Smith is known as the Father of English Geology. Our understanding of the shape and pattern of the history of life depends on the accuracy of fossils and dating methods. Some critics, particularly religious fundamentalists, argue that neither fossils nor dating can be trusted, and that their interpretations are better. Other critics, perhaps more familiar with the data, question certain aspects of the quality of the fossil record and of its dating. These skeptics do not provide scientific evidence for their views.
Early geologists, in the s and s, noticed how fossils seemed to occur in sequences: The first work was done in England and France.
Then, geologists began to build up the stratigraphic column, the familiar listing of divisions of geological time — Jurassic, Cretaceous, Tertiary, and so on. Each time unit was characterized by particular fossils. The scheme worked all round the world, without fail. From the s onwards, geologists noted how fossils became more complex through time.
The oldest rocks contained no fossils, then came simple sea creatures, then more complex ones like fishes, then came life on land, then reptiles, then mammals, and finally humans. Since , paleontologists, or fossil experts, have searched the world for fossils. In the past years they have not found any fossils that Darwin would not have expected. Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since , nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.
All these labors have not led to a single unexpected finding such as a human fossil from the time of the dinosaurs, or a Jurassic dinosaur in the same rocks as Silurian trilobites. Paleontologists now apply sophisticated mathematical techniques to assess the relative quality of particular fossil successions, as well as the entire fossil record.
These demonstrate that, of course, we do not know everything and clearly never will , but we know enough. Today, innovative techniques provide further confirmation and understanding of the history of life. Biologists actually have at their disposal several independent ways of looking at the history of life - not only from the order of fossils in the rocks, but also through phylogenetic trees.
Phylogenetic trees are the family trees of particular groups of plants or animals, showing how all the species relate to each other. Phylogenetic trees are drawn up mathematically, using lists of morphological external form or molecular gene sequence characters.
Modern phylogenetic trees have no input from stratigraphy, so they can be used in a broad way to make comparisons between tree shape and stratigraphy. The majority of test cases show good agreement, so the fossil record tells the same story as the molecules enclosed in living organisms.
Dating in geology may be relative or absolute. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older. The discovery of means for absolute dating in the early s was a huge advance. The methods are all based on radioactive decay:.
The first radiometric dates, generated about , showed that the Earth was hundreds of millions, or billions, of years old. Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.
Age estimates can be cross-tested by using different isotope pairs. Results from different techniques, often measured in rival labs, continually confirm each other. Every few years, new geologic time scales are published, providing the latest dates for major time lines.
Older dates may change by a few million years up and down, but younger dates are stable. For example, it has been known since the s that the famous Cretaceous-Tertiary boundary, the line marking the end of the dinosaurs, was 65 million years old.
What are the different methods of dating fossils
Repeated recalibrations and retests, using ever more sophisticated techniques and equipment, cannot shift that date. It is accurate to within a few thousand years. The fossil record is fundamental to an understanding of evolution.
Fossils document the order of appearance of groups and they tell us about some of the amazing plants and animals that died out long ago. Thus dating that particular tree does not necessarily indicate when the fire burned or the structure was built. For this reason, many archaeologists prefer to use samples from short-lived plants for radiocarbon dating.
The development of accelerator mass spectrometry AMS dating, which allows a date to be obtained from a very small sample, has been very useful in this regard. Other radiometric dating techniques are available for earlier periods. One of the most widely used is potassium—argon dating K—Ar dating. Potassium is a radioactive isotope of potassium that decays into argon The half-life of potassium is 1. Potassium is common in rocks and minerals, allowing many samples of geochronological or archeological interest to be dated.
Argon , a noble gas, is not commonly incorporated into such samples except when produced in situ through radioactive decay.
The date measured reveals the last time that the object was heated past the closure temperature at which the trapped argon can escape the lattice. K—Ar dating was used to calibrate the geomagnetic polarity time scale.
Thermoluminescence testing also dates items to the last time they were heated. This technique is based on the principle that all objects absorb radiation from the environment. This process frees electrons within minerals that remain caught within the item. Heating an item to degrees Celsius or higher releases the trapped electrons , producing light. This light can be measured to determine the last time the item was heated.
Radiation levels do not remain constant over time.NEET Biology Evolution : Dating of Fossils
Fluctuating levels can skew results — for example, if an item went through several high radiation eras, thermoluminescence will return an older date for the item.
Many factors can spoil the sample before testing as well, exposing the sample to heat or direct light may cause some of the electrons to dissipate, causing the item to date younger. It cannot be used to accurately date a site on its own. However, it can be used to confirm the antiquity of an item. Optically stimulated luminescence OSL dating constrains the time at which sediment was last exposed to light. During sediment transport, exposure to sunlight 'zeros' the luminescence signal.
Upon burial, the sediment accumulates a luminescence signal as natural ambient radiation gradually ionises the mineral grains. Careful sampling under dark conditions allows the sediment to be exposed to artificial light in the laboratory which releases the OSL signal.
The amount of luminescence released is used to calculate the equivalent dose De that the sediment has acquired since deposition, which can be used in combination with the dose rate Dr to calculate the age. Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings , also known as growth rings.
Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. Dendrochronology has three main areas of application: In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11, years from present. Amino acid dating is a dating technique      used to estimate the age of a specimen in paleobiology , archaeology , forensic science , taphonomy , sedimentary geology and other fields.
This technique relates changes in amino acid molecules to the time elapsed since they were formed. All biological tissues contain amino acids. All amino acids except glycine the simplest one are optically active , having an asymmetric carbon atom. This means that the amino acid can have two different configurations, "D" or "L" which are mirror images of each other. With a few important exceptions, living organisms keep all their amino acids in the "L" configuration.
When an organism dies, control over the configuration of the amino acids ceases, and the ratio of D to L moves from a value near 0 towards an equilibrium value near 1, a process called racemization. Thus, measuring the ratio of D to L in a sample enables one to estimate how long ago the specimen died.
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American Journal of Archaeology. Annual Review of Earth and Planetary Sciences.