The Record of Life on the Early Earth

1. The Record of Life on the Early Earth Stanley M. Awramik Department of Earth Science University of California Santa Barbara

2. 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): .

3. Candidates for Early Archean Fossils

4. The record of life: Is it a 3.45 billion-year history? Or is it a 1.9 billion-year history? Ga

5. 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

6. Microfossils Stromatolites Isotopes Biomarkers Morphology Chemistry Evidence of Early Life on Earth

7. The Record • Microfossils • 14 localities • 40 morphotypes • Stromatolites • 48 localities • 6 of 7 known major shapes

8. Modified from Awramik and McNamara, in press

9. Awramik and McNamara, in press

12. 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

13. 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

14. Controversy Not New • The pre-Cambrian record of life has been “a problem” since 1859

15. 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

16. 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.

17. 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

18. 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

19. Entered an Age of Discovery Biogenicity wasn’t usually a major issue – except for the Early Archean 100 um Grey

20. 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

21. Archean Phase II: Things Heat Up Establishment of NASA’s Astrobiology Institute (NAI) in 1998 1996, Science paper reporting on martian meteorite microfossils

22. Archean Phase II: Things heat up • Hofmann, et al., 1999, Geol. Soc. Am. Bull.

23. Archean Phase III: Hot Debates • Brasier et al., 2002, Nature • Schopf et al., 2002, Nature

24. 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)

25. Knoll’s Law1 • A good biosignature is something that is difficult to make through inorganic processes 1 from Kirschvink and Weiss (2001)

26. Archean Stromatolites Tumbiana Formation, Neoarchean Steep Rock Group, Neoarchean Kelly Group, Paleoarchean

27. Pilbara, Western Australia http://www.doir.wa.gov.au/GSWA/03AEC693C1144AE18E8877105F822F66.asp

28. Processes involved with laminated structures Biological (skeletal) Chemical Mechanical (clastic) Biological (non-skeletal) Modified from Hofmann (2000)

29. 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

30. 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

31. 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

32. 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

33. 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)

34. 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

35. 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

36. 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

37. 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.

38. 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

39. 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

40. 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!

41. Cartoon by Carol Jones

42. 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.

43. 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.