Origin Of Life

Evolutionary Biology is the scientific study of the history of life forms on Earth. Understanding evolution requires considering the vast timescale and the context of the origin of the universe and Earth.

The universe is estimated to be approximately 20 billion years old. The Big Bang theory describes the origin of the universe from a massive explosion, leading to expansion and cooling. This resulted in the formation of hydrogen and helium, which condensed under gravity to form galaxies.

Our solar system, including Earth, is believed to have formed about 4.5 billion years ago within the Milky Way galaxy.

Early Earth conditions were very different from today:

• There was no atmosphere initially.
• Gases like water vapor, methane ($\textsf{CH}_4$), carbon dioxide ($\textsf{CO}_2$), and ammonia ($\textsf{NH}_3$) were released from molten mass and covered the surface.
• UV rays from the sun split water into hydrogen and oxygen. Lighter hydrogen escaped. Oxygen combined with methane and ammonia to form water and carbon dioxide.
• An ozone layer was formed.
• As Earth cooled, water vapor condensed and fell as rain, forming oceans.

Life is estimated to have appeared on Earth approximately 4 billion years ago, about 500 million years after Earth's formation.

---

Chemical Evolution

The prevailing scientific hypothesis for the origin of the first life form on Earth is based on the proposals by Oparin (Russia) and Haldane (England).

• They suggested that the first life arose from pre-existing non-living organic molecules (like RNA, proteins, etc.).
• This formation of life was preceded by a process called chemical evolution, where inorganic constituents combined under early Earth conditions to form diverse organic molecules.

The conditions on early Earth conducive to chemical evolution included high temperature, volcanic activity, and a reducing atmosphere containing gases like $\textsf{CH}_4$, $\textsf{NH}_3$, $\textsf{H}_2\textsf{O}$ vapor, etc.

---

Miller'S Experiment

In 1953, S.L. Miller, an American scientist, experimentally tested the Oparin-Haldane hypothesis. He created laboratory conditions simulating early Earth's atmosphere and energy sources.

• He set up a closed flask containing a mixture of gases ($\textsf{CH}_4$, $\textsf{H}_2$, $\textsf{NH}_3$, and water vapor) representing the likely early atmosphere.
• Electric discharge was passed through the flask to simulate lightning, a source of energy, at a high temperature ($800^\circ\textsf{C}$).
• After a week, he analysed the contents and observed the formation of amino acids.

Similar experiments by others produced sugars, nitrogenous bases, pigments, and fats, supporting the idea that organic molecules could form abiotically (from non-living matter) under early Earth conditions. Analysis of meteorites also revealed similar compounds, suggesting these chemical processes might be common in the universe.

---

First Life Forms

The origin of the first self-replicating metabolic structures or 'capsules' of life is still not fully understood.

• The first non-cellular forms of life likely appeared around 3 billion years ago. These would have been large organic molecules (macromolecules) capable of self-replication, such as RNA or proteins.
• The first cellular forms of life are thought to have originated much later, around 2000 million years ago (2 billion years ago). These were likely single-celled organisms, confined to the aquatic environment.

The scientific view of the origin of life is that it arose slowly from non-living molecules through gradual evolutionary processes (a biogenesis). This is in contrast to earlier ideas like 'Panspermia' (life came from outer space as spores) or 'Spontaneous Generation' (life arose from decaying matter), the latter disproven by Louis Pasteur's experiments.

Once formed, the evolution of these simple cellular forms into the immense biodiversity seen today is the subsequent focus of evolutionary study.

Evolution Of Life Forms - A Theory

---

Theory Of Special Creation

Conventional religious views often propose the Theory of Special Creation, which states that all living organisms were created exactly as they are seen today. This theory suggests that the diversity of life has always been constant and will remain so, and that Earth is very young (around 4000 years old).

These ideas were challenged significantly by scientific observations in the 19th century.

---

Darwin'S Theory Of Natural Selection

Charles Darwin, based on observations during his voyage on the H.M.S. Beagle, concluded that existing life forms share similarities with each other and with extinct life forms found as fossils. He noted that different life forms have appeared and disappeared over Earth's history (extinction and origination of new forms).

Darwin proposed that evolution is a gradual process. His key insights were:

1. Within any population, there is inherent variation in characteristics among individuals.
2. Individuals with characteristics that make them better able to survive in the prevailing natural conditions (climate, food availability, predators, etc.) are considered 'more fit'.
3. According to Darwin, fitness primarily refers to reproductive fitness; those better adapted survive and leave more offspring than those less adapted.
4. These better-adapted individuals are selected by nature. This process is called Natural Selection.

Darwin implied natural selection as the primary mechanism driving evolution. Alfred Wallace, working independently, arrived at similar conclusions around the same time.

Over many generations, the traits that are selected for become more common in the population, leading to changes in the population's characteristics and the emergence of apparently new types of organisms.

All existing life forms are believed to share common ancestors, though these ancestors lived at different points in geological history. The geological history of Earth (epochs, periods, eras) and the biological history of life are closely interconnected. Scientific evidence indicates that Earth is billions of years old, not just a few thousand.

What Are The Evidences For Evolution?

Evidence supporting the theory of evolution comes from various scientific disciplines:

---

Paleontological Evidence

Fossils are the preserved remains or traces of ancient life forms, typically found in sedimentary rocks. Different layers of rock sediments, laid down over millions of years, contain fossils of organisms that lived during those periods.

• Studying fossils in sequential sedimentary layers reveals that life forms have changed over time.
• Certain life forms are restricted to specific geological time periods (e.g., dinosaurs were present during the Mesozoic era).
• Some fossils are similar to modern organisms, while others represent extinct species.
• Fossils provide a historical record of life on Earth and demonstrate that new forms of life have arisen at different times.

The study of fossils is called Paleontology. The age of fossils can be determined using methods like radioactive dating.

---

Embryological Evidence

Ernst Haeckel proposed embryological support for evolution based on the observation that embryos of all vertebrates share certain common features that are absent in adults (Biogenetic Law: 'Ontogeny recapitulates phylogeny').

Example: Vertebrate embryos (including humans) develop vestigial gill slits behind the head during their early stages. These are functional gills only in adult fish and disappear in other adult vertebrates.

However, this proposal was later challenged and disproved by Karl Ernst von Baer, who showed that embryos do not simply pass through the adult stages of other animals but share common developmental patterns in early stages.

---

Comparative Anatomy (Homology And Analogy)

Comparing the anatomy and morphology of different organisms (living and extinct) reveals similarities and differences that provide clues about evolutionary relationships.

• Homology: Structures that have similar anatomical origin and basic structure, but may perform different functions due to adaptation to different environments. This indicates divergent evolution from a common ancestor.
  • Example: The forelimbs of whales, bats, cheetahs, and humans have the same basic bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges), although they are used for different purposes (swimming, flying, running, grasping).
  • Example in plants: The thorn of Bougainvillea and the tendril of Cucurbita are both modified axillary buds, hence homologous structures.
• Analogy: Structures that perform similar functions but have different anatomical origins and basic structures. This indicates convergent evolution, where unrelated organisms evolve similar features in response to similar environmental pressures or needs.
  • Example: The wings of a butterfly (chitinous exoskeleton) and the wings of a bird (bony skeleton covered with feathers) both function in flight but have different structural origins.
  • Other examples: Eye of octopus and mammal; flippers of penguins and dolphins; sweet potato (root modification) and potato (stem modification).

---

Biochemical Evidence

Similarities in the basic biochemical molecules and metabolic pathways among diverse organisms provide evidence for common ancestry. For instance, the basic structure of DNA, RNA, genetic code, and many metabolic processes (like glycolysis) are remarkably conserved across different life forms.

Comparing the sequences of proteins (like cytochrome c or hemoglobin) and genes across different species reveals genetic similarities that correlate with evolutionary relatedness. Organisms with more similar molecular sequences are generally more closely related.

---

Artificial Selection

Humans have practiced selective breeding (artificial selection) for centuries, creating new varieties of plants and animals with desirable traits (e.g., different breeds of dogs, varieties of crops from wild mustard). This demonstrates that significant changes in traits can occur within a relatively short period (hundreds of years) under selective pressure.

This provides a microevolutionary analogy for how natural selection could cause much larger evolutionary changes over geological timescales (millions of years).

---

Industrial Melanism

A classic example of natural selection in action is observed in populations of the peppered moth (Biston betularia) in England, particularly in the context of industrialisation.

• Before industrialisation, trees in rural areas were covered with light-colored lichens. The white-winged form of the moth was well-camouflaged against this background and survived better, while the dark-winged (melanised) form was easily spotted by predators.
• After industrialisation, pollution killed the lichens, and tree trunks became dark with soot. The white-winged moths were now conspicuous and preyed upon, while the dark-winged moths were better camouflaged and survived better.

This shift in the moth population demonstrates that the environment (in this case, affected by human activity) acts as a selective pressure, favouring traits that increase survival and reproduction. It shows how a trait (melanism) that was initially disadvantageous in one environment became advantageous in a changed environment, leading to a change in allele frequency in the population.

---

Evolution By Anthropogenic Action

Examples like industrial melanism or the rapid evolution of resistance in organisms due to human activities (anthropogenic action) further illustrate evolutionary processes.

• Excessive use of herbicides and pesticides has led to the selection and rapid increase in resistant varieties of weeds and pests.
• The use of antibiotics and drugs has resulted in the evolution of drug-resistant microbes (e.g., antibiotic-resistant bacteria) and eukaryotic pathogens.

Resistant strains appear relatively quickly (months or years). These instances highlight that evolution is not necessarily a directed, predetermined process but rather a stochastic process influenced by chance events (mutations) and chance environmental factors that act as selective pressures.

What Is Adaptive Radiation?

---

Examples Of Adaptive Radiation (Darwin'S Finches, Australian Marsupials)

Adaptive Radiation is the process by which different species evolve from a common ancestral stock in a given geographical area, radiating into different habitats or ecological niches and developing adaptations for those specific environments.

This process starts from a single point (ancestor) and leads to diversification into multiple forms.

Examples:

• Darwin's Finches: On the Galapagos Islands, Darwin observed numerous varieties of small black birds (finches). He hypothesised that all these varieties evolved from a single ancestral seed-eating finch that colonised the islands. Different populations on different islands or parts of islands adapted to different food sources, leading to variations in beak shape and size, resulting in insectivorous, vegetarian, and other types of finches.
• Australian Marsupials: In Australia, a diverse array of marsupials (pouched mammals) evolved from a single ancestral marsupial stock. Examples include the Koala, Kangaroo, Wombat, Sugar glider, Tasmanian devil, etc., each adapted to a specific ecological niche.

Convergent Evolution can occur when adaptive radiations in different isolated geographical areas result in the evolution of species that appear similar because they have adapted to similar ecological niches or lifestyles, despite having different evolutionary origins. For instance, placental mammals in Australia underwent their own adaptive radiation, and some placental mammals have evolved forms that are strikingly similar in appearance and lifestyle to certain Australian marsupials (e.g., Placental wolf and Tasmanian wolf). This similarity is due to convergent evolution, not shared recent ancestry.

Biological Evolution

---

Darwinian Theory And Natural Selection

Biological evolution is the process of change in the inherited traits of populations over successive generations. According to Darwin's theory, this process is driven by natural selection.

Natural selection began operating more effectively once cellular forms of life with varying metabolic capabilities emerged. Microbes, with their short life cycles and rapid reproduction, demonstrate rapid evolutionary changes in response to environmental shifts. Populations with variations better suited to new conditions are selected for, outgrowing less fit populations. The fitness being selected for must have a genetic basis; traits that are heritable are passed on to the next generation.

Fitness is the outcome of an organism's ability to adapt to its environment and subsequently be favored by natural selection.

The two core concepts of Darwin's theory of evolution are:

1. Branching Descent: All organisms are related and have descended from common ancestors, with species diverging over time.
2. Natural Selection: The process by which individuals with advantageous heritable traits survive and reproduce more successfully, leading to an increase in the frequency of these traits in the population over generations.

---

Lamarckian Theory

Prior to Darwin, Jean-Baptiste Lamarck proposed a theory of evolution based on the use and disuse of organs and the inheritance of acquired characteristics. His famous example was the giraffe, suggesting that their long necks evolved as giraffes stretched to reach leaves on tall trees, and this acquired trait of an elongated neck was passed down to offspring.

Lamarck's theory of inheritance of acquired characteristics is now widely discredited because acquired traits (changes in an individual during its lifetime due to environmental influence or behaviour) are generally not heritable unless they affect the germline DNA.

Evolution can be viewed both as a process (the ongoing changes in populations over time) and as the result of processes (the current biodiversity and adaptations are consequences of past evolutionary events). Natural selection is a major process driving this change.

Darwin was likely influenced by Thomas Malthus's work on population growth, particularly the idea that populations tend to increase exponentially while resources are limited, leading to competition for survival. Darwin recognised that variations within a population would give some individuals a competitive edge, allowing them to survive and reproduce more effectively, thus passing on those advantageous variations.

Mechanism Of Evolution

While Darwin described the pattern of evolution through natural selection, the source of variation and the precise mechanisms driving rapid evolutionary change and speciation were clarified later.

---

Mutation Theory (DeVries)

In the early 20th century, Hugo de Vries, based on his work with the evening primrose (Oenothera lamarckiana), proposed the concept of mutations as the primary source of evolutionary change. He defined mutations as large, sudden, and inheritable differences appearing in a population.

• De Vries believed that evolution occurred due to these large mutations, which he called saltation (single step large mutation), leading directly to speciation.
• This contrasted with Darwin's view that evolution was gradual, driven by the accumulation of small, heritable variations.

Modern genetics and population genetics integrate mutation with other factors. Mutations are random and directionless (in terms of their effect on fitness), but natural selection acts on these mutations, favouring those that are advantageous in a given environment. Population genetics studies have provided a clearer understanding of how changes in allele frequencies within populations lead to evolution.

Hardy - Weinberg Principle

The Hardy-Weinberg principle describes the genetic makeup of a non-evolving population. It states that in a large, randomly mating population, the frequencies of alleles and genotypes remain constant from generation to generation, provided that no evolutionary influences are acting upon the population.

This state of constant allele and genotype frequencies is called genetic equilibrium or Hardy-Weinberg equilibrium.

If the frequency of allele A in a population is 'p' and the frequency of allele a is 'q', then:

• The frequency of genotype AA is $\textsf{p}^2$.
• The frequency of genotype aa is $\textsf{q}^2$.
• The frequency of genotype Aa is $2\textsf{pq}$.

According to the principle, the sum of these genotype frequencies equals 1: $\textsf{p}^2 + 2\textsf{pq} + \textsf{q}^2 = 1$. This is the binomial expansion of $(\textsf{p}+\textsf{q})^2 = 1$, where $\textsf{p} + \textsf{q} = 1$ (the sum of allele frequencies is 1).

Deviations from the expected frequencies predicted by the Hardy-Weinberg principle indicate that evolutionary change is occurring in the population.

---

Factors Affecting Equilibrium (Gene Flow, Genetic Drift, Mutation, Genetic Recombination, Natural Selection)

The Hardy-Weinberg equilibrium is maintained only if five conditions are met. Disruptions to these conditions cause changes in allele frequencies, leading to evolution. The five factors known to affect Hardy-Weinberg equilibrium and cause evolution are:

1. Gene Migration (Gene Flow): The movement of individuals (and thus alleles) into or out of a population. Migration introduces new alleles to the recipient population and removes them from the source population, changing allele frequencies in both.
2. Genetic Drift: Random fluctuations in allele frequencies from one generation to the next, especially significant in small populations. Chance events can cause certain alleles to become more or less common, or even lost.
   • Founder Effect: A specific type of genetic drift that occurs when a small group of individuals separates from a larger population and establishes a new colony. The allele frequencies in the new founding population may differ significantly from the original population, potentially leading to the evolution of a new species over time.
3. Mutation: Changes in the DNA sequence. Mutations introduce new alleles into the gene pool, altering allele frequencies. While individual mutation rates are low, they are the ultimate source of all genetic variation.
4. Genetic Recombination: The shuffling of alleles during sexual reproduction (crossing over and independent assortment). This creates new combinations of alleles on chromosomes and in offspring, increasing genetic variation within the population (though it doesn't change overall allele frequencies by itself, it provides raw material for selection).
5. Natural Selection: Differential survival and reproduction of individuals based on their phenotype. Individuals with advantageous traits leave more offspring, increasing the frequency of the alleles for those traits in the population. Natural selection is non-random and acts on existing variation.

Natural selection can operate in different ways on a trait distribution within a population, as shown by population genetics studies:

• Stabilising Selection: Favours individuals with intermediate trait values, reducing variation. The distribution curve becomes narrower around the mean.
• Directional Selection: Favours individuals at one extreme of the trait distribution, shifting the mean trait value in one direction.
• Disruptive Selection: Favours individuals at both extremes of the trait distribution, selecting against intermediate values and potentially leading to the formation of two distinct subpopulations or even speciation.

A Brief Account Of Evolution

Key events in the history of life on Earth:

• Approx. 2000 million years ago (mya): First cellular forms of life appeared.
• Some early cells evolved the ability to perform photosynthesis, releasing oxygen into the atmosphere.
• Slowly, single-celled organisms evolved into multi-cellular forms.
• By 500 mya: Invertebrates emerged and became active.
• Approx. 350 mya: Jawless fish likely evolved. Seaweeds and some plants were present.
• Plants were among the first organisms to colonise land.
• Animals invaded land after plants were established. Fish with strong fins (lobefins) could move onto land and back to water (approx. 350 mya). A living fossil example of a lobefin is the Coelacanth, rediscovered in 1938 after being thought extinct.
• Lobefins are believed to have evolved into the first amphibians, capable of living on both land and water. Modern frogs and salamanders are their descendants.
• Amphibians evolved into reptiles. Reptiles laid thick-shelled eggs, adapted to dry conditions on land. Modern descendants include turtles, tortoises, and crocodiles.
• Over the next 200 million years (approx. 350 mya to 150 mya), reptiles diversified greatly, including the dominant dinosaurs. Some land reptiles evolved to live back in water (e.g., Ichthyosaurs, approx. 200 mya).
• Giant ferns were widespread and formed coal deposits.
• Approx. 65 mya: Dinosaurs suddenly disappeared (extinction event). The exact reasons are debated, possibly climate change or impact events. Some smaller reptiles and birds (potentially evolved from dinosaurs) survived.
• The first mammals evolved during the age of reptiles; they were small, shrew-like. Mammals were viviparous (giving birth to live young) and provided parental care, perhaps contributing to their survival. Mammals became dominant after the dinosaur extinction.
• Mammals in South America diversified. Continental drift played a significant role in distributing and isolating mammal populations. When South America joined North America, South American fauna faced competition from North American species.
• Pouched mammals (marsupials) survived and diversified particularly in Australia due to geographical isolation caused by continental drift, which limited competition from placental mammals.
• Some mammals adapted completely to aquatic life (whales, dolphins, seals).
• The evolution of specific animal lineages like horses, elephants, and dogs are detailed examples studied in higher classes.
• The evolution of humans, marked by the development of language skills and self-consciousness, is a significant evolutionary story.

---

Origin And Evolution Of Man

The evolutionary history of humans (hominin evolution) is based on fossil evidence and genetic studies.

• Approx. 15 mya: Primates like Dryopithecus and Ramapithecus existed. They were hairy and walked somewhat like gorillas and chimpanzees. Ramapithecus is considered more man-like, and Dryopithecus more ape-like.
• Fossils suggest that about 3-4 mya, man-like primates (hominids) with bipedal locomotion (walked upright) existed in eastern Africa. They were relatively short (around 4 feet).
• Approx. 2 mya: Australopithecines lived in East African grasslands. Evidence suggests they used stone weapons for hunting but primarily ate fruit.
• The first human-like being, Homo habilis, appeared around 2 mya. They had a brain capacity of $650-800$ cc. They likely did not eat meat.
• Homo erectus emerged about 1.5 mya. Fossils were found in Java in 1891. Homo erectus had a larger brain capacity, around $900$ cc, and likely ate meat.
• The Neanderthal man lived in the Near East and Central Asia between 1,00,000 and 40,000 years ago. They had a large brain size of $1400$ cc, used hides for clothing, and buried their dead.
• Homo sapiens (modern humans) arose in Africa and migrated to other continents, eventually developing into distinct races.
• Modern Homo sapiens appeared during the last ice age, between 75,000 and 10,000 years ago.
• Pre-historic cave art (e.g., Bhimbetka rock shelter in India) indicates the development of advanced cognitive and communication skills, appearing about 18,000 years ago.
• The development of agriculture around 10,000 years ago led to human settlements and the rise of civilisations.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: