Definition of Fossil Record
The fossil record is an instrumental tool for exploring the history of life, which stretches back millions of years into Earth's past. Small clues from a fossil can tell us a lot about the organism such as when it died and most importantly where it came from.
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Fossil Record
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The fossil record is the ordering of fossils throughout geological time in layers of rock that accumulate and form. Recent layers are near the top, and older layers are toward the bottom. By observing the composition of older layers, we can discover what the Earth was like in the ancient past. However, fossilization is rare, and it can only give us snapshots of moments in time. Imagine the history of Earth laid out like chronological photographs on a wall. The fossil record is not perfect, but it can give us an accurate picture.
Geochronology
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The fossil record can be ordered according to geological segments of time. The longest period, an eon, may be up to a billion years in length. The Hadean, Archean, and Proterozoic eons make up about 89 percent of Earth's 4.5 billion year history. The Phanerozoic eon encompasses the last 545 million years, around when complex life such as trilobites began to appear. Eons can be further broken down into eras, periods and epochs. The current era, the Cenozoic, began 65 million years ago with the extinction of dinosaurs. The Quaternary period covers the previous 2.5 million years. And the Holocene epoch is the last 10,000 years of human civilization.
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Dating
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Scientists use radiometric dating to obtain the age of a fossil layer. When half of a substance decays into another element, it is called a half life. Potassium-argon dating is popular for old samples because it takes half of potassium about 1.25 billion years to decay into argon. When something like an igneous rock crystallizes, it is "zeroed," meaning that it has pure potassium trapped within. By measuring the ratio of potassium to argon, the age of a layer can be determined.
Layers
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Knowing the approximate age of a fossil is often a matter of knowing the layer that it is trapped in. Looking in a layer that corresponds to the Jurassic period, for instance, can give paleontologists a window in which the fossil could appear. An accurate analysis needs to be done to discover a more exact age. However, it is the layer itself that is being dated, which requires rocks with elements that can easily be dated.
Tree of LIfe
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The entire fossil record is well-ordered according to evolutionary history. It can tell us when a species lived and usually the kind of environment in which it lived. It can sometimes even offer us a better idea of how organisms evolved. The evolution of species is represented by a vast tree of life. Like a family tree, it is really about the flow of genetic information. Individuals are related to each other by genetic similarities but are always in the process of branching away from each other with further genetic isolation. Species are just a larger genetic divergence.
Relatedness
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The fossil record allows us to understand how organisms relate: where they are similar or different. It also allows biologists to make predictions. Whales were thought to have evolved in the Eocene epoch about 55 to 34 million years ago. Using this prediction, many fossils were found that gave biologists a more accurate view of whale evolution. Over time, these fossils appeared to lose more and more use of their hind limbs until whales eventually inherited very small limbs. Because the tree of life is constantly branching, however, relatedness doesn't necessarily mean direct ancestry, just as a nephew is closely related but is not progeny.
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References
Resources
- Photo Credit trilobite image by Alison Bowden from Fotolia.com
