HOUSTON - Scientists have found traces of bacteria living more than 3.7 billion years ago, an age that would make them — if confirmed — the oldest-known fossils and bolster the idea that life got off to a running start on Earth, and perhaps elsewhere.
The bacteria lived near hydrothermal vents, cracks in the seafloor that gush hot, mineral-laden water into the open ocean, say the scientists who identified the fossils. Verification of the finding could make ocean vents on other planets in the solar system a key target in the search for extraterrestrial life.
“It’s exciting to find life had managed to get a grip and start to evolve on Earth so quickly after the planets formed,” says Matthew Dodd of Britain’s University College London, co-author of a study in this week’s Nature that describes the fossils. “It gives me … high hopes of finding life elsewhere in the universe.”
The rock in which the fossils reside is at least 3.77 billion years old and could be 4.28 billion years old. The next-oldest confirmed fossil is 3.5 billion years old, the study says. Last year, scientists reported the discovery of stromatolites — formations built by ancient microbes — dating to 3.7 billion years ago.
When Dodd began examining slivers of rock collected from northeastern Quebec, he saw something strange: thread-like filaments half as wide as a human hair and slightly thicker cylinder-shaped tubes. Both filaments and tubes were composed of an iron-rich mineral.
Similar filaments are excreted by modern bacteria that consume iron, says co-author Dominic Papineau, also of University College London. Such filaments and tubes have also been seen in much younger “microfossils,” named because of their microscopic size, found in Norwegian rocks.
The researchers also spotted rosette-shaped formations, which the scientists argue could have blossomed through a chemical process that began with rotting bacteria. The rosettes are freckled with dots and shards of other chemicals linked to life, such as phosphorus, a key ingredient for biological activity.
The rock formations around the fossils hint that the microbes lived on the seafloor around hydrothermal vents or in the water near vents. Like modern iron-dependent bacteria, they “would have literally ‘eaten’ the iron … in the same sense that we eat cake,” Papineau says, adding the bacteria could have formed rust-colored mats like those seen today.
The researchers collected multiple forms of evidence to back their claim, which strengthens their case, says Christopher House of the Pennsylvania State University, who was not involved in the study. Though the data are less definitive than the evidence for life in younger rocks, he says, this “may be as good as it gets for as old as these rocks are.”
“There’s enough in here to get me interested but not enough to make me convinced,” says Michael Tice of Texas A&M University, who also wasn’t part of the study. He questions, for instance, the study’s claim that the microfossils lived near a hydrothermal vent, which could influence where scientists send robots to look for signs of life on Mars. The rocks’ chemistry doesn’t necessarily signal a location near a vent, he says.
Dodd says the rocks around the fossil-containing layers contain chemical signatures of a hydrothermal vent. The evidence so far, he says, raises the possibility that life arose on Jupiter’s watery moon Europa or even closer to home.
“You have life on Earth at a time when we believe there was liquid water on the surface of Mars, and the atmosphere wouldn’t have been too drastically different,” Dodd says. “Maybe there was life on Mars in the past and we have yet to find it.”