294 BIOZONES thus been interpreted as the oldest evidence for life on Earth However, in the absence of original sedimentary features and context, the interpretation of these signatures is contentious Clearly, the fossil that sets the irrefutable starting point for life on Earth has not been found However, if the interpreted complexity of microbial communities seen in the fossil record at around 3.5 Ga is anything to go by, it seems likely that life on Earth began well before then The remains of the very first microbes, or the rocks that they were part of, have been destroyed or metamorphosed beyond recognition Glossary biogenic Of biological origin Ga Billion years before present/billion years of age lacustrine Associated with lake environments metamorphism Pressure- and temperature-related alteration of rocks during burial (involving processes that result in new textural and mineralogical characteristics) See Also Diagenesis, Overview Geysers and Hot Springs Origin of Life Precambrian: Eukaryote Fossils; Vendian and Ediacaran Pseudofossils Sedimentary Rocks: Limestones Further Reading Banfield JF and Nealson KH (1997) Geomicrobiology: Interactions Between Microbes and Minerals Washing ton: Mineralogical Society of America Brocks JJ and Summons RE (2003) Sedimentary hydro carbons, biomarkers for early life In: Holland HD and Turekian K (eds.) Treatise in Geochemistry, pp 63 115 Amsterdam: Elsevier Buick R (1990) Microfossil recognition in Archaean rocks: an appraisal of spheroids and filaments from a 3500 M.Y old chert barite unit at North Pole, Western Australia Palaios 5: 441 459 Ehrlich HL (2000) Geomicrobiology New York: Marcel Dekker Grotzinger JP and Knoll AH (1999) Stromatolites in Pre cambrian carbonates: evolutionary mileposts or environ mental dipsticks? Annual Reviews, Earth and Planetary Science Letters 27: 313 358 Hoffman HJ (2000) Archaean stromatolites as microbial archives In: Riding R and Awramik S (eds.) Microbial Sediments, pp 315 327 Berlin, Heidelberg: Springer Verlag Lowe DR (1994) Abiological origin of described stromato lites older than 3.2 Ga Geology 22: 387 390 Schopf JW (1983) Earth’s Earliest Biosphere Princeton, NJ: Princeton University Press Schopf JW and Packer BM (1987) Early Archaean (3.3 billion to 3.5 billion year old) microfossils from Warrawoona Group, Australia Science 237: 70 73 Walter MR (1976) Stromatolites Amsterdam: Elsevier BIOZONES N MacLeod, The Natural History Museum, London, UK Copyright 2005, Natural History Museum All Rights Reserved Introduction The contemporary concept of biozone is that of a stratigraphical interval defined on the basis of its biotic content This deceptively simple definition serves as the door to vast realms of complexity, access to which is important not only because those realms contain much of the early history of geology, but also because they encompasses the single most important set of tools yet devised for reconstructing earth history The biozone concept is built on the work of William Smith, who first demonstrated the importance of fossils for establishing sequences of geological events Smith’s ideas were extended by many nineteenth-century geologists and were instrumental in creating the geological time-scale (see Time Scale): one of the greatest scientific achievements of that century For the last 100 years, various biozone types – range biozones of diverse types, assemblage biozones, interval biozones, acme biozones – have been employed throughout the Earth sciences where they have proven their worth in fields as diverse as palaeoceanography, palaeogeography, palaeoecology, and palaeoclimate studies; wherever there is a need for relative time correlation In the twenty-first century, use of biozones remains at the forefront of stratigraphical analysis, with methodological improvements being made through use of the concept in quantitative stratigraphy (see Stratigraphical Principles) In order to understand the concept of a biozone, one must understand its development As noted above, foundations for an understanding of biozones were laid by William Smith who, in 1796, realised that the rock layers cropping out around the southern English town of Bath always occurred in the same