Samples of lava that erupted onto the ocean floor almost 3.5 billion years ago contain microscopic tubes that may have been created by microbes, researchers say. That scenario puts these structures among the oldest known physical remnants of life.
When lava oozes out at midocean ridges where Earth’s tectonic plates spread apart, water quickly chills the molten material as it moves across the ocean floor. The pasty rock often solidifies into rounded formations dubbed pillow lava. Marine microorganisms soon colonize the pillow-lava surfaces, where they exploit chemical energy to fuel their metabolism (SN: 11/15/03, p. 315: Attack of the Rock-Eating Microbes!). Many studies over the past decade have found that microbes thrive throughout the uppermost few hundred meters of the ocean floor, says Harald Furnes of the University of Bergen in Norway.
Now, analyses of ancient pillow lavas—reported by Furnes and his colleagues in the April 23 Science—suggest that such microbial colonization has been going on for billions of years.
Using microscopes, the researchers examined South African rock samples that formed on an ancient ocean floor between 3.48 billion and 3.22 billion years ago. The outermost centimeter or so of those pillow lavas is riddled with tubular structures that range from 1 to 9 micrometers in diameter and up to 200 µm in length.
The tubular structures, once filled with seawater, are now packed with fine-grained silicate minerals. X-ray analyses indicate that the tubules are lined with carbon, an element that appears in smaller concentrations elsewhere in the rocks. The ratio of the isotopes of carbon-13 and carbon-12 in the material coating the tubules is lower than that normally found in Earth’s crust, a clue that the substance was produced by—or is what’s left of—ancient microorganisms.
Both the presence of the carbon lining and its relative dearth of carbon-13 could be explained by other phenomena, says Tom Chacko, a geologist at the University of Alberta in Edmonton. Together, however, these characteristics—as well as the size and shape of the tubules themselves—strongly suggest that the features were created and populated by ancient microbes. Chacko notes that although previous studies have identified lower-than-normal carbon-13 concentrations in 3.85-billion-year-old rocks from Greenland, suggesting the presence of life at that time (SN: 11/9/96, p. 292: http://www.sciencenews.org/pages/sn_arch/11_9_96/fob1.htm), the more detailed physical evidence described by Furnes and his colleagues ranks among the oldest trace fossils ever reported.
The possibility that signs of ancient life on Earth can persist in rocks for more than 3 billion years makes the find “very exciting,” Chacko continues. He notes that some areas of Mars today are littered with volcanic rocks that may have an underwater origin.