Download THE ORIGIN AND EVOLUTION OF LIFE THEORIES OF

THE ORIGIN AND EVOLUTION OF LIFE
THEORIES OF THE ORIGIN OF EVOLUTION
1. Ancient theory: THEORY OF SPONTANEOUS GENERATION
It is the idea that life can appear from inanimate objects. For example, in Ancient Egypt, they
believed that the Sun’s heat on the sediment in the Nile produced snakes and crocodiles, or in
the Middle Ages, people had recipes to create living things from materials (clay, food remains…).
That is because for exampleº, they saw that from a piece of meat left in a warm, damp place for
some days flies and larvae appeared.
In the 17th C experiments were more controlled and they could prove that
this theory was wrong.
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In 1668 Francesco Redi demonstrated that the larvae found in
rotting meat came from fly eggs by covering the meat.
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Later on, in the second half of the 19th C, Louis Pasteur demonstrated that
microorganisms did not come from spontaneous generation with the following experiment:
The theory of spontaneous generation was finally rejected.
2. OPARIN’S THEORY
At the beginning of the 20th Allexander Ivanovich Oparin suggested a theory of the origin of life that is
still the basis of the actual one. He stated that life had appeared from physical-chemical processes
that occurred in the atmosphere of the primitive Earth:
1. A series of spontaneous chemical reactions occurred between the
atmosphere’s components (methane, ammonia, hydrogen and water vapor)
4000 million years ago (there wasn’t oxygen)
2. The reaction were started by electric charges from storms, intense ultraviolet
radiation from the Sun and continuous volcanic eruptions, which provided
the huge amounts of energy needed to start these reactions. Organic
molecules started to synthesize.
3. The temperature fell and the water vapor in the atmosphere condensed,
creating clouds. There’d be intense rain which led to the creation of
primitive seas. The recently-formed organic molecules accumulated there.
These seas were much warmer and less deep than the actual ones. Oparin called them
“primordial soup”.
4. The organic molecules joined together and formed bigger molecules (such as
proteins and polysaccharides, which’d be the future components of living things).
5. Theses biological molecules were insulated from water inside structures called
coacervates. However, this insulation wasn’t complete and they were able to
exchange molecules between the water and the coacervates. The more stable
coacervates survived and the less stable disappeared.
6. In order to divide and multiply themselves, the stable coacervates synthesized
molecules (nucleic acids) that could replicate them. Cells appeared.
3. PRESENT DAY HYPOTHESIS
The first stages of Oparin’s theory have been proved, though no one has been able to provide
evidence that confirms the complete model.
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In the 20th C Stanley Miller conducted an experiment that confirmed
Oparin’s hypothesis by reproducing the conditions of the primitive
atmosphere. Several organic compounds (such as amino acids, the
basic chemical components of living beings) were created.
Oparin’s theory has been also modified and improved, adding that:
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Some organic molecules will join together to form large biological
molecules (like proteins) when certain minerals are present. This is why
scientists think this process probably happened in areas with clay or
muddy soil instead of water.
The first nucleic acid to appear probably would have been RNA. DNA is believed to have
evolved later because it is more complicated and stable.
Despite the modifications, the appearance of the first organism is still very unclear.
THEORIES OF EVOLUTION
1. Carl Linnaeus (1707-1778) stated that different species had been created separately
and independently by God and that these hadn’t changed throughout the years.
2. At the end of the 18th C, George Cuvier used scientifically reasoned arguments to
reinforce the theory that species hadn’t changed. For Cuvier, fossils from species that
were now disappeared where there due to the catastrophes that had happened during
the Earth’s history, the last one being the great flood from the Bible.
3. The theory of Carl Linnaeus was accepted until the beginning of the 19th C, when these
ideas were questioned and the first hypothesis of evolution appeared. However, as the
process of evolution is slow and cannot be perceived by a human generation, it took
long for them to change the standard theory (as well as for religious reasons).
4. LAMARKISM AND DARWINISM
5. PRESENT DAY THEORIES OF EVOLUTION
Darwin couldn’t explain how species changed over time. His theory has been developed
and improved with the advance of science (mainly the development of genetics).
 Neo-Darwinism: Mendel’s laws, which explained hereditary traits and their
transmission mechanism, sexual reproduction, which gives rise to gene
combinations that are different from those of the parents, genetic recombination,
and mutations are mechanisms that cause variety between individuals. Thanks to
them this theory has been developed.
Neo-Darwinism considers that natural selection does not act on the individual in
isolation, but on the population it belongs to, because, due to mutations, the
population is modified and the frequency of the mutations (if they are favorable)
increase in the population. When many changes accumulate, the specie evolves.
This is not the only current theory of evolution.
 The synthetic theory of evolution: Theodosius Dobzhansky led this theory,
which combined the principles of Darwin’s theory with genetics and gene
frequencies in populations.
 The theory of the selfish gene: proposed by Richard Sawkins. It establishes the
gene as the unit of evolution (not the population nor the individual).
Genes compete between themselves. The individuals are mere receptors and
transmitters of genes. The final aim of the evolutionary process is to increase the
frequency of some genes over others in a population.
 Endosymbiotic theory: was put forward by Lynn
Margulis (1938-2011). The hypothesis of the theory are:
o Eukaryotic cells originated from the fusion of
two types of bacteria.
o This primitive eukaryote acquired the capacity of
incorporate other cells, which became cellular
organelles.
o The interdependence between these cells caused them to behave as a
single organism.
o The symbiotic relationship between organisms is a key mechanism for the
evolution of living things
 Theory of punctuated equilibrium: proposed by Niles Elredge and Stephen Jay
Gould. Their hypothesis was that the process of evolution has not always been
slow and gradual but that in many cases the appearance of new species has
happened quickly. They based their findings of fossil groups without links to earlier
forms or transitions from previous groups. They thought this happened due to the
development of macromutations which affected the genes that regulate other
genes,
 Neutral theory of molecular evolution: proposed by Motoo Kimura. This theory
maintains that the majority of the mutations originate from genes that are neither
advantageous nor disadvantageous for the individuals who have them, so natural
selection doesn’t apply. According to it, a new species can appear if these
individuals have offspring and they are isolated from the other members of the
population.
EVIDENCE OF EVOLUTION
 Anatomical and morphological evidence: support the theory of evolution.
 Homologous organs: organs with similar structure but different
function. (For example, limbs in a lot of mammals). They prove
divergent evolution or adaptive radiation.
 Analogous organs: organs with the same function but different structure
(like the different types of wings)..They prove convergent evolution.
 Vestigial organs: organs that tend to disappear because they no longer
have a function (kiwi’s wings).
 Fossil evidence: show that species change over time.
 Fossils from related species have been found from different
eras, which showed slow but clear changes (this is
studied in a field called phylogenetics). E.g: horse.
 Present day species are more complex
and have more diversity compared to fossils.
 Fossils with intermediate characteristics that later evolved in a
different way proof evolution too. The different characteristics evolved or
disappeared as the differences between species became greater. (Archaeopteryx
in the left, an intermediate organism between reptiles and birds)
 Embryonic evidence: some embryos of different species are similar.
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Biogeographical evidence: animals’ and plants’ geographical distribution can be
explained according to theories of evolution:
Over time, groups of organisms that are isolated geographically evolve differently to
form new species.
For instance, in islands in the middle of the ocean there are very different species to
those of the continents (sometimes they have even evolved and created new species,
although with similar characteristics ) though species in nearby islands are more
similar. However, if the islands are close to a continent, or if they are new islands,
organisms are very similar to those in the continent.
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Molecular evidence: the greater the molecular similarity between two groups of
organisms, the closer their evolutionary relationship.
All living things have proteins and DNA, which are made up of smaller molecules
(amino acids and nucleotides respectively). They follow specific sequences and
characteristics for each organism and, when we compare these sequences in different
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species and groups of organisms, we can establish the evolutionary relationship
between them.
Other evidence: parasites, for example, have to modify their organs and morphology
to adapt to their host’s way of life (like tapeworms and leeches though they don’t
share a common ancestor they have similar organs). It is an example of convergent
evolution.
Also, some animal behavior is based on genetic inheritance. This happens between
species that have evolved in a similar way.
THE ORIGIN OF NEW SPECIES:
A species is a set of individuals with a similar morphology, with the same or very similar
anatomical and physiological characteristics, that can reproduce with each other and create
fertile offspring.
How does a new species appear?
1. Production of evolutionary changes in populations: some individuals are favored and
others not. Hence, the new populations are different from the original ones though
they are from the same species.
2. Genetic isolation of the new population: for the isolation to be complete, the new
species must not reproduce with the new one. This genetic isolation can occur because
of different barriers, that could be:
 Geographical barriers prevent physical contact
 Sexual barriers maybe by anatomical differences which prevent mating or a lack of
synchronization between fertile periods.
 Physiological barriers: are incompatibilities in the function of the
gametes which prevent fertilization.
 Chromosomal barriers: changes in the structure or number of
chromosomes which prevent the affected individual from having
offspring with the rest of the population.
 Ethological barriers: caused by the appearance of new types of
behavior that result in the rejection of some individual ¡s by other.
3. Gradual differentiation : after the isolation the population
accumulates changes due to new mutations and becomes more and
more different from the original population.
4. Speciation: genetic changes produce such significant physical differences that it is no
longer possible for the two populations to produce fertile offspring.
MICROEVOLTUTION AND MACROEVOLUTION:
Microevolution: process of evolution that causes the
appearance of new species.
Macroevolution: drastic changes that causes the
appearance of large groups of new species. For example,
how from reptiles appeared birds and mammals. The type
of evolution is more intense and last longer. However, this
has been a controversy because they believe that great evolutionary changes also happen
quicker. Some consider macro evolution as microevolution accumulated.
Regulatory genes produce proteins that control the
expression of other genes (like the gene Operón
lactose). These genes mutated and affected a lot
number of genes producing macroevolution.
VOCABULARY:
Divergent: moving away from each other (divergent evolution is for example, several species
that came from a common ancestor but that have different characteristics)
Convergent: moving towards each other (for example, two species that have totally different
antecessors but that end up with similar characteristics).
Adaptive radiation: how a species has diversified into different forms.