new geologic evidence. The discovery helps fill in the gaps about Earth’s early climate and the evolution of life on the planet.

Researchers extracted methane gas from hydrothermal dikes in Western Australia and say that microbes produced the gas, which is evidence of some of Earth’s earliest life. Photo is by Yuichiro Ueno.

Life is thought to have emerged during the Archaean era, between 3.8 and 2.5 billion years ago, based on rocks from the time that contain carbon from a living source. Geochemist Yuichiro Ueno of the Tokyo Institute of Technology in Japan and colleagues have been studying ancient rocks from this period, looking for any traces living organisms might have left. Success came in 2001, when Ueno’s group found organic material in thin veins of rock from a region in Western Australia called North Pole (oddly enough, one of the hottest places on Earth).

The organic material’s origin was still a mystery, but Ueno says that he had a hunch it came from methane-producing microbes, which had existed in the latter part of the Archaean era, according to some geochemical evidence. So Ueno’s team sought to find what these critters would>> have left behind: methane gas.

They examined gas in tiny bubbles trapped within growing quartz rocks dated to 3.5 billion years ago. The bubbles contained mainly carbon dioxide and water, with tiny amounts of methane. Before attributing this gas to microbes, however, researchers had to rule out other potential methane sources, including the breakdown of organic material and natural processes, such volcanic or geyser eruptions.

Methane from microbial metabolism has a distinct geochemical fingerprint — it contains more lightweight carbon isotopes than methane from other sources. Measuring the isotopic contents of the methane’s carbon atoms, Ueno and co-workers found that 56 percent of the gas contained lightweight carbon atoms, a percentage consistent with the methane having come from a living source, the team reported March 23 in Nature.

They also checked to be sure that the gas was a byproduct of the formation of hydrocarbons, which are created by the compression of organic material over time. Hydrocarbon levels in the bubbles were low, however, reaffirming the researchers’ assertion that the methane came from the microbial metabolism.

The discovery, Ueno says, sheds light on the evolution of Earth’s earliest life. The three main branches of the evolutionary tree are the domains known as Eukarya, Bacteria and Archaea, the third group including methane-producing microbes and other “extremophiles.” Knowing now that Archaea existed so early in the evolution of life on Earth allows scientists to “clarify the minimum age of the two branches,” Ueno says. That information could help the researchers determine when the branches split off from each other and began forming the modern tree of life.

Some researchers caution that this evidence for microbes is only preliminary, noting that contamination and inaccurate dating of the source rocks can easily come into play when studying such old and delicate specimens. Martin Brasier, a paleobiologist at the University of Oxford in the United Kingdom says that the study is “promising, but not a confirmation of life.”

If accurate, the find also helps explain an early Earth paradox: During the Archaean, the young sun did not shine as brightly or as hot as it does today, yet Earth was still hot. “There must have been a different, a stronger greenhouse effect to counteract the effects of the weaker young sun,” says Roger Buick, an astrobiologist at the University of Washington in Seattle.

Scientists had previously thought carbon dioxide was responsible for some of the greenhouse effect at this time, but this microbial find and other evidence point toward methane, a greenhouse gas tens of times more powerful than carbon dioxide. Ueno’s find, Buick says, “provides evidence that methane was around at an appropriate time” to keep Earth toasty, and perhaps pave the way for other forms of life to evolve.

Katie Unger
Geotimes contributing writer

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