Reassessing the evidence for the earliest traces of life

Abstract

The isotopic composition of graphite is commonly used as a biomarker in the oldest (>3.5 Gyr ago) highly metamorphosed terrestrial rocks. Earlier studies on isotopic characteristics of graphite occurring in rocks of the approximately 3.8-Gyr-old Isua supracrustal belt (ISB) in southern West Greenland have suggested the presence of a vast microbial ecosystem in the early Archean. This interpretation, however, has to be approached with extreme care. Here we show that graphite occurs abundantly in secondary carbonate veins in the ISB that are formed at depth in the crust by injection of hot fluids reacting with older crustal rocks (metasomatism). During these reactions, graphite forms from the disproportionation of Fe(II)-bearing carbonates at high temperature. These metasomatic rocks, which clearly lack biological relevance, were earlier thought to be of sedimentary origin and their graphite association provided the basis for inferences about early life. The new observations thus call for a reassessment of previously presented evidence for ancient traces of life in the highly metamorphosed Early Archaean rock record.

La vida temprana en la Tierra y los primeros ecosistemas terrestres

“…Y aún en este registro tan antiguo, potenciales biofirmas (carbonatos precipitados biogénicamente) pudieran estar presentes (Nutman et al, 2010), lo cual sugiere que la biósfera podría ser varios millones de años más antigua que los estromatolitos y microfósiles más antiguos conocidos (~3500 Ma). Otras biofirmas putativas de más de 3500 Ma (glóbulos grafitizados asociados a apatita; ver McKeegan et al, 2007;Papineau et al, 2010aPapineau et al, , 2010b son también polémicas (véase Myers, 2001;van Zuilen et al, 2002;Fedo y Whitehouse, 2002;Papineau et al, 2011) dada su dudoso tiempo de formación, que pudo ser más reciente. Firmas biológicas de particular interés son las asociadas a las llamadas Banded Iron Formations (i.e.…”

The Search for Organic Molecules on Mars

“…Unlike Earth, Mars has no plate tectonics to speak of. On Earth, the oldest rocks that contain any traces of life are 3.77 billion yr old and are found in Isua, West Greenland (5). On the other hand, the surface of Mars has remained relatively unchanged for 4 billion yr.…”

Revising the fundamentals of ecological knowledge: the biota–environment interaction

“…There is only one way to ensure that life persists on a practically infinite time scale comparable to that of the Universe itself (more than 3 billion years (Hayes, 1996;van Zuilen et al, 2002) versus 11-20 billion years (Krauss and Chaboyer, 2003)). …”

“…If the rate of decomposition exceeded the rate of synthesis, the global biota would be able to destroy itself completely in equally short periods of time. That is, on a time scale eight orders of magnitude shorter than the age of life (Hayes, 1996;van Zuilen et al, 2002) and 10 5 to 10 6 times shorter than the average period of individual species existence (Stanley, 1979).…”