The Geologic Time Scale is the history of the Earth broken down into spans of time marked by various events. There are other markers, like the types of species and how they evolved, that distinguish one time from another on the Geologic Time Scale.
There are four main time spans the generally mark the Geologic Time Scale divisions. The first, Precambrian Time, is not an actual era on the Geologic Time Scale because the lack of diversity of life, but the other three divisions are defined eras. The Paleozoic Era, Mesozoic Era, and Cenozoic Era saw many great changes.
(4.6 billion years ago - 542 million years ago)
The Precambrian Time Span began at the beginning of the Earth 4.6 billion years ago. For billions of years there was no life on Earth. It wasn't until the end of this time period that single celled organisms came into existence. No one knows for sure how life on Earth began, but there are several theories like the Primoridal Soup Theory, Hydrothermal Vent Theory, and Panspermia Theory.
The end of this time span saw the rise of a few more complex animals in the oceans like jellyfish. There was still no life on land and the atmosphere was just beginning to accumulate the oxygen needed for higher order animals to survive. It wasn't until the next era that life really began to take off and diversify.
(542 million years ago - 250 million years ago)
The Paleozoic Era began with the Cambrian Explosion. This relatively rapid period of large amounts of speciation kicked off a long time span of flourishing life on Earth. This great amounts of life in the oceans soon moved onto land. First plants made the move and then invertebrates. Not long after that, vertebrates moved to land as well. Many new species appeared and thrived.
The end of the Paleozoic Era came with the largest mass extinction in the history of life on Earth. The Permian Extinction wiped out about 95% of marine life and nearly 70% of life on land. Climate changes were most likely the cause of this extinction as the continents all drifted together to form Pangaea. The mass extinction paved the way for new species to arise and a new era to begin.
3. Mesozoic Era
(250 million years ago - 65 million years ago)
The Mesozoic Era is the next era on the Geologic Time Scale. After the Permian Extinction caused so many species to go extinct, many new species evolved and thrived. The Mesozoic Era is also known as the "age of the dinosaurs" because dinosaurs were the dominant species for much of the era. Dinosaurs started off small and got larger as the Mesozoic Era went on.
The climate during the Mesozoic Era was very humid and tropical and many lush, green plants were found all over the Earth. Herbivores especially thrived during this time period. Besides dinosaurs, small mammals came into existence. Birds also evolved from the dinosaurs during the Mesozoic Era.
Another mass extinction marks the end of the Mesozoic Era. All dinosaurs, and many other animals, especially herbivores, completely died off. Again, niches were needing to be filled by new species in the next era.
4. Cenozoic Era
(65 million years ago - Present)
The last and current time period on the Geologic Time Scale is the Cenozoic Period. With large dinosaurs now extinct, the smaller mammals that survived were able to grow and become dominant life on Earth. Human evolution also all happened during the Cenozoic Era.
The climate has changed drastically over the relatively short amount of time in this period. It got much cooler and drier than the Mesozoic Era climate. There was an ice age where most temperate parts of the Earth was covered in glaciers. This made life have to adapt rather rapidly and increased the rate of evolution.
All life on Earth evolved into their present day forms. The Cenozoic Era has not ended and most likely will not end until another mass extinction period.