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. Biology textbooks usually mention that the planet is ~4.6 billion years old but neither tell how that was determined nor give evidence that the method (radiometric dating) is reliable. Students are therefore given insufficient reason to doubt that the Earth is any older than the 6000 years that the Genesis account 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. Earth’s age is relevant to biology because an old Earth is necessary for macroevolution to occur and also because some dating methods (e.g., dendrochronology, varves) use materials of biological origin.
Less than half the population of the United States accepts evolutionary theory (Mazur, 2005; Miller et al., 2006). An important contributor to the American resistance to evolutionary theory is widespread acceptance of the Genesis account, according to which the Earth is only about 6000 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.
Geologic Dating: The Basics
In the 1600s, the Danish anatomist Nicolas Steno noticed that flowing water deposits new layers of sediment over old layers, and he formulated the “principle of superposition”: a stratum (layer) that lies over another is the younger of the two, and the underlying stratum is older (Wicander & Monroe, 1993). In the late 1700s, 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. This is the “principle of fossil succession” (Wicander & Monroe, 1993). Smith and several scientists of the 1800s used the principles of fossil succession and superposition to correlate strata from different areas, thus piecing together the geological history of wide regions. By the end of the 1800s, geologists could describe the changes in the fossil fauna and flora through different time spans in the rock record and had given names to most time spans (Wicander & Monroe, 1993) (Figure 1). 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 to determine the absolute ages of rocks.
Radiometric dating, a method to determine the ages of rocks, was developed in the early 1900s. The method makes use of radioisotopes (radioactive isotopes), materials in which the atoms emit particles from the nucleus and thereby become atoms of a different isotope, often a different element. This process is called “radioactive decay.” The original isotope is called the “parent isotope,” and the decay product is called the “daughter isotope.” The ratio of the amount of parent to daughter isotope in a material can be used to determine its age when the radioactive decay rate is known. 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-238 and its daughter isotope lead-206 or the parent radioisotope uranium-235 and its daughter isotope lead-207; K-Ar uses the parent radioisotope potassium-40 and its daughter isotope argon-40. Dating that uses the radioisotope carbon-14 is often simply called “radiocarbon dating” or “carbon dating.” It is useful only for samples younger than 70,000 years, because of the high decay rate of carbon-14. For older samples, other radiometric methods such as U-Pb or K-Ar are necessary (Wicander & Monroe, 1993).
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-14 from the atmosphere, and the remaining carbon-14 decays into nitrogen-14. The ratio of carbon-14 to the nonradioactive isotope carbon-12 in biological remains can therefore be used to date the death of the remains (Wicander & Monroe, 1993).
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. Radiometric dating therefore reveals the dates of ancient volcanic eruptions and, thus, the age of any stratum that contains minerals that were ejected by an eruption (Wicander & Monroe, 1993).
English geologist Arthur Holmes used the U-Pb decay system to obtain the first radiometric ages of rocks in the early 20th century. His work was the first to show that certain rocks are >1 billion years old (Holmes, 1911). 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).
Age of the Earth
U-Pb yields an age of 4.35 billion years for the oldest known rocks from Earth (Wyche et al., 2004). However, older rocks may once have existed and remelted in tectonic events. Therefore, to determine the age at which the Earth’s crust solidified, scientists have radiometrically dated meteorites that were once part of an asteroid belt that solidified at the same time as the Earth’s crust. Such meteorites consistently yield Pb-Pb ages of 4.55 billion years (Patterson, 1956). Moon rocks, which also solidified at approximately the same time as the Earth’s crust, yield radiometric ages of 4.53 billion years (Kleine et al., 2005). The Earth is therefore a little more than 4.5 billion years old.
Nonradiometric Evidence for an Old Earth
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 used 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 12,410 years long (Friedrich et al., 2004), demonstrating that Earth is much older than 6000 years.
Varves are thin sedimentary layers, typically a few millimeters and often <1 mm thick, that are deposited annually in lakes and certain marine environments, as suspended particles settle slowly to the bottom. 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., 1995; Thunell et al., 1995; Kitagawa & van der Plicht, 1998). 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. Using this method a continuous sequence of varves representing ~13,000 years has been constructed from sediments of ancient Swedish lakes (Wohlfarth et al., 1995).
In some cases, sediment from a single lake yields thousands of varves. A continuous series of 9662 varves is known from the sediment at the bottom of Lake Gósciąż in Poland (Goslar et al., 1995). A series of >12,000 varves is known from North America’s Lake Erie (Sears, 1948). A continuous series of 29,100 varves comes from Lake Suigetsu in Japan (Kitagawa & van der Plicht, 1998). These lakes have therefore been accumulating sediment for at least 9662, 12,000, and 29,100 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. The Green River Formation, a lake deposit in Wyoming, Colorado, and Utah from the Eocene Epoch, contains too many varves to count. Using average varve thicknesses in various beds of this formation and the total thickness of those beds, Bradley (1929) calculated that ~6.5 million varves are present, which indicates that the lake accumulated sediment for 6.5 million years. Using the same method, Stamp (1925) calculated that ~2.17 million varves are present in an Oligocene–Miocene deposit in Myanmar, and Rubey (1930) calculated that ~2 million varves are present in the Upper Cretaceous Graneros Shale, a marine deposit from the American Midwest. 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 number 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, 2008). 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., 1995).
Polar ice has annual growth layers that are visually identifiable. In an ice core from Greenland, 40,500 such layers were visually counted (Alley et al., 1993), showing that the area has been accumulating ice – and has therefore existed – for more than 40,000 years. Alley et al. (1993) confirmed that the layers were indeed annual by counting not only the visible boundaries of the layers but also the variations in dust accumulation and chemical properties of Arctic ice that are known to vary annually. Other Arctic ice cores record time spans of ~40,000 years (Johnsen et al., 1992).
Polar ice cores show patterns of changes in chemical signatures, 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 number of years between given climatic events. Each dating method therefore confirms the accuracy of the other. For example, all three methods confirm that average temperatures rose dramatically in Europe about 11,450–11,390 years ago (Björck et al., 1996). 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., 1992).
An ice core from Lake Vostok, Antarctica, records a much longer span of time than 40,000 years. The Vostok ice core is 3623 m deep, approximately half again the depth of the 40,000-year Greenland ice cores, which are ~2300 m deep (Johnsen et al., 1992; Alley et al., 1993). Annual ice layers at great depths are compressed into smaller thicknesses by the weight of the overlying layers. Using known values for the magnitude of such compression, the time span recorded by the Vostok ice core is estimated as ~420,000 years (Petit et al., 1999).
Radiometric Dating: Evidence for Accuracy
The hypothesis that the results of radiometric dating are reliable makes several testable predictions. (1) Different samples that represent the same period should have similar radiometric ages. (2) Radiometric ages should be greater in older strata and lesser in younger strata. (3) Radiometric ages should agree with ages found by other methods. (4) For given samples, methods using different radioisotopes should yield similar ages. (5) For an event or object of known age, radiometric dating should yield the correct age. (6) Radioactive decay rates should be constant. All these predictions have been tested and confirmed.
1. Samples that represent the same period have the same radiometric age
This prediction is confirmed by a plethora of radiometric dates from different sites. For example, deposits at or immediately above the Cretaceous–Tertiary boundary – which is diagnosed nonradiometrically by fossil assemblages and the presence of a high concentration of the element iridium (from meteorite fallout) at the boundary – consistently give dates of 60–65 million years, even in different geographic locations, including Alberta, Saskatchewan, Montana, Mexico, Haiti, and India (Wellman & McElhinny, 1970; Baadsgaard et al., 1988; Swisher et al., 1992).
2. Radiometric ages are greater in older strata and lesser in younger strata
This prediction is confirmed by a century’s worth of radiometric dates from various stratigraphic levels (positions in the sequence of rock layers). Ogg (2004) 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 (2004) provide a similar list for a series of deposits from the Cambrian Period, as do Cooper and Sadler (2004) for the Ordovician Period, House and Gradstein (2004) for the Devonian Period, Davydov et al. (2004) for the Carboniferous Period, Wardlaw et al. (2004) for the Permian Period, and Luterbacher et al. (2004) for the early Cenozoic Era. 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 1.7 million years according to Ar-Ar dates (Renne et al., 1999).
3. Radiometric ages agree with ages found by other methods
Radiometric ages agree with ages found by other methods. For corals whose growth rings indicate an age between 250 and 300 years, U-Th dating was accurate to within 10% (Bard et al., 1993). U-Th dating also agrees well with dendrochonological dates (Bard et al., 1990). The radiocarbon record in varves agrees with that found in tree rings. Moreover, radiocarbon dates from lake varves, marine varves, tree rings, and polar ice cores all agree with each other (Kitagawa & van der Plicht, 1998).
4. Different radioisotopes yield similar ages
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-14 and beryllium-10 in lake and marine sediments (caused by elevated bombardment by cosmic rays) at approximately 28,000 and 33,000 B.P. (Kitagawa & van der Plicht, 1998). K-Ar and Rb-Sr both date the Laidlaw Volcanics, a Silurian deposit in Australia, with close agreement at ~421 million years (Wyborn et al., 1982). K-Ar, Rb-Sr, and U-Pb all date a Late Cretaceous bentonite layer in Saskatchewan with close agreement at ~72.5 million years (Baadsgaard et al., 1993). U-Pb, Ar-Ar, and Rb-Sr all yield dates agreeing to within 1.5 million years from the mean age of 64.3 million years for a bentonite deposit immediately above the Cretaceous–Tertiary boundary in Alberta, Saskatchewan, and Montana (Baadsgaard et al., 1988). The U-Pb date of 125.2 million years in an Early Cretaceous deposit in China fits neatly between the Ar-Ar ages of 128.4 million and 121.1 million years, respectively, for beds below and above it (Zhou et al., 2003). Rb-Sr and Ar-Ar methods both give close agreement at ~4.5 billion years for the age of the Olivenza meteorite and the Gurarena meteorite; Rb-Sr, Ar-Ar, and Sm-Nd have all yield an age of ~4.5 billion years for the St. Severin meteorite (Dalrymple, 1991).
In cases in which disagreement occurs between ages found using different radioisotopes, the disagreement is usually <4% and can be accounted for by imprecise knowledge of decay constants of some radioisotopes and by differences in calibration methods (Renne et al., 1998a; Villeneuve, 2004). For example, decay constants of the argon-40 decay series are less precisely known than those for the U-Pb system, but even so, for the Fish Canyon Tuff of Colorado, an Oligocene volcanic deposit, the argon-40 series yields ages that differ by <1 million years from the U-Pb age (27.5 million years; Villeneuve et al., 2000). Improvements in calibration for argon-40 ages continue to be made in order to minimize discrepancies, which are already small to begin with (Renne et al., 1998b; Villeneuve et al., 2000).
5. For an event or object of known age, radiometric dating should yield the correct age
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.D. 79, Ar-Ar yielded a date of 1,925 ± 94 years before the study (that is, a date of A.D. 72, give or take 94 years; Renne et al., 1997).
6. Radioactive decay rates are constant
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. Also, experimental evidence now confirms that radioactive decay rates are changed insignificantly or not at all by subjecting samples to extremes of heat (24–1280°K) and pressure (≤200 atm) and strong magnetic fields (up to 83,000 Gauss) (Emery, 1972). 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 4.6-billion-year overestimate of the age of a 6000-year-old Earth, the surface of the Earth 6000 years ago would have been an order of magnitude hotter than the surface of the sun (Meert, 2002). The Garden of Eden would have had no solid ground, because no part of Earth would have been solid. Even Noah’s Flood would have been impossible, because at the time that it supposedly occurred (1646 years after the creation, according to Genesis), Earth’s surface temperature would still have been too high for water to exist as a liquid (Meert, 2002).
Dendrochronology, the varve record, and the polar ice-core record all confirm that the Earth is much older than 6000 years. According to radiometric dates, the Earth is >4.5 billion years old. 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.