370 PRECAMBRIAN/Prokaryote Fossils isotopically light carbon formed through late-stage remobilization of hydrocarbons with hydrothermal silica Complexity per se is not a valid criterion for the recognition of life Biological populations may be distinguished from abiogenic ones by the lack of a continuous spectrum of morphological variation, limited by the DNA-controlled genetic constraints This narrow range can be contrasted with complex abiogenic artefacts, which will tend towards a ‘symmetry-breaking cascade’ of forms, ranging from a few biological look-alikes with rotational symmetry (Figure 4C) to highly information-rich forms (Figures 4A and 4B) with poor symmetry Research into the earliest biosphere is now served by a marvellous range of new tools, including molecular (RNA and DNA) analysis of living-prokaryote phylogeny, biogeochemical analysis of ancient Earth rocks, and digital-image analysis of putative microfossils These techniques should not be allowed to prosper, however, at the expense of more traditional techniques such as mapping and thin-section petrography The latter are also vital for providing a context for early-life studies at a range of scales, from satellite images of greenstone belts, through field mapping, to microfabric mapping of thin sections Future research will also require a much better understanding of the nature of complexity itself and of the multitudinous ways in which seemingly complex structures can self-organize (Figure shows a range of microfossil-like and stromatolite-like objects of inferred non-biological origin.) This better understanding of complexity is needed, not only to understand the ways in which pseudofossils and pseudo-stromatolites can form, but also to comprehend the conditions in which life-forms and their building blocks were able to assemble and develop on the early Earth, and perhaps beyond See Also Biosediments and Biofilms Earth Structure and Origins Origin of Life Precambrian: Eukaryote Fossils Pseudofossils Sedimentary Rocks: Banded Iron Formations Solar System: Mars Further Reading Brasier MD, Green OR, Jephcoat AP, et al (2002) Ques tioning the evidence for Earth’s oldest fossils Nature 416: 76 81 Farquhar J, Bao HM, and Thiemens M (2000) Atmospheric influence of Earth’s earliest sulfur cycle Science 289: 756 768 Furnes HR, Banerjee NR, Muehlenbachs K, et al (2004) Early life recorded in Archean pillow lavas Science 304: 578 581 Garcia Ruez JM, Hyde ST, Carnerup AM, et al (2003) Self assembled silica carbonate structures and detection of ancient microfossils Science 302: 1194 1197 Hofmann HJ, Grey K, Hickman AH, and Thorpe RI (1999) Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia Bulletin of the Geological Society of America 111: 1256 1262 Kazmierczak J and Altermann W (2002) Neoarchean bio mineralization by benthic cyanobacteria Science 298: 2351 Kerr R (2004) New biomarker proposed for earliest life on earth Science 304: 503 Knoll AH (2003) Life on a Young Planet Princeton, NJ: Princeton University Press Mojsis SJ, Arrenhius G, McKeegan KD, et al (1996) Evi dence for life on Earth before 3,800 million years ago Nature 384: 55 59 Rasmussen B (2000) Filamentous microfossils in a 3,250 million year old volcanogenic massive sulphide Nature 405: 676 679 Rothman DH and Grotzinger JP (1996) An abiotic model for stromatolite morphogenesis Nature 383: 423 425 Schopf JW (1999) The Cradle of Life New York: Princeton University Press Simpson S (2003) Questioning the oldest signs of life Scientific American April 2003: 70 77 Westall FM, de Wit MJ, Dann J, et al (2001) Early Archean fossil bacteria and biofilms in hydrothermally influenced sediments from the Barberton greenstone belt, South Africa Precambrian Research 106: 93 116 Woese CR, Kandler O, and Wheelis ML (1990) Towards a natural system of organisms: proposals for the domains of Archaea, Bacteria and Eucarya Proceed ings of the National Academy of Sciences USA 87: 4576 4579