NASA scientists recently proposed a new explanation for the rise of oxygen in Earth's early atmosphere - an event that may have jumpstarted the evolution of complex life.
The idea is suggested in two research papers from NASA's Ames Research Center in the heart of California's Silicon Valley. Both papers address how the Earth got its oxygen-rich atmosphere. One uses theoretical models and the other measurements of 'microbial mats,' communities of microorganisms similar to those found on early Earth. The papers are "The Role of Microbial Mats in the Production of Reduced Gases on Early Earth," by Tori Hoehler, et al., and "Biogenic Methane, Hydrogen Escape, and the Irreversible Oxidation of Early Earth," by David Catling, et al. They were published in the July 19 issue of Nature and the Aug. 3 issue of Science, respectively.
Catling's team argues that oxygen increased in Earth's atmosphere more than 2 billion years ago because hydrogen atoms from water hitched a one-way ride into space inside methane gas produced by primitive microbes. This irreversible loss of hydrogen, they say, left behind an excess of oxygen, which gradually filled the Earth's crust and then flooded Earth's atmosphere.
"Without oxygen, the most sophisticated life on Earth would have been green microbial scum," said Catling. "Fortunately, some bacteria in the early oceans were able to separate water into hydrogen and oxygen. The hydrogen was lost to space, leaving the oxygen behind."
Before 2.4 billion to 2.2 billion years ago, the Earth's atmosphere contained almost no oxygen and could support only single-celled forms of life. The first complicated cells, like the ones that make up today's plants and animals, appear in 2.1 billion-year-old fossils just after the rise of oxygen.
Hoehler and his team measured gases released from modern microbial mats in Baja, Mexico, under conditions simulating the early atmosphere. These mats are close cousins to those that once made up much of the early Earth's biosphere. The team found that the mats released large amounts of hydrogen at night. "If the Earth's early microbial mats acted similarly to the modern ones we studied, they may have pumped a thousand times more hydrogen into the atmosphere than did volcanoes and hydrothermal vents, the other main sources," Hoehler said.
Hoehler and his co-authors suggest that some of the hydrogen might have escaped directly to space, while the remainder could have provided an important food source for other microbes - such as those that produce methane. "We found that the elevated levels of hydrogen within the mats favor the biological production and release of methane. This supports the premise of Dr. Catling's work," Hoehler said. But either way, hydrogen escaped and the Earth became more oxidized.
Questions of how and why oxygen built up in the Earth's atmosphere have been controversial for decades. Although scientists have ample evidence that oxygen first appeared in the atmosphere a little more than 2 billion years ago, why this happened has long been the subject of speculation. Fundamentally, the oxygen in the air is a byproduct of photosynthesis. In photosynthesis, plants and microbes use sunlight to steal hydrogen from water. The hydrogen is mostly used to make organic matter from carbon dioxide and the unwanted oxygen is released. But microbes that make oxygen in photosynthesis were living on Earth at least a half-billion years before oxygen first flooded the atmosphere.
For oxygen to stay in the atmosphere, the hydrogen and oxygen (or the organic matter made from the hydrogen) must be kept apart. Otherwise, they will react with each other and the oxygen will disappear. Conventional theories have focused on the burial of dead organic matter deep in the Earth, where it is 'hidden' from atmospheric oxygen. The possibility that a lot of hydrogen might escape to space was largely ignored.
According to Catling, his theory of high levels of hydrogen-containing methane gas, which acquired its hydrogen indirectly from water, also would account for why early Earth didn't freeze. "Three billion years ago, the sun was only 4/5ths as bright as it is now. The Earth should have frozen over," he said. But methane, a powerful greenhouse gas, would have kept the Earth warm.
Other authors, all of NASA Ames Research Center, include Drs. Brad Bebout and David Des Marais on the Nature paper and Drs. Kevin Zahnle and Christopher McKay on the Science paper.
NASA's Exobiology and Astrobiology Programs provided funding for both projects.
"The advent of oxygenic photosynthesis on Earth may have increased global biological productivity by a factor of 100–1,000, profoundly affecting both geochemical and biological evolution. Much of this new productivity probably occurred in microbial mats, which incorporate a range of photosynthetic and anaerobic microorganisms in extremely close physical proximity. The potential contribution of these systems to global biogeochemical change would have depended on the nature of the interactions among these mat microorganisms."
"The low O2 content of the Archean atmosphere implies that methane should have been present at levels ~102 to 103 parts per million volume (ppmv) (compared with 1.7 ppmv today) given a plausible biogenic source. CH4 is favored as the greenhouse gas that countered the lower luminosity of the early Sun. But abundant CH4 implies that hydrogen escapes to space orders of magnitude faster than today. Such reductant loss oxidizes the Earth. Photosynthesis splits water into O2 and H, and methanogenesis transfers the H into CH4. Hydrogen escape after CH4 photolysis, therefore, causes a net gain of oxygen [CO2 + 2H2O --- CH4 + 2O2 --- CO2 + O2 + 4H (space)]. Expected irreversible oxidation (~1012 to 1013 moles oxygen per year) may help explain how Earth's surface environment became irreversibly oxidized."