This is a very brief description of how the organisms that we have today evolved from the early, single-celled organisms that lived in water two billion years ago. We must remember that it took an extremely long period of time for the modern-day organisms to appear and that there were many more organisms that today’s organisms evolved from. These species were forced to adapt to Earth’s constantly changing environment.
Geologic Time Scale: Divides the history of the Earth into different time periods based on the climate and the types of organisms living on Earth at the time.
Mass Extinction: When some sort of geologic or climate change causes a large number of species to go extinct.
Extinction: When all organisms of a particular species die; this can be a result of some natural phenomenon, such as a meteor falling.
The geologic time scale organizes Earth’s history by major changes in geology and climate and by important events in the evolution of life. These are some of the key eras organized in chronological order:
Occurred two billion years ago to about half a billion years ago.
There were a lot of geologic and climate changes to the Earth.
About a billion years ago, the first multicellular organisms evolved.
This era ended half a billion years ago with a mass extinction, due to some geologic or climate change. Even though we don’t know the real cause of this mass extinction, we do know that this mass extinction paved the way for a lot of new species to form in the next era: the Paleozoic Era.
The extreme burst of life that occurred right before the Paleozoic Era. This happened as a result of the mass extinction that occurred at the end of the Precambrian era. When the surviving species no longer have as much competition as it did before, it is allowed to thrive and adapt in its own way.
Occurred around 544-245 million years ago; so much happened in this era that it had to be divided into 6 periods
This was a mass extinction at the end of the Permian Period, marking the end of the Paleozoic Era.
Occurred around 245-65 million years ago; known as the age of dinosaurs, the era was divided into three main periods.
Occurred around 65 million years ago and still continues today; this is the era during which mammals appeared and flourished and is divided into two periods
There has been five mass extinctions in Earth’s history. This is not a bad thing, as this sudden change in species can allow the surviving species to flourish with less competition.
Single-celled organisms often found it beneficial to live close to one another in colonies and perform different and specialized tasks. Over time, these formerly independent single-celled organisms became dependent on one another and thus the colonies of single-celled organisms became multicellular organisms.