Moorbath, S.

RERO DOC Digital Library

as BIF. Instead of iron oxide, the dark bandsconsisted of pyroxene (an alumino-silicateof calcium, magnesium and iron) andresembled igneous rocks from very close by. Genuine BIF occurs in quantity some 150 km to the northeast in the famous Isuaregion, where the best-preserved 3.7–3.8-billion-year-old rocks on Earth occur. TheAkilia rocks looked nothing like the IsuaBIFs. Detailed chemical and mineralogicalevidence soon convinced Fedo and White-house4 that the rocks were not sediments but strongly metamorphosed, metasoma-tized and deformed igneous rocks, and thus irrelevant for biology.The zircon uranium–lead date of 3.85 bil-lion years or older for the Akilia rocks wasalso questioned, because individual zircongrains are complex, possessing zones ofquite different ages within a single grain thatrepresent its long and complex crystalliza-tion history. The oldest zones indeed gavenearly 3.85 billion years, but this could be an ‘inherited’ age, older than the age of thehost granitic vein, which some claimed to be no more than 3.65 billion years5. Otherworkers6 then objected that the granitic veinwas not cross-cutting at all, but structurallyconcordant with its adjacent rock. Thatmeant that the measured age of the graniticvein was irrelevant to the crucial Akilia rock,because any previous field relationshipbetween the two had been obliterated by tectonic forces.Even more worrying was the proposal7that graphite inclusions in apatite grainsresulted from thermal dissociation (at temperatures of about 450 C) of iron-richcarbonates in the rock to iron oxide, carbondioxide and elemental carbon, the latter witha 13C/12C ratio overlapping values indicativeof a biogenic origin. This mechanism wassuggested to counter claims for supposedlybiogenic carbon in the genuine BIFs fromIsua8, which often contain iron-rich carbon-ate. In fact, only one locality now remains inthe Isua region where association of graphitewith sedimentary rocks leaves the possibilityof a biogenic interpretation open9. Ex-changes between ‘pro-life’ and ‘anti-life’factions on Akilia have now been publishedfor eight years. However, for many observersthe ancient Akilia rocks have graduallyappeared as less and less plausible domicilesfor biogenic tracers.Lepland et al.1 investigated many thinsections of Akilia rocks containing apatitenews and viewsNATURE | VOL  434 | 10 MARCH 2005 | www.nature.com/nature 155crystals by optical and scanning electronmicroscopy, combined with energy-dis-persive spectrometry. This work failed to reveal any graphite inclusions in apatitecrystals from the problematic banded rocks or surrounding rocks. Even the mostwidely discussed sample (known as G91-26),used in the original study2, proved free ofgraphite.This persuasive discovery seems analmost inevitable, yet highly problematic,consequence to the increasing scientificdoubts about the original claim.We may wellask what exactly was the material originallyanalysed and reported? What was the apatitegrain with supposed graphite inclusions thatfigured on the covers of learned and popularjournals soon after the discovery? Thesequestions must surely be answered and, ifnecessary, lessons learned for the more effec-tive checking and duplication of spectacularscientific claims from the outset.To my regret, the ancient Greenlandrocks have not yet produced any compel-ling evidence for the existence of life by 3.8 billion years ago. The reader is remindedthat another debate on early life is currentlyin progress on 3.5-billion-year-old rocks inWestern Australia, where chains of cell-like structures, long identified as genuinefossils10, have recently been downgraded by some workers11 to the status of artefacts produced by entirely non-biological pro-cesses. To have a chance of success, it seemsthat the search for remnants of earliest lifemust be carried out on sedimentary rocksthat are as old, unmetamorphosed, unmeta-somatized and undeformed as possible.That remains easier said than done. For thetime being, the many claims for life in thefirst 2.0–2.5 billion years of Earth’s historyare once again being vigorously debated:true consensus for life’s existence seems to be reached only with the bacterial fossilsof the 1.9-billion-year-old Gunflint Forma-tion of Ontario12. ■Stephen Moorbath is in the Department of EarthSciences, University of Oxford, Parks Road,Oxford OX1 3PR, UK.e-mail: stephenm@earth.ox.ac.uk1. Lepland, A., van Zuilen, M. A., Arrhenius, G., Whitehouse, M. J.& Fedo, C. M. Geology 33, 77–79 (2005).2. Mojzsis, S. J. et al. Nature 384, 55–59 (1996).3. Nutman, A. P., Mojzsis, S. J. & Friend, C. R. L.Geochim. Cosmochim. Acta 61, 2475–2484 (1997).4. Fedo, C. M. & Whitehouse, M. J. Science 296, 1448–1452 (2002).5. Whitehouse, M. J., Kamber, B. S. & Moorbath, S. Chem. Geol.160, 201–224 (1999).6. Whitehouse, M. J. & Fedo, C. M. Precambr. Res. 126, 259–271(2003).7. van Zuilen, M. A., Lepland, A. & Arrhenius, G. Nature 418,627–630 (2002).8. van Zuilen, M. A. et al. Precambr. Res. 126, 331–348 (2003).9. Rosing, M. T. & Frei, R. Earth Planet. Sci. Lett. 217, 237–244(2004).10.Schopf, J. W. Science 260, 640–646 (1993).11.Brasier, M. D. et al. Nature 416, 76–81 (2002).12.Knoll, A. H. Life on a Young Planet (Princeton Univ. Press,2003).PalaeobiologyDating earliest lifeStephen MoorbathClaims that 3.8-billion-year-old rocks from Greenland containcarbonaceous remnants of very early life have been the subject ofargument for several years. The latest analyses look like settling matters.How, where and when did life start?Geologists are helping to addressthese questions by diligently search-ing the oldest-known sedimentary rocks onEarth for traces of primitive life, either in theform of cellular microfossils, or as chemicaland isotopic tracers characteristic of biologi-cal processes.Since 1996, attention has focused on asmall outcrop of supposedly sedimentaryrocks, claimed to be more than 3.85 billionyears old, on the tiny island of Akilia,just off the west coast of Greenland. These rocks have been too strongly recrystallized in subsequent geological eras by heat andpressure (metamorphism), penetration ofdeep crustal fluids (metasomatism) and tectonic deformation to preserve fossils.Hence, claims for life there had to be basedon the carbon isotope composition of tinyinclusions of graphite (carbon) in grains of apatite (calcium phosphate). Writing inGeology, Lepland et al.1 now report that they cannot substantiate claims for the presence of graphite in any of the crucialAkilia apatite grains.The story began with the publication oftwo papers2,3 claiming that 13C isotope de-pletion (that is, low 13C/12C ratios) in Akiliagraphite supported a biological origin. Thehost rocks were identified as a banded ironformation (BIF), a chemically precipitatedmarine sediment normally consisting ofalternating bands of iron oxide (magnetiteor haematite) and silica (quartz) that might,in principle, harbour remnants of early life. Such rocks are difficult to date directly,but an age of 3.85 billion years or more wasclaimed from uranium–lead dating of zircon(zirconium silicate) grains in a granitic vein cross-cutting — and therefore younger than — the rock that hosted the BIF. Thisdate provided ammunition for the wide-spread popular claim that life on Earth began around 4 billion years ago. It alsoimplied that earliest life coexisted with devastating global meteorite impacts,known as the Late Heavy Bombardment,which shaped the Moon’s surface and,most likely, that of the Earth until about 3.85 billion years ago.Following the appearance of the twopapers2,3, scientific debate commenced inearnest. I myself visited Akilia twice, in com-pany with several participants in the unfold-ing controversy, but I could not understandwhy these banded rocks had been identified10.3 n&v 147 MH  4/3/05  5:27 pm  Page 155Nature Publishing Group© 2005

Dating earliest life

Abstract

Similar works

Full text

Available Versions

RERO DOC Digital Library