Paleontology is the study of plant and animal life in the geologic past. Paleontologists use fossils to study past life forms. They research what early organisms looked like, as well as their environments, their relationships to other life forms, and their correlations with modern species. Paleontology was crucial in building our current understanding of Earth's history. Prior to the birth of paleontology in the early nineteenth century, geologists had no way of determining the relative ages of rocks, but by comparing fossils that represented short-lived life forms, geologists were able to date rock strata more accurately, and put together a chronology of the earth. Paleontology also provided evidence for the theory of evolution.
The French biologist Georges Cuvier (1769-1832) is considered the father of paleontology. Cuvier worked in the Museum of Natural History in Paris around the turn of the eighteenth century, and there he developed a keen interest in comparative anatomy. He was able to reconstruct partial skeletons, using his skill and intuition to posit the shape of missing bones. He perfected the classification system of Carl Linnaeus (1707-1778), grouping animals together according to their internal structure instead of their outer form. After completing his work with modern animals, Cuvier turned his attention to fossils. These skeletons of unknown animals frozen in stone still seemed to Cuvier to fit into his classification system. In 1796, he posited that a certain fossil skeleton was an extinct species of elephant, and later he related a giant fossil sloth to the smaller sloth species still living in South America. He identified and named the pterodactyl, though he apparently misidentified the first known discovery of dinosaur teeth as belonging to an extinct rhinoceros.
Unable to believe that ancient life forms could have evolved into modern ones, Cuvier argued instead that several prehistoric catastrophes, including the flood described in Genesis, had destroyed all life on Earth, and it had been created anew in differing forms. Cuvier sparked worldwide interest in fossils that eventually led to a non-Biblical interpretation of the formation of the earth and its creatures. Another Frenchman, Éduard Lartet (1801-1871) found a fossil mammoth tooth inscribed with a drawing in a cave in France, giving clear evidence that humans too had existed in the era of these strange extinct animals. Lartet's discovery made Cuvier's theory untenable, and opened the door for Charles Darwin (1809-1882).
Further studies unearthed fossils found to be over 600 million years old. These were the oldest fossils known until 1965, when an American paleontologist, Elso Sterrenberg Barghoorn (1913-1984) found bits of carbonized material in ancient rocks that seemed to be fossil bacteria. Barghoorn's microfossils were found to be perhaps 3,500 million years old, dating back to a billion years after the formation of the earth.
Prior to modern dating methods, paleontologists used relative dating techniques to determine the approximate age of a fossil. Stratigraphy is the study of layers of rocks or the objects embedded within those layers. It is based on the assumption (which nearly always holds true) that deeper layers were deposited earlier, and thus are older, than more shallow layers. The sequential layers of rock represent sequential intervals of time. Seriation is the ordering of objects according to their age. Artifact styles such as pottery types are seriated by analyzing their abundances through time. Seriation assumes that all differences in artifact styles are the result of different periods of time, and are not due to the immigration of new cultures into the area of study. Faunal dating uses animal bones to determine the age of sedimentary layers or objects such as cultural artifacts embedded within those layers. Faunal dating works best if the animals belonged to species which evolved quickly, expanded rapidly over a large area, or suffered a mass extinction. Another relative dating technique that utilizes that fact that plants annually spread pollen over a given area and, in certain areas such as lake beds, that pollen is more likely to be preserved is pollen dating. Using pollen dating or palynology, scientists can develop a pollen chronology by noting which species of pollen were deposited earlier in time and fossilized within rock layers.
Modern paleontology uses the most advanced chemistry and microbiology, including deoxyribonucleic acid (DNA) research, to date samples. Known as absolute dating, such techniques include amino acid racimization, cation-ratio dating, thermoluminescence dating, tree-ring dating, radioactive decay dating, and uranium series dating. Popular during the 1970s, amino acid racimization is based on the principle that amino acids (except glycine) exist in two mirror image forms called stereoisomers. Living organisms synthesize and incorporate only the L-form into proteins. This means that the ratio of the D-form to the L-form is zero (D/L=0). When these organisms die, the L-amino acids are slowly converted into D-amino acids in a process call racimization.. The reversible reaction eventually creates equal amounts of L- and D-forms (D/L=1.0), which can help date a sample. Since amino acids react at different rates, accurate dating is often difficult.
Cation-ratio dating is a technique that examines how long rock surfaces have been exposed. Since cations move throughout the environment at different rates and the ratio of different cations to each other changes over time, cation ratio dating relies on the principle that the cation ratio (K + + Ca2+)/Ti4+ decreases with increasing age of the sample.
Thermoluminescence dating is used to determine the age of pottery. When electrons from the minerals withing pottery clay are exposed to radioactive substances that are present in the clay or burial medium, they are bumped out of their normal positions. When the pottery is then heated at a very high temperature (over 932°F or 500°C), the electrons fall back to their normal state, emitting light. The longer the exposure to radioactive material, the more light is emitted. By measuring the amount of emitted light, scientists calculated how much time has passed since the pottery was fired.
Also known as dendrochronology, tree-ring dating is based on the fact that trees produce one growth ring per year. The tree-ring patterns are all the same for a given species and geographical area. With the patterns from trees of different ages, a master pattern can be created and used to date buildings and archaeological sites. Tree rings are also used to date climate changes because each ring's thickness indicates the type of climate present for that particular year.
Radioactive decay is a process in which a radioactive form of an element is converted to a nonradioactive product at a constant rate. Radioactive decay dating techniques include potassium-argon dating and radiocarbon dating. When volcanic rocks are heated at very high temperatures, they release argon gas. As they cool, argon-40 (40Ar) begins to accumulate. Argon-40 forms in the rocks from the radioactive decay of potassium-40 ( 40K). The amount of 40Ar formed is proportional to the decay rate of 40K, which is 1.3 billion years. Since it takes a very long time to create a measurable amount of 40Ar, this technique is only appropriate for rocks greater than three million years old. Radiocarbon dating is used to date charcoal, wood, and other biological materials. The range of conventional dating is 30,000-40,000 years, but with sensitive equipment this range can be extended to 70,000 years. Radiocarbon (14C) spontaneously decays into nitrogen-14 (14N). Both plants and animals contain carbon, and while alive, they sustain the same ratio of 14C/12C that is present in the atmosphere. When an organism dies, however, the ratio begins to change as the 14C decays into 14N. The rate at which the decay occurs is called half-life and refers to the time required for 14C to decay into 14N. The half-life of 14C is 5,730 years, and by comparing the ratio of 14C to 14N within the organism's remains, the amount of time since the organism's death can be determined.
Uranium series dating utilizes that fact that radioactive uranium and thorium isotopes decay into a series of unstable, radioactive "daughter" isotopes until a stable lead isotope is formed. Parent isotopes have half-lives of several thousand million years, while daughter isotopes have half-lives ranging from a few hundred thousand years to only a few years. Daughter deficiency or daughter excess methods can be used to date a sample. In daughter deficiency situations, the parent radioisotope is initially deposited by itself. As the parent decays, it reaches the point where it contains the same amount as the daughter, and they are in equilibrium. The age of the deposit can be calculated by measuring how much the daughter has formed, providing that neither isotope has entered or exited the deposit after its formation. In the case of a daughter excess, a larger amount of the daughter is initially deposited than the parent. Over time, the excess daughter disappears as it is converted back into the parent, and by measuring the extent to which this has occurred, scientists can date the sample. Another form of uranium series dating is fission tracking dating, which considers the tracks made in volcanic minerals and glass from the fission of uranium-238 (238U) atoms. The rate ate which fission occurs is proportional to the decay rate of 238U and its measurement is determined by the half-life of the element. The half-life of 238U is 4.47 x 109 years.
Presently, DNA has be examined by scientists in order to date fossil specimens. For example, paleontologists in the 1990s examined the rare carbon 12 and carbon 13 isotopes buried in ancient ocean sediment to trace early climactic activity. Other scientists have been using animal DNA as a "molecular clock" that can date when common species began to diverge. In many cases, the fossil record and the DNA record are at odds, sparking competitive theories between microbiologists and traditionally fossil-bound paleontologists. Much is still unknown about Earth's history. Thus paleontology, with ever more sophisticated tools at its disposal, is still a vital and intriguing field of study.
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