The older and smaller a fossil is, the harder it becomes to recognize as a true record of life. But now a group of geoscientists has found microscopic tubules in 3.5-billion-year-old sea-floor rock they say must have been created by tiny organisms. If the ancient borings indeed had a biological origin, life was firmly established just a few hundred million years after giant impacts had sterilized the young Earth for the last time.
This new contender for oldest signs of life on Earth comes from South African rock that formed as lava oozing across a sea floor 3.5 billion years ago. Geologists Harald Furnes and Neil Banerjee of the University of Bergen, Norway, and their colleagues have found tubular structures sprouting from mineral-filled fractures in what was once the glassy rind of submarine lavas. These ancient microtubules, which average 4 micrometers in width and 50 micrometers in length, bear a striking resemblance to microtubules created by microorganisms in modern sea-floor pillow basalts.
There are several similarities. In addition to their size and shape, some of the ancient microtubules contain lingering organic matter. The carbon in the lava rind is isotopically lighter, which could be life's doing, they report in the 23 April issue of Science. That suite of features convinces Furnes and his colleagues that they have found a “3.48-billion-year-old biomarker.” Others agree. “I think they've got the best evidence I've seen for life at that time," says petrologist Martin Fisk of Oregon State University in Corvallis. But some are a bit more cautious. “This isn't a smoking gun,” says microbial geochemist Jennifer Roberts of the University of Kansas in Lawrence. Abiotic chemical reactions, she notes, might move a few elements and isotopes around while carving out tubules in glass.