LESSON - Geologic Time
Students focus on the long eras of time and how biological life evolved through the eras.
Fossils Provide Evidence of Earth's Geological History
Students, The main point of this chapter is that the many layers of earth directly below our feet have remains of ancient organisms that tell us much regarding what lived in the past and what the earth was like millions of years ago. Students, scientists use the fossil record to reconstruct events in Earth’s history. For example, when the same fossils were found on the western coast of Africa and the eastern coast of South America scientists theorized that the two land masses where once joined. Or scientists have discovered whale bones in the Andes mountains in South America which indicates that those mountains were once under the ocean. Or when scientists discovered that the fossil records of dinosaurs sudddenly stopped at a certain point in time indicating that a mass extinction of the dinosaurs had occured.. Fossils are useful for indicating the past events in the Earth’s geology. In this lesson on the Geologic Time Scale we will learn the major events in nature provided the change from one major era to another, for exampe, the era of dinosaurs changed to the era of mammals by a drastic change in the earth. The Earth is 4.6 Billion Years Old Four and a half billion years is a tremendously long period of time. Because of fossils scientists are able to put together a general idea of the various stages and changes that have occurred to the continents and the species of animals and plants for those 4.6 billion years. The Geologic Time Scale (GTS) is the time line of the entire history of the Earth. Because the earth is 4.6 billion years old the Geologic Time Scale (GTS) is measured in millions of years. It is broken down into divisions of time from eons, the very longest, to the epochs the shortest. The segments or divisions of the GTS are determined by drastic changes on the Earth, or by the emergence or extinction of biological species. The divisions of time are not just based on a certain amount of time passed. It is not based on equal intervals of time. The divisions are based on changes that occur in the natural world. The Idea of Evolution Before we jump into the GTS first it is important to understand what scientists think about with regards to the evolution of animal and plant species. Species are defined as any animal or plant that can reproduce offspring from within its own species. Animals of different species cannot produce offspring. Scientists say that the history of all biological life is a sequence of species of plants and animals changing into other new species over long periods of time.
In other words, animals just don’t appear out of the nowhere. Organic Evolution and Natural Selection Species change and evolve. Evolve means to change naturally over time. This is called organic evolution. Species of plants and animals evolve into new species. There is a law of nature the makes evolution happen. It is the Law of Natural Selection. It is a simple law. It states that within every species of animals and plants there is a variety of strengths and weaknesses among its members. It is the stronger characteristics that continue the species over time and the weaker characterists that do not pass the test of time. Surving Characteristics in Hardships Strengths and weaknesses represents a variety of characteristics among a species members. When a drastic hardship occurs in the environment some of the members of the species survives because their characteristics helped them survive, but other members in their species don’t survive. Those that survive the hardships will continue to reproduce offspring and continue their special characteristics while those that did not survive the hardship do not pass on their characteristics. This process is called Natural Selection. As this happens over and over again over thousands and millions of years the species changes drastically into different species. ![]() For example, giraffes who had longer necks than other giraffes may have survived drought better and, therefore, they survived to pass on their longer neck characteristics to their offspring. Overall, the entire giraffe population becomes taller. Horses used to be very small animals. Now they are large and strong animals. Over many thousands of years it seems the stronger, larger members the horse species survived better and their offspring reflect this.
Check For Understanding
1. Since scientists do not use a specific number of years to demarcate ages in the Earth's history, then what do scientists use from nature to designate the divisions in the Geologic Time Scale? 2. What does the idea of Evolution state? 3. What is the law of natural selection as it applies to evolution of species? How is the giraffe an example? Ok so now you know how Natural Selection effects the evolution of species. Let’s look at the Geological Time Scale Look carefully at this picture of the Geologicl Time Scale, (GTS). ![]() First look at the years. They are indicated in units of millions. The 4.6 billion years is 4,600 million years. Note that the Precambrian Time at the bottom of this chart spans 4 BILLION years. The Precambrian is the longest span of time covering almost 90% of the Earth's history. Eons, Eras, Periods and Epochs The GTS is broken down into four types of divisions of time. Eons are the longest division of time. Eons are subdivided into Eras. Eras are subdivided further into Periods. Periods are subdivied into Epochs. So the scale of time divisions from longest time span to shortest time apan are: Eons, Eras, Periods and Epochs, where Eons are the longest time span and Epochs are the shortest. Now look at the Precambrian time at the bottom of the chart. It is called the Precambrian Age and it is the very longest division of time on the GTS. It is actually three Eons and it spans 4 billion years! It is clearly the longest time segment of the Earth's history. If it is 4 billions years long then all the remaining eras and periods total 0.5 billion (500 million). Most of the dynamic biological evolution on the planet has occurred within the last 500 million years. This is because very little life forms existed the first 2 billion years of the earth's existence. It took the process of evolution a long time to bring about complex living animals. Mammals were the last advanced type of animal to appear.
Check For Understanding
1. Using the GTS chart above: In terms of time, what is unique to the Precambrian Eon compared to the remaining Geologic Timeline divisions? 2. What era was characterized by the dinosaurs? 3. What main type of animals dominate earth after the dinosaurs vanished? 4. List the divisions in the Geologic Time Scale from the longest division to the shortest. 4 billion Years in a Nutshell The first one-billion years of the earth had no life on it and therefore no fossil record. The second one-billion years only had first single cell bacteria and algae. This is virtually inactive compared to the last 500 million years. The algae released oxygen into the atmosphere which future life would need. It also provided the ozone layer which protects the earth from the dangerous sun rays. ![]() The first 4 billion years of the planet (Precambrian) had no or little life. The atmosphere and land masses were developing. The 3rd one-billion years had multi-celled organisms appear named Edianans. The single cell organism became more advanced and then small invertebrate animals with no bones or shells evolved from those complex single cells. Volcanoes where still spewing out lava creating land. Continents and land masses were developing. The atmosphere was developing all those 3 billion years in preparation for the sudden burst of diverse animal and plant life that was to come during the 4th one-billion years.. The 4th One-Billion Years - The Paleozoic Era - Diverse Plants/Animals A new Era was born at the start fo the 4th one-billion years of the Earth's history. That was the Paleozoic Era and it was characterized by many plant and animal speicies. The plant and animal explosion occurred. Paleozoic Era started when plants and animals had grown in diversity, complexity and in size. Animals evolved shells and bones. First invertebrates evolved and later they become vertebrate (with bones and shells). Trilobites appeared all over the earth, a small shelled animal that was an index fossil (hence is a fossil that indicates a stage of evolution in the Paleozoic Era). ![]() A late Precambrian fossil, very rare. Then amphibians evolved who lived in water and land. There were sharks and large sea creatures. Reptiles evolved and where able to live entirely on land. Reptiles have scaly skin for living out of water and their eggs had shells for protection. Animal and plant life become more complex and evolved. ![]() Paleozoic animals of the sea. As the animal and plant life evolved, the continents, mountains and climate where changing with it. Pangaea (Pan gee uh) Scientists know that the continents drift because of plate tectonics, which means large sections of the earth's crust move and the continents that sit on the plates move with them. There was a time 230 million years ago where all the continents converged into one super landmass we call Pangaea. This land mass caused the final stage of the rising of the Appalachian Mountains. Now 260 million years later the Appalachian Mountains have weathered into rolling hills. ![]() Continents merged into one land. It caused the destruction of many species This event changed the oceans and the land to such a degree that many species of plants and animals did not survive and therefore an new era started, the Mesozoic Era. Pangaea made the oceans more shallow and they had glaciers on them. When Pangaea started to break up it broke into two main lands: Laursia and Gondawanaland. Laursia traveled north and became the northern hemisphere continents and Gondawanaland traveled south and become the continents of the southern hemisphere. ![]() Pangaea broke up into north and southern continents. Mesozoic Era and Dinosaurs After Pangaea in the Mesozoic Era reptiles dominated the sea and land. In the Triassic Period reptiles got bigger. As we know reptiles evolved into large dinosaurs which lived in the Jurassic and Cretaceous Periods. Also some dinosaurs grew wings and become flying dinosaurs and later evolved into the birds we have today. Small mammals appeared but they did not dominate. ![]() Many types of dinosaurs lived during the 83 million years of the Mesozoic Era
Check For Understanding
1. In what era did more complex animals appear on the Earth? 2. What is Pangaea? 3. What is Laurasia and Gondawanaland 4. How long ago did our modern human form appear on Earth? Closure From this Geologic Time Scale Lesson you have learned that the Earth is around 4.6 Billion years old, which is an enormous spanse of time. Therefore, Geologists divide Geologic Time into Eons, Eras, Periods and Epochs, all according to the many drastic changes that has occured in the evolution of life on Earth. These drastic changes were demarcations of a new age in the grand Time Scale. You've also seen how the first 4 billion years of the Earth's history there was no life on Earth, but just the slow development of single cell organisms. The remaining 600 million years contain the more rapid explosion of fish, birds, reptiles, dinosaurs, mammals, large mammals and eventually the human species. That is the history of the Earth in a nut shell. Next week we will learn an extended lesson on water erosion, was we watch a documentary on the great Mississippi Flood of 1993. |