Fossil

Fossils are the preserved remains of a once-living organism. They form in many different ways and they can provide us with information on the climate, geology, and geography of ancient Earth. Fossils also provide strong evidence for evolution (the process by which living things change over generations).

Fossils are usually thought of as the remains of once-living things that have been mineralized or turned into rock. While this describes a fossil, the notion of fossilization includes several other methods of preserving the evidence of ancient life. The use of fossils to study ancient life and its development, or evolution, is called paleontology. The word fossil comes from the Latin word fossilis, meaning "something dug up." This is actually how most fossils are discovered. Most are found below the surface of Earth in a preserved form, since they had been covered up at one time or another. A fossil can be the partial or complete bodily remains of a plant or animal, or it can be the more common "trace fossil" which is more like some evidence of the organism's life or activities. Trace fossils are things like trails, coprolites (fossilized animal dung), or footprints.

THE FORMATION OF FOSSILS

Fossils can be formed in many ways, but they all have one process in common. They all replace the relatively fragile organic structure of a living thing with something that is harder and which lasts longer. This can only happen when certain conditions apply. Under normal conditions, if an animal falls dead on the ground, it will eventually begin to rot, or decompose and will be reduced to its basic organic compounds, which are then recycled. In the end, it completely and totally disappears. However, if the animal falls dead and slides into a tar pool, asphalt lake, or peat bog, it may be completely preserved because of the lack of oxygen or the absence of bacteria, which are both needed in order for decomposition to occur. In an extremely hot and dry climate, an animal may undergo mummification, or be rapidly dehydrated, under the right conditions; therefore, it will be preserved. In other extreme climates in the north, completely preserved specimens of woolly mammoths and even humans have been recovered after being frozen under glacial conditions for thousands of years. A final way that nature can capture and preserve a complete specimen is in amber (the sticky resin of a pine tree). Insects were usually fossilized this way when they became trapped inside the amber. When the resin itself became fossilized, the insect was forever frozen in a drop of what looks like yellow glass.

Although these are examples are more like preservation than true fossilization, they are all considered to be fossils. True fossils are the remains of once-living organisms that are preserved in stone. When geology (the study of the Earth) became more of a real science in the late eighteenth and early nineteenth centuries, geologists began exploring below Earth's surface. These scientists went deeper into successively older and older layers of what is called sedimentary rock (formed by the gradual settling of sediments). It was in these buried layers that they found entire communities of fossils in each layer. Often, the deeper layers of rock contained animals that were very different from any known animals, since the deeper the geologists dug, the older the fossil. These unknown animal bodies that were preserved were those that had been covered by sediments very quickly before they could start to decay. However, they needed to be more than just covered to become fossils. They also needed to be deprived of oxygen. This often happened when a dead animal sunk to the bottom of a body of water and was deeply covered by mud. As more sediments piled up and pressure increased, the organic material that made up the animal was slowly replaced by minerals and the body eventually became a fossil made of stone. These type of fossils might be described as a replica of the real living thing.

In the rare process known as petrifaction, both the external shape and the internal structure of a plant or animal are perfectly reproduced. The Petrified Forest of Arizona contains trees that were buried by volcanic eruptions and underwent petrifaction. In this rare process, a molecule-by-molecule replacement occurred, with the end result being the replacement of natural wood fibers by silica. This replacement is usually so accurate that even the cell structure of the tree can be determined.

Paleontology, or the study of fossils, informs scientists about the organisms that lived on Earth long before human life evolved. It also tells us a great deal about Earth's climate in those ancient times. The oldest known fossils are 3,500,000-year-old bacteria. They represent not only the oldest known form of life on Earth but help scientists to learn more about the origins of life itself. Fossils also tell scientists about extinction that has taken place. Finally, most of what is known about evolution is based on the fossil record. It is fossils that provide the most reliable evidence for evolution since for some organisms, paleontologists are able to compare their fossils from different layers of rock (which are from different ages) and actually trace how they evolved physically.

[See alsoGeologic Record ]