Bubbles may have sheltered Earth’s early life

For Earth’s early inhabitants, living in a bubble was a good thing.

Pockets of gas trapped along ancient shorelines gave microbes a cozy place to call home about 3.2 billion years ago, scientists suggest December 4 in Geology. Such a snug hideout could have shielded microbes from ultraviolet radiation not only on Earth, but perhaps on Mars as well.

The new work is “exciting and very plausible,” says geologist Frances Westall of the French National Center for Scientific Research in Orléans. “It expands the known habitats for early life.”

Earth was a tough place to live a few billion years ago. No oxygen in the atmosphere meant no ozone, and therefore no protection from the sun’s ultraviolet rays, says study coauthor Alessandro Airo, a geobiologist at the Free University of Berlin in Germany.

Still, microbes found a way to survive. In many places around the world, these organisms glued sand and cells together, forming slimy carpetlike biofilms, or mats, underwater. Iron dissolved in the water could have screened out radiation, Airo says. During low tide, “surface microbes might get zapped by UV and die,” he says, but communities of microbes living below them may have thrived.

Such communities could have colonized trapped bubbles of gas — perhaps a by-product of microbial metabolism. Inside the sheltered bubble chambers, ancient bacteria might have weathered the harsh conditions of early Earth, says study coauthor Martin Homann, also of the Free University of Berlin.

Until now, the oldest evidence of bubble-dwelling microbes came from 2.75-billion-year-old rocks from Western Australia. Homann and colleagues examined even older rocks, from the oldest records of tidal environments on Earth: 3.22-billion-year-old sandstone from South Africa.

From 2011 to 2013, Homann collected 350 kilograms of this sandstone and then cut and polished slices to look for signs of life. The gas pockets, or cavities, that once formed within the mats have long since filled with fine-grained crystals of quartz. But in the quartz, the researchers saw several clues.

What appeared to be ancient biofilms hung down from the tops of shallow cavities, like tiny stalactites dripping from cave ceilings. And the biofilms exhibited a chemical hallmark of life: a ratio of heavy to light forms of carbon that’s typically found only in living organisms. The quartz also contained microfossil imprints of cells. High-powered microscopes revealed ghostly impressions of these cells linked together in chains, just like those formed by bacteria today. 

Taken together, the clues are “hard evidence for the presence of microbes,” Airo says.

Geomicrobiologist Mark Van Zuilen of the Institute of Earth Physics of Paris agrees. The work is a “sound, basic observation that there were traces of life in these rocks,” he says.

If microbes once survived in these pockets on early Earth, they could potentially have done so on other planets too, Airo says. On Mars, in particular, he says, “little cavities below the surface are a wonderful niche for rovers and future missions to hunt for life.”