New research rekindles debate about earliest traces of life on Earth
A new study contradicts a controversial theory that places the earliest traces of life on Earth on an outcrop of rock on the small island of Akilia in South West Greenland. The team of Swedish, Australian and American scientists argues that previous interpretations of the rock's structure and geochronology are incorrect, and that it 'cannot host evidence of Earth's oldest life', erroneously dated back to 3.85 billion years ago. The rock formation, a five-metre wide outcrop on the island, was first studied in 1996. The research then found that it contained graphite with depleted levels of carbon-13 (13C). Since the lightest form of carbon, carbon-12 (12C), is the isotope preferred in biological processes, lower levels of 13C can be interpreted as signs of such processes going on, hence signs of life. In the case of the Akilia rocks, there were cross-cutting intrusions made by igneous rock into the outcrop. Evidently, the rock that is cross-cut must be older than the intruding rock. When the latter was claimed to be at least 3.85 billion years old, it was concluded that the graphite indeed contained the earliest traces of life on Earth. However, the theory has always been a subject of controversy: Earth itself is considered to be about 4.5 billion years old. Life complex enough to fractionate carbon 3.8 billion years ago would have had to have its origin even earlier than that, during the Hadean eon (3.8 to 4.5 billion years ago). Yet, the Hadean eon is thought of as an environment that was extremely hostile to life. The new study, to be published in the Journal of the Geological Society and led by Martin J Whitehouse at the Swedish Museum of Natural History, now argues that the cross-cutting between the different rocks or gneisses result from tectonic activity. They represent a deformed fault and hence do not give any clue as to the age of the graphite itself. 'Multiple deformation and high-grade metamorphic events occurring intermittently over more than a billion years of Earth history have imparted an extremely complex structure to the early Archean rocks on Akilia that have repeatedly been claimed to (1) represent Earth's earliest water-lain sediments and (2) host Earth's earliest evidence of life,' the researchers say. 'As a result of these events, primary cross-cutting relationships between datable intrusions and the gneisses claimed to be of sedimentary origin cannot be seen anywhere on Akilia.' The researchers go on to state that: 'The contact between these gneisses is tectonic so their relative ages are unknown. Multiple episodes of intrusion and deformation in the tonalitic [igneous] gneiss preceded the earliest deformation fabrics and structures seen in the mafic-ultramafic gneiss. Although previously interpreted as a volcano-sedimentary stratigraphy, the mafic-ultramafic gneiss displays no evidence of such an origin, and could be partly or wholly derived from igneous intrusions.' Analysing the graphite-containing rocks themselves did not provide any evidence that they are older than around 3.67 billion years, Mr Whitehouse and his colleagues conclude. According to the researchers, 'The rocks of Akilia provide no evidence that life existed at or before c. 3.82 Ga [Gigaannum, 1,000,000,000 years], or indeed before 3.67 Ga, or even that traces of life occur at all.'
Countries
Australia, Sweden, United States