The best evidence about the origin of life and the evolution of living organisms comes from the fossil record. Fossils
can be preserved remnants of organisms (e.g., bones and teeth) or whole organisms embedded in amber, acid bogs and tar pits, or anywhere bacteria can't decompose a dead body. The fossil record is not complete and has gaps. This is, in part, because not all organisms are equally likely to fossilize. Organisms with hard body parts fossilized more often than those who lacked hard body parts. Organisms that lived near water and in areas of low oxygen were also more likely to form fossils than organisms who lived away from water. Organisms from large populations are typically more common in the fossil record than organisms from small populations; however, if none of the other conditions are in place, large population size is less important. Fossils can also be found in other forms (e.g., an animal's footprint). A scientist who studies fossils is a paleontologist. Fossils are divided into age groups according to a geological time scale, a classification of different periods in Earth’s history. For example, layers of rock bearing evidence of the origin of most modern animal phyla would be classified as belonging to the Cambrian period in the Paleozoic era. Layers of rock bearing fossils suggest a rapid diversification of reptiles took place during the Permian period in the Paleozoic era. You do not need to know the different periods and epochs for this course, but the images below are a nice visual to show the evolution of major life forms.
Relative Dating
The best source of fossils is sedimentary rock, which is formed by layers of minerals that settle out of water. Over time these minerals build up and pack the layers lying beneath them, along with any organisms that have settled with the sediments. The figure below summarizes the major events associated with the formation of sedimentary rock. Note that each stratum, or layer, represents a particular time period in Earth's history and is characterized by a collection of fossils of organisms that lived at that time. The location of fossils in sedimentary rock is indicative of their relative age. This means that one fossil can generally be classified as older than another based on their relative position in the sedimentary rock. Deeper layers were formed earlier, and their fossils are older than those found in more shallow layers of rock. This method of estimating the relative age of a fossil is known as relative dating .
Absolute Dating
A variety of methods are used to estimate a fossil's age in years, rather than simply in relative terms. These absolute dating methods include radiometric dating.
Radiometric Dating
The identity of a particular atom is determined by the number of protons it has in its nucleus. However, a collection of atoms of a particular element can have varying numbers of neutrons. For example, hydrogen atoms can have no neutrons (the most common form, sometimes called protium), one neutron (deuterium) or two neutrons (tritium). These are hydrogen's three isotopes. Moreover, different isotopes have different (but predictable) stabilities. Many elements have radioactive isotopes; their nuclei decay at predictable rates (unique for each isotope) and give off energy and atomic particles. When the decay leads to a change in the number of protons, it transforms the atom to an atom of a different element. For example, potassium-40 decays from a radioactive parent atom to a stable daughter atom of argon-40. Scientists have identified several elements with radioactive isotopes that decay at known rates; they can be used to date various rocks, including those containing fossils. This procedure is known as radiometric dating, and as mentioned earlier, this absolute dating method has been used to date material as old as meteorites. Let's take a closer look at how this method works.
The graph above illustrates how, over time, radioactive parent atoms decay and the number of stable daughter atoms, relative to the number of parent atoms, increases exponentially. Note how the two lines intersect at the first half-life point. In other words, a radioisotope's half-life is the amount of time it takes for one-half of the parental atom population to decay into daughter atoms. It takes 1.25 billion years for half of the potassium-40 in a sample to decay to argon-40; in other words, the half-life of potassium-40 is 1.25 billion years. Because living things contain so much carbon, radiometric dating using carbon-14 (C14 ), also called radiocarbon dating or carbon-14 dating , is quite common.
This short video (from Scientific American) discusses the key features of radiocarbon dating:
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How Do We Know That C-14 Dating is Accurate?
So how does subatomic physics help establish an absolute dating method for fossils? If an organism is alive, it incorporates C14 and C12 into its body. Once an organism dies it no longer takes up new C14 and the C14 already in its tissues radioactively decays. The C12 in its tissues remains the same because it is not radioactive and does not decay. By measuring the ratio of C14 to C12 and comparing it to the ratio of C14 to C12 present originally in the atmosphere, scientists can calculate the age of a fossil. The rate at which carbon decays (its half-life) is approximately 5730 years. After about 50,000 years, the amount of C14 remaining is so small that material can't be dated reliably (although some specialized samples have been dated to about 75,000 years). How do we know this radiometric “clock” is accurate? To have confidence in a scientific fact, corroboration is required. In other words, without an independent means of verifying a fact, its scientific value is questionable. The more a fact is checked using different methods, the more reliable the scientific community judges that fact. Many experiments have verified the usefulness and accuracy of carbon-14 dating. For example, archaeologists know from the historical record the age of various Egyptian tombs. Wood taken from these tombs has been dated using the carbon-14 dating method, and the two dates agree. In addition, the argon/potassium-40 dating method has been used to date the time of the Mount Vesuvius eruption in Italy, and this date coincides with the date reported by Roman historians. Many other corroborative dates have validated the use of radiometric methods.
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