Proterozoic Life and Environments (or: 3 billion years in 70 minutes) Cambrian Explosion Origin of Life Great Oxygenation Event
Origin of Life
Early Life Localities >2.5 Ga Named localities: >3.2 Ga
Isua (Greenland) Earliest Life? Oldest known sedimentary rocks (3.85-3.8 Ga) Carbon isotope composition (of graphite) suggestive of life Alternative possibility: Abiotic Fischer-Tropsch synthesis of hydrocarbons
Pilbara (Australia) Earliest Fossils? Oldest (3.47 Ga) unmetamorphosed sedimentary rocks Contains putative microfossils of ancient bacteria
Fossils are just abiotic features formed as rims around shards of volcanic glass and chert Microfossils may not even occur in sedimentary rocks!
Schopf responded that some were just folded or decayed Also used new techniques of 3D confocal microscopy and laser Raman spetrometry Spherical cells of made of carbon (kerogen)
More evidence - Stromatolites
Stromatolites 3.4 Ga Western Australia Structures composed of alternating layers of sediment, trapped and bound by microbes But are they a sign of life? stromatoloid
It is fairly certain that life was present by 3.4 Ga
Earliest organisms were prokaryotes (bacteria and archaea)
Microbial Metabolism Light Phototrophy Oxygenic photosynthesis (producing oxygen) like in plants and cyanobacteria Anoxygenic photosynthesis, using light to get energy from chemical molecules Chemicals Chemotrophy Based on redox (reduction or oxidation) chemical reactions E.g., oxidizing Fe +2 to Fe +3 Elements used: S, Fe, H, N, P, Mn, As, U, Mo, Hg, and probably many more
Metabolism of Archean microbes Methanogenesis present by 3.47 Ga CO 2 + 4 H 2 CH 4 + 2H 2 O Evidence from carbon isotopes (assuming fluid inclusions are primary) Sulfate reduction also present by 3.47 Ga SO 4 2- + 2 CH 2 O H 2 S + 2 HCO 3 - Evidence from sulfur isotopes ( 36 S, 34 S, 33 S, 32 S)
Metabolism of Archean microbes Anoxygenic photosynthesis likely present by 3.4 Ga (or even 3.8 Ga) Use light to oxidize iron ( photoferrotrophy ) or hydrogen (H 2 ) Iron isotopes of early Archean rocks are enriched, consistent with anoxygenic photosynthesis But isotope signature of abiotic iron oxidation is about the same
Great Oxygenation Event Archean atmosphere was oxygen-free (<0.001% PAL, present atmospheric levels) When (and why) did oxygen levels rise?
Cyanobacteria Oxygenation of the atmosphere (and ocean) is a story of oxygenic photosynthesis and of the evolution of cyanobacteria
Oxygenic Photosynthesis CO 2 + H 2 O + Light = O 2 + Organics Rise of oxygen was a fundamental change to the terrestrial biosphere = 1) Atmospheric oxygen led to development of ozone (O 3 ) layer, blocking damaging ultraviolet radiation 2) Ultimately allowed evolution of large, complex cells When did oxygenic photosynthesis (=cyanobacteria) evolve?
Biomarkers Complex organic molecules diagnostic of a particular group Usually lipids that were part of the cell membrane = 2a-methylhopane indicates presence of cyanobacteria
Cyanobacteria Evolution Bill Schopf argued for 3.5 Ga cyanobacteria But now that conclusion is criticized and generally thought to be wrong = In 1999, cyanobacterial biomarkers were discovered in late Archean (2.7 Ga) oil But in 2008 some of the original authors did additional analyses showing that the oil was not indigenous (actually formed and seeped into the rock sometime after 2.2 Ga)
Cyanobacteria Evolution The oldest confident evidence for cyanobacteria (at this point) is 2.15 Ga fossils from Canada But maybe we can use a more indirect method: looking for the increase in atmospheric oxygen produced by cyanobacteria Evidence: Redox-sensitive elements and minerals can be used as proxies for atmospheric oxygen levels
Banded Iron Formations Sediments with alternating layers of: Chert (SiO 2 ) Oxidized iron minerals When there is no oxygen, iron (Fe +2 ) stays dissolved in water Iron (Fe +3 ) precipitates as minerals when oxygen is present
Vast majority of BIFs formed between 2.4-2.2 Ga Do Archean BIFs indicate local or temporary oxygenation? End of BIF deposition at 1.8 Ga thought to indicate final oxygenation of oceans Actually indicates depletion of Fe in sulfidic deep ocean
Manganese Deposits Like Fe, manganese is soluble in anoxic water but precipitates in oxygenated oceans vast majority formed around 2.4 Ga
Detrital Minerals Minerals like pyrite (FeS 2 ) and uraninite (UO 2 ) are not stable and oxidize in an oxygenated atmosphere Detrital pyrite/uraninite (in river sediment) was common >2.4 Ga Detrital pyrite
Red Beds Red bed paleosols (ancient soils) first appear after 2.2 Ga Red color indicates oxidized iron (Not Proterozoic)
Sulfur Isotopes Archean sulfur isotope signal produced by UV radiation (photolysis) After 2.4 Ga, oxygen increased to form ozone layer, blocking UV radiation Biological effects then were more important
Mass-independent fractionation by UV dominates prior to 2.4 Ga, but is absent after (all fractionation is mass-dependent)
The Oxygen Revolution Proxy evidence suggests that atmospheric oxygen levels increased rapidly (between 2.45-2.32 Ga) from <0.001% PAL to 1-10% PAL Proxies: -Banded iron formations and manganese deposits -Detrital pyrite and uraninite -Paleosol red beds -Molybdenum concentrations in sediment -Sulfur isotopes
The Oxygen Revolution Why did oxygenic photosynthesis evolve so long (>1 Gyr) after life evolved? Oxygen is damaging to living cells, which have had to evolve a number of enzymes to reduce oxidative damage
Deep Ocean Anoxia Deep ocean remained anoxic for another 1.7 Gyr, until the Neoproterozoic-Cambrian transition
But oxygenation of surface ocean also provided evolutionary opportunities allowed evolution of more complex eukaryotic cells Oldest Eukaryote
Eukaryotes the third major domain of life
Eukaryote Diversification Oxygen Revolution Many crown-group eukaryotes had evolved by mid-neoproterozoic (750 Ma)? First Eukaryote Neoproterozoic Testate Amoeba Modern Testate Amoeba
Cambrian Explosion 3. Body fossils Actually lasted 30-60 Ma 2. Small Shelly Fossils (SSF) 1. Trace Fossils
Ediacaran Biota Large, soft-bodied organisms from the late Precambrian
Ediacara Biota Three distinct assemblages from 575 to 540 Ma Nama White Sea Avalon
Failed Experiments: Rangeomorphs Modular construction Fractal branching
Evolutionary Innovation: Mobility First trace fossils indicate mobile bilaterian-grade animals
Evolutionary Innovation: Grazing Scratching and other feeding traces of mobile animals
Disappearance of the Rangeomorphs Their food source (dissolved organic carbon) disappeared But this increased oxygen levels, allowing motile and skeletonized organisms to evolve Rangeomorphs were the most abundant fossil in early Ediacaran communities, but rare later
Trace Fossils Behavioral Complexity Late Proterozoic traces were rare and were exclusively horizontal grazing burrows But in the Cambrian, traces were common, many were vertical, and represented a wider range of behaviors (feeding, dwelling, resting, movement, )
Small Shelly Fossils Small shelly fossils (SSF) are the earliest signs of the rapid diversification of Cambrian animals Common in the earliest Cambrian, 10 Myr before the first large crown group body fossils Some are known crown-group taxa, but many are parts of larger, unknown animals
Body Fossil Diversification Most evidence for the Cambrian explosion comes from body fossils Diversity and disparity increased rapidly from 530-510 Ma
What Caused the Cambrian Explosion? Animal diversity and disparity were low in the Ediacaran and there were very few skeletonized animal taxa What caused the increased diversity and disparity during the Early Cambrian? 1. Changes in ocean water chemistry 2. Increased oxygen concentration 3. Genetic innovations 4. Ecological interactions
Ocean Chemistry Changes Most marine animals make shells from calcium carbonate (CaCO 3 ) Increase in Ca concentration during Proterozoic-Cambrian transition Could the Cambrian explosion have been an explosion of skeletonization, not an actual diversification?
Increased Oxygen Concentration Large motile organisms have high metabolic needs, and making a skeleton is metabolically costly (metabolism requires oxygen) Could an increase in oxygen levels have triggered Early Cambrian diversification? No, but it was an important threshold in the Ediacaran First motile bilaterians after 555 Ma oxygenation event Post-Snowball oxygen rise allowed evolution of large animals
Did the proliferation of body plans result from increased genetic complexity? Hox genes are responsible for large-scale body patterning (segmentation, anterior-posterior and dorso-ventral axes, etc.) Genetic Innovations
However, the Hox gene toolkit had evolved by the late Proterozoic with the Bilateria (some Hox genes present in the Eumetazoa)
Ecological Interactions Predators first evolved in the Early Cambrian
Ecological Interactions Motility was a much more common trait among Cambrian organisms (although was present in Ediacaran) Some organisms evolved to actively move through the sediment
Ecological Interactions Vertical burrowing (bioturbation) changed the nature of the seafloor in the Early Cambrian
When organisms only have to adapt to one constraint (e.g., maximizing food gathering), there are only a few optimal morphologies to satisfy that constraint But as more constraints are added (avoiding predation, maximizing reproduction, adapting to soft substrates), there is no single optimal strategy Ecological feedback cycle
Take-Home Messages 1. Carbon isotopes, microfossils and stromatolites suggest that life evolved sometime during the first billion years of Earth history, at least by 3.5-3.4 Ga, but perhaps earlier. 2. Early life used chemicals (sulfur or hydrogen) for energy or used sunlight to oxidize iron (or H 2 ). Oxygen-producing photosynthesis evolved (much) later. 3. The release of oxygen by cyanobacteria led to a rise in atmospheric oxygen around 2.4 Ga, to 1-10% of present levels. 4. 1) Detrital minerals, 2) red beds, 3) banded iron formations, 4) molybdenum levels, and 5) sulfur isotopes are proxies for atmospheric oxygen concentrations.
Take-Home Messages 5. Cambrian Explosion was stepwise appearance of major animal groups and complex behaviors over 40 million years, indicated by trace fossils, small shelly fossils, and body fossils. 6. Necessary precursors included increased oxygen levels and genetic complexity, but ecological interactions (such as predation) were important causes of diversification.