From Wikipedia, the free encyclopedia
Geological time put in a diagram called a
geological
clock, showing the relative lengths of the
eons
of the Earth's history.
The history of Earth covers approximately 4.6
billion years (4,567,000,000 years), from Earth’s
formation out of the solar
nebula to the present. This article presents a broad overview, summarizing
the leading, most current scientific theories.
[edit]
Origin
-
The Earth formed as part of the birth of the Solar
System: what eventually became the solar system initially existed as a
large, rotating cloud of dust,
rocks,
and gas. It was
composed of hydrogen
and helium
produced in the Big
Bang, as well as heavier elements
ejected by supernovas.
Then, as one theory suggests, about 4.6 billion years ago a nearby star
was destroyed in a supernova
and the explosion sent a shock
wave through the solar
nebula, causing it to gain angular
momentum. As the cloud began to accelerate its rotation,
gravity
and inertia
flattened it into a protoplanetary
disk oriented perpendicularly to its axis of rotation. Most of the mass
concentrated in the middle and began to heat up, but small perturbations
due to collisions and the angular momentum of other large debris created the
means by which protoplanets
began to form. The infall of material, increase in rotational speed and the
crush of gravity created an enormous amount of kinetic
heat at the center. Its inability to transfer that energy away through any
other process at a rate capable of relieving the build-up resulted in the
disk's center heating up. Ultimately, nuclear
fusion of hydrogen
into helium
began, and eventually, after contraction, a T
Tauri star, ignited to create the Sun.
Meanwhile, as gravity caused matter
to condense around the previously perturbed objects outside of the new sun's
gravity grasp, dust particles and the rest of the protoplanetary
disk began separating into rings. Successively larger fragments collided
with one another and became larger objects, ultimately destined to become
protoplanets.[1]
These included one collection approximately 150 million kilometers
from the center: Earth. The solar
wind of the newly formed T Tauri star cleared out most of the material in
the disk that had not already condensed into larger bodies.
Animation (not to scale) of
Theia
forming in Earth’s
L5
point and then, perturbed by gravity, colliding to help form the
moon. The animation progresses in one-year steps making Earth appear not
to move. The view is of the south pole.
-
The origin of the Moon
is still uncertain, although much evidence exists for the giant impact
hypothesis. Earth may not have been the only planet forming 150 million
kilometers from the Sun. It is hypothesized that another collection occurred
150 million kilometers from both the Sun and the Earth, at their fourth or
fifth Lagrangian
point. This planet, named Theia,
is thought to have been smaller than the current Earth, probably about the
size and mass of Mars.
Its orbit may at first have been stable, but destabilized as Earth increased
its mass by the accretion of more and more material. Theia swung back and
forth relative to Earth until, finally, an estimated 4.533 billion years ago,[2]
it collided at a low, oblique angle. The low speed and angle were not enough
to destroy Earth, but a large portion of its crust was ejected into space.
Heavier elements from Theia sank to Earth’s core, while the remaining
material and ejecta condensed into a single body within a couple of weeks.
Under the influence of its own gravity, this became a more spherical body: the
Moon.[3]
The impact is also thought to have changed Earth’s axis to produce the large
23.5° axial
tilt that is responsible for Earth’s seasons. (A simple, ideal model of
the planets’ origins would have axial tilts of 0° with no recognizable
seasons.) It may also have sped up Earth’s rotation and initiated the
planet’s plate
tectonics.
[edit]
The Hadean eon
-
Volcanic eruptions would have been common in Earth's early days.
The early Earth, during the very early Hadean
eon, was very different from the world known today. There were no oceans and
no oxygen in the atmosphere. It was bombarded by planetoids and other material
left over from the formation of the solar system. This bombardment, combined
with heat from radioactive breakdown, residual heat, and heat from the
pressure of contraction, caused the planet at this stage to be fully molten.
During the iron
catastrophe heavier elements sank to the center while lighter ones rose to
the surface producing the layered structure
of the Earth and also setting up the formation of Earth's
magnetic field. Earth's early atmosphere would have comprised surrounding
material from the solar nebula, especially light gases such as hydrogen
and helium,
but the solar
wind and Earth's own heat would have driven off this atmosphere.
This changed when Earth was about 40% its present radius, and gravitational
attraction allowed the retention of an atmosphere which included water.
Temperatures plummeted and the crust of the planet was accumulated on a solid
surface, with areas melted by large impacts on the scale of decades to
hundreds of years between impact. Large impacts would have caused localized
melting and partial differentiation, with some lighter elements on the surface
or released to the moist atmosphere. [4]
The surface cooled quickly, forming the solid crust
within 150 million years;[5]
although new research[6]
suggests that the actual number is 100 million years based on the level of hafnium
found in the geology at Jack
hills in Western Australia. From 4 to 3.8 billion years ago, Earth
underwent a period of heavy
asteroidal bombardment.[7]
Steam escaped
from the crust while more gases were released by volcanoes, completing the
second atmosphere.
Additional water was imported by bolide
collisions, probably from asteroids ejected from the outer asteroid belt under
the influence of Jupiter's gravity. The planet cooled. Clouds formed. Rain gave
rise to the oceans within 750 million years (3.8 billion years ago), but
probably earlier. Recent evidence suggests the oceans
may have begun forming by 4.2 billion years ago[8]
[9].
The new atmosphere probably contained ammonia,
methane, water
vapor, carbon
dioxide, and nitrogen,
as well as smaller amounts of other gases. Any free oxygen would have been
bound by hydrogen or minerals on the surface. Volcanic
activity was intense and, without an ozone
layer to hinder its entry, ultraviolet
radiation flooded the surface.
The replicator in virtually all known life is
deoxyribonucleic
acid. DNA is far more complex than the original replicator and its
replication systems are highly elaborate.
-
Main article: Origin
of life
The details of the origin of life are unknown, though the broad principles
have been established. Two schools of thought regarding the origin of life
have been proposed. The first suggest that organic components may have arrived
on Earth from
space (see “Panspermia”),
while the other argues for terrestrial origins. The mechanisms by which life
would initially arise are nevertheless held to be similar.[10]
If life arose on Earth, the timing of this event is highly
speculative—perhaps it arose around 4 billion years ago.[11]
In the energetic chemistry of early Earth, a molecule (or even something else)
gained the ability to make copies of itself–the replicator. The nature of
this molecule is unknown, its function having long since been superseded by
life’s current replicator, DNA.
In making copies of itself, the replicator did not always perform accurately:
some copies contained an “error.” If the change destroyed the copying
ability of the molecule, there could be no more copies, and the line would
“die out.” On the other hand, a few rare changes might make the molecule
replicate faster or better: those “strains” would become more numerous and
“successful.” As choice raw materials (“food”) became depleted,
strains which could exploit different materials, or perhaps halt the progress
of other strains and steal their resources, became more numerous.[12]
Several different models have been proposed explaining how a replicator
might have developed. Different replicators have been posited, including
organic chemicals such as modern proteins, nucleic acids, phospholipids,
crystals,[13]
or even quantum systems.[14]
There is currently no method of determining which of these models, if any,
closely fits the origin of life
on Earth. One of the older theories, and one which has been worked out in
some detail, will serve as an example of how this might occur. The high energy
from volcanoes, lightning,
and ultraviolet
radiation could help drive chemical reactions producing more complex
molecules from simple compounds such as methane
and ammonia.[15]
Among these were many of the relatively simple organic
compounds that are the building blocks of life. As the amount of this
“organic soup” increased, different molecules reacted with one another.
Sometimes more complex molecules would result—perhaps clay
provided a framework to collect and concentrate organic material.[16]
The presence of certain molecules could speed
up a chemical reaction. All this continued for a very long time, with
reactions occurring more or less at random, until by chance there arose a new
molecule: the replicator.
This had the bizarre property of promoting the chemical reactions which
produced a copy of itself, and evolution
began properly. Other theories posit a different replicator. In any case, DNA
took over the function of the replicator at some point; all known life (with
the exception of some viruses and prions)
use DNA as their replicator, in an almost identical manner (see genetic
code).
A small section of a cell membrane. This modern cell membrane is far
more sophisticated than the original simple phospholipid bilayer (the
small blue spheres with two tails). Proteins and
carbohydrates
serve various functions in regulating the passage of material through
the membrane and in reacting to the environment.
Modern life has its replicating material packaged neatly inside a cellular
membrane. It is easier to understand the origin of the cell membrane than
the origin of the replicator, since the phospholipid
molecules that make up a cell membrane will often form a bilayer
spontaneously when placed in water. Under certain conditions, many such
spheres can be formed (see “The
bubble theory”).[17]
It is not known whether this process preceded or succeeded the origin of the
replicator (or perhaps it was the replicator). The prevailing theory is
that the replicator, perhaps RNA
by this point (the RNA
world hypothesis), along with its replicating apparatus and maybe other
biomolecules, had already evolved. Initial protocells
may have simply burst when they grew too large; the scattered contents may
then have recolonized other “bubbles.” Proteins
that stabilized the membrane, or that later assisted in an orderly division,
would have promoted the proliferation of those cell lines. RNA is a likely
candidate for an early replicator since it can both store genetic information
and catalyze
reactions. At some point DNA
took over the genetic storage role from RNA, and proteins
known as enzymes
took over the catalysis role, leaving RNA to transfer information and modulate
the process. There is increasing belief that these early cells may have
evolved in association with underwater volcanic vents known as “black
smokers”.[18]
or even hot, deep rocks.[19]
However, it is believed that out of this multiplicity of cells, or protocells,
only one survived. Current evidence suggests that the last
universal common ancestor lived during the early Archean
eon, perhaps roughly 3.5 billion years ago or earlier.[20],[21]
This “LUCA” cell is the ancestor of all cells and hence all life on Earth.
It was probably a prokaryote,
possessing a cell membrane and probably ribosomes,
but lacking a nucleus
or membrane-bound organelles
such as mitochondria
or chloroplasts.
Like all modern cells, it used DNA as its genetic code, RNA for information
transfer and protein synthesis, and enzymes
to catalyze reactions. Some scientists believe that instead of a single
organism being the last universal common ancestor, there were populations of
organisms exchanging genes in lateral
gene transfer.[20]
[edit]
Photosynthesis and oxygen
The harnessing of the
sun’s
energy led to several major changes in life on Earth.
It is likely that the initial cells were all heterotrophs,
using surrounding organic molecules (including those from other cells) as raw
material and an energy source.[22]
As the food supply diminished, a new strategy evolved in some cells. Instead
of relying on the diminishing amounts of free-existing organic molecules,
these cells adopted sunlight
as an energy source. Estimates vary, but by about 3 billion years ago[23],
something similar to modern photosynthesis
had probably developed. This made the sun’s energy available not only to autotrophs
but also to the heterotrophs that consumed them. Photosynthesis used the
plentiful carbon
dioxide and water
as raw materials and, with the energy of sunlight, produced energy-rich
organic molecules (carbohydrates).
Moreover, oxygen
was produced as a waste product of photosynthesis. At first it became bound up
with limestone,
iron, and other
minerals. There is substantial proof of this in iron-oxide rich layers in
geological strata that correspond with this time period. The oceans would have
turned to a green color while oxygen was reacting with minerals. When the
reactions stopped, oxygen could finally enter the atmosphere. Though each cell
only produced a minute amount of oxygen, the combined metabolism of many cells
over a vast period of time transformed Earth’s atmosphere to its current
state.[24]
This, then, is Earth’s third atmosphere. Some of the oxygen was
stimulated by incoming ultraviolet radiation to form ozone,
which collected in a layer near the upper part of the atmosphere. The ozone
layer absorbed, and still absorbs, a significant amount of the ultraviolet
radiation that once had passed through the atmosphere. It allowed cells to
colonize the surface of the ocean and ultimately the land:[25]
without the ozone layer, ultraviolet radiation bombarding the surface would
have caused unsustainable levels of mutation
in exposed cells. Besides making large amounts of energy available to
life-forms and blocking ultraviolet radiation, the effects of photosynthesis
had a third, major, and world-changing impact. Oxygen was toxic; probably much
life on Earth died out as its levels rose (the “Oxygen
Catastrophe”).[25]
Resistant forms survived and thrived, and some developed the ability to use
oxygen to enhance their metabolism and derive more energy from the same food.
[edit]
Endosymbiosis and the three domains of life
-
Some of the pathways by which the various
endosymbionts
might have arisen.
Modern taxonomy
classifies life into three
domains. The time of the origin of these domains are speculative. The Bacteria
domain probably first split off from the other forms of life (sometimes called
Neomura),
but this supposition is controversial. Soon after this, by 2 billion years ago[26],
the Neomura split into the Archaea
and the Eukarya.
Eukaryotic cells (Eukarya) are larger and more complex than prokaryotic cells
(Bacteria and Archaea), and the origin of that complexity is only now coming
to light. Around this time period a bacterial
cell related to today’s Rickettsia[27]
entered a larger prokaryotic cell. Perhaps the large cell attempted to ingest
the smaller one but failed (maybe due to the evolution of prey defenses).
Perhaps the smaller cell attempted to parasitize the larger one. In any case,
the smaller cell survived inside the larger cell. Using oxygen,
it was able to metabolize the larger cell’s waste products and derive more
energy. Some of this surplus energy was returned to the host. The smaller cell
replicated inside the larger one, and soon a stable symbiotic
relationship developed. Over time the host cell acquired some of the genes of
the smaller cells, and the two kinds became dependent on each other: the
larger cell could not survive without the energy produced by the smaller ones,
and these in turn could not survive without the raw materials provided by the
larger cell. Symbiosis
developed between the larger cell and the population of smaller cells inside
it to the extent that they are considered to have become a single organism,
the smaller cells being classified as organelles
called mitochondria.
A similar event took place with photosynthetic
cyanobacteria[28]
entering larger heterotrophic
cells and becoming chloroplasts.[29],[30]
Probably as a result of these changes, a line of cells capable of
photosynthesis split off from the other eukaryotes some time before one
billion years ago. There were probably several such inclusion events, as the
figure at left suggests. Besides the well-established endosymbiotic theory of
the cellular origin of mitochondria and chloroplasts, it has been suggested
that cells gave rise to peroxisomes,
spirochetes
gave rise to cilia
and flagella,
and that perhaps a DNA
virus gave rise to the cell
nucleus,[31],[32]
though none of these theories are generally accepted.[33]
During this period, the supercontinent
Columbia
is believed to have existed, probably from around 1.8 to 1.5 billion years
ago; it is the oldest hypothesized supercontinent.[34]
[edit]
Multicellularity
Volvox
aureus is believed to be similar to the first multicellular plants.
Archaeans, bacteria, and eukaryotes continued to diversify and to become
more sophisticated and better adapted to their environments. Each domain
repeatedly split into multiple lineages, although little is known about the
history of the archaea and bacteria. Around 1.1 billion years ago, the supercontinent
Rodinia was
assembling.[35]
The plant, animal,
and fungi lines
had all split, though they still existed as solitary cells. Some of these
lived in colonies, and gradually some division
of labor began to take place; for instance, cells on the periphery might
have started to assume different roles from those in the interior. Although
the division between a colony with specialized cells and a multicellular
organism is not always clear, around 1 billion years ago[36],
the first multicellular
plants emerged, probably green
algae.[37]
Possibly by around 900 million years ago,[38]
true multicellularity had also evolved in animals. At first it probably
somewhat resembled that of today’s sponges,
where all cells were totipotent
and a disrupted organism could reassemble itself.[39]
As the division of labor became more complete in all lines of multicellular
organisms, cells became more specialized and more dependent on each other;
isolated cells would die. Many scientists believe that a very severe ice age
began around 770 million years ago, so severe that the surface of all the
oceans completely froze (Snowball
Earth). Eventually, after 20 million years, enough carbon dioxide escaped
through volcanic outgassing; the resulting greenhouse effect raised global
temperatures.[40]
By around the same time, 750 million years ago,[41]
Rodinia began to break up.
[edit]
Colonization of land
![For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars, one of Earth’s neighboring planets.[citation needed]](http://upload.wikimedia.org/wikipedia/commons/thumb/3/39/Mars_Twin_Peaks_(1024px).jpg/200px-Mars_Twin_Peaks_(1024px).jpg)
For most of Earth’s history, there were no multicellular organisms on
land. Parts of the surface may have vaguely resembled this view of
Mars,
one of Earth’s neighboring planets.
[citation
needed]
As we have already
seen, the accumulation of oxygen in Earth’s atmosphere caused the
formation of ozone
into a layer
that absorbed much of Sun’s ultraviolet
radiation. As a result, 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. Prokaryotes had likely colonized the land as early
as 2.6 billion years ago[42]
even before the origin of the eukaryotes. For a long time, the land remained
barren of multicellular organisms. The supercontinent Pannotia
formed around 600 million years ago and then broke apart a short 50 million
years later[43].
Fish, the earliest
vertebrates,
evolved in the oceans around 530 million years ago[44].
A major extinction
event occurred near the end of the Cambrian period,[45]
which ended 488 million years ago[46].
Several hundred million years ago, plants (probably resembling algae)
and fungi started growing at the edges of the water, and then out of it.[47]
The oldest fossils of land fungi and plants date to 480–460 million years
ago, though molecular evidence suggests the fungi may have colonized the land
as early as 1000 million years ago and the plants 700 million years ago.[48]
Initially remaining close to the water’s edge, mutations and variations
resulted in further colonization of this new environment. The timing of the
first animals to leave the oceans is not precisely known: the oldest clear
evidence is of arthropods
on land around 450 million years ago[49],
perhaps thriving and becoming better adapted due to the vast food source
provided by the terrestrial plants. There is also some unconfirmed evidence
that arthropods may have appeared on land as early as 530 million years ago[50].
At the end of the Ordovician
period, 440 million years ago, additional extinction
events occurred, perhaps due to a concurrent ice
age.[51]
Around 380 to 375 million years ago, the first tetrapods
evolved from fish.[52]
It is thought that perhaps fins evolved to become limbs which allowed the
first tetrapods to lift their heads out of the water to breathe air.
This would let them survive in oxygen-poor water or pursue small prey in
shallow water.[52]
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 million years ago, another period
of extinction occurred, perhaps as a result of global cooling.[53]
Plants evolved seeds,
which dramatically accelerated their spread on land, around this time (by
approximately 360 million years ago).[54],
[55]
Pangaea,
the most recent supercontinent, existed from 300 to 180 million years
ago. The outlines of the modern continents and other land masses are
indicated on this map.
Some 20 million years later (340 million years ago[56]),
the amniotic
egg evolved, which could be laid on land, giving a survival advantage to tetrapod
embryos. This resulted in the divergence of amniotes
from amphibians.
Another 30 million years (310 million years ago[57])
saw the divergence of the synapsids
(including mammals)
from the sauropsids
(including birds
and non-avian, non-mammalian reptiles).
Other groups of organisms continued to evolve and lines diverged—in fish,
insects, bacteria, and so on—but less is known of the details. 300 million
years ago, the most recent hypothesized supercontinent formed, called Pangaea.
The most
severe extinction event to date took place 250 million years ago, at the
boundary of the Permian
and Triassic
periods; 95% of life on Earth died out,[58]
possibly due to the Siberian
Traps volcanic event. The discovery of the Wilkes
Land crater in Antarctica may suggest a connection with the
Permian-Triassic extinction, but the age of that crater is not known.[59]
But life persevered, and around 230 million years ago [60],
dinosaurs
split off from their reptilian ancestors. An extinction event between the Triassic
and Jurassic periods 200 million years ago spared many of the dinosaurs,[61]
and they soon became dominant among the vertebrates. Though some of the
mammalian lines began to separate during this period, existing mammals were
probably all small animals resembling shrews.[62]
By 180 million years ago, Pangaea broke up into Laurasia
and Gondwana.
The boundary between avian and non-avian dinosaurs is not clear, but Archaeopteryx,
traditionally considered one of the first birds,
lived around 150 million years ago.[63]
The earliest evidence for the angiosperms
evolving flowers
is during the Cretaceous
period, some 20 million years later (132 million years ago)[64]
Competition with birds drove many pterosaurs
to extinction, and the dinosaurs were probably already in decline for various
reasons[65]
when, 65 million years ago, a 10-kilometer meteorite
likely struck Earth just off the Yucatán
Peninsula, ejecting vast quantities of particulate matter and vapor into
the air that occluded sunlight, inhibiting photosynthesis. Most large animals,
including the non-avian dinosaurs, became
extinct.[66],
marking the end of the Cretaceous period and Mesozoic
era. Thereafter, in the Paleocene
epoch, mammals rapidly diversified, grew larger, and became the dominant
vertebrates. Perhaps a couple of million years later (around 63 million years
ago), the last common ancestor of primates
lived.[67]
By the late Eocene
epoch, 34 million years ago, some terrestrial mammals had returned to the
oceans to become animals such as Basilosaurus
which later gave rise to dolphins
and whales.[68]
[edit]
Humanity
-
A small African ape living around six million years ago was the last animal
whose descendants would include both modern humans and their closest
relatives, the bonobos,
and chimpanzees.[69]
Only two branches of its family tree have surviving descendants. Very soon
after the split, for reasons that are still debated, apes in one branch
developed the ability to walk
upright.[70]
Brain size
increased rapidly, and by 2 million years ago, the very first animals
classified in the genus Homo
had appeared.[71]
Of course, the line between different species or even genera is rather
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.[69]
The ability to control fire
likely began in Homo
erectus (or Homo
ergaster), probably at least 790,000 years ago[72]
but perhaps as early as 1.5 million years ago.[73]
It is more difficult to establish the origin
of language; it is unclear whether Homo erectus could speak or if
that capability had not begun until Homo sapiens.[74]
As brain size increased, babies were born sooner, before their heads grew too
large to pass through the pelvis.
As a result, they exhibited more plasticity,
and thus possessed an increased capacity to learn and required a longer period
of dependence. Social skills became more complex, language became more
advanced, and tools became more elaborate. This contributed to further
cooperation and brain development.[75]
Anatomically modern humans — Homo
sapiens — are believed to have originated somewhere around 200,000
years ago or earlier in
Africa; the oldest fossils date back to around 160,000 years ago.[76]
The first humans to show evidence of spirituality
are the Neanderthals
(usually classified as a separate species with no surviving descendants); they
buried their dead, often apparently with food or tools.[77]
However, evidence of more sophisticated beliefs, such as the early Cro-Magnon
cave
paintings (probably with magical or religious significance)[78]
did not appear until some 32,000 years ago.[79]
Cro-Magnons also left behind stone figurines such as Venus
of Willendorf, probably also signifying religious belief.[78]
By 11,000 years ago, Homo sapiens had reached the southern tip of South
America, the last of the uninhabited continents.[80]
Tool use and language continued to improve; interpersonal relationships became
more complex.
[edit]
Civilization
-
Throughout more than 90% of its history, Homo sapiens lived in small
bands as nomadic hunter-gatherers.[81]
As language became more complex, the ability to remember and transmit
information resulted in a new sort of replicator: the meme.[82]
Ideas could be rapidly exchanged and passed down the generations. Cultural
evolution quickly outpaced biological
evolution, and history proper began. Somewhere between 8500 and 7000 BC,
humans in the Fertile
Crescent in Middle
East began the systematic husbandry of plants and animals: agriculture.[83]
This spread to neighboring regions, and also developed independently
elsewhere, until most Homo sapiens lived sedentary lives in permanent
settlements as farmers. Not all societies abandoned nomadism, especially those
in isolated areas of the globe poor in domesticable
plant species, such as Australia.[84]
However, among those civilizations that did adopt agriculture, the relative
security and increased productivity provided by farming allowed the population
to expand. Agriculture had a major impact; humans began to affect the
environment as never before. Surplus food allowed a priestly or governing
class to arise, followed by increasing division
of labor. This led to Earth’s first civilization
at Sumer in the
Middle
East, between 4000 and 3000 BC.[85]
Additional civilizations quickly arose in ancient
Egypt and the Indus
River valley.
Starting around 3000 BC, Hinduism,
one of the oldest religions still practiced today, began to take form.[86]
Others soon followed. The invention of writing
enabled complex societies to arise: record-keeping and libraries
served as a storehouse of knowledge and increased the cultural transmission of
information. Humans no longer had to spend all their time working for
survival—curiosity and education drove the pursuit of knowledge and wisdom.
Various disciplines, including science
(in a primitive form), arose. New civilizations sprang up, traded with one
another, and engaged in war
for territory and resources: empires
began to form. By around 500 BC, there were empires in the Middle East, Iran,
India, China, and Greece, approximately on equal footing; at times one empire
expanded, only to decline or be driven back later.[87]
In the fourteenth
century, the Renaissance
began in Italy
with advances in religion, art, and science.[88]
Starting around 1500, European civilization began to undergo changes leading
to the scientific
and industrial
revolutions: that continent began to exert political and cultural dominance
over human societies around the planet.[89]
From 1914 to 1918 and 1939 to 1945, nations around the world were embroiled in
world wars.
Established following World
War I, the League
of Nations was a first step toward a world
government; after World
War II it was replaced by the United
Nations. In 1992, several European nations joined together in the European
Union. As transportation and communication improved, the economies and
political affairs of nations around the world have become increasingly
intertwined. This globalization
has often produced discord, although increased collaboration has resulted as
well.
[edit]
Recent events
Four and a half billion years after the planet's formation, one of
Earth’s life forms broke free of the
biosphere.
For the first time in
history,
Earth was viewed first hand from the vantage of space.
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 more recently, global
warming have risen as the world population increases.
In 1957, the Soviet
Union launched the
first artificial satellite into orbit and, soon afterward, Yuri
Gagarin became the first human in space. Neil
Armstrong, an American,
was the first to set foot on another astronomical object, the Earth's Moon.
Unmanned probes have been sent to all the major planets in the solar system,
with some (such as Voyager)
in the process of leaving the solar system. The Soviet
Union and the United
States of America were the primary early leaders in space exploration in
the 20th Century. Five space agencies, representing over fifteen countries,[90]
have worked together to build the International
Space Station. Aboard it, there has been a continuous human presence in
space since 2000.[91]
[edit]
See also
[edit]
External links
[edit]
References
- ^ Chaisson, Eric J.
(2005). Solar
System Modeling. Cosmic
Evolution. Tufts
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From Wikipedia, the free encyclopedia
Diagram of geological time scale.
The geological time scale is used by geologists
and other scientists
to describe the timing and relationships between events that have occurred
during the history
of Earth. The table of geologic
periods presented here agrees with the dates and nomenclature
proposed by the International
Commission on Stratigraphy, and uses the standard color codes of the United
States Geological Survey.
Evidence from radiometric
dating indicates that the Earth
is about 4.570 billion years old. The geological or deep
time of Earth's past has been organized into various units according
to events which took place in each period. Different spans of time on the time
scale are usually delimited by major geological
or paleontological
events, such as mass
extinctions. For example, the boundary between the Cretaceous
period and the Paleogene
period is defined by the extinction
event, known as the Cretaceous–Tertiary
extinction event, that marked the demise of the dinosaurs
and of many marine species.
Older periods which predate the reliable fossil record are defined by absolute
age.
[edit]
Graphical timelines
The second and third timelines are each subsections of their preceding
timeline as indicated by asterisks.
Millions of Years
The Holocene
(the latest epoch)
is too small to be shown clearly on this timeline.
[edit]
Terminology
The largest defined unit of time is the supereon composed of Eons.
Eons are divided into Eras,
which are in turn divided into Periods,
Epochs
and Stages.
At the same time paleontologists define a system of faunal
stages, of varying lengths, based on changes in the observed fossil
assemblages. In many cases, such faunal stages have been adopted in building
the geological nomenclature, though in general there are far more recognized
faunal stages than defined geological time units.
Geologists
tend to talk in terms of Upper/Late, Lower/Early and Middle parts of periods
and other units , such as "Upper Jurassic",
and "Middle Cambrian".
Upper, Middle, and Lower are terms applied to the rocks
themselves, as in "Upper Jurassic sandstone,"
while Late, Middle, and Early are applied to time, as in
"Early Jurassic deposition"
or "fossils
of Early Jurassic age." The adjectives are capitalized when the
subdivision is formally recognized, and lower case when not; thus "early
Miocene" but "Early Jurassic." Because geologic units occurring
at the same time but from different parts of the world can often look
different and contain different fossils, there are many examples where the
same period was historically given different names in different locales. For
example, in North
America the Lower Cambrian
is referred to as the Waucoban
series that is then subdivided into zones based on trilobites.
The same timespan is split into Tommotian,
Atdabanian
and Botomian
stages in East
Asia and Siberia.
A key aspect of the work of the International Commission on Stratigraphy is to
reconcile this conflicting terminology and define universal horizons that can
be used around the world.
[edit]
History of the time scale
-
-
Earth history mapped to 24 hours
The principles underlying geologic (geological) time scales were laid down
by Nicholas
Steno in the late 17th century. Steno argued that rock layers (or strata)
are laid down in succession, and that each represents a "slice" of
time. He also formulated the principle
of superposition, which states that any given stratum is probably older
than those above it and younger than those below it. While Steno's principles
were simple, applying them to real rocks proved complex. Over the course of
the 18th century geologists realized that:
- Sequences of strata were often eroded, distorted, tilted, or even
inverted after deposition;
- Strata laid down at the same time in different areas could have entirely
different appearances;
- The strata of any given area represented only part of the Earth's long
history.
The first serious attempts to formulate a geological time scale that could
be applied anywhere on Earth
took place in the late 18th century. The most influential of those early
attempts (championed by Abraham
Werner, among others) divided the rocks of the Earth's crust
into four types: Primary, Secondary, Tertiary, and Quaternary. Each type of
rock, according to the theory, formed during a specific period in Earth
history. It was thus possible to speak of a "Tertiary Period" as
well as of "Tertiary Rocks." Indeed, "Tertiary" (now
Paleocene-Pliocene) and "Quaternary" (now Pleistocene-Holocene)
remained in use as names of geological periods well into the 20th century.
In opposition to the then-popular Neptunist
theories expounded by Werner (that all rocks had precipitated out of a single
enormous flood), a major shift in thinking came with the reading by James
Hutton of his Theory of the Earth; or, an Investigation of the Laws
Observable in the Composition, Dissolution, and Restoration of Land Upon the
Globe before the Royal
Society of Edinburgh in March and April 1785, events which "as things
appear from the perspective of the twentieth century, James Hutton in those
reading became the founder of modern geology"[1]
What Hutton proposed was that the interior of the Earth was hot, and that this
heat was the engine which drove the creation of new rock: land was eroded by
air and water and deposited as layers in the sea; heat then consolidated the
sediment into stone, and uplifted it into new lands. This theory was dubbed
"Plutonist" in contrast to the flood-oriented theory.
The identification of strata by the fossils they contained, pioneered by William
Smith, Georges
Cuvier, Jean
d'Omalius d'Halloy and Alexandre
Brogniart in the early 19th century, enabled geologists to divide Earth
history more precisely. It also enabled them to correlate strata across
national (or even continental) boundaries. If two strata (however distant in
space or different in composition) contained the same fossils, chances were
good that they had been laid down at the same time. Detailed studies between
1820 and 1850 of the strata and fossils of Europe
produced the sequence of geological periods still used today.
The process was dominated by British
geologists, and the names of the periods reflect that dominance. The
"Cambrian," (the Roman name for Wales)
and the "Ordovician," and "Silurian", named after ancient Welsh
tribes, were periods defined using stratigraphic sequences from Wales.[2]
The "Devonian" was named for the English
county
of Devon, and
the name "Carboniferous" was simply an adaptation of "the Coal
Measures," the old British geologists' term for the same set of strata.
The "Permian" was named after Perm,
Russia,
because it was defined using strata in that region by a Scottish
geologist Roderick
Murchison. However, some periods were defined by geologists from other
countries. The "Triassic" was named in 1834 by a German geologist Friedrich
Von Alberti from the three distinct layers (Latin
trias meaning triad) —red
beds, capped by chalk,
followed by black shales—
that are found throughout Germany
and Northwest
Europe, called the 'Trias'. The "Jurassic" was named by a French
geologist Alexandre
Brogniart for the extensive marine limestone
exposures of the Jura
Mountains. The "Cretaceous" (from Latin creta meaning 'chalk')
as a separate period was first defined by a Belgian
geologist Jean
d'Omalius d'Halloy in 1822, using strata in the Paris
basin[3]
and named for the extensive beds of chalk (calcium
carbonate deposited by the shells of marine invertebrates).
British geologists were also responsible for the grouping of periods into
Eras and the subdivision of the Tertiary and Quaternary periods into epochs.
When William
Smith and Sir
Charles Lyell first recognized that rock
strata represented successive time periods, time scales could be estimated
only very imprecisely since various kinds of rates of change used in
estimation were highly variable. While creationists
had been proposing dates of around six or seven thousand years for the age
of the Earth based on the Bible,
early geologists were suggesting millions of years for geologic periods with
some even suggesting a virtually infinite age for the Earth. Geologists and paleontologists
constructed the geologic table based on the relative positions of different
strata and fossils,
and estimated the time scales based on studying rates of various kinds of weathering,
erosion, sedimentation,
and lithification.
Until the discovery of radioactivity
in 1896 and the development of its geological applications through radiometric
dating during the first half of the 20th century (pioneered by such
geologists as Arthur
Holmes) which allowed for more precise absolute dating of rocks, the ages
of various rock strata and the age of the Earth
were the subject of considerable debate.
In 1977, the Global Commission on Stratigraphy (now the International
Commission on Stratigraphy) started an effort to define global references
(Global
Boundary Stratotype Sections and Points) for geologic periods and faunal
stages. The commission's most recent work is described in the 2004 geologic
time scale of Gradstein et al.[4].
A UML model for how the timescale is structured, relating it to the GSSP, is
also available[5].
[edit]
Table of geologic time
The following table summarizes the major events and characteristics of the
periods of time making up the geologic time scale. As above, this time scale
is based on the International Commission on Stratigraphy. (See lunar
geologic timescale for a discussion of the geologic subdivisions of
Earth's moon.) The height of each table entry does not correspond to the
duration of each subdivision of time.
| Supereon |
Eon |
Era |
Period[6] |
Series/
Epoch |
Major events |
Start, million
years ago[7] |
| |
Phanerozoic |
Cenozoic |
Neogene
[8] |
Holocene |
The last
glacial period ends and rise of human civilization. |
0.011430 ± 0.00013[9] |
| Pleistocene |
Flourishing and then extinction of many large mammals
(Pleistocene
megafauna). Evolution of anatomically modern humans. |
1.806 ± 0.005 * |
| Pliocene |
Cool and dry climate.
Australopithecines,
many of the existing genera of mammals, and recent mollusks
appear. Homo
habilis appears. Present
ice age begins. |
5.332 ± 0.005 * |
| Miocene |
Moderate climate; Orogeny
in northern
hemisphere. Modern mammal
and bird
families became recognizable. Horses
and mastodons
diverse. First kelp
forests, grasses
become ubiquitous. First apes
appear. |
23.03 ± 0.05 * |
Paleogene
[8] |
Oligocene |
Warm climate; Rapid evolution
and diversification of fauna, especially mammals.
Major evolution and dispersal of modern types of flowering
plants |
33.9±0.1 * |
| Eocene |
Archaic mammals
(e.g. Creodonts,
Condylarths,
Uintatheres,
etc) flourish and continue to develop during the epoch. Appearance of
several "modern" mammal families. Primitive whales
diversify. First grasses.
Ice cap
develops on Antarctica. |
55.8±0.2 * |
| Paleocene |
Climate tropical. Modern plants
appear; Mammals
diversify into a number of primitive lineages following the Cretaceous–Tertiary
extinction event. First large mammals (up to bear or small hippo
size). |
65.5±0.3 * |
| Mesozoic |
Cretaceous |
Upper/Late |
Flowering
plants proliferate, along with new types of insects.
More modern teleost
fish begin to appear. Ammonites,
belemnites,
rudist
bivalves,
echinoids
and sponges
all common. Many new types of dinosaurs
(e.g. Tyrannosaurs,
Titanosaurs,
duck
bills, and horned
dinosaurs) evolve on land, as do modern
crocodilians; and mosasaurs
and modern sharks appear in the sea. Primitive birds
gradually replace pterosaurs. Monotremes,
marsupials
and placental
mammals appear. Break up of Gondwana. |
99.6±0.9 * |
| Lower/Early |
145.5 ± 4.0 |
| Jurassic |
Upper/Late |
Gymnosperms
(especially conifers,
Bennettitales
and cycads)
and ferns
common. Many types of dinosaurs,
such as sauropods,
carnosaurs,
and stegosaurs.
Mammals common but small. First birds
and lizards.
Ichthyosaurs
and plesiosaurs
diverse. Bivalves,
Ammonites
and belemnites
abundant. Sea
urchins very common, along with crinoids,
starfish, sponges,
and terebratulid
and rhynchonellid
brachiopods.
Breakup of Pangaea
into Gondwana
and Laurasia. |
161.2 ± 4.0 |
| Middle |
175.6 ± 2.0 * |
| Lower/Early |
199.6 ± 0.6 |
| Triassic |
Upper/Late |
Archosaurs
dominant on land as dinosaurs,
in the oceans as Ichthyosaurs
and nothosaurs,
and in the air as pterosaurs.
cynodonts
become smaller and more mammal-like, while first mammals
and crocodilia
appear. Dicrodium
flora common on land. Many large aquatic temnospondyl
amphibians. Ceratitic
ammonoids extremely common. Modern
corals and teleost
fish appear, as do many modern insect
clades. |
228.0 ± 2.0 |
| Middle |
245.0 ± 1.5 |
| Lower/Early |
251.0 ± 0.4 * |
| Paleozoic |
Permian |
Lopingian |
Landmasses unite into supercontinent Pangaea,
creating the Appalachians.
End of Permo-Carboniferous glaciation. Synapsid
reptiles
(pelycosaurs
and therapsids)
become plentiful, while parareptiles
and temnospondyl
amphibians
remain common. In the mid-Permian, coal-age
flora are replaced by cone-bearing
gymnosperms
(the first true seed
plants) and by the first true mosses.
Beetles
and flies
evolve. Marine life flourishes in warm shallow reefs; productid
and spiriferid
brachiopods, bivalves, forams,
and ammonoids
all abundant. Permian-Triassic
extinction event occurs 251 mya: 95 percent of life on Earth
becomes extinct, including all trilobites,
graptolites,
and blastoids. |
260.4 ± 0.7 * |
| Guadalupian |
270.6 ± 0.7 * |
| Cisuralian |
299.0 ± 0.8 * |
Carbon-
iferous[10]/
Pennsyl-
vanian |
Upper/Late |
Winged
insects radiate suddenly; some (esp. Protodonata
and Palaeodictyoptera)
are quite large. Amphibians
common and diverse. First reptiles
and coal
forests (scale
trees, ferns, club
trees, giant
horsetails, Cordaites,
etc.). Highest-ever oxygen
levels. Goniatites,
brachiopods, bryozoa, bivalves, and corals plentiful in the seas.
Testate forams
proliferate. |
306.5 ± 1.0 |
| Middle |
311.7 ± 1.1 |
| Lower/Early |
318.1 ± 1.3 * |
Carbon-
iferous[10]/
Missis-
sippian |
Upper/Late |
Large primitive
trees, first land
vertebrates, and amphibious sea-scorpions
live amid coal-forming
coastal swamps.
Lobe-finned rhizodonts
are big fresh-water predators. In the oceans, early sharks
are common and quite diverse; echinoderms
(esp. crinoids
and blastoids)
abundant. Corals,
bryozoa,
goniatites
and brachiopods (Productida,
Spiriferida,
etc.) very common. But trilobites
and nautiloids
decline. Glaciation
in East Gondwana. |
326.4 ± 1.6 |
| Middle |
345.3 ± 2.1 |
| Lower/Early |
359.2 ± 2.5 * |
| Devonian |
Upper/Late |
First clubmosses,
horsetails
and ferns
appear, as do the first seed-bearing
plants (progymnosperms),
first trees
(the progymnosperm Archaeopteris),
and first (wingless) insects.
Strophomenid
and atrypid
brachiopods,
rugose
and tabulate
corals, and crinoids
are all abundant in the oceans. Goniatite
ammonoids
are plentiful, while squid-like coleoids
arise. Trilobites and armoured agnaths decline, while jawed fishes (placoderms,
lobe-finned
and ray-finned
fish, and early sharks)
rule the seas. First amphibians
still aquatic. "Old Red Continent" of Euramerica. |
385.3 ± 2.6 * |
| Middle |
397.5 ± 2.7 * |
| Lower/Early |
416.0 ± 2.8 * |
| Silurian |
Pridoli |
First Vascular
plants (the rhyniophytes
and their relatives), first millipedes
and arthropleurids
on land. First jawed
fishes, as well as many armoured
jawless
fish, populate the seas. Sea-scorpions
reach large size. Tabulate
and rugose
corals, brachiopods
(Pentamerida, Rhynchonellida,
etc.), and crinoids
all abundant. Trilobites
and mollusks
diverse; graptolites
not as varied. |
418.7 ± 2.7 * |
| Upper/Late (Ludlow) |
422.9 ± 2.5 * |
| Wenlock |
428.2 ± 2.3 * |
| Lower/Early (Llandovery) |
443.7 ± 1.5 * |
| Ordovician |
Upper/Late |
Invertebrates
diversify into many new types (e.g., long straight-shelled
cephalopods).
Early corals,
articulate brachiopods
(Orthida, Strophomenida, etc.), bivalves,
nautiloids,
trilobites,
ostracods,
bryozoa,
many types of echinoderms
(crinoids,
cystoids,
starfish,
etc.), branched graptolites,
and other taxa all common. Conodonts
(early planktonic
vertebrates)
appear. First green
plants and fungi
on land. Ice age at end of period. |
460.9 ± 1.6 * |
| Middle |
471.8 ± 1.6 |
| Lower/Early |
488.3 ± 1.7 * |
| Cambrian |
Upper/Late (Furongian) |
Major diversification of life in the Cambrian
Explosion. Many fossils; most modern animal
phyla
appear. First chordates
appear, along with a number of extinct, problematic phyla.
Reef-building Archaeocyatha
abundant; then vanish. Trilobites,
priapulid
worms, sponges,
inarticulate brachiopods
(unhinged lampshells), and many other animals numerous. Anomalocarids
are giant predators, while many Ediacaran fauna die out. Prokaryotes,
protists
(e.g., forams),
fungi
and algae
continue to present day. Gondwana
emerges. |
501.0 ± 2.0 * |
| Middle |
513.0 ± 2.0 |
| Lower/Early |
542.0 ± 0.3 * |
Preca-
mbrian
[11] |
Proter-
ozoic
[12] |
Neo-
proterozoic |
Ediacaran |
Good fossils
of multi-celled
animals. Ediacaran
biota flourish worldwide in seas. Trace
fossils of possible worm-like Trichophycus, etc. First sponges
and trilobitomorphs.
Enigmatic forms include many soft-jellied creatures shaped like bags,
disks, or quilts (like Dickinsonia). |
630
+5/-30 *
|
| Cryogenian |
Possible "snowball
Earth" period. Fossils
still rare. Rodinia
landmass begins to break up. |
850 [13] |
| Tonian |
Rodinia
supercontinent persists. Trace
fossils of simple multi-celled
eukaryotes.
First radiation of dinoflagellate-like
acritarchs. |
1000 [13] |
Meso-
proterozoic |
Stenian |
Narrow highly metamorphic
belts due to orogeny
as supercontinent Rodinia
is formed. |
1200 [13] |
| Ectasian |
Platform
covers continue to expand. Green
algae colonies
in the seas. |
1400 [13] |
| Calymmian |
Platform
covers expand. |
1600 [13] |
Paleo-
proterozoic |
Statherian |
First complex
single-celled life: protists
with nuclei. Columbia
is the primordial supercontinent. |
1800 [13] |
| Orosirian |
The atmosphere
became oxygenic.
Vredefort
and Sudbury
Basin asteroid impacts. Much orogeny. |
2050 [13] |
| Rhyacian |
Bushveld
Formation occurs. Huronian
glaciation. |
2300 [13] |
| Siderian |
Oxygen
Catastrophe: banded
iron formations result. |
2500 [13] |
Archean
[12] |
Neoarchean |
Stabilization of most modern cratons;
possible mantle
overturn event. |
2800 [13] |
| Mesoarchean |
First stromatolites
(probably colonial
cyanobacteria).
Oldest macrofossils. |
3200 [13] |
| Paleoarchean |
First known oxygen-producing
bacteria.
Oldest definitive microfossils. |
3600 [13] |
| Eoarchean |
Simple
single-celled life (probably bacteria
and perhaps archaea).
Oldest probable microfossils. |
3800 |
Hadean
[12][14] |
Lower
Imbrian[15] |
This era overlaps the end of
the Late
Heavy Bombardment of the inner solar system. |
3850 |
| Nectarian[15] |
This era gets its name from
the lunar
geologic timescale when the Nectaris
Basin and other major lunar
basins were formed by large impact
events. |
3920 |
| Basin
Groups[15] |
The first Lifeforms self
replicating RNA
molecules may have evolved on earth around 4 bya during this era. |
4150 |
| Cryptic
era[15] |
Formation of earth
begun close to 4567.17 mya.
Oldest known mineral,
zircon
(4400 mya). |
c.4567.17 |
[edit]
References and footnotes
- ^ John McPhee, Basin
and Range, New York:Farrar, Straus and Giroux, 1981, pp.95-100.
- ^ John McPhee, Basin
and Range, pp.113-114.
- ^ (1974)
Great
Soviet Encyclopedia, 3rd ed. (in Russian), Moscow: Sovetskaya
Enciklopediya, vol. 16, p. 50.
- ^ Felix M. Gradstein,
James G. Ogg, Alan G. Smith (Editors); A Geologic Time Scale 2004,
Cambridge University Press, 2005, (ISBN
0-521-78673-8)
- ^ Cox & Richard, A
formal model for the geologic time scale and global stratotype section
and point, compatible with geospatial information transfer standards,
Geosphere,
volume 1, pp 119-137, Geological Society of America, 2005
- ^
Paleontologists often refer to faunal
stages rather than geologic (geological) periods. The stage
nomenclature is quite complex. See The
Paleobiology Database. Retrieved on 2006-03-19.
for an excellent time ordered list of faunal stages.
- ^ Dates are slightly
uncertain with differences of a few percent between various sources
being common. This is largely due to uncertainties in radiometric
dating and the problem that deposits suitable for radiometric dating
seldom occur exactly at the places in the geologic column where they
would be most useful. The dates and errors quoted above are according to
the International
Commission on Stratigraphy 2004 time scale. Dates labeled with a *
indicate boundaries where a Global
Boundary Stratotype Section and Point has been internationally
agreed upon: see List
of Global Boundary Stratotype Sections and Points for a complete
list.
- ^ a
b
Historically, the Cenozoic
has been divided up into the Quaternary
and Tertiary
sub-eras, as well as the Neogene
and Paleogene
periods. However, the International Commission on Stratigraphy has
recently decided to stop endorsing the terms Quaternary and Tertiary as
part of the formal nomenclature.
- ^ The start
time for the Holocene
epoch is here given as 11,430 years
ago ± 130 years (that is, between 9610 B.C. and 9350 B.C.). For
further discussion of the dating of this epoch, see Holocene.
- ^ a
b In North
America, the Carboniferous is subdivided into Mississippian
and Pennsylvanian
Periods.
- ^ the Precambrian
is also known as Cryptozoic.
- ^ a
b c
The Proterozoic,
Archean
and Hadean
are often collectively referred to as the Precambrian
or Cryptozoic.
- ^ a
b c
d e
f g
h i
j k
l Defined by
absolute age (Global
Standard Stratigraphic Age).
- ^ Though
commonly used, the Hadean
is not a formal eon and no lower bound for the Archean has been agreed
upon. The Hadean has also sometimes been called the Priscoan or the
Azoic. Sometimes, the Hadean can be found to be subdivided according to
the lunar
geologic time scale. These eras include the Cryptic
and Basin
Groups (which are subdivisions of the pre-Nectarian
era), Nectarian,
and Lower
Imbrian eras.
- ^ a
b c
d Since
little or no geological
evidence on Earth
exists from the time spanned by the Hadean Eon, Eras of the Moon have
been used by at least one notable scientific work as unofficial
subdivisions of the terrestrial Hadean
eon.
(W. Harland, R. Armstrong, A. Cox, L. Craig, A. Smith, D. Smith (1990).
A Geologic time scale 1989. Cambridge University Press.)
[edit]
See also
[edit]
External links
![For most of Earth’s history, there were no multicellular organisms on land. Parts of the surface may have vaguely resembled this view of Mars, one of Earth’s neighboring planets.[citation needed]](http://en.wikipedia.org/wiki/Image:Mars_Twin_Peaks_(1024px).jpg)
