Geologist and Palaeontologist divide the past into several different periods called eras, which are further subdivided into periods, each of which lasts for millions of years. The earth can be presumed to be around 4.5 billion years old dating back to the Precambrian era.
The Precambrian Era is the longest geological era beginning 4.5 billion years ago and lasting approximately 4 billion years. The Precambrian era is generally separated into two eons the first of which being the Archaean Eon. spanning from around 4.5 billion years ago to 2.5 billion years ago, during this time air temperatures lowered and the water vapour condensed forming the first primeval oceans. This was a period of great volcanic activity when immense lava flows solidified both on the surface and below were transformed into rocks which were then carried by the first rivers to oceans where they were deposited at the bottom of the oceans forming the first sediments. The second eon was the Proterozoic Eon spanning from 2.5 billion years ago to 500 million years ago.
Palaeozoic literally translates as ‘ancient life’; this era saw the occurrence of the oldest fish, reptiles and insects. During this era animals first began to have hard parts such as shells and carapaces in their bodies. The harder sections of the body fossilize well and it is from this era onwards that scientists have been able to successfully chart the rise and fall of individual groups of animals and plants. Beginning 500 million years ago and ending in an extinction event 248 million years ago, the Palaeozoic Era consists of seven different periods listed below.
Period | Epoch | Beginning | Major events |
Permian | Late | 256,000,000 | Reptiles become large; deserts proliferate; great extinction event devastates life |
Early | 290,000,000 | ||
Carboniferous | Late | 323,000,000 | Complex Land communities occur; coral reefs dominate; oldest reptiles and insects occur |
Early | 354,000,000 | ||
Devonian | Late | 370,000,000 | Oldest sharks and jawed fish; oldest amphibians occur, Lobe- and ray-finned fish evolve |
Middle | 392,000,000 | ||
Early | 417,000,000 | ||
Silurian | Late | 423,000,000 | Oldest land plants and animals; coral reefs are common |
Early | 443,000,000 | ||
Ordovician | Late | 458,000,000 | Complex marine communities; arthropods dominate the sea, jawless fish become common |
Middle | 470,000,000 | ||
Early | 490,000,000 | ||
Cambrian | 543,000,000 | Oldest hard shelled fossils, oldest jawless fishes |
The Mesozoic era is commonly referred to as the ‘age of reptiles’ as during this time dinosaurs evolved into the dominate animal group on earth. Spanning from 248 million years ago to 65 million years ago the Mesozoic era saw the evolution of not only reptiles but also mammals and birds and ended in the extinction of the dinosaurs. This era consist of three periods listed below.
Period | Epoch | Beginning | Major events |
Cretaceous | Late | 99,000,000 | Mammals and birds diversify; pterosaurs decline; dinosaurs become extinct |
Early | 144,000,000 | ||
Jurassic | Late | 159,000,000 | Oldest mammals; dinosaurs dominate; flowering plants and birds; giant marine reptiles in sea |
Middle | 180,000,000 | ||
Early | 206,000,000 | ||
Triassic | Late | 227,000,000 | Oldest Dinosaurs, crocodilians and pterosaurs occur; conifers and cycads become common; extinction event |
Middle | 242,000,000 | ||
Early | 248,000,000 |
A mass extinction event 65 million years ago marks the beginning of the Cenozoic Era or ‘recent’ era which is the current geological era. The extinction event at the end of the Cretaceous period destroyed everything larger then a crocodile including dinosaurs. The small animals that did survive found themselves in a very different setting as this era is also marked by the separation of the continents and the formation of their own unique plants and animals. The Cenozoic Era currently consists of two periods which are subdivided into seven epochs as depicted below.
Period | Epoch | Beginning | Major events |
Quaternary | Holocene | 10,000 | Human Civilisation begins |
Pleistocene | 1,800,000 | Ice Age; Neanderthal and Cro-Magnon man evolve; mammoths roam free | |
Tertiary | Pliocene | 5,000,000 | Oldest hominid fossils; South America joins with North America |
Miocene | 24,000,000 | Grasslands evolve; hoofed animals proliferate; mountain chains form | |
Oligocene | 34,000,000 | Carnivorous and giant mammals evolve; Antarctic ice-sheet forms | |
Eocene | 55,000,000 | First large mammals occur; planet becomes drier; small extinction event | |
Palaeocene | 65,000,000 | Many modern mammal families evolve; jungles cover the earth |
