How The Life Begin

Dark Ages and large-scale  structure emergence , from 377,000 years until about 1 billion years. After  recombination  and  decoupling , the universe was transparent but the clouds of  hydrogen  only collapsed very slowly to form  stars  and  galaxies , so there were no new sources of light. The only photons (electromagnetic radiation, or "light") in the universe were those released during decoupling (visible today as the  cosmic microwave background ) and  21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually  redshifting  to non-visible  wavelengths after about 3 million years, leaving it without visible light. This period is known as the  Dark Ages . Between about 10 and 17 million years the universe's average temperature was suitable for liquid water (273 – 373K) and there has been specula...

The  early universe , lasting around 377,000 years. Initially, various kinds of  subatomic particles  are formed in stages. These particles include almost equal amounts of  matter  and  antimatter , so most of it quickly annihilates, leaving a small excess of matter in the universe. At about one second,  neutrinos decouple ; these neutrinos form the  cosmic neutrino background . If  primordial black holes  exist, they are also formed at about one second of cosmic time.  Composite   subatomic particles  emerge – including  protons and  neutrons  – and from about 3 minutes, conditions are suitable for  nucleosynthesis : around 25% of the protons and all the neutrons  fuse   into heavier elements , mainly  helium-4 . By 20 minutes, the universe is no longer hot enough for fusion, but far too hot for neutral  atoms  to exist or  photons  to travel far. It is ...

Outline For the purposes of this summary, it is convenient to divide the chronology of the universe since it  originated , into five parts. It is generally considered meaningless or unclear whether  time existed before this chronology: The  very early universe  – the first  picosecond  (10 −12 ) of cosmic time. It includes the  Planck epoch , during which currently understood  laws of physics may not apply; the emergence in stages of the four known  fundamental interactions  or  forces  – first  gravity , and later the  strong ,  weak  and  electromagnetic  interactions; and the  expansion of space  and  supercooling  of the still immensely hot universe due to  cosmic inflation , which is believed to have been triggered by the separation of the  strong  and  electroweak interaction . Tiny ripples in the universe at this stage are believed...

The chronology of the universedescribes the history and  future of the universe  according to  Big Bang cosmology. The earliest stages of the universe's existence are estimated as taking place 13.8  billion years  ago, with an  uncertainty  of around 21 million years at the 68% confidence level.

Astronaut  Bruce McCandless II  outside of the  space  shuttle  Challenger  in 1984 Main article:  Modern history See also:  Modernity  and  Future Change has continued at a rapid pace from the mid-1940s to today. Technological developments include  nuclear weapons ,  computers ,  genetic engineering , and  nanotechnology .  Economic globalization , spurred by advances in communication and transportation technology, has influenced everyday life in many parts of the world. Cultural and institutional forms such as  democracy ,  capitalism , and  environmentalism  have increased influence. Major concerns and problems such as  disease ,  war ,  poverty , violent  radicalism , and recently, human-caused  climate change  have risen as the world population increases. In 1957, the Soviet Union launched  the first artificial satellite  into orbit and, soon ...

Main articles:  History of the world  and  Cradle of civilization Further information:  History of Africa ,  History of the Americas ,  History of Antarctica , and  History of Eurasia Vitruvian Man  by  Leonardo da Vinci epitomizes the advances in art and science seen during the Renaissance. Throughout more than 90% of its history, Homo sapiens lived in small bands as  nomadic   hunter-gatherers . As language became more complex, the ability to remember and communicate information resulted, according to a theory proposed by  Richard Dawkins , in a new replicator: the  meme .Ideas could be exchanged quickly and passed down the generations.  Cultural evolution quickly outpaced  biological evolution , and  history  proper began. Between 8500 and 7000  BC , humans in the  Fertile Crescent  in the  Middle East  began the systematic husbandry of plants and animals:...

Main article:  Human evolution A small African ape living around 6 Ma was the last animal whose descendants would include both modern humans and their closest relatives, the  chimpanzees . Only two branches of its family tree have surviving descendants. Very soon after the split, for reasons that are still unclear, apes in one branch developed the ability to  walkupright . Brain size increased rapidly, and by 2 Ma, the first animals classified in the genus  Homo  had appeared.Of course, the line between different species or even genera is somewhat arbitrary as organisms continuously change over generations. Around the same time, the other branch split into the ancestors of the  common chimpanzee  and the ancestors of the  bonobo  as evolution continued simultaneously in all life forms. T he ability to control  fire  probably began in  Homo erectus  (or  Homo ergaster ), probably at least 790,000 ...

Further information:  Evolution of mammal s The first true mammals evolved in the shadows of dinosaurs and other large archosaurs that filled the world by the late Triassic. The first mammals were very small, and were probably nocturnal to escape predation. Mammal diversification truly began only after the Cretaceous-Paleogene extinction event.By the early Paleocene the earth recovered from the extinction, and mammalian diversity increased. Creatures like  Ambulocetus  took to the oceans to eventually evolve into whales, whereas some creatures, like primates, took to the trees. This all changed during the mid to late Eocene when the circum-Antarctic current formed between Antarctica and Australia which disrupted weather patterns on a global scale. Grassless  savannas  began to predominate much of the landscape, and mammals such as  Andrewsarchus  rose up to become the largest known terrestrial predatory mammal ever,and  early whales ...

The first of five great mass extinctions was the  Ordovician-Silurian extinction . Its possible cause was the intense glaciation of Gondwana, which eventually led to a  snowball earth . 60% of marine invertebrates became extinct and 25% of all families. The second mass extinction was the  Late Devonian extinction , probably caused by the evolution of trees, which could have led to the depletion of greenhouse gases (like CO2) or the  eutrophication  of water. 70% of all species became extinct. The third mass extinction was the Permian-Triassic, or the  Great Dying , event was possibly caused by some combination of the  Siberian Traps volcanic event, an asteroid impact,  methane hydrate  gasification, sea level fluctuations, and a major  anoxic event . Either the proposed  Wilkes Land crater in Antarctica or  Bedout structure  off the northwest coast of Australia may indicate an impact connection with the Permian-Triassic...

Further information:  Evolution of tetrapods Tiktaalik , a fish with limb-like fins and a predecessor of tetrapods . Reconstruction from fossils about 375 million years old. At the end of the Ordovician period, 443 Ma, additional  extinction events occurred , perhaps due to a concurrent ice age.Around 380 to 375 Ma, the first  tetrapods  evolved from fish.Fins evolved to become limbs that the first tetrapods used to lift their heads out of the water to breathe air. This would let them live in oxygen-poor water, or pursue small prey in shallow water.They may have later ventured on land for brief periods. Eventually, some of them became so well adapted to terrestrial life that they spent their adult lives on land, although they hatched in the water and returned to lay their eggs. This was the origin of the  amphibians . About 365 Ma, another  period of extinction  occurred, perhaps as a result of  global cooling ....

Artist's conception of  Devonian  flora Oxygen accumulation from photosynthesis resulted in the formation of an ozone layer that absorbed much of the Sun's  ultraviolet radiation , meaning unicellular organisms that reached land were less likely to die, and prokaryotes began to multiply and become better adapted to survival out of the water. Prokaryote lineages  had probably colonized the land as early as 2.6 Ga even before the origin of the eukaryotes. For a long time, the land remained barren of multicellular organisms. The supercontinent Pannotia formed around 600 Ma and then broke apart a short 50 million years later. Fish, the  earliest vertebrates , evolved in the oceans around 530 Ma. A major  extinction event  occurred near the end of the Cambrian period, which ended 488 Ma. Several hundred million years ago, plants (probably resembling  algae ) and fungi started growing at the edges of the water, and then out of it. The...

Main article:  Cambrian explosion Trilobites  first appeared during the Cambrian period and were among the most widespread and diverse groups of Paleozoic organisms. The rate of the evolution of life as recorded by fossils accelerated in the  Cambrian  period (542–488 Ma).The sudden emergence of many new species,  phyla , and forms in this period is called the Cambrian Explosion. The biological fomenting in the Cambrian Explosion was unpreceded before and since that time.Whereas the Ediacaran life forms appear yet primitive and not easy to put in any modern group, at the end of the Cambrian most modern phyla were already present. The development of hard body parts such as shells,  skeletons  or  exoskeletons  in animals like  molluscs ,  echinoderms ,  crinoids  and  arthropods  (a well-known group of arthropods from the lower Paleozoic are the  trilobites ) made the preservation and  fos...

Pangaea  was a  supercontinent  that existed from about 300 to 180 Ma. The outlines of the modern continents and other landmasses are indicated on this map. At the end of the Proterozoic, the supercontinent Pannotia had broken apart into the smaller continents Laurentia,  Baltica ,  Siberia  and Gondwana. During periods when continents move apart, more oceanic crust is formed by volcanic activity. Because young volcanic crust is relatively hotter and less dense than old oceanic crust, the ocean floors rise during such periods. This causes the  sea level  to rise. Therefore, in the first half of the Paleozoic, large areas of the continents were below sea level. Early Paleozoic climates were warmer than today, but the end of the Ordovician saw a short  ice age  during which glaciers covered the south pole, where the huge continent Gondwana was situated. Traces of glaciation from this period are only found on former Gondwana. ...

Main article:  Phanerozoic The Phanerozoic is the current eon on Earth, which started approximately 542 million years ago. It consists of three eras: The  Paleozoic ,  Mesozoic , and  Cenozoic ,and is the time when multi-cellular life greatly diversified into almost all the organisms known today. The Paleozoic ("old life") era was the first and longest era of the Phanerozoic eon, lasting from 542 to 251 Ma.During the Paleozoic, many modern groups of life came into existence. Life colonized the land, first plants, then animals. Two major extinctions occurred. The continents formed at the break-up of Pannotia and Rodinia at the end of the Proterozoic slowly moved together again, forming the supercontinent  Pangaea  in the late Paleozoic. The Mesozoic ("middle life") era lasted from 251 Ma to 66 Ma.It is subdivided into the  Triassic ,  Jurassic , and  Cretaceous  periods. The era began with the  Permian–Triassic...

A 580 million year old fossil of  Spriggina floundensi , an animal from the  Ediacaran  period. Such life forms could have been ancestors to the many new forms that originated in the  Cambrian Explosion . The end of the Proterozoic saw at least two Snowball Earths, so severe that the surface of the oceans may have been completely frozen. This happened about 716.5 and 635 Ma, in the  Cryogenian period . The intensity and mechanism of both glaciations are still under investigation and harder to explain than the early Proterozoic Snowball Earth.Most paleoclimatologists think the cold episodes were linked to the formation of the supercontinent Rodinia.Because Rodinia was centered on the equator, rates of  chemical weathering  increased and carbon dioxide (CO 2 ) was taken from the atmosphere. Because CO 2  is an important greenhouse gas, climates cooled globally.In the same way, during the Snowball Earths most of the continental surf...