I.                    Formation of the Earth

A.    Along with the other bodies of our solar system, the Earth was formed approximately 4.5 billion years ago due to the collision of orbiting matter.

B.     The oldest rocks found on the Earth's surface are measured to be only 4.0 billion years old. However, by studying the age of asteroids, scientists have determined that Earth must share the same age as the rest of the solar system, which is 4.5 billion years old. Rocks that would be as old as the asteroids no longer exist on the surface due to harsh physical and/or chemical weathering.

II.                 The Precambrian Era: 4.5 billion years - 545 million years ago

A.    Radioactivity within the Earth's core gives off continual heat; hence the core remains molten. Also, radioactivity drives the building of the continents. During the early Precambrian, protocontinents were isolated islands of cooling rock, which would collide and form larger bodies. The oldest rocks on earth today exist mainly where the crust is the thickest, which is usually toward the center of the continents.

B.     Oldest known remains of life date to 3 billion years ago. Among these are the Stromatdites, a blue-green algae. Their history is recorded in rock where mucus produced would have trapped sedimentary particles. Eventually, layers upon layers, compaction occurred which in turn, caused the sediment to lithify.

1.      Stromatdites were photosynthesizing organisms and were therefore able to release oxygen into the young atmosphere.

2.      This allowed for the oxidation of iron, which then created Banded Iron Formations. Not to mention, the liberation of oxygen into the atmosphere provided the formation of ozone. This allowed for a layer of ozone to form, thus shielding the Earth from ultraviolet rays radiating from the sun as well as trapping in heat in the atmosphere. Because of the trapped heat, the dark side of the Earth would not freeze; and because of the shield, daylight would not be radioactively toxic to life on the surface.

C.    Early Life

1.      There is some evidence that the first sign of life on Earth occurred in the ocean depths around deep sea vents. Here, gases and nutrients along with heat are released from under the thin crust.

2.      Prokaryotes were perhaps the first life to form. Prokaryotes are bacteria lacking a nucleus containing their DNA. Instead, the DNA is free-floating in the cytoplasm along with few specialized organelles.

3.      Eukaryotes, in contrast, are bacteria that had developed a nucleus to contain DNA and had developed more organelles. Also, eukaryotes commonly displayed symbiosis, unlike prokaryotes that survive individually.

4.      Near the close of the Precambrian Era, there are signs of higher marine life lacking a hard skeleton or shell.

D.    Scientists have formed the Snowball Earth Hypothesis, which suggests that during the Precambrian, the Earth was covered with vast amounts of ice due to the breaking up of the protocontinents and the building up of ice. The Earth could have remained as a "snowball" for millions of years and perhaps longer. However, active volcanism released plenty of greenhouse gases into the atmosphere, which would trap more heat from the sun. This caused massive glacier melting, and the Earth completed a cycle oscillating between cold and warm global temperatures that would continue to repeat.

III.               The Paleozoic Era: 545 - 250 million years ago

A.    During this time dubbed "Ancient Animal Life Era," the Earth experienced an explosion of diversity of life. In actuality, the so-called "Cambrian Explosion" had a long evolutionary fuse; however, when considering what survives in fossil records, the life population of the Earth seemed to have suddenly burst into existence.

B.     During this time period, we start to see the first mineralized crystals in organisms. For example, certain species began to develop hard, protective shells or supportive skeletons out of calcium deposits. Because of this seemingly sudden development, more and more fossils have survived, thus creating a more detailed picture of the Paleozoic biosphere. Before, a soft, meaty body of an ancestor would fail to leave its imprint.

C.    In the seas, where life began and thrived, there were jungles among the coral reefs filled with sponges, mollusks (snails), cephalopods (octopi), and arthropods (crustaceans). Vertebrates, animals with backbones, also appeared in the oceans. By the mid-Paleozoic, there was a wide variety of fish. One variety included a fish with a primitive lung, which permitted the animal to obtain oxygen in low oxygen environments.

D.    As animal life branched in the oceans, plant life had begun to creep up onto land where sunlight was plentiful for photosynthesis. Soon after plant life anchored onto land, animal species started to follow.

1.      First to crawl upon the land were arthropods, ancestors to insects and arachnids. Without predators, such species flourished.

2.      Not too long after the arthropods came the tetrapods, four-legged creatures. These animals could live out of the water and on the land instead; however, like their ancestors, they needed to return to the water to reproduce. These first tetrapods were the amphibians.

3.      A major evolutionary break-through occurred among the tetrapods, creating the branch of the amniotes. The advantage of the amniotes was the ability to lay amniotic eggs, or eggs with a protective, leathery shell. Due to this ability, eggs could be laid in the sand on the land instead. Hence, the rising reptiles no longer needed to return to the water to reproduce.

E.     At the very end of the Paleozoic 250 million years ago the largest mass extinction in Earth's history occurred. Volcanic activity released toxic gases, thus choking huge amounts of species into oblivion. Much of the marine species became extinct, including trilobites and coral.

F.     The mass extinction left voids in the ecosystem as the once dominant species disappeared. In their place, reptilians began to greatly evolve into those recognizable today, such as the turtle, lizard, snake, crocodile and eventually the bird.

IV.              The Mesozoic Era: 250 - 65 million years ago

A.    At this time, the amniotes have started to fare better than amphibians.The amniotes developed skeletal structures that provided the legs to be positioned under the body. This positioning of the legs gave the animal more support and speed, thus allowing a species to grow to a larger size, from as small as a chicken to as large as a dinosaur.

B.     The dinosaurs dominated the Mesozoic Era. Some species had developed protective fibers that resemble primitive feathers. While looking at modern day birds, they share the feather and the wishbone structures of the dinosaurs. Also noteworthy is that crocodiles and birds are both archosaurs, and therefore they are the remaining cousins of the dinosaurs.

C.    Mammals make their debut during the Mesozoic. Seven tenths of mammalian existence on Earth had been spent in the shadow of the dinosaurs. In contrast, the largest Mesozoic mammal grew to be no larger than the size of a modern day badger.

D.    At the close of the Mesozoic Era, a massive asteroid theoretically struck the Earth, causing another mass extinction. The Age of the Dinosaurs came to an end; however, the little mammals stepped forward to claim the planet.

V.                 The Cenozoic Era: 65 million years ago - Today

A.    The dominance of the dinosaurs fell into extinction, and once again, a void had to be filled. Mammals, warm-blooded animals with the ability to produce milk, were next in line and therefore became dominant.

B.     At this time, the Earth's climates grew cooler in some regions, causing plant life to adapt and eventually branch into new species. Grass became dominant, and grasslands became common communities for life, as opposed to the original swamp communities of the Paleozoic.

C.    Grass contains nutrients; however, the cellulose that protectively covers the plant is tough. Hence, herbivores were forced to develop mechanisms in order to extract the nutrition from the food source. In response to the new plant life, mammals in particular evolved highly specialized jaws and teeth. Other animals, amphibians, reptiles, birds, insects, and fish, were evolving just as well as the mammals; however, the Cenozoic earned the reputation for being the age of the mammals.

D.    The Earth continued its cycle between cool and warm global temperatures. During the Ice Ages, when much of the surface water was trapped in gigantic glaciers, the land seemed to grow while the oceans receded. At certain areas between the continents land bridges surfaced. One of the most important land bridges surfaced in the Bering Strait, thus connecting the North American and Asian continents. Across this bridge, following the woolly mammoth along with the beaver and many others, humans were introduced to North America. Going the other way, horses abandoned the continent completely.