The Earth’s history has been recorded in reference to what is called the Geological Time Scale, or GTS.  Units of time descend from Eons, which contain Eras, which contain Periods, which contain Epochs.


Pre-Cambrian is a term used to describe time between the Earth’s creation and the Cambrian Explosion (542 mya). It contains both the Archaean and Proterozoic Eons. During most of the Archaean Eon, the Earth had a turbulent climate involving tectonic movements, volcanic activity, and heavy and enduring rainfall. It was at this time that the oceans and land masses emerged, as well as an atmosphere containing oxygen. The first life forms were prokaryotes, or single celled bacteria. How these pioneer organisms appeared is still uncertain in the scientific community. During the Proterozoic Eon, eukaryotic cells developed, which contained symbolic organelles contained within the cell and controlled by a nucleus. Additionally, soft-bodied invertebrates developed, as well as Blue Green Algae, which is considered to be the first example of a multi-celled organism. Blue Green Algae existed in large colonies that encompassed most the planet’s oceans.


The Paleozoic Era saw some of the greatest advanced in biological development. The Cambrian Explosion was arguably the greatest diversification of flora and fauna in history. New features such as eyes, the hard shells associated with arthropods, and faster modes of movement similar to fins evolved and were seen in marine life (Anomalocaris, trilobites, Opabinia). Over the course of several periods, the Kingdom Animalia and Plantae diversified greatly (development of fish, vascular plants, amphibians, reptiles, trees, insects, flying animals, etc). Other characteristic changes aside from the Cambrian Explosions include the atmospheric changes of the Carboniferous Period and the Permian Extinction. During the Carboniferous Period, the oxygen levels in the Earth’s atmosphere significantly increased as a result of accelerated plant growth, thus causing insects to adapt with larger body sizes, and more swamp-like biomes to emerge. During the Permian Period, there was a widespread and enduring drought which caused the extinction of roughly 95% of marine species. This mass extinction marked the end of the Paleozoic Era.


The Mesozoic Era is often characterized as the ‘Age of Reptiles’, as most orders of Dinosaurs evolved during this era and constituted most of the apex predators within the planet’s biomes. During the Triassic Period, the first mammal-like reptiles appeared, indicating the development of mammalian features such as fur and the use of breast milk. Many climates were still very arid and hot. Earth’s first bird (Archaeopteryx) evolved during the Jurassic Period and had both reptilian and avian traits. Flowers emerged during the Cretaceous Period, as well as one of the most famous dinosaurs, Tyrannosaurus Rex. The Cretaceous-Paleogene Extinction event, or KT extinction, occurred about 65 million years ago. There are multiple theories surrounding this event that eliminated approximately 80% of the world’s animal life. However, the asteroid theory is the most widely accepted by the scientific community. The theory suggests that an asteroid collided with the planet (on Mexico) resulting in an impact that triggered multiple planet-wide climate changes including tidal waves and a global dust cloud that blocked out sunlight and killed plant life.


The Cenozoic Era is the one that we are currently living in, and is considered the ‘Age of Mammals’, as mammals generally came to take the place that dinosaurs held during the Mesozoic Era. Tectonic movements shifted landmasses closer to where they lie today. The Paleogene Period saw the diversification of mammals and plant life similar to modern flora. Neogene Period fossils are very common in California, especially at beaches. This is because the sediments were the most recently deposited so the strata is closer to the surface and easier to find. The Pleistocene Epoch is characterized by the Ice Age, which swept Southward from the Arctic circle and caused a shift in North American, European, and Asiatic life. Many of the species that evolved during this time (2mya-0.01mya) still exist today. Currently we are in the Holocene epoch, which many fear may cause another mass extinction like the Permian or KT extinction. This 6th extinction would be caused by dramatic climate change due to human pollution.