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1. The origin of life
1.1. The concept of life
In biology, we use the word life to refer to the ability living things have to carry out the vital functions: nutrition, interaction and reproduction.
The vital functions are common and essential to all living things, whatever their level of complexity or organisation. This is known as the constancy principle of the vital functions.
1.2. Significance of vital functions
All living things interact with their surroundings or the environment.
The biological significance of vital functions is that they enable permanent interaction between living things and their environment.
The reproduction function means new organisms can form, enabling the species to last and evolve over time.
The interaction function enables organisms to respond to stimuli and adapt to environmental conditions. This determines their survival in the environment.
Through the nutrition function, The living things exchange matter and energy with their surroundings.
1.3. Primordial conditions
a. Geophysical evolution of the earth
The first traces of life on Earth are about 3 800 million years old. Our planet’s
geophysical evolution took place in this 800 million-year gap.
b. Chemical evolution
This process gave rise to the biogenic elements, along with the other chemical elements and compounds that make up matter on Earth.
1.4. Theories of the origin of life
A scientific theory is a set of ideas and a set of rules, or scientific laws, which define the relationships between those ideas. It is expressed as a principle or set of principles, such as cell theory. |
a. The theory of spontaneous generation
Proposed by Aristotle (384-322 BC). This theory assumes that the spontaneous generation of some simple life forms is the result of interaction between inert matter and a vital force or divine intervention, called entelechy.
Scientists such as Francesco Redi, Anton van Leeuwenhoek and Lazzaro Spallanzani began to question this theory in the 17th and 18th centuries, supported by “new technologies such as microscopes.
Then, in the 19th century, Louis Pasteur refuted this theory and provided evidence to disprove it.
Pasteur’s experiments led to the spread of the biogenesis principle: life comes from life”. This matches one of the assumptions of cell theory: cells are the anatomical, functional and reproductive units of living things.
b. Panspermia theory
This theory argues for a cosmic origin of life, stating that life is created in outer space and travels from one location to another.
This theory was developed in the 5th century BC by Anaxagoras. However, it grew in popularity in the 20th century when organic matter, including fossilised bacteria and primitive DNA molecules, was observed in the analysis of certain meteorites.
One of its greatest supporters was Swedish chemist Svante Arrhenius. He proposed that life, in the form of spores or bacteria, travels in cosmic dust or rock fragments, propelled by cosmic radiation. However, even though bacterial life is highly resistant, it has not yet been proven that it could survive the extreme conditions of a journey through space.
Panspermia is considered a hypothesis rather than a scientific theory, because, although it is possible, it does not currently offer any conclusive arguments to explain the origin of life on Earth.
C. Oparine- Haldane Theory
The main supporters of this idea were Russian biochemist Aleksandr Oparin and British geneticist John Haldane. According to their hypothesis, the primitive oceans contained a large amount of dissolved organic compounds, forming what is known as the “primordial soup”.
Earth’s primitive atmosphere was dense and rich in water vapour, ammonia, methane and carbon dioxide, and it lacked free oxygen. There were constant and powerful electric shocks, which had enough energy to give rise to the first organic compounds. These compounds were dissolved in the primitive oceans to form the “primordial soup’ where life began.
The Oparin-Haldane hypothesis is based on three assumptions: the conditions of primitive Earth, the capacity of chemical elements to interact to form compounds and the generation of complex molecules from simple ones.
This theory suggests that life developed over several stages
2. The evolution of living things
Look at these photos. They show different fossils of species which inhabited Earth in the past. They are now either extinct or have been replaced by others that are better adapted to current environmental conditions.
The existence of fossils is some of the main evidence in favour of evolution in living things.
2.1. Fixism vs biological evolution
There was a prevalent general belief up to the 19th century that the current species have remained almost exactly the same throughout the Earth’s history. This is the central idea of the theory of “fixism’. However, as early as the 4th century BC, the discovery of fossils allowed thinkers such as Anaximander to intuitively develop the idea of the evolution of species.
Biological evolution is the continuous process by which species transform and new ones appear, thanks to changes that develop from one generation to the next.
Biological evolution is now considered a real, natural and scientifically proved fact. lt is therefore a scientific theory.
2.2. Evidence of evolution of living things
The theory of evolution is based on evidence provided by different scientific disciplines. Evidence that today’s living things come from a common ancestor is divided into the following areas:
a. Paleontological evidence
This includes the evidence provided by the fossil record, which shows differences between organisms that correspond to different geological periods.
Fossils of organisms that are intermediates between two great groups of living things serve as confirmation of the link between them. For example, the tiktaalik is a fossil with characteristics between those of fish and amphibians. The platypus, which exists today, has intermediate characteristics between birds and mammals.
b. Comparative anatomy evidence
Comparative anatomy shows the changes that occur in a species as a consequence of its adaptation to new environmental conditions, or the emergence of new species from existing ones. lt is based on the comparative study of the organ morphology of living things and fossils. There are three types of organs: homologous organs, which have a common evolutionary origin and have adapted to different functions; analogous organs, which have similar functions but different evolutionary origins; and vestigial organs, which have lost their original function in the course of evolution.
C. Biogeographical evidence
If you look at the distribution of current species, you can see that they are not all found on every continent, even where there are suitable habitats for them. The distribution of species can be explained as a consequence of the organisms spreading out unevenly from their place of origin.
d. Embryological evidence
The comparative study of embryological evidence in living things shows morphological similarities between all the vertebrate groups at the early stages of embryonic development.
This great similarity between the embryos of fish, amphibians, reptiles, birds and mammals in their early stages of development is seen as evidence in favour of the theory of evolution, because it suggests all vertebrates have a common origin. Furthermore, the greater the level of similarity between two species, the longer the development period in which their embryos have morphological similarities.
e. Biochemical or molecular evidence
All living things are made up of the same biological elements and the same biomolecules. These observations, provided by the discipline of molecular biology, are evidence in support of the theory of evolution of living things from a common ancestor.
Also, as you know, genetic code is universal. This is another key fact from molecular biology in support of the idea of evolution in living things. In addition, the molecules of species that are close in evolution are more similar than those of distant species in evolution, and viceversa.
2.3. Lamarckism and Darwinism
Jean-Baptiste de Monet, Chevalier de Lamarck (1744-1829), established Lamarckism, which states that diversity among living things on Earth is a result of the organisms adaptation to different environments.
Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) proposed the theory of evolution by natural selection.
These theories are based on the following principles:
Lamarckism:
– Environmental changes create new needs.
– These needs determine the use or disuse of certain organs which develop or deteriorate accordingly: the principle of use and desuse.
– Acquiered traits are kept and passed down to offspring.
Darwinism:
– Organisms have a high capacity for reproduction, so species tend to produce a large number of offspring.
– The variability of descendants, as descendants from a single line are similar but not identical to one another.
– The fight for survival in adverse conditions. The best-adapted organisms survive and can reproduce. This is how they are selected.
– Natural selection of the best-adapted organisms, because the selected organisms reproduce and pass favourable traits down to their descendants.
– After many generations, the traits that most favour survival in a specific environ- ment will have been naturally selected.
2.4. Neo-Darwinism or synthetic theory
Neo-Darwinism or the synthetic theory of evolution was proposed in the
mid-20th century by Huxley, Dobzhansky and Mayr. lt combines principles of
genetics and molecular biology with the theory of evolution.
The key principles of synthetic theory are:
– Variability between the organisms in a population is a result of genetic re-
combination that occurs in sexual reproduction and genetic mutations.
– Mutations occur randomly. Many of them are negative and even lethal, so
they disappear. Others that may seem innocuous turn out to be favourable
when environmental conditions change.
– Nature selects the mutations that favour survival in the new environmental
conditions. They are called adaptive mutations.
– In this way, populations gradually adapt to their environment.
3. The mechanisms of evolution
3.1 Genetic variability
Genetic variability refers to the number of different genotypes of each species, which produce individual organisms with different traits.
There are two causes or sources of genetic variability in living things:
– Mutations. These are changes that occur in organisms’ genetic material. They may affect the DNA nucleotides, genes, chromosomes or mitochondrial DNA. The changes are expressed in the organism’s phenotype or traits.
Mutations occur randomly and do not usually depend on the environment.
– Genetic recombination. This is the mix of genes produced by gametes in sexual reproduction. This process leads to variability because it allows maternal and paternal genes to combine, producing a large number of individuals with different traits.
3.2 Natural selection
Natural Selection is the process by which the environment selects the organisms that are best adapted to the conditions. These organisms reproduce and pass their favourable traits to their offspring.
3.3.Consequences of the evolutionary process:
The key consequences of the evolutionary process are:
a. Organisms adaptation to their environment
– Living things adapt to new environmental conditions when their surroundings change. These adaptations determine how many descendants of a population will make it to the next generation.
– If the changes are sudden, some species are not able to adapt and they become extinct.
b. Speciation, or the appareance of new species:
Speciation is the process by which new species evolve from a common ancestor.
C. Diversification or increase in biodiversity
Another consequence is the increase in the number and diversity of species, resulting in the biodiversity on our planet today.
Adaptive convergence. This occurs when different species inhabit similar environments, resulting in similarities in their form even though they are not related.
Processes of convergent evolution or adaptive convergence lead to the development of analogous organs. These organs are adapted to the same function but their evolutionary origins are different.
Evolutionary divergence occurs when populations inhabit different environments and so develop different adaptations.
Processes of divergent evolution or evolutionary divergence lead to the development of homologous organs. These organs, which have the same origin, are not obviously similar and are adapted to the species environment.
3.3 Controversy:
As with other complex theories, evolution allows for different points of view:
a. Selectionism vs Neutralism
The controversy between selectionists and neutralists is based on the mechanisms of evolution. Selectionists argue that the key process is natural selection, while neutralists argue that it is chance.
b. Gradualism vs punctuated equilibrium or saltation
In this case, the controversy lies in the way new species appear. Gradualists argue that it is gradual, while punctuated equilibrium states that they appear suddenly and then remain stable for long periods of time.
3.5. Phylogenetic trees
Phylogenetic trees are diagrams that represent evolutionary relationships between species with a common ancestor.