Old rocks found in the Australian Outback have some weighty implications, scientists say: They hint at the environment in which life on Earth originated and suggest a location to search for life on Mars.
Scientists in Australia say they have found biological signatures of life in rocks that also show the presence of a hot spring, lending weight to a theory that the earliest life on Earth might have originated in freshwater hot springs on land rather than in deep-sea hydrothermal vents.
The fossil finding predates the previous oldest evidence for life on land by almost 600 million years, the scientists say. They described their findings in the journal Nature Communications.
For decades, the rock formation known as the Pilbara Craton in Western Australia has provided scientists with early evidence of life, NPR contributor Marcelo Gleiser recently explained:
"The dates there can be pushed to 3.48 billion years ago. How do scientists date such old rocks? Using well-known radioactive decay techniques based on unstable isotopes found on the site. (In other words, they look for radioactive minerals near the old life sites and date them together.) Interestingly, chemical fingerprints from the mineralization process show that those living creatures had already a complex metabolism, indicating that earlier, simpler life existed."
Scientists initially thought the area had been filled with shallow water; later evidence suggested it may have been volcanic.
But samples collected by University of New South Wales scientist Tara Djokic provided what she called a "smoking gun" that the area — now a barren wilderness — was once home to a volcanic hot springs system. She says the rocks there contain deposits of geyserite, a material found only in a hot springs environment.
In those same rocks, researchers found what they call "a suite of microbial biosignatures indicative of the earliest life on land." Those include multiple "textures" associated with life, such as stromatolites — described as "layered rock structures created by communities of ancient microbes."
There are also preserved bubbles in the rock. And while it's not entirely clear how they were formed, Djokic says the fact that they were preserved for billions of years suggests the bubbles were once trapped within a sticky substance associated with microbial activity, rather than simply gas.
Scientists have long debated whether early life it was fueled by chemicals in deep-sea hydrothermal vents or, alternatively, in a hot springs-like environment, as Charles Darwin once suggested in a 1871 letter to a friend:
"But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etcetera present, that a protein compound was chemically formed, ready to undergo still more complex changes."
This recent finding, Djokic says, is "a geological perspective saying actually, really early on we're already seeing life on land. So it just lends weight potentially to an argument suggesting that the origin of life on land might be something to consider."
It could also suggest where to look in the search for life on Mars.
NASA is currently considering where to land the rover on its 2020 Mars Exploration Mission, and one of the sites is a "hot spring-type setting," about the same age as the early Earth, Djokic says.
"If you're going to look for life on Mars, we know it was preserved on hot springs here on the ancient earth," she says. "So there's a good chance if it ever developed on Mars, then it would probably be preserved in hot springs there, too."
Djokic and her colleagues participated in a February NASA workshop that narrowed down the landing-site candidates to three from eight, including the hot springs-like site.
Studies on ancient fossils are often controversial, and this one is no exception. MIT's Tanja Bosak, who specializes in signatures of microbial processes in ancient sedimentary rocks, tells NPR that she is thinks more evidence is needed to prove the bubbles were formed through biological stabilization. She says relating the results to Mars "is taking this a step too far."
But Djokic says the rocks are so well-preserved that the scientists were able to observe multiple clear signs of life in an ancient, volcanic hot springs setting.
These patterns are "exactly what we see in modern hot springs. So it fits the bigger picture of these converging lines of evidence, and that's what makes the argument in the case so strong."