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Information to know |
The basis for the information |
Explanatory comments |
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The oldest known rocks are about 3. 9 Ga |
radiometric dating |
-- rocks much older than this were probably re-melted -- these rocks are metamorphosed sedimentary rocks |
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The oldest mineral grains within rocks are about 4.1 Ga |
radiometric dating of zircon crystals |
-- the mineral grains within a sedimentary rock are older than the rock itself |
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The oldest evidence for life is in the 3.9 Ga rocks |
radiometric dating |
-- carbon isotopes with graphite in the rocks is poor in 13C[1] |
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The oldest fossils known are about 3.5 Ga |
-- radiometric dating of associated igneous rocks -- we know these are biological because of the form of the stromatolites and from individual bacterial cells found in chert |
-- these fossils are stromatolites |
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Life can be divided into bacteria (prokaryotes) and everything else (eukaryotes) |
-- the cells of prokaryotes and eukaryotes differ considerably in their structure |
-- (among other features) eukaryotes (1) have “organelles” such as mitochondria, chloroplasts, ribosomes, etc., (2) have their genetic material enclosed in a cell nucleus, and (3) are typically much larger |
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The earliest known eukaryotes appear in the Proterozoic Eon, about 2+ Ga |
-- relatively large cells preserved in chert |
-- eukaryotes can be difficult to distinguish since organelles and nuclei do not preserve; size is the best available criterion -- later eukaryotes may have tough cyst walls, which are unique to eukaryotes -- the record of eukaryotes has been pushed back to over 2 Ga just in the past decade |
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Bacteria were common in shallow marine environments and increased in diversity through the Precambrian |
Stromatolites and records of individual bacteria increase through most of the Precambrian |
Stromatolites have been recorded for many years in Precambrian rocks throughout the world; records of bacteria were discovered in cherts[2] in the 1950s and have since been widely studied |
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Oxygen increased in the atmosphere to about 1/100 present levels by about 2.2 Ga |
Banded iron formations from this interval Disappearance of minerals such as uraninite |
Banded iron records oxidation of Fe in marine water, forming Fe3O4, which precipitates from solution Minerals such as uraninite do not form in environments with too much oxygen |
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Oxygen is likely to have arisen from photosynthesis of cyanobacteria |
The early atmosphere is not believed to have contained much oxygen The only apparent source for significant amounts of oxygen at this time is increasing numbers of photosynthetic bacteria |
Cyanobateria make stromatolites, which increase in abundance through the Precambrian |
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By the late Proterozoic, about 1.2 – 0.6 Ga, cyst-walled eukaryotes had become more abundant and diverse |
Organic-cysts increase in abundance in sedimentary rocks of this age |
Some kinds of organic cysts preserve readily in rocks; sedimentary rocks of this age, if broken down into their component grains, often have such cysts; they are also preserved in cherts |
Bacteria dominated the first 3 billion years of life in Earth history. Bacteria continue to be a major force in ecology in every environment. From such a perspective, the Earth is the planet of bacteria, and other life forms are relatively strange late-comers.
The Precambrian is known in less detail than any other interval in Earth history, because so much of the record from that time has had a chance to be eroded away or metamorphosed.
The origin of life is not known to be approachable from the fossil record. It is studied by chemists, who have attempted to make in a laboratory increasingly complex, self-replicating molecules using conditions thought to have existed early in Earth’s history.
Hot topic: Much of what is known about the Precambrian has been learned since the 1960s. This is because new techniques of looking for fossils have led to the discovery that more exist in Premabrian rocks than was previously recognized, and new techniques in geochemistry give us new insights into ancient ocean chemistry.
How old is it?
There can’t be a “History of Life” class if we can’t figure out how to order events in history.
Stratigraphy is the science of figuring out the “relative” ages of rocks – which are older and which are younger (as opposed to figuring out a numerical age).
Stratigraphy was developed in Europe for the practical purposes of looking for mineral and energy resources that were known to occur in certain layers and their experience that the ordering of rock layers was the same in different places locally. The first and most obvious technique was simply to characterize the nature of the rocks – the composition, texture, and so on. It was eventually found that over larger distances such layers can change from place to place, or even disappear, and something more general was needed to figure out one’s position in a vertical rock column. Early stratigraphers learned from practical experience that the vertical sequence of types of fossils in the rocks was the same everywhere they looked. This ordering was recognized long before it was understood how old the rocks were and before discussions of the evolutionary significance of organisms changing through time.
Based on fossils, early geologists worked out a “geological time scale” and, for convenience, gave the different sequences of rocks names by which to refer to them. At this time, geological ages in years were completely unknown and of no practical significance. Interest grew, however, in mapping in all industrial countries and gradually other parts of the world the relative ages of rocks found at the surface. Eventually it was found that stratigraphy would be of tremendous use and economic importance for petroleum companies coring through rock at any given point, until they reached layers that were likely to contain oil or gas. Fossils were found consistently to be the most practical value for determining relative ages, creating the field of “biostratigraphy,” and it was quickly found that some groups of fossils were more useful that others.
Characteristics of fossils useful for biostratigraphy:
-- Abundance and likelihood of fossilization is critical. One must have confidence that if the animal lived in the area that you will find it. Organisms are likely to fossilize if they have a hard mineralized skeleton, as do clams, snails, etc.
-- Obviously, to correlate rocks in two different areas, the organisms must be sufficiently widespread to live in both areas. Further, the organisms should be tied too tightly to any particularly environment, or else the absence of a species is more likely to represent local environment change than global extinction of the species. Organisms that fit these characteristics tend to be planktonic, floating across large expanses of ocean. However, some bottom-dwelling organisms can be widespread too, especially if they have larvae (babies) that live in the plankton for a little while before taking up residence on the bottom.
-- Small size is useful. Firstly, small organisms tend to be potentially more abundance. But in addition, fossils that can be found in abundance in a sediment sample as small as a drill core are extremely useful – such fossils are microscopic, collectively called “microfossils.”
-- Finally, individual taxa within the organisms should be reasonably easy to identify and geologically short-lived, to provide precision in relative dating.
It is less easy to correlate rocks – figure out the relative ages of rocks in different places – for rocks of Precambrian age, because the fossils that exist do enable us to identify specific taxa that existed for a short time interval. For example, the cyanobateria present in Precambrian rocks look much like the cyanobacteria present today.
[1] Photosynthetic organisms “fractionate” carbon, which means they selectively use 12C over 13C; for this reason, organic carbon has less 12C than the natural environment. Graphite is a mineral made of carbon, and such carbon accumulations are typically organic.
[2] Chert is “microcrystalline” quartz and seems to have formed from supersaturation of marine water with SiO2 (silica), causing it to precipitate around living bacteria, preserving their shapes and sizes. Internal features of bacteria are not preserved. We don’t understand chert formation well, because it does not seem to be a process commonly occurring in marine environments today.