es have been replaced by minerals. A different process of fossilization occurs with soft tissue when mineral-rich water fills in the spaces normally occupied by liquids or gases. This mineralization process can occur even at the cellular level, leaving behind incredibly detailed fossils. Both the mineralization and imprint processes can take thousands and even millions of years to occur. p> Another form of evidence utilized by paleontologists is preserved organic tissue, usually from small invertebrates such as insects, trapped in fossilized plant resin, or amber, though the organic remains of some more recently extinct species, such as mammoths, have been found in glaciers and bogs. Finally, some paleontologists work with existing life forms. Popularly referred to as "living fossils, "such species - among the best known is the ancient fish species, coelacanth - have existed for up to hundreds of millions of years and are believed to resemble long extinct life forms. (For simplicity sake, all but the latter form of paleontological evidence will be referred to as fossils in this discussion.)
The first step in analyzing fossils is to find them unless, of course, the paleontologist chooses to examine fossils that have already been collected. Fossils of all types are relatively rare. That is because the conditions for fossilization depend on many factors coming together. For mineralization, there has to be just the right combination of minerals and groundwater, while, for the process that leaves imprinted fossils, just the right geological processes have to be at work soon after the organism dies. Thus, paleontologists look for telltale geological formations to guide them to fossil remains. Examination of such formations, known as topology, can also allow paleontologists to date the fossils. This method-now outdated - is known as "relative dating" because it was best for determining the order in which fossils were created and not their precise ages. p> Once fossils have been found they can be analyzed using a variety of methods. The most obvious and earliest of these methods is simple visual observation of the remains. Such observation can help the paleontologist classify the life form. For more complex life forms, such as vertebrates, paleontologist use visual observation to assemble the various parts to recreate the whole organism. p> Analytical tools developed over the past 60 years have moved paleontologists far beyond simple visual observation and comparison of fossils. Perhaps the most important has been radiometric dating, that is, the analysis of the radioactive decay that naturally occurs, to one degree or another, in all elements or, more specifically, within the radioactive isotopes present in elements. Because radioactive isotopes break down at a specific rate-their so-called half-lives-scientists can note the amount of a radioactive isotope in a given element and know when it was created. Since carbon forms the basis of all life, scientists in the mid-twentieth century first focused on the decay of the isotope carbon-14. But carbon-14 proved a useful indicator for relatively short periods of time only-roughly good for about 40,000 years-making it helpful in the study of human remains but largely useless for paleontologists who work in time frames of hundred of thousands to hundreds of millions of years. Scientists soon discovered that potassium-40, a radioactive isotope of potassium, a metal found in all life on Earth-breaks down into the inert gas argon over a period of roughly 1.3 billion years, making it an ideal radiometric marker for paleontologists. p> With the discovery of the structure of DNA in the 1950s, paleontologists were offered a new avenue for the analysis of fossils, at the molecular level, though it took several decades for the tools to be developed to make sense of fossilized DNA, usually found in life forms persevered in amber. Changes in the structure of the DNA molecules found in fossils allow paleontologists to examine very specific evolutionary changes within extinct species as well as the physical and even behavioral traits of those species in a way simple visual and even chemical analysis is incapable of. DNA analysis also provides key insights to evolutionary biologists, that is, scientists who examine the biology of adaptation and extinction.
Paleontologists divide Earth history into eons, eras, periods, and epochs. Eons cover billions of years, eras cover hundreds of millions of years, periods are usually in the tens of millions of years, and epochs, the shortest of these periods, is measured in millions or hundreds of thousands of years. The time spans in these eons, eras, periods, and epochs vary greatly, as they do not signify specific time periods, such as years or millennia. Instead, they are marked by great changes in the fossil records. br/>
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Earth is believed to be about 4.5 billion yea...
