The Oldest Fossils & Prokaryote Fossil History
Definitions of Life
There is no single standard definition of life. Different definitions tend to reflect the priorities of their authors. One that Merck is fond of:
"Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation."
These responses may involve:
- Homeostasis: Negative feedback mechanisms promoting internal stability
- Metabolism: Chemical systems for converting external chemicals and the energy of their bonds into life-system components and energy.
- Reproduction: The ability to produce new living units.
Isua metaconglomerates from Panoramio.com
Life is thought to have arisen (for the last time?) shortly after the end of the Late Heavy Bombardment, between 3.9 and 3.8 Ga. The environment in which this happened differed markedly from what we know today.
- Atmospheric composition.
- Early atmosphere was result of volcanic outgassing of volatiles.
- Highly reducing atmosphere. Rich in CO2.
- Some free oxygen derived from photochemical dissociation by UV in upper atmosphere. 2 H2O + uv --> 2 H2 +O2This would have generated enough oxygen quickly to oxidize chemical building blocks of life near ocean surface.
- Oceanic conditions. Initially strongly acidic, precluding precipitation of carbonate rocks. This was because atmospheric CO2, dissolved in sea water, forming carbonic acid.
- Land conditions: Here is one spot of optimism. By 3.8 Ga, we have continental conglomerates in Isua, Greenland (right), showing that streams of water were flowing on surface.
Requirements for life:
- Energy source
- Proteins: Polymers of amino acids. Structural elements and catalysts.
- Nucleic acids: Regulate synthesis of proteins in proper cells.
- Semipermeable membranes: In which to package and isolate the components of life.
Proteins: These end up being surprisingly easy to form under natural circumstances.
- The simple experiment of Miller and Urey, 1959, in a classic experiment, showed that amino acids are readily synthesized from presumed primordial components of Earth atmosphere.
- Fox et al., 1959 saw that concentrated solutions of amino acids form proteinoids (short polymers of 18 common amino acids) if heated to 140 deg. C. When cooled, proteinoids form suspiciously cell-like spheres. Fox ultimate found "wild" proteinoids in pools associated with Hawaiian volcanoes. It's not a huge stretch to speculate on a similar origin of proper proteins.
- A. G. Cairns-Smith observed that RNA nucleotides can bind to the edges of clay minerals like smectite to form RNA-like polymers. (For an overview, see Genetic Takeover: And the Mineral Origins of Life).
- RNA World? - Altman et al., 1986 demonstrated that RNA is capable of acting not only as a template for protein synthesis, but, in limited ways, as a biochemical catalyst. (Particularly interacting with phospholipids like those occurring in cell membranes). Sidney Altman went on to propose repose an early stage in the origin of life, called RNA World. in which simple "biochemical" processes were carried out entirely by RNA. In this scenario, double-strand DNA is derived from RNA at a later time. Recently, however, Gavette et al., 2016 observed that intermediate forms of nucleic acid that would have figured in this transition are unstable, making the RNA -> DNA transition problematic.
- Only later did nucleic acids and proteins join forces.
- PNAs: Because ribose, the sugar component of the RNA polymer is difficult to synthesize from a Miller and Urey-style primordial soup, but amino acids are easy, some researchers have proposed that the first gene-bearing molecules were "peptide-nucleic acids" that used amino acids instead of ribose. Not crazy: PNAs have been synthesized, and recently identified in cyanobacteria by Banack et al., 2012. And yet, the substitution to RNA seems to have happened early.
- In living cells, these are made of highly impermeable phospholipid bilayers. (Indeed, protein channels regulate transport across the membrane.) the lab of Jack Szostak of Harvard has shown that fatty acids that would have been common in the Archean oceans can form vesicles that are permeable to nucleotide monomers and amino acids, but not to polymers of these. (See Szostak, 2012.)
- Osmotic pressure from nucleic acid polymers causes larger vesicles to "steal" fatty acids from smaller ones that they encounter. Mechanical forces cause larger vesicles to fission.
- The result, the beginning of natural selection, in which fatty acid vesicles that grow faster dominate.
Energy source: The crucial fact in the foregoing is that the both cell membranes and proteins seem to have originated in environments that are at least intermittently hot.
Synergies: Don't assume that the components of life given above evolved independently. Black et al., 2013 show that the simple fatty acid decanoic acid binds preferentially to the four RNA nucleotides (adenine, guanine, cytosine, and uracil). Moreover, in their bound state, the nucleotides buffer decanoic acid against the disruptive effects of salt water. The result is a natural affinity between fatty acids and RNA nucleotides.
Black smoker from NOAA - Ocean Explorer
Location of LUCA (the Last Universal Common Ancestor):Current thinking maintains that life probably originated in hydrothermal vents (right). These environments were:
- Sheltered from free oxygen, which is toxic
- Rich in thermal energy
- Live only at near-boiling temperatures
- Obtain energy from exotic reactions involving materials readily available in minerals, esp sulfur.
- Find oxygen to be toxic.
- LUCA lived in an anoxic environment rich in hydrogen and carbon dioxide
- It was a chemosynthetic autotroph, employing sulfur compounds as an energy source but was unable to feed heterotrophically
- It's environment was hot
- It possessed the biochemical machinery for the translation of DNA to proteins
- It relied heavily on molecules in its external environment for many metabolic functions that are handled internally by living cells.
Apex chert from Schropf, 1993
- Oldest strong evidence of life is Rosing, 1999 reports 3.7 Ga fractionated carbon in deep sea sediments from Isua, Greenland. This is a mere 100 my after the planet-sterilizing Late Heavy Bombardment.
- Schopf and Barghoorn, 1967 reported the oldest body fossils in the ~ 3.4 Ga Fig Tree Cherts of South Africa, however recently, Nutman et al., 2016 have reported on 3.7 Ga stromatolitic bacterial mats, also from Isua, Greenland.
- Mojzsis et al., 1996 reported a 3.85 Ga banded-iron formation with biogenically fractionated carbon from Akila island, Greenland. The rocks are considered by Fredo and Whitehouse, 2002 to be a
metavolcanic with its carbon being an abiotic metasomatic product.
- Schopf, 1993 suggests the Apex Chert (Australia 3.485 Ga) contains "microfossils." Probably bits of organic matter in a hot-springs solution deposit. Brasier et al., 2002 skeptically note that shapes grade from reasonable bacterial shapes to wholly inorganic, suggesting supposed biological forms are just part of shape spectrum.
- More recently Wacey et al., 2011 note sulfur-metabolizing microfossils from the Strelley Pool Formation of Australia, on morphological and geochemical evidence.
Cyanobacteria from Plant Science 4 U
What we definitely know:
Photosynthesis: Organisms change Earth chemistry
For a while, organisms got away with chemosynthesis in vent environments, and heterotrophically absorbing the organic materials that were floating around in the ocean. As these started to get scarce, one group, the cyanobacteria, came up with a new method of autographically capturing energy from the environment - Photosynthesis,
We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for the rock record
Living cyanobacteria probably provide a good picture of ancient photosynthesizers. Indeed, all other photosynthesizing organisms ultimately rely (directly or indirectly) on cyanobacterial symbionts. Phylogenetic studies of living cyanobacteria suggest a minimum divergence age of roughly 2.8 Ga.
Great Oxidation Event: Between 2.4 and 2.3 Ga atmospheric oxygen concentrations rise to roughly 1%. Indicates that cyanobacteria had become widespread by this point.
The Gunflint Chert (1.88 Ga - Michigan) represents a well preserved Paleoproterozoic cyanobacterial flora containing forms that can be directly compared with living ecomorphs.
Banded iron formation
- BIFs first appear when photosynthesizers start cranking out free oxygen. This oxygen reacts with oxygen sinks like ferrous (2+) iron to form mineral oxides. Early BIFs formed in localized deep marine environments, but spread to encompass a wider environmental range.
- Brocks et al., 1999 report hopanes in 2.7 Ga rock from West Australia that seemed to clinch the presence of cyanobacteria at that time. More recent work by Rasmussen et al., 2008 has called this into question. They maintain that the oldest unambiguous cyanobacterial body fossils are roughly 2.15 ga.
- The disappearance of BIFs around 1.8 ga indicates the saturation of oxygen sinks and signals the beginning of accumulation of high concentrations of oxygen in atmosphere.
Bar River formation red bed
- Terrestrial red bed deposits begin to appear at around 2.2 ga. and start to become common around 1.8 ga. This tells us that the oceanic oxygen-sinks had become saturated and free oxygen was now building up in the atmosphere. (This roughly coincides with the onset of the "boring billion," an interval of unprecedented geochemical stability that lasted until the beginning of the "snowball Earth" episode of the Neoproterozoic.)
- Ozone: As it accumulated, free oxygen in upper atmosphere recombined to form ozone layer (O3). Ozone is opaque to ultraviolet light. The appearance of the ozone layer thus allowed life to colonize surface waters.
- Oceanic acidity: Of course, by eating up atmospheric CO2, photosynthesizers caused the acidity of the oceans to diminish, allowing the direct precipitation of carbonate rocks for the first time. Once that was possible, CO2 concentrations fell very rapidly. as carbon became locked up in rock.
- Respiration: The earliest organisms used nitrate or sulfur as electron receptors in the synthesis of ATP. As oxygen began to appear in the environment, some organisms evolved the ability to use it instead. To such aerobic critters, oxygen became a necessity rather than a poison. Today anaerobic organisms are restricted to marginal environments.C6H12O6+ 6 O2 ---> 6 CO2 + 6 H2O + energyLook familiar? It's just photosynthesis run backwards. In this case, the energy powers cell activities.
Stromatolite in cross-section
Stromatolites: Beginning about 3.0 Ga, we begin to see abundant fossil stromatolites - laminated bacterial mats. The earliest are from the 3.43 Ga Strelley Pool Chert of Australia (Allwood et al., 2006) and possibly the 3.7 Ga Isua metasediments (Nutman et al., 2016). These were very common for most of the Proterozoic, but declined during the Neoproterozoic, when, presumably, critters appeared that could eat them showed up.
- Stromatolites form when sediment falls onto a thin film of bacteria. The bacteria bind the sediment, and grow up through it. At any moment, only the top layer is alive.
- When there was nothing around to eat them, stromatolites were very common. Today, they only live in hypersaline environments that exclude other critters, like Shark Bay, Australia
Eukaryote - prokaryote comparison from Wikipedia
Eukaryota:Complex cells characterized by:
- Typically much larger than prokaryotic cells. > 60 microns as opposed to < 20.
- DNA contained in nucleus (phylogenetically related to the archaean genome)
- Specialized organelles bound by double-layer cell membranes, possessing their own genome. Especially:
- Chloroplasts: photosynthesizers (derived form cyanobacteria)
- Mitochondria: aerobic respirers (related to Rickettsia, an obligate endocellular parasite that causes typhus.)
- Eubacteria (proper bacteria)
- Oldest biochemical markers of eukaryotes are steranes ~2.7 Ga. (Brocks et al., 1999)
- Oldest eukaryotic body fossils: 2.2 Ga of South Africa, reported from a terrestrial paleosol (!) by Retallack et al., 2013.
Acritarch from Dinopedia
- Common Proterozoic eukaryote body fossils, include Shuiyousphaeridiumfrom the 1.6 Ga Ruyang Group of Henan, China (Yin, 1997).
- Represent encysted "resting stage" of organism.
- Broadly similar to dinoflagellates.
- Oldest acritarchs 1.6 Ga. Maximum diversity 850 Ma Marked decline during Snowball earth episode (850-600Ma) Survivors straggled into the Ordovician.
A Reasonable but Speculative Time-Line
- 3.8 Ga: End of Late Heavy Bombardment
- 3.7 Ga: Life present (geochemical evidence)
- 3.4 Ga: Recognizable prokaryote-grade cells present
- 3.0 Ga: Photosynthesis present, first BIFs.
- 2.7 Ga: Eukaryotes present (geochemical evidence)
- 2.4-2.3 Ga: Great oxidation event. Photosynthesizers widespread.
- 2.2 Ga: Eukaryotes present (fossil evidence)
- 1.8 Ga: Last BIFs.
- 1.6 Ga: Acritarchs present
- 1.2 Ga Multicellular organisms unambiguously present.
- Abigail C. Allwood, Malcolm R. Walter, Balz S. Kamber, Craig P. Marshall and Ian W. Burch. 2006. Stromatolite reef from the Early Archaean era of Australia. Nature 441, 714-718.
- Sandra Anne Banack, James S. Metcalf, Liying Jiang, Derek Craighead, Leopold L. Ilag, Paul Alan Cox. 2012. Cyanobacteria Produce N-(2-Aminoethyl)Glycine, a Backbone for Peptide Nucleic Acids Which May Have Been the First Genetic Molecules for Life on Earth. PlosOne November 2012.
- Roy A. Black, Matthew C. Blosser, Benjamin L. Stottrup, Ravi Tavakley, David W. Deamer, and Sarah L. Keller. 2013. Nucleobases bind to and stabilize aggregates of a prebiotic amphiphile, providing a viable mechanism for the emergence of protocells. Proceedings of the National Academy of Sciences 110(33), 13272–13276.
- Martin D. Brasier, Owen R. Green, Andrew P. Jephcoat, Annette K. Kleppe, Martin J. Van Kranendonk, John F. Lindsay, Andrew Steele, and Nathalie V. Grassineau. 2002. Questioning the evidence for Earth's oldest fossils. Nature 416, 76-81.
- Jesse V. Gavette, Matthias Stoop, Nicholas V. Hud, and Ramanarayanan Krishnamurthy. 2016. RNA–DNA Chimeras in the Context of an RNA World Transition to an RNA/DNA World. Angewandte Chemie September 2016 preprint.
- Allen P. Nutman, Vickie C. Bennett, Clark R. L. Friend, Martin J. Van Kranendonk, and Allan R. Chivas. 2016. Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature online 31 August 2016.
- Gregory J. Retallack, Evelyn S. Krull, Glenn D. Thackray, Dula Parkinson. 2013. Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa. Precambrian Research 235, 71-87.
- David Wacey, Matt R. Kilburn, Martin Saunders, John Cliff, and Martin D. Brasier. 2011. Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia. Nature Geoscience 4, 698-702.
- Madeline C. Weiss, Filipa L. Sousa, Natalia Mrnjavac, Sinje Neukirchen, Mayo Roettger, Shijulal Nelson-Sathi, and William F. Martin. 2016. The physiology and habitat of the last universal common ancestor. Nature Microbiology 1, Article number: 16116 .
- Tom A. Williams, Peter G. Foster, Tom M. W. Nye, Cymon J. Cox, T. Martin Embley. 2012. A congruent phylogenomic signal places eukaryotes within the Archaea. Proceedings of the Royal Society B. October 2012.