Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada

The authors provide evidence for the existence of life on Earth in the earliest known sedimentary rocks and suggest that the presence of organic carbon, and low stable-isotope values of graphite from sedimentary rocks in Labrador pushes back the existence of organic life to beyond 3.95 billion years. The beginning of organic life on Earth is being pushed back by evidence from the earliest known sedimentary rocks. Tsuyoshi Komiya and colleagues argue that the presence of organic carbon and stable-isotope excursions in graphite from sedimentary rocks in Labrador, Canada, pushes back the existence of organic life to more than 3.95 billion years ago. Together with recent work showing evidence for a diverse range of living organisms around 3.7 billion years ago, including stromatolites living in sunlit surface waters and bacteria living in deep-sea hydrothermal vents, the work shows that life has been around almost as long as there has been a planet that it can call home. The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible1,2,3, and biogenic graphite has thus far been found only in the 3.7–3.8 Ga (gigayears ago) Isua supracrustal belt4,5,6,7. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador8,9. Some pelitic rocks have low δ13Corg values of −28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.