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The Record of Life on the Early Earth. Stanley M. Awramik Department of Earth Science University of California Santa Barbara. An Opinion.
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The Record of Life on the Early Earth Stanley M. Awramik Department of Earth Science University of California Santa Barbara
An Opinion • “…true consensus for life’s existence seems to be reached only with the bacterial fossils of the 1.9-billion-year-old Gunflint Formation…” Moorbath, S., 2005. Nature, 434, p. 155): .
The record of life: Is it a 3.45 billion-year history? Or is it a 1.9 billion-year history? Ga
The Archean Earth System • Archean: 4.03 to 2.5 Ga • Late Heavy Bombardment (4.1 to 3.8 Ga) • Atmosphere: hardly any molecular oxygen (O2) • UV radiation (no ozone shield) • Sun less luminous (20-30%) • Length of day ~15 hours (~584 days per year) • CO2 and CH4 -rich atmosphere • Likely warm surface conditions (80o to 45oC) • High rate of heat transfer from core to surface, possible thin crust, plate tectonics just being established • Magnetic field by 3.2 Ga • Fossils
Microfossils Stromatolites Isotopes Biomarkers Morphology Chemistry Evidence of Early Life on Earth
The Record • Microfossils • 14 localities • 40 morphotypes • Stromatolites • 48 localities • 6 of 7 known major shapes
Some Meaningless Search Statistics • Google (Number of hits on 14 April 2007) • Stromatolite 339,000 • Archean life 193,000 • Archean stromatolite 34,300 • GeoRef (March 7, 2007) • Stromatolite 5,043
Early Earth’s Fossil Record is Controversial • A “poster child” for “uncertain science” • Few have feared to write with authority on the subject • With this and other uncertainties of science • Done carefully • Knowledge is not static • The process of doing science • Balancing act of multiple lines of evidence, none of which are compelling
Controversy Not New • The pre-Cambrian record of life has been “a problem” since 1859
Hints of Life: The Controversies Begin • Eozoon canadense * Dawson 1864 • Lyell (1864) “one of the greatest discoveries of his time” • By 1894, all but Dawson considered E. canadense to be inorganic Dawson E. canadense: Layered serpentine and calcite *Discovered in 1858 by Logan
Benchmark Papers • Walcott, C.D. • 1899: Pre-Cambrian fossiliferous formations. Bulletin of the Geological Society of America, 10: 199-244 • 1914: Pre-Cambrian Algonkian algal flora. Smith. Misc. Coll., 64:74-156.
Backward Steps • A.C.Seward in 1931, in Plant Life through the Ages • Rejected Cryptozoan and similar structures (stromatolites) as “algal” in origin and rejected their biogenicity • Rejected fossil bacteria described by Walcott
Emergence of a New Field* • Tyler, S.A. and Barghoorn, E.S., 1954. Occurrence of structurally preserved plants in pre-Cambrian rocks. Science, 119:606-608. • ~1900 Ma Gunflint Formation • From Lower Algal Member (stromatolites) *Schopf, 2000, used this phrase
Entered an Age of Discovery Biogenicity wasn’t usually a major issue – except for the Early Archean 100 um Grey
Archean Phase I: The Early Phase • Lowe, 1980, Nature • Walter, Buick, and Dunlop, 1980, Nature • Byerly, Lowe, and Walsh, 1986, Nature Kelly Group, Western Australia Swaziland Supergroup, South Africa Kelly Group, Western Australia
Archean Phase II: Things Heat Up Establishment of NASA’s Astrobiology Institute (NAI) in 1998 1996, Science paper reporting on martian meteorite microfossils
Archean Phase II: Things heat up • Hofmann, et al., 1999, Geol. Soc. Am. Bull.
Archean Phase III: Hot Debates • Brasier et al., 2002, Nature • Schopf et al., 2002, Nature
Brasier and others’ Null Hypothesis • “…very ancient/alien microfossil-like structures (or stromatolites or geochemical and isotopic signals older than ~3.0 Ga) should not be accepted as being of biological origin until possibilities of their non-biological origin have been tested and can be falsified...” (Brasier et al., 2004, p. 259)
Knoll’s Law1 • A good biosignature is something that is difficult to make through inorganic processes 1 from Kirschvink and Weiss (2001)
Archean Stromatolites Tumbiana Formation, Neoarchean Steep Rock Group, Neoarchean Kelly Group, Paleoarchean
Pilbara, Western Australia http://www.doir.wa.gov.au/GSWA/03AEC693C1144AE18E8877105F822F66.asp
Processes involved with laminated structures Biological (skeletal) Chemical Mechanical (clastic) Biological (non-skeletal) Modified from Hofmann (2000)
Perception that stromatolites are unreliable indicators of life • Why? • No valid or appropriate living analog for Archean and Proterozoic examples • Similarities superficial • No unified ‘theory’ of stromatolite morphogenesis • Definition of stromatolite is contentious • There appears to be a lack of substantive information, rigor, and critical thinking
Perception that stromatolites are unreliable indicators of life • Why? • No valid or appropriate living analog for Archean and Proterozoic examples • Similarities superficial • No unified ‘theory’ of stromatolite morphogenesis • Definition of stromatolite is contentious • There appears to be a lack of substantive information, rigor, and critical thinking
Perception that stromatolites are unreliable indicators of life • Why? • No valid or appropriate living analog for Archean and Proterozoic examples • Similarities superficial • No unified ‘theory’ of stromatolite morphogenesis • Definition of stromatolite is contentious • There appears to be a lack of substantive information, rigor, and critical thinking
Perception that stromatolites are unreliable indicators of life • Why? • No valid or appropriate living analog for Archean and Proterozoic examples • Similarities superficial • No unified ‘theory’ of stromatolite morphogenesis • Definition of stromatolite is contentious • There appears to be a lack of substantive information, rigor, and critical thinking
Descriptive Definition “Attached, laminated, lithified sedimentary growth structures, accretionary away from a point or limited surface of initiation” (Semikhatov et al., 1979) Definitions of stromatolite Bioconfusion • Genetic Definition “An organosedimentary structure produced by sediment trapping, binding, and/or precipitation as a result of the growth and metabolic activity of microorganisms, principally cyanobacteria” (Awramik and Margulis, 1976)
Perception that stromatolites are unreliable indicators of life • Why? • No valid or appropriate living analog for Archean and Proterozoic examples • Similarities superficial • No unified ‘theory’ of stromatolite morphogenesis • Definition of stromatolite is contentious • There appears to be a lack of substantive information, rigor, and critical thinking
Preliminary Criteria for Biogenicity • Occur in sedimentary rocks • Metasedimentary OK, but confidence decreases with metamorphism • Provenance • Age • Indigeneous to rock • Synsedimentary http://www7.nationalacademies.org/ssb/nanopanel3schopf.html
Main Criteria for Biogenicity: Stromatolites • Convex-upward structures predominate • Laminae thicken over flexures • Laminae wavy, wrinkled, several orders of curvature • Inhabited a plausibly livable environment • Organic matter in laminae • Biogenic isotopic signature of organic matter • Microfossils present • Morphology reflects microfossil assemblage changes • Microfossils indicate trapping/binding or precipitation
Main Criteria for Biogenicity: Microfossils • Composed of organic matter (or shown to be mineral replacements) • Complex enough in structure to rule out plausible nonbiologic origins • Numerous specimens • A multicomponent assemblage • Exhibit a range of taphonomic degradation consistent with their mode of preservation • Exhibit morphological variability • Inhabited a plausibly livable environment • Grown and reproduced by biologic means of cell division • Exhibit a biogenic isotopic signature.
Descending Scale of Credibility • Compelling evidence • Abundant evidence that permits only one reasonable interpretation • Presumptive evidence • The preponderance of evidence suggests a most likely interpretation but for which less probable interpretations also merit consideration • Permissive evidence • Evidence that seems consistent with at least two more or less equally tenable competing interpretations • Suggestive evidence • Evidence that although weak, is at least consistent with the interpretation See Awramik and Grey, 2005, for discussion
Gunflint Formation; 1.9 Ga Mt. Ada Basalt; 3.467 Ga Click and drag arrow below Atar Group; 0.85 Ga Strelley Pool Chert; 3.45 Ga Malachite from C.Klein
An “Objective Yardstick”? • Smoking gun?? • Probably not • “Black and white” approaches cloud issue • Paleobiology ain’t physics! • Paleobiology, like biology, is an autonomous science • Unequivocal proof is something on which paleobiology is moot • Compelling evidence is an obtainable goal • Lesser degrees of confidence are constructive • Comparisons with other, better known fossils are useful • Modern analogs are important • Morphology matters!
References • Awramik, S.M. and Grey, K., 2005. Stromatolites: Biogenicity, Biosignatures, and Bioconfusion. Proc. of SPIE, 5906: 59060P-1-59060P-9 [doi: 10.1117/12.625556] • Awramik, S.M. and Margulis, L., 1976. In page 1 of Introduction by M.R.Walter to Stromatolites. M.R.Walter, Ed., Elsevier, Amsterdam, pp. 1-3. • Awramik, S.M. and McNamara, K.J., in press. The evolution and diversification of life. In: W.T.Sullivan and J.A.Baross, Eds., Planets and Life: The Emerging Science of Astrobiology. Cambridge University Press, Cambridge, pp. 313-334. • Awramik, S.M., Schopf, J.W., and Walter, M.R., 1983. Filamentous fossil bacteria from the Archean of Western Australia. Precambrian Research, 20: 357-374. • Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., Van Kranendonk, M.J., Lindsay, J.F., Steele, A., and Grassineau, N.V., 2002. Questioning the evidence for Earth's oldest fossils. Nature, 416: 76-81. • Brasier, M.D., Green, O.R., Lindsay, J.F., and Steele, A., 2004. Earth's oldest (approximately 3.5 Ga) fossil and the "Early Eden hypothesis": Questioning the evidence. Origins of Life and Evolution of the Biosphere, 34: 257-269. • Byerly, G.R., Lowe, D.R., and Walsh, M.M., 1986. Stromatolites from the 3,300-3,500-Myr Swaziland Supergroup, Barberton Mountain Land, South Africa. Nature, 319: 489-491. • Grotzinger, J.P. and D. H. Rothman, D.H., 1996. An abiotic model for stromatolite genesis. Nature, 383: 423-425. • Hofmann, H.J., 2000. Archean stromatolites as microbial archives. In: R.Riding and S.M.Awramik (Eds.), Microbial Sediments. Springer, Berlin, pp. 315-327.
References • Hofmann, H.J., Grey, K., Hickman, A.H., and Thorpe, R.I., 1999. Origin of 3.45 Ga coniform stromatolites in Warrawoona Group, Western Australia. Geological Society of America Bulletin 111: 1256-1262. • Kirschvink, J.L. and Weiss, B.P., 2001. Carl Sagan Memorial Lecture 2001: Mars, panspermia, and the origin of life: Where did it all begin? retrieved http://www.gps.caltech.edu/users/jkirschvink/ • Lowe, D.R., 1980. Stromatolites 3,400-Myr old from the Archaean of Western Australia. Nature, 284: 441-443. • Moorbath, S., 2005. Dating earliest life. Nature, 434: 155. • Schopf, J.W., 2000. Solution to Darwin’s dilemma: Discovery of the missing Precambrian record of life. Proc. Nat. Acad. Sci., 97: 6947-6953. • Schopf, J.W., Kudryavtsev, A.B., Agresti, D.G., Wdowiak, T.J., and Czaja, A.D., 2002. Laser-raman imagery of Earth’s earliest fossils. Nature, 416: 73–76. • Semikhatov, S.M., Gebelein, C.D., Cloud, P., Awramik, S.M., and Benmore, W. C., 1979. Stromatolite morphogenesis - progress and problems. Canadian Journal of Earth Sciences, 16: 992-1015. • Tyler, S.A. and Barghoorn, E.S., 1954. Occurrence of structurally preserved plants in pre-Cambrian rocks. Science, 119: 606-608. • Walcott, C.D., 1899. Pre-Cambrian fossiliferous formations. Bulletin of the Geological Society of America, 10: 199-244. • Walcott, C. D., 1914. Cambrian geology and paleontology III; No. 2, Pre-Cambrian Algonkian algal flora. Smithsonian Miscellaneous Collections, 67: 77-156 • Walter, M.R., Buick, R., and Dunlop, J.S.R., 1980. Stromatolites 3,400-3,500 Myr old from the North Pole area, Western Australia. Nature, 248: 443-445.