A melting snow patch in Greenland recently revealed tiny mound-like structures. They don’t look like much. But they could just be the oldest signs of life on Earth. And they are a mind-boggling 3.7 billion years old. By studying them, scientists might learn about the rise of the first microbes shortly after Earth formed some 4.5 billion years ago.
Allen Nutman is a geologist at the University of Wollongong in Australia. His team has just reported on its discovery of those mini-mounds in the August 31 issue of Nature.
Unlike dinosaur bones, the new fossils are not preserved bits of an ancient critter. They are mounds of minerals. Called stromatolites (Stroh-MAT-oh-lytes), each is but a few centimeters tall. Scientists suspect clusters of microbes deposited the minerals. And it would have happened very long ago — perhaps a mere 800 hundred million years after Earth first formed.
Their shape and chemical recipe match those of mounds made by some modern bacteria. These current microbe mineral mounds appear in shallow seas, Nutman’s team notes.
But Abigail Allwood warns that it’s in no way certain that ancient microbes made the old mini mounds. An astrobiologist, Allwood works at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Something other than living organisms might have caused the minerals to clump onto the seafloor, she points out. Indeed, she argues, the new data are “a little bit thin for what you would usually want to see for such an extraordinary claim.”
Yet if they are super-ancient, the mounds would demonstrate that complex microbes arose early in Earth’s history. And that would support other genetic and chemical studies. Some of them had suggested basic life got its start more than 4 billion years ago. Indeed, Allwood says, if Earthly life emerged that early, then Mars might once have hosted life, too. Early conditions on both planets, she explains, were likely similar in terms of being able to support life.