Bubbles may have sheltered Earth’s early life
The fossils offer signs of microbes from 3.2 billion years ago
Earth offered a harsh environment for its earliest inhabitants. It now appears that some of them survived by living in a bubble. A beach bubble.
The earliest life would have been single-celled microbes. Pockets of gas trapped along ancient shorelines would have given them a cozy place to call home about 3.2 billion years ago. Or so scientists reported December 4 in Geology. Such a snug hideout could have shielded microbes from ultraviolet (UV) radiation. And not only on Earth, the scientists say, but perhaps on Mars as well.
The new work is “exciting and very plausible,” says Frances Westall. She’s a geologist at the French National Center for Scientific Research in Orléans. This environment “expands the known habitats for early life,” she says.
Today, high in the stratosphere, a thick layer of ozone blankets the planet. Ozone, made from three bound oxygen atoms, helps filter the sun’s harmful UV rays from Earth’s surface. But back a few billion years go, the atmosphere lacked oxygen, notes study coauthor Alessandro Airo. He’s a geobiologist at the Free University of Berlin in Germany.
Yet even without the protective ozone layer, microbes found a way to survive back then. In many places around the world, these organisms glued sand and cells together. This formed slimy carpetlike underwater mats, called biofilms. 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. Yet communities of microbes living below them may have thrived.
Such communities could have colonized trapped bubbles of gas. Those bubbles may have been a product of the microbes’ metabolism. Inside the sheltered bubble chambers, ancient bacteria might have survived the harsh conditions of early Earth, says study coauthor Martin Homann. He, too, works at the Free University of Berlin.
Until now, the oldest evidence of bubble-dwelling microbes came from rocks that were 2.75 billion years old. They were found in Western Australia. The rocks that Homann and his colleagues are describing now are even older. They date back to the oldest records of tidal environments. These sandstone deposits are 3.22 billion years old.
From 2011 to 2013, Homann collected 350 kilograms (770 pounds) of this sandstone in South Africa. His team cut and polished slices of the rock, looking for signs of life. The gas pockets, or cavities, that had formed in the biofilms are long gone. Over time, they filled with fine-grained crystals of a type of crystalline rock known as quartz. But in the quartz, the researchers saw several clues.
They saw what looked like tiny icicles of rock hanging down from the tops of the shallow cavities. And the remnants of these biofilm fossils showed a chemical hallmark of life. It was a mix of heavy and light forms of carbon. This proportion of heavy-to-light carbon typically occurs only in living organisms.
The quartz also contained microfossil imprints of cells. High-powered microscopes revealed ghostly impressions of these cells. They appeared linked together in chains, just like those that bacteria form today.
Taken together, the clues are “hard evidence for the presence of microbes,” Airo says.
Geomicrobiologist Mark Van Zuilen works at the Institute of Earth Physics of Paris, France. And he 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.”
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bacterium (plural bacteria) A single-celled organism. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.
biofilm A gooey community of different types of microbes that essentially glues itself to some solid surface. Living in a biofilm is one way microbes protect themselves from stressful agents (such as poisons) in their environment.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
cavity (in geology or physics) A large open region surrounded by tissues (in living organisms) or some rigid pocketlike structure.
crystal (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.
fossil Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils. The process of forming fossils is called fossilization.
geobiology The study of how living things interact with Earth, its atmosphere and other geological features (such as rock, soil and waterways). People who work in this field are known as geobiologists.
habitat The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.
Mars The fourth planet from the sun, just one planet out from Earth. Like Earth, it has seasons and moisture. But its diameter is only about half as big as Earth’s.
metabolism The set of life-sustaining chemical reactions that take place inside cells and bigger structures, such as organs. These reactions enable organisms to grow, reproduce, move and otherwise respond to their environments.
microbe Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.
niche A small or narrow pocket that sets something apart, or perhaps offers a region of protection.
ozone A colorless gas that forms high in the atmosphere and at ground level. When it forms at Earth’s surface, ozone is a pollutant that irritates eyes and lungs.
oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.
quartz A clear mineral made of silica crystals — composed of silicon and oxygen. It is one of the most common minerals on Earth’s surface.
rover A carlike vehicle, such as those designed by NASA to travel across the surface of the moon or some planet without a human driver. Some rovers also can perform computer-driven science experiments.
sandstone A type of sedimentary rock. It formed as sand-size grains of mineral grit became compacted or glued together over time.
tides (adj. tidal) The alternate rising and falling of the sea, usually twice in each lunar day at a particular place, due to the attraction of the moon and sun.
ultraviolet A portion of the light spectrum that is close to violet but invisible to the human eye.