Evidence for Early Life Microfossils biomarkers and preservation

Evidence for Early Life: Microfossils, biomarkers, and preservation Tracy Quan September 19, 2006

The Big Questions When did life start? What type of organisms were first? When did photoautotrophs develop? When did the Earth become oxygenated? How do environmental conditions control organism development/distribution? How did modern organisms/distributions evolve?

Evidence of Life Microfossils preserved cell walls, skeletons, cysts Biomarkers compounds specific to certain organisms Geologic evidence isotopic shifts correspond to enzymatic systems microbially mediated deposits Molecular clocks backtracking based on mutations in genes

Microfossils Direct evidence of the presence of organisms Can be soft cell walls that are remineralized, hard shells/tests, or cysts and resting stages Particular forms are diagnostic of specific species Presence or absence can also be indicative of certain environmental conditions Abundance relative to total can also indicate environmental conditions or shifts due to evolution

diatom Dinoflagellate cyst Prasinophyte phycoma Fossil cyanobacteria

Fossil Preservation Not all organisms make easily preserved parts Even silica and carbonate shells can dissolve Not all parts are specific to certain taxa Environmental conditions can influence Variations in species evolution can make interpretation difficult Older fossils can be hard to identify as such Can also be altered by temperature and geologic processes

Coccoid cyanobacteria can be thermally altered to look like filamentous fossils due to kerogenization of mucus envelopes rather than individual cells 440 Mya Kazmierczak and Kramer, 2002

What are Biomarkers? Biomarkers are molecules that are characteristic of certain organisms and/or processes pigments reflect photosynthesis membrane lipids can indicate species These compounds are often derived from lipids and can be very stable Diagenesis alters the compounds into more stable forms Not all organisms leave identifiable biomarkers; some only generate during certain conditions need to know source and diagenetic pathways

2 -methyl bacteriohopanepolyols characteristic of cyanobacteria Summons et al. , 1999 n Trimethylarylisoprenoids > carotenoid breakdown products o Carotenoid okenane >purple sulfur bacteria (<20 m) i, c carotenoids -> planktonic green and brown anerobic phototrophs 1. 6 Gyr Barney Creek Formation Brocks et al. , 2005

The Gamble of Preservation Organic matter passes through a series of oxidation reactions in the water and sediments: O 2 respiration: CH 2 O + O 2 -> H 2 O + CO 2 Denitrification: 4/5 H+ + CH 2 O + 4/5 NO 3 - -> CO 2 + 2/5 N 2 + 7/5 H 2 O Mn red: 4 H+ + CH 2 O + 2 Mn. O 2 -> 2 Mn 2+ + 3 H 2 O + CO 2 Fe red: 8 H+ + CH 2 O + 4 Fe. OOH -> 4 Fe 2+ + CO 2 + 7 H 2 O Sulfate red: H+ + CH 2 O + 1/2 SO 42 - -> CO 2 + 1/2 H 2 S + H 2 O ~5% of OM reaches the sediment; <1% is buried Proteins & sugars lost first; lipids remain, eventually geopolymerized to kerogen Watch for contamination!

Geologic and Isotopic Evidence Biological processes deplete carbon relative to the source Modern phytoplankton between -20 and -25‰ It has been shown that some high T and P processes can deplete C to -35‰ Schopf 1993 Organisms can also imprint on the geologic record precipitated stromatolites, microbial mat-like textures, structures of trapped and bound particles Use age of surrounding rocks to confine age of fossils

No Markers ≠ No Life The absence of fossils, biomarkers, etc. does not mean those organisms did not exist Preservation requires specific circumstances Some organisms don’t have identifiable markers Tectonic and geologic processes can eliminate or alter signals Sampling location and biases may affect findings Molecular clocks indicate longer times than fossil record It can generally be assumed that an organism existed long before it appears in the fossil record

Life Through the Ages Archean (2500 -3800 Mya) data is very limited: early cyanobacteria evolution possible by 3500 Mya first prokaryotes (DNA, RNA, cell membranes)? In the Proterozoic (545 -2500 Mya) Great Oxidation Event at ~2300 Mya cyanobacteria and microeukaryotes (1900 Mya) were present limited primary photosynthetic endosymbiosis started about 1200 Mya green algae primary production increasing

The Paleozoic (250 -545 Mya) had: minor dinoflagelletes and heterokonts larger presence of red and green algae and cyanobacteria largest extinction event at the Permian-Triassic boundary (250 Mya); 96% marine, 70% terr. extinct Modern phytoplankton arose during the Mesozoic (65 -250 Mya) dominance of diatoms, dinoflagellates and coccoliths as primary producers on shelves cyanobacteria primary in the open ocean

The Oldest Fossils? Need to know: provenance and age, indigenous to and deposited with rock surround, biologic origin Schopf found 11 diverse species of filamentous cyanobacterium-like organisms in Early Archean Apex chert from Western Australia (~3465 Mya) presence of distinct septa O 2 -producing photoautotropy seen 1300 My earlier Assume similarity in form to modern organisms equals similar function Isotopic C signature indicates biological C fixation Laser-Raman spectra seem to be consistent with presence of kerogen

Anabena sp. Spirulina sp. Schopf 1993

But Are They Fossils? Braiser et al 2002 looked at same rocks and found no evidence for fossils some shapes follow crystal ghosts, or are part of complex branching structures ‘septa’ made by quartz interspersed into graphite Both structures and isotopes can be explained by nonbiogenic hydrothermal processes Laser-Raman spectra are not characteristic of kerogen only (Dill Pasteris and Wopenka, 2002) Could these Archean samples contain fossils of a different type? - see Kazmierczak and Kramer, 2002

‘Fossil’ structures part of larger forms that extend in 3 dimensions changing focal depth can show additional structures

Conclusions Several techniques and lines of evidence must be combined to determine how and when life evolved All have their limitations; care must be taken to prevent contamination and misinterpretation Absence of physical and chemical evidence does not mean life did not exist; preservation is limiting! The earliest life is hard to identify: lack of samples, low preservation, inconclusive evidence The debate on the Apex chert ‘fossils’ is science in action!