Geology is a complex and difficult subject, especially for the novice. I recommend that beginners first concentrate on the four major divisions of geologic history: the Geologic Eras. While professional and academic geologists divide the eras into periods and the periods into epochs and these into subdivisions, etc., such classification can overwhelm those who have had no formal education in the field. This blog entry thus provides a basic overview of the four Geologic Eras.
The Precambrian Era (4.6 billion years ago to 600 million years ago) encompasses the great majority of Earth's geologic history, stretching from the formation of our planet to the appearance of shelled marine life. Highlights include the formation of the oceans, the development of the atmosphere and, of course, the evolution of life. The first life forms are thought to have been chemoautotrophic bacteria, which appeared about 3.6 billion years ago.
The evolution of cyanobacteria and photosynthetic algae gradually enriched the atmosphere with oxygen and, eventually, led to the development of the vital ozone layer.
The Paleozoic Era (600 to 225 million years ago) stretches from the appearance of shelled marine life to the evolution of mammal-like reptiles. Highlights include the colonization of the land by plants and animals and the evolution of sharks, fish, insects, spiders, amphibians, ancestral reptiles, ferns and early conifers. The fern forests of this Era would later yield the vast coal deposits that fed the industrial revolution.
The Mesozoic Era (225-65 million years ago) covers the reign of dinosaurs. Other new life forms of this Era include turtles, crocodiles, ancestral birds, primitive mammals, flowering plants, snakes, lizards and social bees.
The Cenozoic Era, which continues today, began 65 million years ago. This Age of Mammals has been characterized by the spread and diversification of mammals, including the appearance of bats, whales and primates. Man belongs to the latter group but did not appear until 125,000 years ago. The last 10,000 years of the Cenozoic, following the end of the Pleistocene Ice Age, have witnessed the rise of human culture, the domestication of animals, the cultivation of plants, the development of industry and man's widespread impact on the ecosystems of our planet.