The details of the origin of life are unknown, but the basic
principles have been established. There are basically two schools of
thought which are further divided into many about the origin of life.
One suggests that organic components arrived on Earth from space, while
the other argues that they originated on Earth.
10. Panspermia
Panspermia is the hypothesis that life exists throughout the
Universe, distributed by meteoroids, asteroids and planetoids.
Panspermia proposes that life that can survive the effects of space,
such as extremophile bacteria, become trapped in debris that is ejected
into space after collisions between planets that harbor life and Small
Solar System Bodies (SSSB). Bacteria may travel dormant for an extended
amount of time before colliding randomly with other planets or
intermingling with protoplanetary disks. If met with ideal conditions on
a new planets’ surfaces, the bacteria become active and the process of
evolution begins.
Recent probes inside comets show it is overwhelmingly likely that life began in space, according to a new paper by Cardiff University scientists.
9. Biopoesis
In natural science, abiogenesis or biopoesis is the study of how
biological life arises from inorganic matter through natural processes,
and the method by which life on Earth arose. Most amino acids, often
called “the building blocks of life”, can form via natural chemical
reactions unrelated to life, as demonstrated in the Miller–Urey experiment
and similar experiments that involved simulating some of the conditions
of the early Earth in a laboratory. In all living things, these amino
acids are organized into proteins, and the construction of these
proteins is mediated by nucleic acids, that are themselves synthesized
through biochemical pathways catalysed by proteins. Which of these
organic molecules first arose and how they formed the first life is the
focus of abiogenesis.
8. Cosmogeny
Cosmogeny, is any theory concerning the coming into existence or
origin of the universe, or about how reality came to be. In the
specialized context of space science and astronomy, the term refers to
theories of creation of (and study of) the Solar System. Attempts to
create a naturalistic cosmogony are subject to two separate limitations.
One is based in the philosophy of science and the epistemological
constraints of science itself, especially with regards to whether
scientific inquiry can ask questions of “why” the universe exists.
Another more pragmatic problem is that there is no physical model that
can explain the earliest moments of the universe’s existence because of
a lack of a testable theory of quantum gravity, although string
theorists and researchers in loop quantum cosmology believe they have the formulas to describe it within their field equations.
7. Endosymbiosis
The endosymbiotic
theory was first articulated by the Russian botanist Konstantin
Mereschkowski in 1905. According to this theory, certain organelles
originated as free-living bacteria that were taken inside another cell
as endosymbionts. Mitochondria developed from proteobacteria (in
particular, Rickettsiales or close relatives) and chloroplasts from
cyanobacteria. It suggests that multiple forms of bacteria entered into
symbiotic relationship to form the eukaryotic cell. The horizontal
transfer of genetic material between bacteria promotes such symbiotic
relationships, and thus many separate organisms may have contributed to
building what has been recognised as the Last Universal Common
Ancestor (LUCA) of modern organisms.
6. Spontaneous Generation
Until the early 19th century, people generally believed in the
ongoing spontaneous generation of certain forms of life from non-living
matter. This was paired with the belief in heterogenesis, e.g. that one
form of life derived from a different form (e.g. bees from
flowers). Classical notions of spontaneous generation, held that certain
complex, living organisms are generated by decaying organic substances.
According to Aristotle it was a readily observable truth that aphids
arise from the dew which falls on plants, flies from putrid matter, mice
from dirty hay, crocodiles from rotting logs at the bottom of bodies of
water, and so on. Spontaneous generation or Equivocal generation is
considered obsolete by many, regarding the origin of life from inanimate
matter, which held that this process was a commonplace and everyday
occurrence, as distinguished from univocal generation, or reproduction
from parent(s). The theory was synthesized by Aristotle, who
compiled and expanded the work of prior natural philosophers and the
various ancient explanations of the appearance of organisms; it held
sway for two millennia. It is generally accepted to have been
ultimately disproven in the 19th Century by the experiments of Louis
Pasteur. The disproof of ongoing spontaneous generation is no longer
controversial, now that the life cycles of various life forms have been
well documented. However, the question of biopoesis or abiogenesis, how
living things originally arose from non-living material, remains
relevant today
5. Clay Theory
A model for the origin of life based on clay was forwarded by A.
Graham Cairns-Smith of the University of Glasgow in 1985 and explored as
a plausible illustration by several other scientists, including Richard
Dawkins. Clay theory postulates that complex organic molecules arose
gradually on a pre-existing, non-organic replication platform—silicate
crystals in solution. Complexity in companion molecules developed as a
function of selection pressures on types of clay crystal is then
exapted to serve the replication of organic molecules independently of
their silicate “launch stage”.
4. Theory of Consecutive Creations
The idea of extinction paved the way for the theory of catastrophism
or “consecutive creations”, one of the predecessors of the evolution
theory. Catastrophism is the idea that Earth has been affected in the
past by sudden, short-lived, violent events, possibly worldwide in
scope. This view holds that the present is the key to the past, and that
all things continue as they were from the beginning of the world.
According to this theory, since each catastrophe completely destroyed
the existing life, each new creation consisted of life form different
from that of previous ones. French scientists Georges Cuvier (1769-1832)
and Orbigney (1802 to 1837) were the main supporters of this theory.
3. Materialistic Theory
According this theory, the origin of life on earth is the result of a
slow and gradual process of chemical evolution that probably occurred
about 3.8 billion years ago. Chemical evolution refers to molecular
evolution is the process of evolution at the scale of DNA, RNA, and
proteins. Molecular evolution emerged as a scientific field in the 1960s
as researchers from molecular biology, evolutionary biology and
population genetics sought to understand recent discoveries on the
structure and function of nucleic acids and protein. Some of the key
topics that spurred development of the field have been the evolution of
enzyme function, the use of nucleic acid divergence as a “molecular
clock” to study species divergence, and the origin of noncoding DNA.
2. Organic Evolution
Speciation stretches back over 3.5 billion years during which life has existed on earth. It
is thought to occur in multiple ways such as slowly, steadily and
gradually over time or rapidly from one long static state to another.
Evolution (also known as biological or organic evolution) is the change
over time in one or more inherited traits found in populations of
organisms. Inherited traits are particular distinguishing
characteristics, including anatomical, biochemical or behavioural
characteristics, that are passed on from one generation to the next.
Evolution has led to the diversification of all living organisms, which
are described by Charles Darwin as “endless forms most beautiful and
most wonderful”.
1. Theory of Special Creation
According to this theory, all the different forms of life that occur
today on planet earth, have been created by God, the almighty. Adam and
Eve were, according to the Book of Genesis, Bible and Quran the first
man and woman created by the God. Life on earth began from them
according to Christians, Muslims and Jews. The 3 religions have a common
agreement on the fact God created the universe in seven days, reserving
for his sixth-day labor the climax of creation: man and woman. On the
seventh day God rests and so establishes the holiness of the Sabbath.
God fashioned a man fom the dust and blows the breath of life into his
nostrils, then planted a garden (the Garden of Eden) and caused to grow
in the middle of the garden the Tree of Knowledge of Good and Evil and
the Tree of Life. God set the man in the garden “to work it and watch
over it,” permitting him to eat from all the trees in the garden except
the Tree of Knowledge, “for on the day you eat of it you shall surely
die.” God brought the animals to the man for him to name. None of them
were found to be a suitable companion for the man, so God caused the
man to sleep and created a woman from a part of his body (Tradition
describes the part as a rib). The Quran says that Adam initiated the
fruit eating and that both Adam and Eve (Hawa) ate the forbidden fruit,
for which God later forgave them, and then sent both of them down to
earth as his representatives. The Hadith (the prophetic narrations) and
literature shed light on the Muslim view of the first couple. The
concept of original sin does not exist in Islam, as Adam and Eve were
forgiven after they repented on Earth, according to the Quran. One of
the differences between the Qur’an and the book of Genesis is that it
does not recount the Genesis narrative in which Eve leads Adam to
transgress God’s laws; they are simply both held responsible and thus
sent to earth.
source : http://www.smashinglists.com/top-10-theories-on-beginning-of-life-on-earth/
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