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History of Life on Earth

Phanaerozoic Eon

The Earth began to form from the solar dust cloud into a spherical planet around 4.6 billion years ago. It was very hot, and liquid water could not form. The atmosphere would have been an unbreathable mixture of hydrogen cyanide (HCN), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2), hydrogen sulphide ('rotten egg gas', H2S), and water (H2O). By 3.8 billion years ago, it had cooled enough for oceans to begin to form, and the surface became gradually more and more hospitable.


Paleontology is a branch of science concerned with ancient life, technically up to 11,700 years ago, which saw the beginning of the current Holocene Epoch.

From Greek palaios meaning "ancient", on(-tos) meaning "living thing", and logos meaning "study of".

Paleontologists are concerned with the interpretation of fossils within a geological context, to build an evolutionary history of life, dating back to the start of life, 3.8 billion years ago.

Paleontology is multi-disciplinary, combining the knowledge and techniques of many scientific disciplines. One of the tools it uses to date remains is radiometric dating.

Through a mechanism not fully understood by paleontologists, molecules began to self-replicate in early Earth's early goop. The current thinking is that first life's energy source were the hot springs which leaked out a nutrient cocktail through crustal fractures, called hydrothermal vents, deep in the oceans. Only much later did photosynthesis, utilising the energy of the Sun, become the dominant energy source for most life. We know this because photosynthesis produces oxygen, and there seems to have been very little oxygen in the atmosphere at the time the first signs of life emerged. In addition, chemosynthesis (chemosynthetic bacteria using chemical energy rather than solar energy to grow with) still exists around black smokers on the ocean floor today.

Precambrian life

From about 3.5 billion years ago till 541 million years ago, life was very simple, gradually adding a membrane to become unicellular, and there was little oxygen in the atmosphere. It was only shortly before the Phanerozoic Eon that more complex, multicellular life began to emerge in marine environments. But quite suddenly, life became far more abundant, and brought about many rapid changes to the planet, including the composition of the atmosphere. This was the beginning of the Phanaerozoic Eon [Ancient Greek: fanerós = visible, and zoí = life].

Phanerozoic Eon

541 Mybp - present

It was once believed that all life started in the Phanerozoic (or Phanaerozoic) Eon. However, we have since discovered that simple forms of life existed as long ago as 3.5 billion years before present (Mybp). This early phase of life, known as Protozoic ('early life') has a boundary with the Phanerozoic Eon, defined by the relatively sudden start of an age in which life diversified greatly. Plant life and hard-shelled animals began to appear, and the number of phyla grew. Plants grew more complex, and fish, insects, tetrapods, and eventually modern fauna evolved.

The Phanerozoic Eon is divided into three eras: the Pal(a)eozoic ('old'), the Mesozoic ('middle') and the Cenozoic ('recent') Eras. These eras are in turn divided into different periods, defined by distinct geological phases, and named after the geographical location where the fossils were first discovered.

Paleozoic Era

541 Mybp - 245 Mybp

The Paleozoic (or Palaeozoic) Era is when complex life appears and began to use oxygen, and left the seas.

By the end of the Paleozoic, great forests covered the continents. These are now the coal beds exploited across Europe and North America.

Cambrian Period

541 Mybp - 485.4 Mybp

The Cambrian Explosion was a starting pistol for complex organisms to experiment like crazy, producing a very large number of creatures. From out of this 55 million year trial and error phase came algae, all of the marine phyla, arthropods, such as trilobites (armoured vehicle type creatures that scurried along the ocean floor), fish, amphibians, reptiles and synapsids. However, dry land species were sparse and very simple.

The Cambrian was named by Adam Sedgwick, after the Latin name for Wales, where he discovered rocks from this era. The sedimentary layers of the Cambrian are particularly rich in fossils revealing soft parts of animals as well as hard shells.

Mean atmospheric O2 = 12.5% by volume (today = 21%). Mean atmospheric CO2 = 4500 ppm (today 400 ppm). Mean surface temperature = 21°C (today = 14°C). Sea level = 30m - 90m above today's level.

Ordovician Period

485.4 Mybp - 443.8 Mybp

The Ordovician is named after an ancient Celtic tribe, the Ordovices, by Charles Lapworth in 1879. Mean atmospheric O2 = 13.5% by volume (today = 21%). Mean atmospheric CO2 = 4200 ppm (today 400 ppm). Mean surface temperature = 16°C (today = 14°C). Sea level = 140m - 220m above today's level.

Although the majority of species are still marine, such as fish, cephalopods, coral, trilolites, snails, and shellfish, some anthropids ventured onto dry land. By this period, the super-continent Pannotia had broken up to the continent of Gondwana, which drifted by the end of the Ordovician to the South Pole.

The Ordovician-Silurian Extinction killed off 60% of marine invertebrates, and 25% of all families. It was caused by the continent of America colliding with Europe, causing the Atlantic Ocean to close, with resulting glaciation of Africa during the so-called 'snowball earth', and a fall in sea level. Many species had evolved along the coastline of Gondwana, so the fall in sea level was fatal.

Silurian Period

443.8 Mybp - 419.2 Mybp

Mean atmospheric O2 = 14% by volume (today = 21%). Mean atmospheric CO2 = 4500 ppm (today 400 ppm). Mean surface temperature = 17°C (today = 14°C). Sea level = 180m above today's level.

In this period seal levels rose again, allowing many species of jawless, then also jawed, fish to evolve, including freshwater species. Terrestrial species also evolved, including spiders, funghi, giant centipedes, and vertical growing (vascular) plants.

Four continents had formed by the Silurian: Gondwana, Laurentia, Baltica, and Siberia.

Devonian Period

419.2 Mybp - 358.9 Mybp

Mean atmospheric O2 = 15% by volume (today = 21%). Mean atmospheric CO2 = 2200 ppm (today 400 ppm). Mean surface temperature = 20°C (today = 14°C). Sea level = 189m - 120m above today's level.

This period is nicknamed 'the Age of Fish' for its diversity in marine species, but terrestrial life also continued to diversify. The Devonian Explosion saw the evolution of the first trees and seeds. The Late Devonian Extinction brought a cataclysmic reduction in species to close the period.

Carboniferous Period

358.9 Mybp - 298.9 Mybp

Mean atmospheric O2 = 32.5% by volume (today = 21%). Mean atmospheric CO2 = 800 ppm (today 400 ppm). Mean surface temperature = 14°C (today = 14°C). Sea level = 120m - 0m - 80m above today's level.

The temperature varied greatly during this period, starting at 20°C and falling to 10°C in the middle. Consequently there were many tropical swamps and large swathes of forest, which are a large part of today's coal seams. in fact, the name Carboniferous refers to the vast quantities of carbon trapped in the geological strata laid down in this period.

Amphibians moved inland and could lay ambiotic eggs, and reptiles and synapsids enjoyed the goodlife afforded by the swamps.

By the end of the period, a glaciation of Gondwana led to the Carboniferous Rainforest Collapse.

Permian Period

298.9 Mybp - 252.2 Mybp

Mean atmospheric O2 = 23% by volume (today = 21%). Mean atmospheric CO2 = 900 ppm (today 400 ppm). Mean surface temperature = 16°C (today = 14°C). Sea level = 60m above to 20m below today's level.

In contrast to the lush, wet Carboniferous, the Permian Period was dry. The continents ran together into one super-continent, Pangaea, with one surrounding ocean, Panthalassa. The interior had no large water body so became desertified.

Reptiles and synapsids evolved to adjust to the dry climate, and new species, such as conifer trees, dominated the land. In the desert, species like Scutosaurus and gorgonopsids flourished. However, the end of the Permian, and the end of the Paleozoic Eon, was the venue for the most severe mass extinction the planet has known: called the 'Great Dying'. 95% of all life died in the Permian extinction.

Mesozoic Era

252.2 Mybp - 65 Mybp

The Mesozoic Era could be nicknamed the 'Age of the Dinosaurs', but many other types of life existed as well, including mammals. The Mesozoic (the middle realm of life) is divided into three periods:

Triassic Period

252.2 Mybp - 210 Mybp

Pangaea remained a single landmass, so was a dry wasteland for the most part. Following the mass extinction at the transition from the Paleozoic to the Mesozoic Eons, the Triassic saw the rise of a completely different set of flora and fauna. As Pangaea broke up, a large inland sea, the Tethys Sea, aided the development of a major ecosystem, and reptiles began to grow in size.

Pine forests sprang up, as well as insects we would recognise today: mosquitoes and fruit flies. As the world warmed, dinosaurs came into being. However, yet another mass die-out, the Triassic-Jurassic extinction event, devastated the diversity once again.

Jurassic Period

210 Mybp - 145 Mybp

Named after the Jura Mountains in France/Switzerland. Giant reptiles and tiny mammals arose in the humid Jurassic tropics. The oceans became filled with plesiosaurs and ichthyosaurs, and dinosaurs roamed the land, while the swamps gave crocodiles free range.

By the Middle Jurassic (175-163 Mybp) there were large herds of sauropods (e.g. Brachiosaurus) and conifer forests. This golden age for the reptiles ended in the Jurassic-Cretaceous extinction, accompanied by rising sea levels, and the opening up of the Atlantic Ocean. This division of the landmasses diversified dinosaur species even further.

Cretaceous Period

145 Mybp - 65 Mybp

As the seas opened up, the climate changed, creating new biomes, and island, sea and inland habitats. Globally, seasons created migratory routes, and the poles became seasonally colder. Birds evolved alongside changing diversity of dinosaurs.

The late Cretaceous (100-65 Mybp) experienced a cooling planet and the tropics shrank to the equatorial regions. The world had extreme seasonal variations. Dinosaurs were still dominant, but birds, mammals, marsupials and flowering plants were also finding their niches, as the less favourable conditions saw some dinosaur species go extinct.

The Indian subcontinent's movement and the creation of the Deccan Traps, and other volcanic activity poured methane, sulphur and carbon dioxide into the atmosphere. 75% of life collapsed when a meteor struck in the K-T Extinction.

Cenozoic Era

65 Mybp - present

Even though it is now known that mammals began to evolve during the Mesozoic Era, the Cenozoic Era is distinctive in being the current age, beginning when the dinosaurs went extinct, and in which mammals are the dominant phyla.

Palaeogene Period

65 Mybp - 25 Mybp

The Pal(a)eogene Period is sub-divided into three epochs: Paleocene, Eocene and Oligocene.

In the Paleocene (65 - 55 Mybp), the Earth recovered its diversity, albeit without the dinosaurs. The continents as we know them began to take shape and separate. The planet warms, and the tropical zone extends as far as the poles. Other species of reptiles take up the power vacuum left by the extinct dinosaurs, and large sharks roam the seas. The land was populated by small species of mammals.

In the Eocene (55-33 Mybp) the first small primates, horses and other mammals appear and thrive. Whales develop, but birds make it to the top of the predator hit parade for the first and last time. In the first half of the Eocene, temperatures soared to a global average of 30°C (14°C above today's), with little gradient from the equator to the poles. By mid-Eocene, however, a global cooling effect shrank the tropical jungles, which had the effect of enlarging mammls. Mammals now had their turn as the dominant predators. By the end of the Eocene, seasons have rebooted and grass has evolved.

The Oligocene (33-23 Mybp) sees grass become widespread and consequently ruminant species, as well as the modern species, such as cats, dogs, elephant and marsupials. Plants diversified into biome-related vegetation types, such as evergreens and deciduous.

Neogene Period

23 Mya - 2.58 Mya

The Neogene is sub-divided into:


23-5 Mya In the Miocene, apes, grazers and seed plants flourish as the biomes of jungle, desert and savannah are shaped by continental shifts.


5 - 2.58 Mya. The Pliocene experiences very dramatic climatic changes, and the evolution of modern flora and fauna. The ice collects over the poles, and features such as the Himalayas are formed by tectonic plate collisions, and Earth's geography takes its modern shape. The first hominids arise in Africa, and Australopithecus, the first hominin, suggest a place for humans is now available.


2.58 Mya - Present

This period has seen cyclic patterns of ice sheet growth and decay. Oxygen levels have been stable at 20.8% and CO2 till the industrial pollution phase was at around 250-280 ppm. Currently, mankind has raised CO2 to over 400 ppm and triggered an irreversible climate change which will have catastrophic consequences for all species, especially large mammals like homo sapiens.

Linnaeus Classification of Species

Carl Linnaeus was Swedish and lived from 1707 - 1778. He developed a system for classifying species which is still widely used today.

Common Name:
























Tribe (sub-family):







Homo sapiens

Canis lupis

e.g. Quercus pyrineica

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