Hadean Eon: 4.5-3.8 Billion Years Ago
Archean Eon: 3.8-2.5 Billion Years Ago
Proterozoic Eon: 2.5 Billion to 542 Million Years Ago
Our museum is designed to explore permineralized plant material through geologic time. The evolution of plants that we recognize as trees starts in the Devonian, thus we focus our attention on the Phanerozoic eon, which includes the Paleozoic, Mesozoic and Cenozoic eras. Before starting our adventure in the Cambrian, let’s note some important fossil evidence found in the Archean and Proterozoic eons, once known as the Precambrian. The Precambrian also includes the Hadean eon, which is represented by meteorites and dates from 4.5 to 3.8 billion years. The rock record for earth starts at around 3.8 billion years in the Archean eon.
Evidence of Life
The Archean eon stretches from 3.8 Ga to 2.5 Ga. The symbol Ga is used to represent a giggannum, a unit equal to 10 raised to the 9th power or one billion years. The symbol Ma is used to denote a megaanum, a unit equal to 10 raised to the 6th power or 1 million years. Evidence for the first life and photosynthesis comes from multiple lines of evidence, which include: organic chemical signatures, fossils, and rock structures representing biogenic activity.
Organic chemical signatures or biomarkers are chemical fossils that may be in the form of isotopic ratios or molecular fossils. The first chemical evidence of photosynthesis, in the form of C-12 to C-13 ratios, can be found in Archean rock of 3.8 Ga from Isua, Greenland (Kenrick & Davis, 2004, pp 10-11; Johnson & Stucky, 1995, p 22). The process of photosynthesis preferentially utilizes C-12 over C-13 when removing CO2 from the air to synthesize carbohydrates, creating ratios of these carbon isotopes that differ from normal background ratios. Thus, carbon compounds processed by photosynthetic organisms are enriched with C-12 (Rich & Fenton, 1996, p 91). The enrichment of C-12 in rocks is a test for the presence of life. Carbon isotope ratios consistent with presence of cyanobacteria are widespread in rock dated at 3.5 Ga (Johnson & Stucky, 1995, p. 22). One problem with the evidence above is the fact that chemical pathways in non-photosynthetic autotrophs and nonautotrophs can produce C-12 enrichment (Blankenship, Sadekar, & Raymond, 2007, pp 22-23).
Stromatolites & Oncolites
The most common fossil structures attributed to cyanobacteria are stromatolites and oncolites (Stinchcomb, 2008, p. 16). The earliest evidence for life from structural fossils comes from rock outcrops in Australia (dated at 3.5 Ga) and Africa (dated at 3.2-3.4 Ga). Both rock outcrops contain cherts with stromatolite structures. Living stromatolites are communities of cyanobacteria and other microbes, which form a mat-like structure. As filaments of this structure become inundated with sediment the living material migrates to the top to form a new photosynthetic mat-like layer. In this way a mound-shaped structure made of alternating layers of organic matter and sediment is created. Structures resembling prokaryotes are associated with both locations. However, nonbiological processes can produce similar structures and the biogenic origin of these stromatolites is debated (Nudds & Selden, 2008, p. 23). There are examples of stromatolites from the Proterozoic eon (2.5 Ga-542 Ma) that contain microfossils similar to modern forms. Stromatolites are also found in geologic environments consistent with biologic activity (Kenrick & Davis, 2004, p. 14). Oncolites are similar to stromatolites, but form as spherical concretions formed by layers of calcium carbonate.
No consensus is to be found concerning the definition of what constitutes a reef. For the purpose of our museum we will use a definition modified from Rachael Wood. Reefs form as a result of organic activity, developing due to the aggregation of sessile organisms living on the sea-floor, with a resulting higher concentration of carbonate production than the surrounding sea sediments (Wood, 1998, p. 182). Reef structures often serve as a habitat for a community of organisms. Reef structures eventually build rock, often in the form of large carbonate platforms. Stromatolites formed the first reef structures, the oldest of which are late Archean (Webb, 2001, p. 172).
Banded Iron Formations
Banded iron formations, known as BIF's, are ancient sedimentary deposits consisting of alternating layers of iron rich and silica rich layers. BIF's started out as sediments deposited on ancient ocean floors. BIF deposits are huge and can extend for hundreds of kilometers with uninterrupted banding (Perkins, 2009, p. 24).
In 1965 Preston Cloud realized that banded iron formations (BIF) provided clues to Earth’s early atmosphere and oceans. Evidence of low oxygen levels in the atmosphere and high levels of dissolved iron in the oceans is provided by the formation of red beds. Banded iron formations were formed only during a narrow range of time; dating from 2.8 Ga to 1.8 Ga. BIF formation reached a peak at 2.5 Ga and is rare in rocks younger than 2.0 Ga (Cowen, 2005, pp. 25-27). During this time, iron was deposited into the oceans by deep sea hydrothermal vents and terrestrial erosion. Iron dissolves in water that has no oxygen. In the presence of oxygen, iron chemically reacts forming an insoluble iron oxide, which precipitates out of solution. In the organic model for BIF formation oxygen produced from the process of photosynthesis, carried out by cyanobacteria, accounts for the formation of the iron rich bands.
The first photoautotrophs expelled oxygen into the seawater, where it was free to oxidized iron. The precipitates from this oxidation process settled into layers forming the distinctive red formation rich with hematite and magnetite, alternating with silicates, such as jasper and chert. The banding pattern may represent seasonal temperature variations. During the summer photosynthesis and thus oxygen levels would increase, chemically reacting with the dissolved iron and precipitating to form the iron rich sediment layers. During winter as photosynthesis decreased silicate rich layers formed (Perkins, 2009, p. 24).
These banded iron formations are a testament to that fact that biological activity was rusting the planet (Nudds & Selden, 2008, pp. 21-22). Today, the modern steel industry mines the iron laid down by bacteria over 1.8 billion years ago. The steel frames of cars and skyscrapers are made possible due to the biologic activity of Precambrian photosynthetic bacteria (Cowen, 2005, pp. 25-26).
Oxygen in the Atmosphere
BIF's record the transition from an early Earth that had low levels of free oxygen and high amounts of dissolved iron to an Earth with high levels of free oxygen and low levels of dissolved iron. Multiple lines of evidence suggest an atmosphere free of oxygen before and during early BIF formation. Ancient deposits of sand and gravel record that iron and sulfur were in reduced states. Pyrite (iron disulfide), siderite (iron carbonate), and uraninite (uranium dioxide) exist as sedimentary grains in river deposits older than 2.2 Ga. These minerals break down in the presence of oxygen and today are not found among sediments on coastal floodplains (Knoll, 2003, pp. 96 & 97).
The formation of BIF's does indicate that the Earth underwent a global redox reaction. The last of the banded iron formations occurs at around 1.8 Ga. After a staggering 1 billion years of forming red beds around the world there was less free iron to chemically react with the oxygen produced through photosynthesis. At some point during the billion year period of red bed formation the production of oxygen outpaced the production of chemicals that consumed oxygen. Thus, oxygen could flood the atmosphere.
The rock record indicates that Earth has experienced two major increases in oxygen levels. The first rise in oxygen levels is known as the Great Oxidation Event (GOE) and occurred at around 2.4 Ga (Catling, 2008, p. 17). Chemical signatures for oxygen build up in the atmosphere on a global scale occur between 2.4 Ga and 2.3 Ga (Blankenship, Sadekar, & Raymond, 2007, p25). As we have already mentioned oxygen sensitive minerals could exist as eroded grains in sedimentary deposits aged 2.2 Ga and older. As these oxygen sensitive minerals fade from sedimentary deposits sedimentary rock requiring oxygen rose to prominence (Knoll, 2003, p. 97). Red beds, which are terrestrial sedimentary deposits rich in iron oxides, start forming at around 2.3 Ga and become increasingly common after 2.2 Ga (Cowen, 2005, p. 27). Sulfur isotopes in rocks also indicate an increase in the amount of sulfates in ancient sediments, a sure sign that sulfur was being oxidized. Oxygen levels rose from less than 1 ppm to 0.3 to 0.6% during GOE (Catling, 2008, p. 18).
Thus, biological activity started to change an atmosphere made mostly of methane, carbon dioxide, and nitrogen to one that was enriched with oxygen. Methane, produced by Archaea bacteria, acted as a greenhouse gas, warming the Earth during the Archean Eon. Methane breaks down in the presence of free oxygen. So, the build up of oxygen cooled the planet by lowering methane levels. Evidence for large ice ages occur between 2.4 and 2.2 Ga. Atmospheric oxygen also started to form the ozone layer high in the stratosphere. The ozone layer absorbs harmful ultraviolet radiation (UV), critical for evolving life on Earth (Cowen, 2005, pp 27-28).
Cyanobacteria were creating an "oxygen revolution" by converting the early reducing atmosphere into an oxidizing atmosphere. A second spike in oxygen levels took place around 580 Ma. At this time atmospheric oxygen increased up to 15% or more of present levels. This second increase is closely correlated with the first appearance of animals in the fossil record (Catling, 2008, p. 18).
Prokaryotic Structural FossilsThe first universally accepted occurrence of prokaryotic structural fossils comes from the ~2 Ga Gun Flint Formation of Ontario, Canada. This banded iron formation was recently dated at 1.878 Ga. The BIF is made of alternating layers of iron oxides and cherts. The cherts contain stromatolite structures and a variety of microfossils that resemble present day cyanobacteria (autotrophs) and iron and manganese-loving bacteria (chemotrophs). The Gun Flint Formation most likely represents deposition on a continental shelf (Nudds & Selden, 2008, pp. 13-21).
Eukaryotic Structural Fossils
The first undisputed occurrence of fossil eukaryotes comes form the Huntington Formation of Summerset Island, Arctic Canada. The formation is dated at 1.2 Ga. Tidal flat deposits preserve red algae, stromatolite structures, and cyanobacteria (Nudds & Selden, 2008, p.23). The eukaryotic fossils display patterns of thallus organization, cell division, and differentiation that represent bangiophyte red algae (Knoll, Summons, Waldbauer, & Zumberge, 2007, p.150). Chert nodules in the 700-800 million year old Draken Conglomerate Formation of Spitsbergen contain microfossils of cyanobacteria as well as a diverse biota of eukayotes. The Draken Conglomerate Formation represents a tidal flat/lagoon complex (Nudds & Selden, 2008, p. 24). The oldest sponge spicules date to 800 Ma. The first whole sponge dates to 570 Ma (Cooper, 2001, p. 98).
The First Primary Producers
Although eukaryotes are present in the Proterozoic, evidence from fossils and biomarkers indicate that primary production on Earth started with and was dominated by cyanobacteria. Eukaryotes, in the form of green algae, played a limited role in primary production for the last 600 million years of the Proterozoic eon (2.5 Ga-542 Ma) (Knoll, Summons, Waldbauer, & Zumberge, p. 152). Cyanobacteria, green algae, and red algae would continue to be the major primary producers in the oceans during the Paleozoic eon. It could be that cyanobacteria dominated the role of primary producers because ocean conditions favored photoautotrophs capable of nitrogen fixation. To this day cyanobacteria are still important for nitrogen fixation and take on the role of primary producers in some parts of the oceans.
In addition to creating Earth’s oxidizing atmosphere and taking on the role of the first primary producers there is strong evidence that the double membrane bound organelles found in eukaryotes formed as a result of endosymbiotic relationships among prokaryotes. The now well supported Endosymbiotic Theory was proposed by the American Biologist Lynn Margulis in 1966. Multiple lines of evidence support the theory. To name just a few, organelles, such as mitochondria and chloroplasts are the same size as bacteria, retain functional bacteria-like DNA, make some of their own proteins with ribosomes identical to bacteria, possess an inner membrane that is similar in structure to that found in bacteria, and replicate via binary fission independently of the cell. Although not the first to propose this evolutionary explanation it was the hard work and persistence of Margulis and many other scientists that led to the current theory, which is the best plausible explanation for the evolution of eukaryotes. The Endosymbiotic theory was finally popularized in Margulis 1981 book Symbiosis in Cell Evolution.
The period of time spanning 670 Ma to about 540 Ma is referred to as Ediacaran or Venadian period and is characterized by some very interesting soft-bodied multicellular organisms. The first embryos of multicellular organisms also occur in Venadian deposits. Cherts and shales in the Doushantuo Formation of Southern China, dated at 600 Ma, contain microfossils of red algae and spheres interpreted as animal embryos (Nudds & Selden, 2008, p. 24). Some of the embryos look very similar to embryos of invertebrate animals with bilateral symmetry (Palmer, 1999, p. 57).
The first multicellular animals are well represented by the Ediacara biota, which consist of soft-bodied marine organisms. Ediacara type fossils are found in deposits ranging from 670 Ma to 540 Ma. Although first found in the Ediacara hills of Australia north of Adelaide similar deposits are now known from the UK, Africa, Canada, U.S., China, etc. (Selden & Nudds, 2004, p. 11).
The Ediacara represents a Fossil-Lagerstatten. Lagerstatten is a German word used in mining that denotes a particularly rich seam (Selden & Nudds, 2004, p. 7). Organisms with hard parts that habitat environments where sedimentary deposition occurs are most likely to form fossils. Fossil deposits with soft-bodied organisms well preserved or with terrestrial animals, such as dinosaurs in the Morrison formation are termed Lagerstatten because they give us a window into past environments seldom preserved in the fossil record. Two types of fossil lagerstatten are recognized. Deposits that contain vast numbers of fossils represent Concentration Lagerstatten. The preservation may not be exceptional, but the great numbers can be very informative. Conservation Lagerstatten contain fossils with soft body preservation, impressions of soft tissue or fossils of well-articulated skeletons without soft tissue preservation. Conservation Lagerstatten allow paleontologists to better understand the morphology and phylogentic relationships of organism, provide knowledge of soft-bodied organisms, and reconstruct paleoecosystems (Nudds & Selden, 2008, pp. 8-9).
Some of the Ediacara biota seem to represent the first occurrences of jellyfish, sea pens, arthropods and flatworms. Other members of this fauna represent extinct groups with no descendants. Some scientists place the Ediacara biota in their own kingdom called the Vendozoa, removing them from Metazoa (Selden & Nudds, 2004, p16).
The Ediacara biota represents a shallow marine environment unlike any known today. These organisms had no mouths or stomachs and bodies with large surface to volume ratios. Nutrients and wastes were probably exchanged with the seawater through diffusion. It is possible that some of the Ediacara fauna obtained food through a symbiotic relationship with algae (Johnson & Stucky, 1995, p. 12).
The First Mass Extinction
Evidence of the first mass extinction occurs during the Proterozoic eon. Acritarchs are durable life stages of single-celled algae. Acritarchs appear around 1.4 Ma and become greatly diversified by 700 Ma. Towards the end of the Proterozoic eon at around 650 Ma many Acritarch species meet extinction. The extinction event coinsides with widespread glaciation (Stanley, 1987, p. 52-53).
Towards the end of the Proterozoic eon organisms evolved exoskeletons and
shells. The development of these hard structures greatly increased the likelihood
of preservation. It is the evolution of these hard parts that defines our
next time period, the Cambrian.
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