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History of life
| Life timeline | ||
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| −4500 —–—–−4000 —–—–−3500 —–—–−3000 —–—–−2500 —–—–−2000 —–—–−1500 —–—–−1000 —–—–−500 —–—–0 — | WaterSingle-celled lifePhotosynthesisEukaryotesMulticellular lifeP l a n t sArthropods MolluscsFlowersDinosaursMammalsBirdsPrimatesH a d e a n A r c h e a n P r o t e r o z o i cP h a n e r o z o i c | ←Earth formed←Earliest water←LUCA←Earliest fossils←LHB meteorites←Earliest oxygen←Pongola glaciation*←Atmospheric oxygen←Huronian glaciation*←Sexual reproduction←Earliest multicellular life←Earliest fungi←Earliest plants←Earliest animals←Cryogenian ice age*←Ediacaran biota←Cambrian explosion←Andean glaciation*←Earliest tetrapods←Karoo ice age*←Earliest apes / humans←Quaternary ice age* |
| (million years ago)*Ice Ages |
Main article: History of life
The history of life on Earth traces the processes by which organisms have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on Earth, both living and extinct, descended from a last universal common ancestor that lived about 3.5 billion years ago. The dating of the Earth’s history can be done using several geological methods such as stratigraphy, radiometric dating, and paleomagnetic dating. Based on these methods, geologists have developed a geologic time scale that divides the history of the Earth into major divisions, starting with four eons (Hadean, Archean, Proterozoic, and Phanerozoic), the first three of which are collectively known as the Precambrian, which lasted approximately 4 billion years. Each eon can be divided into eras, with the Phanerozoic eon that began 539 million years ago being subdivided into Paleozoic, Mesozoic, and Cenozoic eras. These three eras together comprise eleven periods (Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary, and Quaternary) and each period into epochs. # ISO certification in India
The similarities among all known present-day species indicate that they have diverged through the process of evolution from their common ancestor. Biologists regard the ubiquity of the genetic code as evidence of universal common descent for all bacteria, archaea, and eukaryotes. Microbal mats of coexisting bacteria and archaea were the dominant form of life in the early Archean epoch and many of the major steps in early evolution are thought to have taken place in this environment. The earliest evidence of eukaryotes dates from 1.85 billion years ago, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 billion years ago, multicellular organisms began to appear, with differentiated cells performing specialised functions.
Algae-like multicellular land plants are dated back even to about 1 billion years ago, although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 billion years ago. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician period. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event.
Ediacara biota appear during the Ediacaran period, while vertebrates, along with most other modern phyla originated about 525 million years ago during the Cambrian explosion. During the Permian period, synapsids, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252 million years ago. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates; one archosaur group, the dinosaurs, dominated the Jurassic and Cretaceous periods. After the Cretaceous–Paleogene extinction event 66 million years ago killed off the non-avian dinosaurs, mammals increased rapidly in size and diversity. Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. # ISO certification in India