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1
The
evolutionary history of life and origin of life are fields on ongoing geological and biological research. Although not necessary conditions for the acceptance of evolution by
natural selection, the
origin of life and its evolutionary history can nonetheless help shed light on evolutionary processes. The current scientific consensus is that the complex biochemistry that makes up life came from simpler chemical reactions, but it is unclear how this occurred. Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any
last universal common ancestor or ancestral gene pool. Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as
RNA, and the assembly of simple cells. The first simple, sea dwelling organic structures appeared about 3,400 million years ago. It is considered that they may have formed when certain chemical (organic) molecules joined together.
Prokaryotes, single-celled micro-organisms like
blue green algae, were able to photosynthesize and produce oxygen. Around thousand million years later, sufficient oxygen had built up in the atmosphere and hence it allow multicellular organisms to proliferate in the
Precambrian seas.
2
Deoxyribonucleic acid (
DNA) is a
nucleic acid that contains the
genetic instructions used in the
development and functioning of all known
living organisms. The main role of DNA
molecules is the long-term storage of
information. DNA is often compared to a set of blueprints, since it contains the instructions needed to construct other components of
cells, such as
proteins and
RNA molecules. The DNA segments that carry this genetic information are called
genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information. Chemically, DNA is a long
polymer of simple units called
nucleotides, with a backbone made of sugars and phosphate groups joined by
ester bonds. Attached to each sugar is one of four types of molecules called
bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the
genetic code, which specifies the sequence of the
amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called
transcription. Most of these RNA molecules are used to synthesize proteins, but others are used directly in structures such as
ribosomes and
spliceosomes.
3
The
Cretaceous–Tertiary extinction event was the large-scale
mass extinction of animal and plant species in a geologically short period of time, approximately
65.5 million years ago (mya). It is widely known as the
K–T extinction event and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the
K–T boundary.
K is the traditional abbreviation for the
Cretaceous Period, and
T is the abbreviation for the
Tertiary Period. The event marks the end of the
Mesozoic Era, and the beginning of the
Cenozoic Era. Non-
avian dinosaur fossils are only found below the K–T boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the K-T boundary, but they have been explained as
reworked, that is, fossils that have been eroded from their original locations then preserved in later
sedimentary layers.
Mosasaurs,
plesiosaurs,
pterosaurs and many
species of
plants and
invertebrates also became extinct.
Mammalian and bird
clades passed through the boundary with few extinctions, and
radiation from those
Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms. Many scientists theorize that the K-T extinctions were caused by one or more catastrophic events such as massive
asteroid impacts or increased
volcanic activity. Several
impact craters and massive volcanic activity in the
Deccan traps have been dated to the approximate time of the extinction event. These geological events may have reduced
sunlight and hindered
photosynthesis, leading to a massive disruption in Earth's
ecology. Other researchers believe the extinction was more gradual, resulting from slower changes in
sea level or
climate.
4
Evolutionary thought has roots in antiquity as philosophical ideas known to the
Greeks,
Romans,
Indians,
Chinese and
Muslims. Until the 18th century, however,
Western biological thought was dominated by
essentialism, the idea that living forms are static and unchanging in time. During the
Enlightenment, evolutionary
cosmology and the
mechanical philosophy spread from the physical sciences to
natural history, and naturalists such as
Maupertuis and
Georges-Louis Leclerc, Comte de Buffon focused on the variability of species. The emergence of
paleontology (and with it the notion of
extinction), as well as the dramatic expansion of known species, helped undermine the traditional static view of nature. The first full theory of evolution was proposed by
Jean-Baptiste Lamarck in the early 19th century;
Lamarck's theory was based on the idea that species had an innate drive that pushed them up the
great chain of being and that the mechanism of
inheritance of acquired characteristics helped them adapt to local conditions. The evolutionary theory often referred to as
Darwinism was first publicly put forth by
Charles Darwin and
Alfred Russel Wallace and discussed in detail in
On the Origin of Species, published by Darwin in 1859. Darwinism, which unlike Lamarck's theory proposed
common descent and a branching
tree of life, was based on
natural selection, and synthesized a wide range of evidence from
animal husbandry,
biogeography,
geology,
morphology, and
embryology. The debate over
Origin would play a key role in the displacement of
natural theology by
methodological naturalism in the life sciences, and raised profound questions about human nature and the place of humanity in the natural world.
5
In
evolutionary ecology,
mimicry describes a situation where one organism, the
mimic, has
evolved to share common outward characteristics with another organism, the
model, through the
selective action of a
signal-receiver or "
dupe". Collectively this is known as a
mimicry complex. The model is usually another species except in cases of
automimicry. The signal-receiver is typically another intermediate organism, e.g the common
predator of two species, but may actually be the model itself (such as an
orchid resembling a female
wasp). As an
interaction, mimicry is in most cases advantageous to the mimic and harmful to the receiver, but may increase, reduce or have no effect on the
fitness of the model depending on the situation. Models themselves are difficult to define in some cases, for example eye spots may not bear resemblance to any specific organism's eyes, and camouflage often cannot be attributed to any particular model.
Camouflage, in which a species appears similar to its surroundings, is essentially a form of visual mimicry, but usually is restricted to cases where the model is non-living or abiotic. In between camouflage and mimicry is
mimesis, in which the mimic takes on the properties of a specific object or organism, but one to which the dupe is indifferent. The lack of a true distinction between the two phenomena can be seen in animals that resemble twigs, bark, leaves or flowers, in that they are often classified as camouflaged (a plant constitutes its "surroundings"), but are sometimes classified as mimics (a plant is also an organism).
Crypsis is a broader concept that encompasses all forms of detection evasion, such as mimicry, camouflage, hiding etc.
6
Charles Darwin's
The Origin of Species (publ.
1859) is a seminal work in
scientific literature and arguably
the pivotal work in
evolutionary biology. The book's full title is
On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. It introduced the
theory that populations
evolve over the course of generations through a process of
natural selection. It was controversial because it contradicted
religious beliefs which underlay the then
current theories of
biology. Darwin's book was the culmination of evidence he had accumulated on
the voyage of the Beagle in the
1830s and expanded through continuing investigations and experiments since his return. The book is readable even for the non-specialist and attracted widespread interest on publication. The book was controversial, and generated much discussion on
scientific,
philosophical, and
religious grounds. The scientific theory of
evolution has
itself evolved since Darwin first presented it, but
natural selection remains the most widely accepted scientific model of how species evolve. The at-times bitter
creation-evolution controversy continues to this day.
7
Natural selection is the process by which favorable
traits that are
heritable become more common in successive
generations of a population of reproducing
organisms, and unfavorable traits that are heritable become less common. Natural selection acts on the
phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and
reproduce than those with less favorable phenotypes. If these phenotypes have a
genetic basis, then the
genotype associated with the favorable phenotype will increase in
frequency in the next generation. Over time, this process can result in
adaptations that specialize organisms for particular
ecological niches and may eventually result in the
emergence of new species. Natural selection is one of the cornerstones of modern
biology. The term was introduced by
Charles Darwin in his groundbreaking
1859 book
The Origin of Species in which natural selection was described by analogy to
artificial selection, a process by which animals with traits considered desirable by human breeders are systematically favored for reproduction. The concept of natural selection was originally developed in the absence of a valid theory of
inheritance; at the time of Darwin's writing, nothing was known of modern
genetics.
8
In biology and ecology,
extinction is the cessation of existence of a
species or group of
taxa, reducing
biodiversity. The moment of extinction is generally considered to be the death of the last individual of that species (although the
capacity to breed and recover may have been lost before this point). Because a species' potential
range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as
Lazarus taxa, where a species presumed extinct abruptly "re-appears" (typically in the
fossil record) after a period of apparent absence. Through
evolution, new species arise through the process of
speciation — where new varieties of organisms arise and thrive when they are able to find and exploit an
ecological niche — and species become extinct when they are no longer able to survive in changing conditions or against superior competition. A typical species becomes extinct within 10 million years of its first appearance, although some species, called
living fossils, survive virtually unchanged for hundreds of millions of years. Only one in a thousand species that have existed remain today.
9
Punctuated equilibrium is a theory of
evolutionary biology. Its principle is that most
sexually reproducing populations experience little change for most of their geological history, and that when phenotypic evolution does occur, it is localized in rare, rapid events of branching speciation (called
cladogenesis). Punctuated equilibrium is commonly contrasted against the theory of
phyletic gradualism, which states that evolution generally occurs uniformly and by the steady and gradual transformation of whole lineages (
anagenesis). In this view, evolution is seen as generally smooth and continuous. In
1972 paleontologists
Niles Eldredge and
Stephen Jay Gould published a landmark paper developing this idea. Their paper was built upon
Ernst Mayr's theory of
geographic speciation, I. Michael Lerner's theories of developmental and genetic homeostasis, as well as their own empirical research. Eldredge and Gould proposed that the degree of gradualism championed by
Charles Darwin was virtually nonexistent in the fossil record, and that stasis dominates the history of most
fossil species.
10
Clinton Richard Dawkins,
FRS (born
March 26,
1941) is a
British ethologist,
evolutionary biologist and
popular science writer. He holds the
Charles Simonyi Chair for the Public Understanding of Science at the
University of Oxford. Born in
Nairobi,
Kenya, Dawkins moved to
England with his parents at the age of eight, and completed his education at the University of Oxford. He first came to prominence with his 1976 book
The Selfish Gene, which popularised the
gene-centered view of evolution and introduced the term
meme, playing a significant role in the foundation of
memetics as a scientific field of study. In 1982, he made a widely cited contribution to
evolutionary biology with the theory, presented in his book
The Extended Phenotype, that
phenotypic effects are not limited to an organism's body but can stretch far into the environment, which includes the bodies of other organisms. He has since written several best-selling popular books, and made regular appearances on television and radio programmes discussing
evolution,
creationism,
intelligent design, and
religion. In addition to his biological work, Dawkins is well-known for his views on religion. He is an outspoken
antitheist and
atheist; a
secular humanist,
sceptic, scientific
rationalist, and a supporter of the
Brights movement.
