Over the past several years researchers have explored new kinds of biomarkers, molecules and isotopes, in a quest for the holy grail of fossils in ancient rocks - when did life first appear on earth? But interpreting both new and old kinds of markers has proven trickier than expected as science does its painstaking job of challenging hypotheses.

In the May 24, 2002 issue of Science, Fedo and Whitehouse (1) challenge the interpretation (2) that a quartz-pyroxene rock containing carbon-13-depleted graphite from the island of Akilia, Southwest Greenland is a banded iron formation (BIF). This finding would deny the claim (2) that this formation represents the oldest fossils (>3850 million years ago) of life on Earth. This is after the biological origin for morphological fossils from the ~3465 million-year-old Apex chert in Western Australia, long thought to represent the oldest physical evidence for life on Earth (3), has been recently questioned (4). Taken together, these two recent challenges are a set back that distinct shapes and organic chemical signatures provide adequate evidence of biomarkers of fossils of the oldest life on Earth. With these challenges, the remaining best evidence for the earliest life on Earth is carbon-13-depleted graphite particles in deep-sea clastic sedimentary rocks from the Isua greenstone in Greenland (5). These sediments, dating to 3700 to 3800 million years ago, correspond to the end of the late heavy asteroid bombardment, when Earth's surface conditions were more stable and retention of a life-sustaining hydrosphere was favored (6).

(1) Fedo CM, Whitehouse MJ. Metasomatic origin of quartz-pyroxene rock, Akilia, Greenland, and implications for Earth's earliest life. Science 2002 May 24;296(5572):1448-52 [PubMed]

(2) Mojzsis SJ, Arrhenius G, McKeegan KD, Harrison TM, Nutman AP, Friend CR. Evidence for life on Earth before 3,800 million years ago. Nature 1996 Nov 7;384(6604):55-9

(3) Schopf JW. Microfossils of the Early Archean Apex chert: new evidence of the antiquity of life. Science 1993 Apr 30;260:640-6; PMID: 11539831 [PubMed]

(4) Brasier MD, Green OR, Jephcoat AP, Kleppe AK, Van Kranendonk MJ, Lindsay JF, Steele A, Grassineau NV. Questioning the evidence for Earth's oldest fossils. Nature. 2002 Mar 7;416(6876):28. [PubMed]

(5) Minik T. Rosing. 13C-Depleted Carbon Microparticles in >3700-Ma Sea-Floor Sedimentary Rocks from West Greenland.Science 283, 674 (1999) [PubMed]

(6) Arrhenius G, Lepland A. Accretion of Moon and Earth and the emergence of life. Chem Geol 2000 Aug 15;169(1-2):69-82 [PubMed]