The history of the Earth is categorized into four different geological time eras beginning with, the Precambrian Time (4. 6 billion-544 million years ago), the Paleozoic Era (544-248 million years ago), the Mesozoic Era (248-65 million years ago), and the Cenozoic Era (65 million years ago to the present). Eras are broken down further into smaller units called periods, which help scientists detect changes in Earth’s history. Geologists’s main sources of evidence for geological eras, and periods are the ages of rocks.
There are several methods geologists’t use to determine the age of rocks such as, relative age, the law of superposition, index fossils, relative dating, and radioactive dating. Through scientific reasoning and evidence this research paper discusses two milestone events from each geological era with the date scientists believe it occurred, the evidence used to determine the milestone events, and an explanation for how the scientists reached their conclusion. Prior to the late 1800s one of the most sought after answers, and common mystery question was, “When was Earth formed? Many theories were developed based on religion, culture, and reasoning.
However, in 1913 the first geologic time scale was presented by Arthur Holmes; this was deciphered after the discovery of radioactivity by Henri Becquerel in 1896. Holmes used radioactivity to deduce that the Precambrian Era was between 4. 6 billion-544 million years ago; the first milestone is the formation of Earth, and the second milestone is the beginning of life. As science and technology progresses scientists have a more exact date, which is, that the Earth is 4. 543 billion years old.
Scientists formulated the Precambrian era by practicing radioactive dating: knowing the number of radioactive parent atoms present when a rock formed and the number present now. The number of parent atoms initially present is the number present now plus the number of daughter atoms formed by the decay, both of which are amounts that can be measured. Moreover, another method that was used to determine when Earth formed was by reporting the proportion of lead types located in meteorites and comparing them to lead proportions located in other rocks on the Earth and other meteorites.
Clearly, using radioactive dating and the comparison of rocks was essential factors in answering the questions on Earth’s beginnings. Scheimreif 3 Another, major milestone event in the Precambrian era, is the emergence of life; the first one-celled organisms were formed in the ocean. These one-celled organisms helped to make the air and water around the Earth full of oxygen, forming new life. Next, photosynthetic organisms became part of Earth by using carbon dioxide from the atmosphere, and releasing oxygen.
The last life form to develop in this era were simple soft bodied animals, since soft bodies do not have vertebrates there are not many fossils as evidence from this era. Scientists know that onecelled organisms, photosynthetic organisms, and soft bodied animals were the first species on earth by the following information, “The amount of C-14 in any sample of carbon containing material can be found by measuring the level of radioactive decay, and comparing that with the decay rate observed in a carbon sample exposed to the continual mixing at the surface of the earth of C-12 and C-14 produced in the upper atmosphere.
Using the ratio of C-14 to total carbon, one can determine the age of the sample. ” As a culmination, carbon dating, examining index fossils, and using relative dating reveal to scientists the milestone of the first organisms in the Precambrian era. The second era of Earth’s history is the Paleozoic era (544-248mya) with six dynamic periods including, Cambrian period, Ordovician period, Silurian period, Devonian period, Carboniferous period, and the Permian period in which plants, nd more complex animals manifested due to the many changes occurring on Earth. First, was the Cambrian Period (544-505mya), where most major animal groups appeared which is confirmed by findings of fossils using relative dating and index fossils. This was a significant milestone in this period leading up to modern day species.
In this period, animals such as, metazoans, sponges, corals, molluscs, echinoderms, bryozoans, brachiopods, arthropods, trilobites, archaeocythas, and small shelly animals encompassed the Earth starting to make a very diverse animal kingdom. Moreover, this period is known for the first record of vertebrates appearing as fossils of jawless fish. Next, was the Ordovician Period (505-440mya) with massive marine life diversification that was specialized. Again, index fossils and relative dating are the proof of the creatures that inhabited the
