Overview • If you were able to travel back to visit the Earth during the Archaean Eon (3.8 to 2.5 bya), you would likely not recognize it • The atmosphere was very different from what we breathe today; a reducing atmosphere of methane, ammonia, and other gases which would be toxic to most life on our planet today. • Also during this time, the Earth's crust cooled enough that rocks and continental plates began to form. • It was early in the Archaean Eon that life first appeared on Earth. • Our oldest fossils date to roughly 3.8 billion years ago, and consisted of bacteria microfossils.
Earth’s Oldest Rocks • Rocks older than 3.5 billion years are very rare on Earth • Indeed, there may have been only tiny patches of continental crust on the early Earth; over 90% of the crust would have been oceanic rock • The oldest rocks on Earth are from northern Canada; they are 3.9 billion years old and • There are also some very old rocks in the Isua area of west Greenland, dating at approximately 3.85 bya • Rocks from both sites contain no evidence of life. Major areas of exposed Precambrian rock
Carbon isotope evidence at 3.8 bya from Isua, Greenland • The Isua rocks do, however, contain quite a lot of carbon in the form of the mineral graphite (a type of elemental carbon). • The carbon at Isua is in the form of “light” graphite; as if it had been produced by RUBSICO via photosynthesis • It is indirect evidence that there may have been life on earth before 3.85 bya
The Oldest Rocks with Life • Although there is some indirect evidence that there may have been life on Earth before 3.85 bya, there is clearer isotopic evidence for biological carbon life at about 3.5 bya; the first fossils are from these rocks • Archaen districts in both Australia and Africa provide evidence of stromatolites - low mounds or domes of finely laminated sediment composed of either calcium carbonate (CaCO3) or chert (SiO2). • Represent fossilized microbial mats formed mainly by photosynthetic blue-green "algae") called cyanobacteria
Living Stromatolites • We can observe them being formed today in both marine and freshwater systems. • Stromatolites contain a consortium, a complex associations of interacting organisms; interwoven mats of slime-covered, filamentous cyanobacteria and other bacteria. • At the top, cyanobacteria do oxygenic photosynthesis. • Below the surface, bacteria that do photosynthesis without producing oxygen occur. Finally, deeper in the mat, heterotrophic bacteria feed on the decaying organic matter produced by photosynthesis at the top of the mat. • The minerals, along with grains of sediment precipitating from the water, are trapped within the sticky layer of mucilage that surrounds the bacterial colonies, which later continued to grow upwards through the sediment to form a new layer. • As this process occurred over and over again, the layers of sediment were created. Present day columnar stromatolites in Australia
Summary • There appear to be several lines of morphologic and geochemical evidence for Archean life. • These data clearly document the presence of living things, but the record is extremely spotty. • Unfortunately, very little rock of Archean age is preserved at the Earth’s surface at present, and the rock that does out crop is often severely metamorphosed and unfossiliferous. • Archaen populations were probably kept in check by natural disasters - storms, heating, drying, and starvation due to lack of nutrients • As a consequence, there must have been very rapid fluctuations in bacterial populations as conditioned changed from day to day or season to season
Banded Iron Formations (BIFs) • The geologic record indicates a rather peculiar rock type between 3.5 to 2.0 bya • Banded iron formations or BIFs - sedimentary rocks formed from alternating bands of chert (SiO2) and iron oxide. • There is no evidence of these kinds of deposits being formed today BIFs with red bands of hematitie and interbedded chert
Where does the iron come from? • Iron is and was probably dumped into the oceans from erosion down rivers and from deep-sea volcanic vents • Why are there no BIFs around in present geologic time? • Iron readily precipitates out of solution in the presence of oxygen and organisms subsequently extract and use iron and silica (silica that could have gone into chert, SiO2) in building protective shells and skeletons • Iron dissolves readily in water that has no oxygen and that there was apparently little or no free oxygen when BIFs were being formed • Iron can only have precipitated from seawater in the amounts observed in the BIFs by an oxidizing chemical reaction
Evidence of no oxygen on the early Earth • Pyrite (iron sulfide) and Uranite (uranium oxide) occurring in riverbeds from 3 to 2 bya. • These minerals are not stable when O2 levels are high. • Their presence in rivers confirms our suspicions that oxygen levels were very low on the early Earth.
How did iron precipitate out of solution when there was apparently little or no free oxygen available? • The alternating iron oxide/chert beds indicate that there must have been periodic waves of O2 available • In an oxygen-poor ocean, iron is soluble in water, so chert dominates the sediments on the ocean floor. • In an oxygen-rich ocean, iron is oxidized (it rusts), forming minerals that are insoluble in water, so iron oxide dominates the ocean-floor sediments. • Oxygen could have been supplied by the photosynthetic cyanobacteria present in stromatolites • Ultimately, this oxygen is used up by the "rusting" of this iron, and the ocean reverts to its ocean-poor state.
The Oxygen Revolution • Q. Why might photosynthesis have established itself on the early earth? • For bacteria, the advantages of photosynthesis may have occurred as soon as simple organic molecules began to run low, and fermenters began to run low of food • Autotrophic cells could store food and have a buffer against times of low food supply • The earliest photosynthetic cells probably used H from H2, H2S, or lactic acid • Some of the bacteria may have began to break up the strong H bonds of water molecules • H2O + CO2 + light (CH2O) + 2 O
Note: • Any bacteria that became capable of successfully breaking down water rather than H2S would immediately have multiplied their energy supply • However, there certainly would have been a cost with this switch • The waste product of H2O photosynthesis is monatomic oxygen (O), which is a poison to a cell because it can break down vital organic molecules by oxidizing them • Thus cells needed to evolve a natural antidote to this oxygen poison before they could consistently operate the new photosynthesis • We and other organisms evolved superoxide dismutases to serve as antidotes • Presumably as soon as cyanobacteria evolved an antidote to oxygen poisoning, they could control the use of it, including the use in new processes such as respiration
The Advantage of Respiration • Aerobic respiration extracts considerably more energy from organic molecules (C6H12O6) than does fermentation (=anaerobic respiration) • Fermentation yields lactic acid which still has a great deal of energy • By using oxygen to break up a series of by-products all the way down t water and carbon dioxide, a cell can release up to 18X more energy from a sugar molecule via respiration than it can via simple fermentation
Conclusions about Stromatolites • Cyanobacteria, especially those in stromatolites, appear to be the dominant forms along the early ocean shorelines • Their success was likely due to the control over oxygen, which gave them an abundant and reliable energy supply in 2 ways: • 1) by mastering photosynthesis based on water and • 2) by breaking down food molecules in respiration rather than • fermentation • Note: • Stromatolites increase dramatically in the rock record with the beginning of the Proterozoic Era at about 2500 mya
Conclusions regarding the Oxygen Revolution • The increased oxygen supply by stromatolites in shallow water produced the first great masses of BIFs • It is probable that by oxidizing the iron, the BIFs served as a sort of buffer, allowing oxygen tolerance and utilization to evolve among some bacteria • But eventually BIF formation slackened and the oceans and atmosphere began to accumulate small amounts of oxygen • After about 2000 mya oxygen levels in the ocean reached a permanent level so high that sea water could no longer hold dissolved iron and BIFs could no longer form
Continental Red Beds • There is other geological evidence that confirms the oxygenation of the oceans around 2 bya • Beginning 2.3 Bya, iron minerals in soils on land began to be oxidized (rusted) during weathering; soils turned red. • The atmosphere must have contained O2 for this to occur. • Based on the types of oxide minerals present in early Proterozoic soils, it has been estimated that O2 levels were from 2 to 10% of modern. • Today, O2 comprises 20% of the atmosphere, so in the late Proterozoic, it may have comprised 0.4 to 2% of the atmosphere.
Red-Bed Blouberg Formation • The appearance of red beds, characterized by red iron oxide minerals, in the geological record marks the first appearance of significant quantities of oxygen in the Earth's atmosphere.
An Ozone Shield • One important environmental effect of higher O2 levels is an Ozone Shield • With O2 levels in the 1% range, the stratosphere would begin to develop an effective ozone (O3) layer. • Although this ozone shield is not especially important to aquatic organisms, who are protected by water, it would be extremely important to living things trying to colonize land