History of Earth
The history of Earth concerns the
development of planet Earth from
its formation to the present
day.[1][2] Nearly all branches of
natural science have contributed to
understanding of the main events of
Earth's past, characterized by
constant geological change and
biological evolution.
The geological time scale (GTS), as
defined by international
convention,[3] depicts the large
spans of time from the beginning of
the Earth to the present, and its
divisions chronicle some definitive
events of Earth history. (In the
graphic: Ga means "billion years
ago"; Ma, "million years ago".)
Earth formed around 4.54 billion
years ago, approximately one-third
the age of the universe, by accretion
from the solar nebula.[4][5][6]
Volcanic outgassing probably
created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of the Earth was
molten because of frequent collisions with other bodies which led to extreme volcanism. While the Earth was in its earliest stage
(Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, the
Earth cooled, causing the formation of a solidcrust, and allowing liquid water on the surface.
The Hadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0
billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution.
The succeeding eon is the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the first life on land;
the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic, which saw the
rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale.
The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago,[7][8][9] during the Eoarchean Era, after a
geological crust started to solidify following the earlier molten Hadean Eon. There are microbial mat fossils such as stromatolites
found in 3.48 billion-year-old sandstone discovered in Western Australia.[10][11][12] Other early physical evidence of a biogenic
substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland[13] as well as "remains of
biotic life" found in 4.1 billion-year-old rocks in Western Australia.[14][15] According to one of the researchers, "If life arose
relatively quickly on Earth … then it could be common in theuniverse."[14]
Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life
remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over
time, and culminated in the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms produced
most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated
,that 99 percent of all species that ever lived on Earth, over five billion,[16] have gone extinct.[17][18] Estimates on the number of
Earth's current species range from 10 million to 14 million,[19] of which about 1.2 million are documented, but over 86 percent have
not been described.[20] However, it was recently claimed that 1 trillion species currently live on Earth, with only one-thousandth of
one percent described.[21]
The Earth's crust has constantly changed since its formation, as has life has since its first appearance. Species continue to evolve,
taking on new forms, splitting into daughter species, or going extinct in the face of ever
-changing physical environments. The process
of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor. Human activity is now a dominant
force affecting global change, harming the biosphere, the Earth's surface, hydrosphere, and atmosphere with the loss of wild lands,
over-exploitation of the oceans, production of greenhouse gases, degradation of the ozone layer, and general degradation of soil, air,
and water quality.
Contents
Eons
Geologic time scale
Solar System formation
Hadean and Archean Eons
Formation of the Moon
First continents
Oceans and atmosphere
Origin of life
Proterozoic Eon
Oxygen revolution
Snowball Earth
Emergence of eukaryotes
Supercontinents in the Proterozoic
Late Proterozoic climate and life
Phanerozoic Eon
Tectonics, paleogeography and climate
Cambrian explosion
Colonization of land
Evolution of tetrapods
Extinctions
Diversification of mammals
Human evolution
See also
Notes
References
Further reading
External links
Eons
In geochronology, time is generally measured in mya (megayears or million years), each unit representing the period of
approximately 1,000,000 years in the past. The history of Earth is divided into four great eons, starting 4,540 mya with the formation
of the planet. Each eon saw the most significant changes in Earth's composition, climate and life. Each eon is subsequently divided
into eras, which in turn are divided intoperiods, which are further divided intoepochs.
, Time
Eon Description
(mya)
The Earth is formed out of debris around the solarprotoplanetary disk. There is no life.
4,540– Temperatures are extremely hot, with frequent volcanic activity and hellish environments.
Hadean
4,000 The atmosphere is nebular. Possible early oceans or bodies of liquid water. The moon is
formed around this time, probably due to aprotoplanet's collision into Earth.
Prokaryote life, the first form of life, emerges at the very beginning of this eon, in a process
4,000–
Archean known as abiogenesis. The continents of Ur, Vaalbara and Kenorland may have been
2,500
formed around this time. The atmosphere is composed of volcanic and greenhouse gases.
Eukaryotes, a more complex form of life, emerge, including some forms ofmulticellular
organisms. Bacteria begin producing oxygen, shaping the third and current of Earth's
2,500– atmospheres. Plants, later animals and possibly earlier forms of fungi form around this time.
Proterozoic
541 The early and late phases of this eon may have undergoneSnowball
" Earth" periods, in
which all of the planet suffered below-zero temperatures. The early continents ofColumbia,
Rodinia and Pannotia may have formed around this time, in that order .
Complex life, including vertebrates, begin to dominate the Earth's ocean in a process
known as the Cambrian explosion. Pangaea forms and later dissolves intoLaurasia and
Gondwana. Gradually, life expands to land and all familiar forms of plants, animals and
541–
Phanerozoic fungi begin appearing, including annelids, insects and reptiles. Severalmass extinctions
present
occur, among which birds, the descendants of dinosaurs, and more recently mammals
emerge. Modern animals—including humans—evolve at the most recent phases of this
eon.
Geologic time scale
The history of the Earth can be organized chronologically according to the geologic time scale, which is split into intervals based on
stratigraphic analysis.[2][22] The following four timelines show the geologic time scale. The first shows the entire time from the
formation of the Earth to the present, but this gives little space for the most recent eon. Therefore, the second timeline shows an
expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, and the most recent
period is expanded in the fourth timeline.
Millions of Years
The history of Earth concerns the
development of planet Earth from
its formation to the present
day.[1][2] Nearly all branches of
natural science have contributed to
understanding of the main events of
Earth's past, characterized by
constant geological change and
biological evolution.
The geological time scale (GTS), as
defined by international
convention,[3] depicts the large
spans of time from the beginning of
the Earth to the present, and its
divisions chronicle some definitive
events of Earth history. (In the
graphic: Ga means "billion years
ago"; Ma, "million years ago".)
Earth formed around 4.54 billion
years ago, approximately one-third
the age of the universe, by accretion
from the solar nebula.[4][5][6]
Volcanic outgassing probably
created the primordial atmosphere and then the ocean, but the early atmosphere contained almost no oxygen. Much of the Earth was
molten because of frequent collisions with other bodies which led to extreme volcanism. While the Earth was in its earliest stage
(Early Earth), a giant impact collision with a planet-sized body named Theia is thought to have formed the Moon. Over time, the
Earth cooled, causing the formation of a solidcrust, and allowing liquid water on the surface.
The Hadean eon represents the time before a reliable (fossil) record of life; it began with the formation of the planet and ended 4.0
billion years ago. The following Archean and Proterozoic eons produced the beginnings of life on Earth and its earliest evolution.
The succeeding eon is the Phanerozoic, divided into three eras: the Palaeozoic, an era of arthropods, fishes, and the first life on land;
the Mesozoic, which spanned the rise, reign, and climactic extinction of the non-avian dinosaurs; and the Cenozoic, which saw the
rise of mammals. Recognizable humans emerged at most 2 million years ago, a vanishingly small period on the geological scale.
The earliest undisputed evidence of life on Earth dates at least from 3.5 billion years ago,[7][8][9] during the Eoarchean Era, after a
geological crust started to solidify following the earlier molten Hadean Eon. There are microbial mat fossils such as stromatolites
found in 3.48 billion-year-old sandstone discovered in Western Australia.[10][11][12] Other early physical evidence of a biogenic
substance is graphite in 3.7 billion-year-old metasedimentary rocks discovered in southwestern Greenland[13] as well as "remains of
biotic life" found in 4.1 billion-year-old rocks in Western Australia.[14][15] According to one of the researchers, "If life arose
relatively quickly on Earth … then it could be common in theuniverse."[14]
Photosynthetic organisms appeared between 3.2 and 2.4 billion years ago and began enriching the atmosphere with oxygen. Life
remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over
time, and culminated in the Cambrian Explosion about 541 million years ago. This sudden diversification of life forms produced
most of the major phyla known today, and divided the Proterozoic Eon from the Cambrian Period of the Paleozoic Era. It is estimated
,that 99 percent of all species that ever lived on Earth, over five billion,[16] have gone extinct.[17][18] Estimates on the number of
Earth's current species range from 10 million to 14 million,[19] of which about 1.2 million are documented, but over 86 percent have
not been described.[20] However, it was recently claimed that 1 trillion species currently live on Earth, with only one-thousandth of
one percent described.[21]
The Earth's crust has constantly changed since its formation, as has life has since its first appearance. Species continue to evolve,
taking on new forms, splitting into daughter species, or going extinct in the face of ever
-changing physical environments. The process
of plate tectonics continues to shape the Earth's continents and oceans and the life they harbor. Human activity is now a dominant
force affecting global change, harming the biosphere, the Earth's surface, hydrosphere, and atmosphere with the loss of wild lands,
over-exploitation of the oceans, production of greenhouse gases, degradation of the ozone layer, and general degradation of soil, air,
and water quality.
Contents
Eons
Geologic time scale
Solar System formation
Hadean and Archean Eons
Formation of the Moon
First continents
Oceans and atmosphere
Origin of life
Proterozoic Eon
Oxygen revolution
Snowball Earth
Emergence of eukaryotes
Supercontinents in the Proterozoic
Late Proterozoic climate and life
Phanerozoic Eon
Tectonics, paleogeography and climate
Cambrian explosion
Colonization of land
Evolution of tetrapods
Extinctions
Diversification of mammals
Human evolution
See also
Notes
References
Further reading
External links
Eons
In geochronology, time is generally measured in mya (megayears or million years), each unit representing the period of
approximately 1,000,000 years in the past. The history of Earth is divided into four great eons, starting 4,540 mya with the formation
of the planet. Each eon saw the most significant changes in Earth's composition, climate and life. Each eon is subsequently divided
into eras, which in turn are divided intoperiods, which are further divided intoepochs.
, Time
Eon Description
(mya)
The Earth is formed out of debris around the solarprotoplanetary disk. There is no life.
4,540– Temperatures are extremely hot, with frequent volcanic activity and hellish environments.
Hadean
4,000 The atmosphere is nebular. Possible early oceans or bodies of liquid water. The moon is
formed around this time, probably due to aprotoplanet's collision into Earth.
Prokaryote life, the first form of life, emerges at the very beginning of this eon, in a process
4,000–
Archean known as abiogenesis. The continents of Ur, Vaalbara and Kenorland may have been
2,500
formed around this time. The atmosphere is composed of volcanic and greenhouse gases.
Eukaryotes, a more complex form of life, emerge, including some forms ofmulticellular
organisms. Bacteria begin producing oxygen, shaping the third and current of Earth's
2,500– atmospheres. Plants, later animals and possibly earlier forms of fungi form around this time.
Proterozoic
541 The early and late phases of this eon may have undergoneSnowball
" Earth" periods, in
which all of the planet suffered below-zero temperatures. The early continents ofColumbia,
Rodinia and Pannotia may have formed around this time, in that order .
Complex life, including vertebrates, begin to dominate the Earth's ocean in a process
known as the Cambrian explosion. Pangaea forms and later dissolves intoLaurasia and
Gondwana. Gradually, life expands to land and all familiar forms of plants, animals and
541–
Phanerozoic fungi begin appearing, including annelids, insects and reptiles. Severalmass extinctions
present
occur, among which birds, the descendants of dinosaurs, and more recently mammals
emerge. Modern animals—including humans—evolve at the most recent phases of this
eon.
Geologic time scale
The history of the Earth can be organized chronologically according to the geologic time scale, which is split into intervals based on
stratigraphic analysis.[2][22] The following four timelines show the geologic time scale. The first shows the entire time from the
formation of the Earth to the present, but this gives little space for the most recent eon. Therefore, the second timeline shows an
expanded view of the most recent eon. In a similar way, the most recent era is expanded in the third timeline, and the most recent
period is expanded in the fourth timeline.
Millions of Years
