“From so simple a beginning, endless forms most beautiful and most wonderful have been and are being evolved.” ~ Charles Darwin
Life appeared on this planet as soon as it could. Life proliferated into every possible niche where metabolic energy could be got, whether by molecular manipulation or by radiation intake. Numerous extinction events – a few of searing intensity – have pounded life on Earth and transposed its mix. Yet life has persevered.
Earth scientists divide the geologic time scale of our planet’s 4.55 billion-year history into a nested hierarchy of eons, eras, periods, epochs, and ages. Eons have eras, eras comprise periods, and so on. The division is ostensibly based upon stratigraphy: a branch of geology related to rock layers (stratification).
Earth’s surface was supposedly hellish in the Hadean eon (Greek for “underworld.”) After that, each succeeding eon marked a milestone in predominant life forms, based upon now outdated estimates.
The Proterozoic was the eon of oxygen buildup: the by-product of plants’ progenitors. The Cambrian period’s ostensible explosion of diversity, marked by the onset of a prolific fossil record, initiated the Phanerozoic. Each era in the Phanerozoic is marked by a major extinction event. Period demarcations are slightly less pronounced in demonstration of drastic change. Nonetheless, where once there was a regatta of biota, a period ends with an extinction event that shuffled the deck of life on Earth.