# Science: An Elementary Teacher’s Guide/A History of Planet Earth

The estimated age of the universe is approximately 13.7 billion years old. According to radiometric dating and other methods, our planet Earth is estimated to be 4.56 billion years old. Uniformitarianism is the assumption that the same natural laws and processes that operate in the universe today have always operated in the universe in the past and apply everywhere. Hence, what we learn about physics, chemistry, earth sciences, and biology today gives us insights into the processes that formed and shaped this planet. The history of the planet is divided into 3 eons, and each eon is subdivided into eras, which are further subdivided into periods. The timeline has not been divided up equally, as you will see.

## Precambrian Time (4.6 bya - 541 mya)

The Precambrian is the earliest eon of Earth's history and accounts for 88% of its total geologic time (technically it is a "super eon" and there are 3 eons within the Precambrian, but for our purposes we are just considering it as one giant block of time). It spans from the formation of the earth, about 4.6 billion years ago, up to the Cambrian Period 541 million years ago (which is marked by the appearance of abundant hard-shelled creatures in the fossil record). Even though this is the largest period of the Earth's history, little is known about it, due largely to the fact that most rocks from this time period have undergone metamorphosis and are not in their original state, or are very deeply buried, or have been eroded away. Oceans and continents have formed and reformed during this long period.
The very early earth was very hot, with no liquid water. As it cooled the crust formed, water condensed and oceans formed. Life began in the oceans by 3.5 billion years ago (perhaps as early as 4.3 bya). The earliest bacterial life did not rely on sunlight, but instead harvested energy from volcanic vents in the ocean. When photosynthesis eventually evolved, some bacteria became able to use sunlight as an energy source. This greatly increased the amount of energy available to life, but also resulted in a "waste" product of oxygen (in photosynthesis energy from light is used to split water molecules, generating Oxygen as a waste product). While this formed an atmosphere that allowed for more complex life forms to evolve, the Oxygen Revolution also caused the first mass extinction because most early bacteria could not live in the presence of concentrated oxygen (even today there are bacteria that cannot tolerate Oxygen--they can live deep in the soil, under oceans, or in animal intestines). During the last half-billion years of Precambrian time multi-cellular sea life appeared, including jellyfish and worms. The fossil record is sparse for the Precambrian because Precambrian rocks are rare and because bacteria and soft-bodied invertebrates do not fossilize well.
* The Earth is 4.6 billion years old, having coalesced around the sun (chunks of matter smashed together, probably over a period of 10-20 million years). Within the first 100 million years it was hit by a Mars-sized asteroid/planetoid, which is believed to have broken off a chunk of early earth and formed the moon.
* Earth's crust cooled and formed a hard outer shell of rock, but this crust is very thin--comparable to the skin of an apple. The interior of the earth remains incredibly hot and molten! When the crust was sufficiently cool the water vapor started to collect as liquid, eventually forming seas.
* Earth has been spinning since its beginning and continues to spin simply because of inertia (think of it as a ridiculously giant top that is spinning without friction).
* Even though we think of a day as 24 hours, because that is how long it takes for the earth to make one rotation on its axis, days used to be shorter because the planet was spinning faster. It continues to slow because of the moon's gravity creating a bit of drag, but it is only slowing by a small fraction of a second per year.
* Only during last 1/2 billion years of the Precambrian eon did worms, jellyfish, and other multicellular life develop. Hard-shelled life marks the end of the Precambrian eon and beginning of the Cambrian Period of the Paleozoic Era (everything after Precambrian Eon is in the Phanerozoic eon, but we will to talking about the eras and periods).

## The Layers of the Earth

The crust, the mantle, and the core. All three of these words refer to the Earth's layers. These layers could be further subdivided (for example, there is an inner core and an outer core, a stiffer mantle and a rigid mantle). To start we begin with the crust which is the shell of the Earth. Below the crust is the "meat" of the Earth which we call the mantle, and last we have the the center of it all, the core. Fun fact about the crust, Geologists think that the temperature within the deepest part of the crust might be as high as 1,600 degrees Fahrenheit or 870 degrees Celsius, which is hot enough to melt iron.

## Paleozoic Era (541 mya - 252 mya)

The Paleozoic Era, meaning "ancient life" is the first of three geologic eras stretching from the Precambrian to the present. The other two eras are the Mesozoic Era (252 mya - 66 mya) and the Cenozoic (66 mya - present). Each era is subdivided into periods: the Paleozoic has 6 periods, while the later two eras each have 3 periods.
The Paleozoic is bracketed by two of the most important events in the history of animal life. At its beginning, multicellular animals underwent a dramatic increase in diversity (the "Cambrian Explosion"), and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90% of all marine animal species (the "Permian-Triassic Extinction"). The causes of both these events are still not fully understood and the subject of much research and controversy. Roughly halfway in between, animals, fungi, and plants colonized the land, the insects took to the air, and limestone formed from skeletal fragments of marine organisms such as coral, forams and molluscs.
By the end of the Ordovician, life was no longer confined to the seas. Plants had begun to colonize the land, closely followed in the Silurian by invertebrates, and in the Upper Devonian by vertebrates. The early tetrapods of this time were amphibian-like animals that eventually gave rise to the reptiles and synapsids by the end of the Paleozoic.
The Paleozoic Era had plenty of fossils and it was divided into 6 periods.
* Cambrian Period- mollusks, Trilobites, Jawless fishes
* Ordovician Period- the first vertebrates
* Silesian Period- first land plants
* Donovan period- first amphibians and first insects (animals conquering land)
* Carboniferous Period- the origin of reptiles, the first seed plants. Tropical swamps that eventually formed into coal.
* Permian Period- supercontinent Pangaea grew drier and hotter. Massive volcanic activity, huge releases of hydrogen sulfide gas, and abrupt global warming associated with "The Great Dying" mass extinction event.

## Mesozoic Era

* This era lasted about 179 million years, extending from about 252 million years ago to about 66 million years ago. This era includes 3 well known periods called the Triassic, Jurassic, and Cretaceous periods.
* During this era the first turtles, crocodiles, mammals, birds, and flowering plants appeared.
* The Mesozoic era is known as the age of the reptiles. They began during the Triassic period, flourished and reached their largest size during the Jurassic period, and mostly died out at the end of the third and final period, the Cretaceous.
* The environment of the Triassic period was generally warm. Polar ice caps did not yet exist and the difference in temperature between the poles and the equator was much less graded than it is today. The continents were still connected as one, which caused desert environment to prevail near the center regions of Pangaea. This made it difficult for life to exist in the center of the continent, which is why most dinosaurs and other life forms lived near the seaboards.
* Fauna from Triassic Jurassic Period Wellcome M0008776During the Jurassic Period, the supercontinent Pangaea split apart. The northern half, known as Laurentia, was splitting into landmasses that would eventually form North America and Eurasia, opening basins for the central Atlantic and the Gulf of Mexico. The Jurassic period is known for an increase in the numbers and diversity of dinosaurs. At the beginning of the period, dinosaurs such as the bipedal and carnivorous theropods were small and lightly boned, feeding on insects or other small dinosaurs. Mammals continued to diversify during the Jurassic, but remained small and nocturnal, possibly to avoid competition with the dinosaurs. These early mammals were almost all herbivores, insectivores, and frugivores (fruit eaters).
* The Cretaceous is usually noted for being the last portion of the "Age of Dinosaurs", but that does not mean that new kinds of dinosaurs did not appear then. It is during the Cretaceous that the first ceratopsian and pachycepalosaurid dinosaurs appeared. Also during this time, we find the first fossils of many insect groups, modern mammal and bird groups, and the first flowering plants.
* The Cretaceous-Paleogene extinction event has been attributed to an asteroid impact near the Yucatan peninsula of Mexico approximately 66 mya.Impact event "The Great Extinction" marked the end of the dinosaurs as the Cenozoic era began.

## Cenozoic Era

* This era is the era in which we are living and extends from about 66 million years ago to the present time.
* With dinosaurs gone, mammals flourished and increased greatly in diversity. Dogs, whales, horses, cats, bears, raccoons, and primates (monkeys, apes, humans) all appear during this era. Flowering plants also greatly diversified during this era.
* The term Cenozoic is derived from the Greek phrase meaning “recent life.”
* The Cenozoic Era is divided into two major periods:
* Birds, insects, mammals, flowering plants now resemble those of today
* Early ancestors of human appeared towards end of Tertiary period
* The last 1.6 million years of Earth's history.
* Known as the age of Homo sapiens (humans).
Cave lions, saber-toothed cats, cave bears, giant deer, woolly rhinoceroses, and woolly mammoths were prevailing species of the Quaternary Period. Without the dinosaurs, plant life had an opportunity to flourish during the Cenozoic Era. Nearly every plant living today had its roots in the Cenozoic Era.

## Quiz

Try this quick quiz and test what you have learned by reading this chapter!
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