The Earth and the history of Life
The Earth is a unique and dynamic planet, with a rich history that spans billions of years. The study of Earth's history, or geology, seeks to understand how the planet formed and how it has evolved over time.
Because the history of the Earth is so long, it is measured in Geological Time. This is made up of eons, divided into eras, which are in turn divided into periods, epochs, ages, and chrons. Traditionally there are four eons: the Hadean, Archean, Proterozoic, and Phanerozoic eons.
A fifth eon, the Chaotian, has been informally suggested for the formation of the Solar System as a whole, beginning about 4.55 billion years ago. This situates the Earth and Solar System in the wider context of the history of the Cosmos. Alternatively, this can be considered a substage of the Hadean.
The first formal eon, known as the Hadean eon, saw the formation of the planet and the first solid crust, oceans, and atmosphere.
This was followed by the Archean eon, during which life first appeared, and the Earth's atmosphere lacked free oxygen. During the Archean eon, other planets in the solar system such as Mars and Venus, experienced early Earth-like conditions (or perhaps rather Titan like conditions, resembling the largest moon of Saturn, which still retains its primordial atmosphere) and may have been home to microbial life just as on the early Earth.
During the Proterozoic eon, there was a build-up of oxygen, as a result of photosynthetic bacterial (also called blue-green algae), which caused conditions to change dramatically, including the extinction of most of the original anaerobic organisms. The Proterozoic eon saw the evolution of the eukaryotes, which have a more complex cell structure, and eventually the first multicellular Eukaryotes.
This whole period, of some four billion years, is referred to as the Precambrian. For the most part, the world was inhabited by microbes and algae mats.
Archean and Proterozoic eras are generally around three or four hundred million years long each, although the longest, the Paleoproterozoic is a whopping nine hundred million. Such long expanses of time indicate that not much was happening (due to unicelluar organisms evolving slowly), which ties in with the idea of accelerating change.
It was only following an extraordinary coincidence of events such as a really big ice age (Snowball Earth), the break-up of a supercontinent (called Rodinia) a rise in oxygen, and change in ocean chemistry, complex life suddenly emerged, the result being the Cambrian explosion and the start of the Phanerozoic. From the Cambrian to now was little more than a tenth of the age of the Earth and Solar System. Yet so much happens that we need finer divisions, and eons, eras and geons are replaced by eras (the famous Paleozoic, Mesozoic, and Cenozoic), periods, epochs, ages, -and megayears. Hence the story of the succession of all sorts of amazing plants and animals.
Over the past several billion years, the Earth has experienced numerous changes, including periods of extreme heat and cold, mass extinctions, and the shifting of continents due to tectonic activity. Despite these challenges, life has continued to thrive on Earth, adapting and evolving to survive and thrive in an ever-changing environment.
For the equivalent time span projected into the future, see the very far distant future. Here the sun becomes a red giant and swallows the inner planets including the Earth. Perhaps a future very technologcally advanced civilisation will migrate life on Earth to other star systems, assuming this hadn’t already been done.
This timeline covers mostly the Precambrian Eon, which represents some 88% of the Earth's history. This long period was marked by the emergence of life, the formation of continents and supercontinents, and the evolution of multicellular organisms. These geological and biological milestones laid the foundation for the evolution of complex life forms in the Phanerozoic Eon, which continues to this day.
Page by M Alan Kazlev, 2023