Earth's History
Geologic Time Scale
Earth is 4.6 billion years old. We have rocks that date back to almost that time period all the way to today. When we find a rock or fossil, how do we know when it formed? Well, we can use something called relative and/or absolute dating, but we'll get to those later. Right now, we're focusing on how we have divided the history of the planet into chunks of time that allow us to talk and learn about the distant past. We divide the Earth's history first into eons, then into eras, eras into periods, and, finally, periods into epochs. There are no set lengths of time for these divisions like there are for others (year = 365 days; day = 24 hours; hour = 60 minutes; etc). We have created these divisions by looking at the dominant life forms and the changes in these life forms. So, in this way, the geologic time scale is a combination of biology and geology (rocks and fossils) into something called paleontology.
Geologic Time Scale Click on the link to see a version of the geologic time scale.
You will notice on the time scale that there are 4 major divisions: Cenozoic, Mesozoic, Paleozoic, and Precambrian. These are the eras. However, I must take this space to complain that the Precambrian is not a valid era as it is made of 3 eons...and eras are parts of eons, not the other way around. But, chances are likely that you'll see Precambrian on the test before you see Proterozoic Eon. The Paleozoic Era is when most complex life got started on the planet and started about 540 million years ago. The end of the Paleozoic and beginning of the Mesozoic Eras is marked by a huge mass extinction, the biggest ever on Earth. Almost 95% of life died off in this extinction. Luckily our distant ancestors survived. The Mesozoic Era is dominated by dinosaurs. Another extinction ended their reign of the planet 65 million years ago. Then, the final era got started: the Cenozoic. We live in the Cenozoic. In fact, we live in the Anthropocene Epoch (the test might say Holocene) of the Quaternary Period in the Cenozoic Era of the Phanerozoic Eon.
Earth is 4.6 billion years old. We have rocks that date back to almost that time period all the way to today. When we find a rock or fossil, how do we know when it formed? Well, we can use something called relative and/or absolute dating, but we'll get to those later. Right now, we're focusing on how we have divided the history of the planet into chunks of time that allow us to talk and learn about the distant past. We divide the Earth's history first into eons, then into eras, eras into periods, and, finally, periods into epochs. There are no set lengths of time for these divisions like there are for others (year = 365 days; day = 24 hours; hour = 60 minutes; etc). We have created these divisions by looking at the dominant life forms and the changes in these life forms. So, in this way, the geologic time scale is a combination of biology and geology (rocks and fossils) into something called paleontology.
Geologic Time Scale Click on the link to see a version of the geologic time scale.
You will notice on the time scale that there are 4 major divisions: Cenozoic, Mesozoic, Paleozoic, and Precambrian. These are the eras. However, I must take this space to complain that the Precambrian is not a valid era as it is made of 3 eons...and eras are parts of eons, not the other way around. But, chances are likely that you'll see Precambrian on the test before you see Proterozoic Eon. The Paleozoic Era is when most complex life got started on the planet and started about 540 million years ago. The end of the Paleozoic and beginning of the Mesozoic Eras is marked by a huge mass extinction, the biggest ever on Earth. Almost 95% of life died off in this extinction. Luckily our distant ancestors survived. The Mesozoic Era is dominated by dinosaurs. Another extinction ended their reign of the planet 65 million years ago. Then, the final era got started: the Cenozoic. We live in the Cenozoic. In fact, we live in the Anthropocene Epoch (the test might say Holocene) of the Quaternary Period in the Cenozoic Era of the Phanerozoic Eon.
Evolution
This is not biology, it's earth science. But, we are just as concerned about the history of life as biologists are. As mentioned above, the geologic history of the Earth is determined by the changes in life. Life changes through evolution. Evolution is merely change. It is a scientific fact (an observable piece of data). The theory of evolution is about the mechanism bringing about this change: natural selection. So, I will present evolution against the backdrop of geologic history. This is by no means an exhaustive study on evolution, but it should give you an idea that not only has life changed, so has the Earth.
4.6 billion years ago - The Earth forms.
Around 4.2 billion years ago - The Moon forms.
3.5 billion years ago - the first life appears
Between 3 billion years ago and 2 billion years ago - oxygen slowly accumulates in the atmosphere as a byproduct of photosynthetic single-celled organisms
560 million years ago - a possible beginning to complex life evolved in the form of the Ediacara Fauna but they go extinct
540 million years ago - the "Cambrian Explosion" occurs; a large number of complex, multicellular creatures appear on Earth...but only in the water
440 million years ago - life makes its move to land (insects were first, followed by vertebrates)
About 300 million years ago - reptiles evolve and begin to dominate the landscape; Pangaea begins to form
250 million years ago - the largest mass extinction in Earth's history: approximately 90%-95% of all life dies; there was probably no single cause, but the massive volcanic eruptions of the Siberian Traps most likely got things started
225 million years ago - Dinosaurs evolve and soon rule the planet; mammals also evolve during the Mesozoic Era, as did birds and flowers
65 million years ago - the dinosaurs and many other life forms go extinct in another mass extinction that killed off about 65%-70% of all life
About 30 million years ago - the first primates evolve
About 200,000 years ago - modern humans (Homo sapiens) evolve and rapidly expand across the globe
This is not biology, it's earth science. But, we are just as concerned about the history of life as biologists are. As mentioned above, the geologic history of the Earth is determined by the changes in life. Life changes through evolution. Evolution is merely change. It is a scientific fact (an observable piece of data). The theory of evolution is about the mechanism bringing about this change: natural selection. So, I will present evolution against the backdrop of geologic history. This is by no means an exhaustive study on evolution, but it should give you an idea that not only has life changed, so has the Earth.
4.6 billion years ago - The Earth forms.
Around 4.2 billion years ago - The Moon forms.
3.5 billion years ago - the first life appears
Between 3 billion years ago and 2 billion years ago - oxygen slowly accumulates in the atmosphere as a byproduct of photosynthetic single-celled organisms
560 million years ago - a possible beginning to complex life evolved in the form of the Ediacara Fauna but they go extinct
540 million years ago - the "Cambrian Explosion" occurs; a large number of complex, multicellular creatures appear on Earth...but only in the water
440 million years ago - life makes its move to land (insects were first, followed by vertebrates)
About 300 million years ago - reptiles evolve and begin to dominate the landscape; Pangaea begins to form
250 million years ago - the largest mass extinction in Earth's history: approximately 90%-95% of all life dies; there was probably no single cause, but the massive volcanic eruptions of the Siberian Traps most likely got things started
225 million years ago - Dinosaurs evolve and soon rule the planet; mammals also evolve during the Mesozoic Era, as did birds and flowers
65 million years ago - the dinosaurs and many other life forms go extinct in another mass extinction that killed off about 65%-70% of all life
About 30 million years ago - the first primates evolve
About 200,000 years ago - modern humans (Homo sapiens) evolve and rapidly expand across the globe
Relative Dating
In order to interpret the past from the rocks, we need to be able to read them. We use two methods to tell the story of the past: relative and absolute (or radiometric) dating. Relative dating uses the rocks and their positions relative to each other in a sequence to reveal the hidden story. We use a set of geologic laws to make sense of a rock sequence: Law of Original Horizontality; Principle of Lateral Continuity; Law of Superposition; and Principle of Cross-Cutting Relationships.
In order to interpret the past from the rocks, we need to be able to read them. We use two methods to tell the story of the past: relative and absolute (or radiometric) dating. Relative dating uses the rocks and their positions relative to each other in a sequence to reveal the hidden story. We use a set of geologic laws to make sense of a rock sequence: Law of Original Horizontality; Principle of Lateral Continuity; Law of Superposition; and Principle of Cross-Cutting Relationships.
Law of Original Horizontality - This law states that all sedimentary layers are originally deposited in a horizontal (flat) position and that if they are no longer horizontal, then some tectonic force altered their position.
Principle of Lateral Continuity - This law states that sedimentary layers are assumed to continue in all directions until encountering a natural barrier such as a change in environment. Simply put, if you see a sandstone layer on one side of an igneous intrusion and a similar sandstone layer on the other side, then the assumption is that they are the same sandstone layer.
Principle of Lateral Continuity - This law states that sedimentary layers are assumed to continue in all directions until encountering a natural barrier such as a change in environment. Simply put, if you see a sandstone layer on one side of an igneous intrusion and a similar sandstone layer on the other side, then the assumption is that they are the same sandstone layer.
Law of Superposition - This very simple law states that, in a sequence of rocks, the rocks at the bottom are older than the rocks at the top. My analogy is a skyscraper. When a skyscraper is being built, they don't start with the 31st floor and then fill in underneath; they start at the bottom and work up. So does nature.
Principle of Cross-Cutting Relationships - This law is easier shown than stated, but I'll try. When a rock cuts across another rock layer, the rock that is cutting is younger than the rock layer getting cut. Think of a doughnut. The doughnut is not made around the hole, the hole is cut out after the doughnut already exists.