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A New Contender for the Title of Oldest Fossil
The discovery of fossilized microbes within ancient rocks that formed around submarine-hydrothermal vents provides evidence that life existed at least 3,770 million years ago. If confirmed, these would be the earliest known microfossils on Earth.
In a new paper, DCO Deep Life Community members Matthew Dodd, Dominic Papineau (both at University College London, UK) and colleagues describe microfossils discovered in the Nuvvuagittuq belt in Quebec, Canada . These newly found ancient fossils are similar in appearance to fossils of filamentous microbes in younger rocks as well as cells in modern hydrothermal vent environments. The researchers also provide isotopic and mineral evidence supporting a biological origin of these structures. Their findings appear in the journal Nature.
The current record holder for the oldest confirmed sign of life is a set of microfossils located in Australian rocks dating to about 3,500 million years ago. Due to the metamorphic processes that have heated and squeezed the oldest sedimentary rocks, intact fossils from this time period in Earth’s history are incredibly rare. The DCO researchers discovered the microfossils within an iron formation in the Nuvvuagittuq belt, which may be the oldest suite of sedimentary rocks on Earth.
“These rocks are under quite a bit of debate among the scientists who dated them. Some argue for an age around 3.8 billion years ago and others argue for an age of 4.2 billion years ago,” said Dodd. “I don’t think many people expected microfossils in these rocks.”
Under the microscope, the researchers observed tubes made of the iron mineral hematite. These tubes match the shape and size of ones made by filamentous bacteria in modern hydrothermal vents, which use iron for their metabolism.
The researchers also observed carbonate rosettes, concentric circles of minerals that form when organic matter from dead cells reacts with nearby ferric iron. These rosettes also contained particles of graphite, which likely formed from organic matter during metamorphism. Isotopic analyses show that both the graphite and carbonate minerals contain reduced levels of a heavy carbon isotope, 13C, a telltale signature that cellular life processed the carbon.
Some of the rosettes also contained apatite, a phosphorus mineral that forms when cells decay and release their phosphorus compounds to the environment.
“It is the characteristic association of graphite with these minerals, carbonate and apatite, that enable us to suggest that the graphitized carbon has a biological origin rather than a non-biological origin,” said Dodd.
Dodd presented some of this work at a DCO-sponsored meeting Carbon at Extreme Conditions, in Lugano, Switzerland in 2015, and also at the 2016 DCO Summer School in Yellowstone National Park in Wyoming. Both experiences led to interactions that helped Dodd build his case that the structures in the rock represent early life forms.
“These kinds of studies need quite a lot of interdisciplinary research,” said Dodd. “Getting scientists together and sharing their knowledge can really move things forward.”
Next, Dodd hopes to be involved with research that uses the new mass spectrometer, Panorama, developed through the DCO by Edward Young and Douglas Rumble at the University of California, Los Angeles. The instrument has incredible resolving power that can help researchers distinguish between molecules formed by biological or physical means. He is also interested in other DCO projects looking at modern hydrothermal vent ecosystems and how they can be used to trace the microbes living in these environments from life to death and preservation in the rock record. Coupling modern ecosystems and state-of-the-art techniques to measure chemical fingerprints of biology may enable us to find evidence for life on other planets in our solar system.
Top left: Hematite tubes from the Nuvvuagittuq Supracrustal Belt hydrothermal vent deposits, estimated to be at least 3,770 million years old. Credit: Matthew Dodd, courtesy of Springer Nature. Middle: This microscopic image of a rosette from the Nuvvuagittuq Supracrustal Belt shows the iron-carbonate (white) with concentric layers of quartz inclusions (grey) and a single quartz crystal at the center with tiny inclusions of red hematite. Credit: Matthew Dodd
1. Dodd MS, Papineau D, Grenne T, Slack JF, Rittner M, Pirajno F, O’Neil J, Little CTS (2017) Evidence for early life in Earth’s oldest hydrothermal vent precipitates. Nature 543:60 doi:10.1038/nature21377