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The American Biology Teacher 1 April ; 75 4 : — Biology textbooks tend to assert the correctness of evolutionary concepts but mention very little of the evidence that supports them. This gives the impression that evolutionary theory is poorly supported, which discourages acceptance of the theory. A case in point is the age of the Earth. Students are therefore given insufficient reason to doubt that the Earth is any older than the years that the Genesis suggests. Here, therefore, I review the evidence for an old Earth, to provide a concise but thorough reference for teachers who wish to supplement the meager information in textbooks with further details.
Less than half the population of the United States accepts evolutionary theory Mazur, ; Miller et al. An important contributor to the American resistance to evolutionary theory is widespread acceptance of the Genesisaccording to which the Earth is only about years old and the various kinds of organisms were created independently of each other.
Another problem is that biology textbooks mention very little of the wealth of information supporting evolutionary theory. This gives the reader the impression that such evidence is scant, which in turn discourages acceptance of the theory. For example, biology textbooks usually omit the evidence that the Earth is old enough for all organisms to have had time to descend from a common ancestor.
It is important to present this information, to demonstrate that macroevolution is plausible. For the teacher who wishes to introduce such evidence in the classroom, I provide the following review. In the late s, the English civil engineer William Smith noticed that different strata have different assemblages of fossils and that the sequence of these assemblages from older to younger rocks is the same in different areas.
Smith and several scientists of the s used the principles of fossil succession and superposition to correlate strata from different areas, thus piecing together the geological history of wide regions. However, although the time spans had been named and their sequence was known, no one knew how much time each span represented or how long ago it had occurred, because no one had yet discovered How does radioactive dating help in the study of evolution to determine the absolute ages of rocks.
The geological time scale, showing the named time spans and the dates in millions of years ago of the borders of each, as determined by radiometric dating. Data are from Ogg et al. A Geological time scale, including the Precambrian Supereon.
B Geological time scale, with breakdown of time spans within the Phanerozoic Eon. Radiometric dating, a method to determine the ages of rocks, was developed in the early s. The method makes use of radioisotopes radioactive isotopesmaterials in which the atoms emit particles from the nucleus and thereby become atoms of a different isotope, often a different element. Each radioisotope has a different decay rate; for many radioisotopes, the decay rates are known, having been measured in the lab, and these known rates are used in radiometric dating.
Geologists often use a shorthand way to specify which pair of isotopes is used in a particular dating method. For example, U-Pb uses the parent radioisotope uranium and its daughter isotope lead or the parent radioisotope uranium and its daughter isotope lead; K-Ar uses the parent radioisotope potassium and its daughter isotope argon Radiocarbon is used to date the death of once-living material such as animal remains, plant fibers, or wood.
At death an organism stops taking in carbon from the atmosphere, and the remaining carbon decays into nitrogen Other radioisotopes are usually applied to date the solidification of igneous rock, which is rock that was once molten magma. As magma cools, the minerals in it crystallize. As they crystallize, some of these minerals incorporate parent radioisotopes into their crystal structure.
Upon cooling, the radioisotopic clock is set at zero for such minerals, and from that time on, daughter isotopes begin to accumulate in the mineral crystals. This accumulation is measured in the lab to determine when these rocks cooled. English geologist Arthur Holmes used the U-Pb decay system to obtain the first radiometric ages of rocks in the early 20th century. Since then, geologists have determined the ages of many strata around the world, including those that define the borders between the named geological time spans Figure 1.
U-Pb yields an age of 4. However, older rocks may once have existed and remelted in tectonic events. Such meteorites consistently yield Pb-Pb ages of 4. The Earth is therefore a little more than 4. Dendrochronology is the use of data from annual tree rings to date samples. Tree rings are wider in wetter years and narrower in dryer years. In trees from overlapping periods, patterns of variations in ring thickness through the years can be How does radioactive dating help in the study of evolution to correlate periods represented by rings from different trees, and periods can even be matched in trees from different locations.
Using such correlation methods with European trees, dendrochronologists have pieced together a continuous tree-ring sequence 12, years long Friedrich et al. Typically, a varve consists of a light-colored layer deposited during spring and summer and a darker layer deposited during autumn and winter. The difference in colors is due to a higher accumulation of the shells of microscopic organisms during the spring and summer months, when these organisms are more abundant Goslar et al.
Series of chemical differences in varves from one year to the next can be matched in sediments from different lakes. This allows correlation between varve sequences of different ancient lakes. In some cases, sediment from a single lake yields thousands of varves.
These lakes have therefore been accumulating sediment for at least12, and 29, years, respectively, which can only have happened if the Earth is at least that old. The varve series mentioned above are from sediments that represent only the Holocene and Pleistocene epochs of the Neogene Period see Figure 1. Varves in sedimentary rock from some earlier periods record even longer stretches of time. Varves therefore provide evidence that Earth is millions of years old. A typical young-Earth creationist objection to varve evidence for long expanses of time is that a large of thin layers can represent a short time span; for example, ash layers produced during a single day by the eruption of Mount St.
Helens contains many fine laminations Whitmore, However, this objection is nonsensical, because volcanic ash laminations are not varves. Volcanic ash lacks the shells of aquatic microorganisms that color the summer layer of a varve, and experiments using sediment traps demonstrate that a single varve takes a year to accumulate Thunell et al. Polar ice has annual growth layers that are visually identifiable. In an ice core from Greenland, 40, such layers were visually counted Alley et al.
Alley et al. Polar ice cores show patterns of changes in chemical atures, dust accumulation, and pollen accumulation that vary across centuries and can be matched from one ice core to the next. Such changes can also be matched with corresponding changes across tree rings in the dendrochronological record and across varves in the lake sediment record. The ice record, the dendrochronological record, and the lake sediment record all record the same of years between given climatic events. Each dating method therefore confirms the accuracy of the other. That the time estimates produced by such methods are correct is confirmed by the presence, in ice layers from the expected periods, of fallout from volcanic eruptions of known times Johnsen et al.
An ice core from Lake Vostok, Antarctica, records a much longer span of time than 40, years. Annual ice layers at great depths are compressed into smaller thicknesses by the weight of the overlying layers. The hypothesis that the of radiometric dating are reliable makes several testable predictions. All these predictions have been tested and confirmed. This prediction is confirmed by a plethora of radiometric dates from different sites. Ogg lists radiometric dates found by various methods for deposits from all over the globe, spanning the entire Mesozoic Era and including examples from each subdivision of each period of the Mesozoic.
Shergold and Cooper provide a similar list for a series of deposits from the Cambrian Period, as do Cooper and Sadler for the Ordovician Period, House and Grtein for the Devonian Period, Davydov et al. Among and between all these examples, dates are indeed greater for stratigraphically lower older deposits and lesser for stratigraphically higher younger deposits. This is true even within narrow stratigraphic ranges such as the series of 12 volcanic deposits in the Sagantole Formation of Ethiopia, which spans only 1.
Radiometric ages agree with ages found by other methods. U-Th dating also agrees well with dendrochonological dates Bard et al. The radiocarbon record in varves agrees with that found in tree rings. Often only one pair of radioisotopes is used to date a given sample, but in some cases more than one pair is used, and ages found using different pairs of radioisotopes generally agree.
For example, radiocarbon and U-Th both date a pair of spikes in carbon and beryllium in lake and marine sediments caused by elevated bombardment by cosmic rays at approximately 28, and 33, B. The U-Pb date of Severin meteorite Dalrymple, Improvements in calibration for argon ages continue to be made in order to minimize discrepancies, which are already small to begin with Renne et al.
Radiometric dating is not often applied to events or objects for which history records the exact ages, because their ages are not in question. However, a few examples exist, and the radiometric dates are correct. For example, for ejecta from the eruption of Mount Vesuvius, known from historical records to have taken place in A. One could argue that radiometric methods find ages of millions and billions of years only because radioactive decay was once faster and has now slowed.
However, if that is correct, then radiometric dating should consistently overestimate the ages of events and objects of known age. As shown above, it does not. Arguments that such processes have changed radioactive decay rates in buried strata are therefore invalid. In addition, radioactive decay releases heat.
For the rate of radioactive decay to have been high enough in the past to yield a 4. The Garden of Eden would have had no solid ground, because no part of Earth would have been solid. Dendrochronology, the varve record, and the polar ice-core record all confirm that the Earth is much older than years. Numerous lines of evidence confirm the reliability of radiometric dating. The Earth has therefore experienced the immense time span necessary for macroevolution to have taken place.
I thank Alan Deino Berkeley Geochronology Center and two anonymous reviewers for helpful reviews of the manuscript. User Tools. In. Skip Nav Destination Article. Close mobile search Article. Volume 75, Issue 4. Article Next Article. Geologic Dating: The Basics. Age of the Earth. Nonradiometric Evidence for an Old Earth. Radiometric Dating: Evidence for Accuracy.How does radioactive dating help in the study of evolution
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