Earth history
The Precambrian (Age of Early Life) is the earliest part of Earth's history. It accounts for 88% of the Earth's geologic time and subdivided into three eons (Hadean, Archean, Proterozoic). Many Precambrian rocks have been highly metamorphosed, hiding their origins, or have been destroyed by erosion and are now deeply buried beneath Phanerozoic strata. Hadean refers to the first eon of the Precambrian period. It started with the genesis of the Earth. When the oldest igneous rocks developed in the Earth's crust, most likely during the Hadean Era, there were no creatures on Earth. The Archaean period marked the development of the continents. During the Archean period, the earliest living cells most likely developed in the waters. Sponge, jellyfish, and hydra are examples of animals having more than one cell that evolved some 600 million years ago during the late Proterozoic period.
Stromatolites are microfossil specimens that remain relatively large in Precambrian sediments. These organisms are the best producers of Precambrian sedimentary rocks, so in stratigraphic studies, stromatolite fossils are used as indicator fossils of the pre-Cambrian period, and the oldest traces of stromatolites are also used at the ages of 4.3 - 5.3. Cyanobacteria were the origin and primary constructors of stromatolites; stromatolite life, which should be attributed to oxygen in the Earth's atmosphere, continues to this day in some areas of the world, particularly off the coast of Australia; thus, stromatolites are the oldest examples of life that still exist on Earth.
The Paleozoic Era is the first of three geological periods of the Phanerozoic Eon. Paleozoic is a term that means "ancient life". It is the longest of the Phanerozoic eras, and is divided into six geologic periods (from oldest to youngest): Cambrian, Ordovician. 3. Silurian, 4. Devonian. 5. Carboniferous and 6. Permian periods. The Paleozoic era saw significant changes in geology, climate, and evolution. A notable feature of Paleozoic life is the abrupt arrival of nearly all of the invertebrate animal phyla in large numbers at the start of the Cambrian. The first vertebrates arose in the form of rudimentary fish, which considerably evolved during the Silurian and Devonian periods. While macroscopic plant life developed early in the Paleozoic Era, plants generally remained aquatic until the Silurian Period, which occurred around 420 million.
The Mesozoic Era, which followed the Paleozoic, spanned approximately 186 million years, from around 252 million years ago to 66 million years ago. This era is divided into three distinct geological periods: the Triassic, the Jurassic, and the Cretaceous. Often referred to as the Age of Reptiles and the Age of Conifers, the Mesozoic is marked by the prevalence of archosaurian reptiles, including dinosaurs, a rich variety of conifers and ferns, a predominantly warm greenhouse climate, and the gradual fragmentation of the supercontinent Pangaea.
In contrast, the Cenozoic Era, encompasses the last 66 million years of Earth's history. This era is characterized by the rise of mammals, birds, and flowering plants, alongside a trend towards a cooler and drier climate, as well as the present-day arrangement of continents. The Cenozoic is typically subdivided into three periods: the Paleogene (from 66 million to 23 million years ago), the Neogene (from 23 million to 2.6 million years ago), and the Quaternary (from 2.6 million years ago to the present).
