Evolution

Origin Of Life

The origin of life on Earth is one of the most fundamental and complex questions in biology. It is widely accepted that life originated from non-living matter through a series of chemical and physical processes.

Origin of the Universe:

The universe is vast, containing galaxies, stars, and planets. The commonly accepted theory for the origin of the universe is the Big Bang theory. According to this theory, the universe originated about 20 billion years ago from a singular, extremely hot and dense point that exploded, leading to the expansion of the universe and the formation of galaxies, stars, and eventually planets.

Earth was formed about 4.5 billion years ago in the solar system.

Theories on Origin of Life:

Several theories have been proposed to explain the origin of life:

• Theory of Special Creation: States that life was created by a divine power. Has no scientific basis.
• Theory of Panspermia (Cosmozoic theory): Suggests that life came from outer space, possibly in the form of spores. While life might exist elsewhere, this doesn't explain the origin itself.
• Theory of Spontaneous Generation: States that life arose spontaneously from decaying and rotting matter. Disproven by Louis Pasteur's experiments.
• Theory of Biogenesis: States that life comes only from pre-existing life. However, this doesn't explain the origin of the very first life form.
• Oparin-Haldane Theory (Theory of Chemical Evolution): The most widely accepted scientific theory. Proposed independently by A.I. Oparin (Russia) and J.B.S. Haldane (England) in the 1920s. It states that life originated from simple inorganic molecules through a series of chemical reactions under the specific conditions of the primitive Earth.

Chemical Evolution (Oparin-Haldane Theory):

According to this theory, the conditions on primitive Earth were very different from today. The early atmosphere was reducing (lacked free oxygen) and contained gases like methane ($CH_4$), ammonia ($NH_3$), hydrogen ($H_2$), and water vapour ($H_2O$). Energy sources included lightning, UV radiation, and volcanic activity.

Steps proposed by Oparin and Haldane:

1. Formation of simple inorganic molecules: From elements present on early Earth.
2. Formation of simple organic molecules (monomers): Simple inorganic molecules reacted to form simple organic molecules like amino acids, simple sugars, nitrogenous bases, fatty acids. This occurred spontaneously in the reducing atmosphere with energy from lightning, UV radiation, etc.
3. Formation of complex organic molecules (polymers): Monomers polymerised to form complex organic molecules like proteins, polysaccharides, nucleic acids (RNA and DNA).
4. Formation of protobionts (primitive cell-like structures): These complex molecules aggregated and organised into structures with a boundary (membrane-like structure), capable of simple metabolic reactions and reproduction. Examples include coacervates (Oparin) and microspheres (Sidney Fox). These were not true cells but precursors.
5. Evolution of true cells: Protobionts developed into true cells with a genetic material (likely RNA initially, then DNA) and metabolic machinery.

The environment of early Earth, with its reducing atmosphere and abundant energy, provided the necessary conditions for the synthesis of organic molecules from inorganic precursors.

Miller's Experiment

In 1953, Stanley Miller and Harold Urey provided experimental evidence supporting the chemical evolution part of the Oparin-Haldane theory. They created an apparatus to simulate the conditions of the primitive Earth's atmosphere and energy sources.

Miller's Apparatus and Experiment:

• An airtight glass apparatus contained a mixture of gases (methane, ammonia, hydrogen, water vapour) thought to be present in the early atmosphere.
• Water was heated in a flask to simulate evaporation from primitive oceans.
• Electrodes were used to generate electric sparks (simulating lightning) through the gas mixture.
• A condenser cooled the gases, causing water and dissolved substances to condense and collect in a flask, simulating rain accumulating in oceans.
• The experiment was run for several weeks.

Results:

• They analysed the chemical composition of the liquid collected in the flask.
• They found that simple organic molecules, including several amino acids (like glycine, alanine, aspartic acid), simple sugars, and nitrogenous bases, were formed spontaneously from the inorganic gases.

*(Image shows a diagram of Miller's apparatus with a flask for boiling water, a chamber with electrodes for sparks, a condenser, and a collection flask, illustrating the circulation of gases and water)*

Significance:

• Miller's experiment provided strong experimental support for the idea that organic molecules, the building blocks of life, could have formed spontaneously from inorganic matter under primitive Earth conditions.
• Similar experiments by other scientists later produced other organic molecules like nucleotides and complex lipids.

While Miller's experiment didn't create life itself, it validated a key step in the chemical evolution hypothesis.

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First Life Forms

The first life forms are believed to have originated in the oceans. They were likely simple, single-celled organisms.

• These first cells were probably heterotrophs, obtaining energy and organic molecules from the organic 'soup' in the primitive oceans.
• They were likely anaerobic, as the early atmosphere lacked free oxygen.
• Over time, some of these early cells evolved the ability to produce their own food through photosynthesis. The earliest photosynthetic organisms were likely prokaryotes, perhaps similar to modern cyanobacteria.
• The evolution of photosynthesis led to the release of oxygen into the atmosphere, gradually changing it from a reducing to an oxidising atmosphere. This paved the way for the evolution of aerobic respiration and eukaryotic cells.
• The earliest undisputed fossil evidence of life dates back about 3.5 billion years, found in the form of stromatolites (layered structures formed by cyanobacteria).

The origin of life was a gradual process, starting with chemical evolution leading to the formation of complex organic molecules, followed by their self-organisation into primitive cell-like structures, and eventually the evolution of the first true cells capable of metabolism and reproduction.

Evolution Of Life Forms - A Theory

Once life originated, it began to diversify and change over time, leading to the vast array of organisms we see today. This change in the inherited traits of populations over successive generations is called evolution.

Evolution is not just a hypothesis; it is considered a theory in the scientific sense, meaning it is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. The theory of evolution explains the process by which life has changed and diversified since its origin.

Theory Of Special Creation

This is a non-scientific explanation for the diversity of life. It states that:

• All living organisms were created by a divine power (God) in their present form.
• Life forms have remained unchanged since creation.
• The diversity we see today is the same as it was at the time of creation.

This theory is not supported by scientific evidence and is outside the realm of scientific investigation.

Darwin'S Theory Of Natural Selection

The most widely accepted scientific theory explaining the mechanism of evolution is the Theory of Natural Selection, proposed by Charles Darwin. Darwin conducted extensive observations during his voyage on H.M.S. Beagle and compiled his ideas in his book "On the Origin of Species by Means of Natural Selection" (1859).

Key components of Darwin's theory:

1. Overproduction (High reproductive potential): Organisms tend to produce more offspring than can possibly survive.
2. Struggle for existence: Due to limited resources (food, shelter, space) and environmental challenges (predators, diseases), organisms compete with each other.
3. Variation: Individuals within a population exhibit variations in their traits. These variations are heritable (passed to offspring).
4. Survival of the fittest: Individuals with traits that are more advantageous (better adapted) to the specific environment are more likely to survive and reproduce successfully. The term "fitness" here refers to reproductive fitness.
5. Natural Selection: Nature selects individuals with favourable variations, allowing them to survive and pass their traits to the next generation. Over generations, the frequency of these favourable traits increases in the population.
6. Origin of New Species (Speciation): Accumulation of favourable variations over long periods, coupled with reproductive isolation, can lead to the formation of new species.

Darwin saw evolution as a gradual process of 'descent with modification' driven by natural selection.

Note: Alfred Russel Wallace also independently proposed a similar theory of natural selection.

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The theory of evolution by natural selection is a unifying principle in biology, explaining the diversity and adaptation of life forms over geological time scales. This theory is supported by extensive evidence from various fields of biology.

What Are The Evidences For Evolution?

The theory of evolution is supported by a vast body of evidence from various scientific disciplines. These evidences demonstrate that life has changed over time and that diverse organisms share common ancestry.

Paleontological Evidence

• Paleontology is the study of fossils. Fossils are the preserved remains or traces of organisms that lived in the past, found in sedimentary rocks.
• Fossil records provide a historical sequence of life forms, showing that organisms in the past were different from those living today.
• Older rock layers generally contain fossils of simpler organisms, while younger rock layers contain fossils of more complex organisms.
• Fossils of intermediate or transitional forms demonstrate links between different groups of organisms (e.g., *Archaeopteryx*, a fossil showing features of both reptiles and birds).
• Dating of fossils (using radiometric methods like carbon dating) helps in determining the geological time periods when organisms lived.

*(Image shows examples of different types of fossils or a simplified geological time scale with corresponding life forms)*

Embryological Evidence

• Comparing the embryonic development of different organisms reveals similarities that suggest common ancestry.
• Example: The embryos of vertebrates (fish, amphibians, reptiles, birds, mammals) show striking similarities during early stages of development, such as the presence of gill slits and a tail, even though these structures may be modified or absent in the adult.
• Ernst Haeckel's Biogenetic Law ("Ontogeny recapitulates phylogeny"): Proposed that the developmental history of an individual (ontogeny) repeats the evolutionary history of its ancestors (phylogeny). While this is an overstatement and not strictly true, the similarities in early embryonic development do provide evidence of common ancestry.

*(Image shows rows of embryos from different vertebrates, illustrating the similarities in their early stages)*

Comparative Anatomy (Homology And Analogy)

• Comparing the anatomy (structure) of different organisms provides evidence of common ancestry and adaptation.
• Homologous structures: Structures that have a common evolutionary origin and similar basic structural plan but may have different functions due to adaptation to different environments (divergent evolution). Indicate common ancestry.

  Example: The forelimbs of humans (grasping), whales (swimming), bats (flying), cheetahs (running). The bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges) is similar, indicating they evolved from a common ancestor.

  *(Image shows diagrams comparing the bone structure of vertebrate forelimbs)*

• Analogous structures: Structures that have different evolutionary origins and different basic structural plans but perform similar functions due to adaptation to similar environmental pressures (convergent evolution). Do not indicate common ancestry.

  Example: The wings of insects and birds (both for flying, but structurally very different), eye of octopus and mammal, flippers of penguins and dolphins.

  *(Image shows illustrations of insect wing and bird wing)*

• Vestigial organs: Reduced and non-functional organs in an organism that were functional in its ancestors. Provide evidence of evolutionary history.

  Example in humans: Vermiform appendix, wisdom teeth, body hair, nictitating membrane (third eyelid).

Biochemical Evidence

• Comparing biochemical molecules and metabolic processes in different organisms reveals similarities that reflect evolutionary relationships.
• Similarities in the basic genetic material (DNA), genetic code, and the central dogma of molecular biology (DNA $\rightarrow$ RNA $\rightarrow$ Protein) across diverse organisms are strong evidence for common ancestry.
• Comparing the amino acid sequences of homologous proteins (e.g., haemoglobin, cytochrome c) or the nucleotide sequences of homologous genes in different species. More similar sequences indicate closer evolutionary relationships.
• Similar metabolic pathways (e.g., glycolysis, Krebs cycle) in diverse organisms suggest a common origin.

Artificial Selection

• Humans have selectively bred plants and animals for desirable traits for thousands of years (e.g., domestication of wild animals, development of different crop varieties from wild ancestors, different breeds of dogs, cows, chickens).
• Artificial selection is a powerful demonstration of how differential reproduction based on specific traits can lead to significant changes in populations over relatively short periods. It provides a parallel to natural selection.

Example: Different varieties of cabbage, broccoli, cauliflower, kale, kohlrabi were developed from a common wild mustard ancestor through artificial selection for different plant parts.

*(Image shows illustrations of wild cabbage and different derived varieties like cabbage, broccoli, cauliflower, kale)*

Industrial Melanism

• A classic example of natural selection in action, observed in populations of peppered moths ($Biston \: betularia$) in England during the industrial revolution.
• Before industrialisation, the environment was clean, and the predominant form of the moth was light-coloured (white with dark spots), which was well camouflaged against lichen-covered tree bark. Dark-coloured (melanic) moths were rare.
• Industrial pollution caused tree trunks to become covered with soot, and lichens died. The dark-coloured moths became better camouflaged against the dark bark, while the light-coloured moths were easily spotted by predators (birds).
• Natural selection favoured the dark-coloured moths, and their population increased significantly, while the population of light-coloured moths decreased.
• After pollution control measures were implemented, tree trunks became lighter, and the trend reversed, with light-coloured moths increasing again.

*(Image shows a light-coloured and a dark-coloured peppered moth on a light, lichen-covered tree bark and on a dark, soot-covered tree bark, illustrating camouflage and predation)*

Evolution By Anthropogenic Action

• Evolution can also be driven by human activities other than deliberate artificial selection.
• Example: Resistance to pesticides in insects, resistance to antibiotics in bacteria, resistance to herbicides in weeds.
• When antibiotics are used, susceptible bacteria are killed, but resistant variants survive and reproduce, increasing the frequency of resistance genes in the bacterial population. This is a form of natural selection driven by human use of antibiotics.
• These are rapid examples of evolution occurring due to human influence on the environment.

These diverse lines of evidence strongly support the theory that life on Earth has evolved over millions of years, with existing species arising from pre-existing ones through processes like natural selection.

What Is Adaptive Radiation?

Adaptive radiation is an evolutionary process where a single ancestral species or group of species rapidly diversifies into multiple new species. These new species adapt to occupy different ecological niches (ways of life or roles in the environment).

Adaptive radiation typically occurs when organisms colonise a new environment with diverse resources and opportunities, or after a mass extinction event clears ecological niches.

It involves the evolutionary divergence of traits that enable the descendants to utilise different resources, reducing competition and allowing co-existence.

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

• Darwin's Finches: A classic example observed by Charles Darwin on the Galapagos Islands. A single ancestral species of finch colonised the islands. The different islands presented diverse environments and food sources (seeds of different sizes, insects, buds, etc.).
  • Over time, the finch population diversified into about 14 different species.
  • Each species evolved beak shapes and sizes adapted to its specific food source, reducing competition for food.
  • Example: Thick beaks for cracking large seeds, slender beaks for probing insects, finch with a beak like a woodpecker.

  *(Image shows illustrations of different finch species from the Galapagos Islands with various beak types and examples of the food they eat)*

• Australian Marsupials: Australia isolated for a long time. Numerous marsupial species evolved from a single ancestral marsupial. They diversified to occupy various ecological niches available on the continent.
  • Examples: Kangaroo, Koala, Wombat, Tasmanian devil, Bandicoot, Marsupial mole, Marsupial mouse.
  • These marsupials show convergent evolution with placental mammals that evolved on other continents, occupying similar niches (e.g., Marsupial mole vs. Placental mole, Marsupial mouse vs. Placental mouse). This is an example of convergent evolution resulting from adaptive radiation in isolated regions.

  *(Image shows illustrations of several Australian marsupials and possibly pairs illustrating convergence with placental mammals)*

Adaptive radiation is a powerful evolutionary process that can lead to the formation of significant biodiversity in relatively short geological periods.

Biological Evolution

Biological evolution is the process of cumulative change in the heritable characteristics of biological populations over successive generations. It is the central concept in biology, explaining the history of life and the relationships between different species.

Darwinian Theory And Natural Selection

As discussed earlier, Charles Darwin proposed natural selection as the primary mechanism driving biological evolution (Descent with Modification). The key aspects are:

• Populations vary in their inherited traits.
• More offspring are produced than can survive.
• Individuals with traits better suited to the environment survive and reproduce at higher rates.
• Favourable traits accumulate in the population over generations.

Darwin's theory highlighted the role of variation and environmental selection in shaping populations over time.

Lamarckian Theory

Before Darwin, Jean-Baptiste Lamarck proposed a theory of evolution known as the Theory of Inheritance of Acquired Characteristics.

Key ideas of Lamarck's theory:

• Use and Disuse: Organs that are used more frequently become stronger and larger, while those that are used less frequently become weaker and smaller.
• Inheritance of Acquired Characteristics: Characteristics acquired by an individual during its lifetime (due to use or disuse or environmental influence) are passed on to its offspring.

Example given by Lamarck: The long neck of giraffes evolved because ancestral giraffes stretched their necks to reach leaves on tall trees (use), and this acquired long neck was inherited by their offspring.

Lamarck's theory of inheritance of acquired characteristics has been disproven by experiments (e.g., August Weismann's experiments with cutting tails of mice for many generations - the offspring still had tails). Changes acquired during an organism's lifetime are generally not heritable unless they somehow affect the germ cells (which is not the mechanism proposed by Lamarck).

Modern evolutionary theory (Neo-Darwinism) combines Darwin's ideas of natural selection with genetics, recognising mutation as the source of variation and understanding the molecular basis of inheritance.

Mechanism Of Evolution

Evolution occurs at the level of populations, not individuals. It is essentially a change in the genetic makeup of a population over time, specifically changes in the frequencies of different alleles of genes.

The primary factors or mechanisms that cause changes in allele frequencies in a population and thus drive evolution are:

• Mutation: Original source of new alleles and variation.
• Gene Flow (Gene Migration): Movement of alleles between populations due to migration of individuals or gametes. Can introduce new alleles or change existing allele frequencies.
• Genetic Drift: Random fluctuations in allele frequencies from one generation to the next, especially significant in small populations. Can lead to the loss of alleles or fixation of others.
  • Bottleneck Effect: A drastic reduction in population size (e.g., due to a natural disaster) leading to a non-representative sample of alleles surviving.
  • Founder Effect: A small group of individuals from a larger population colonises a new area, carrying only a subset of the original population's alleles.
• Genetic Recombination: Reshuffling of alleles during sexual reproduction (crossing over and independent assortment), creating new combinations of alleles on chromosomes and in offspring.
• Natural Selection: Differential survival and reproduction of individuals based on their phenotype, leading to the increase in frequency of advantageous alleles.

Mutation Theory (DeVries)

Hugo de Vries, one of the scientists who rediscovered Mendel's work, proposed the Mutation Theory of Evolution based on his observations of the evening primrose (*Oenothera lamarckiana*).

Key ideas of De Vries's theory:

• Evolution is driven by mutations.
• Mutations are sudden, discontinuous changes in the genetic material.
• Mutations are the source of variation.
• Evolution is a jerky and discontinuous process (saltation - single large mutation).

De Vries's theory emphasised the role of mutations as the source of new traits, contrasting with Darwin's focus on small, continuous variations. Modern evolutionary theory integrates both concepts, recognising mutations as the source of variation and natural selection acting on this variation (along with other factors) as the main driver of evolutionary change.

Evolutionary change is the result of these mechanisms altering the genetic composition of populations over successive generations.

Hardy - Weinberg Principle

The Hardy-Weinberg principle (or equilibrium) is a fundamental concept in population genetics. It states that in a large, randomly mating population, the allele frequencies and genotype frequencies will remain constant from generation to generation in the absence of evolutionary influences.

Hardy-Weinberg Equation:

Consider a gene with two alleles, a dominant allele 'A' and a recessive allele 'a', in a population.

Let the frequency of allele 'A' in the population be represented by p.

Let the frequency of allele 'a' in the population be represented by q.

According to the principle, the sum of allele frequencies for all alleles of a gene in a population is 1.

$ p + q = 1 $

In a randomly mating population, the frequencies of the three possible genotypes (AA, Aa, aa) will remain constant and can be represented by the equation:

$ p^2 + 2pq + q^2 = 1 $

Where:

• $p^2$ = Frequency of the homozygous dominant genotype (AA)
• $2pq$ = Frequency of the heterozygous genotype (Aa)
• $q^2$ = Frequency of the homozygous recessive genotype (aa)

This equation describes the genotype frequencies in a population that is in Hardy-Weinberg equilibrium.

*(Image shows a Punnett square illustrating how random mating of individuals with allele frequencies p and q leads to genotype frequencies p^2, 2pq, and q^2)*

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

The Hardy-Weinberg principle describes an ideal, hypothetical situation where no evolution is occurring. In reality, populations are rarely in perfect Hardy-Weinberg equilibrium because evolutionary influences are always at play. Deviation from the Hardy-Weinberg equilibrium indicates that evolution is occurring.

The factors that can cause a deviation from the equilibrium (i.e., cause evolution) are the same mechanisms of evolution discussed earlier:

• Gene Flow: Introduction or removal of alleles from a population due to migration. Changes allele frequencies.
• Genetic Drift: Random changes in allele frequencies, particularly significant in small populations. Can lead to fixation or loss of alleles.
• Mutation: Creates new alleles, changing allele frequencies.
• Genetic Recombination: Although it creates new combinations of alleles, recombination itself does not change allele frequencies unless coupled with other factors like selection.
• Natural Selection: Differential survival and reproduction based on phenotype, leading to non-random changes in allele frequencies.

For a population to be in Hardy-Weinberg equilibrium, the following conditions must be met:

• No mutation
• No gene flow
• Large population size (to avoid genetic drift)
• Random mating
• No natural selection

Since these conditions are rarely met perfectly in natural populations, allele and genotype frequencies usually change over time, meaning evolution is occurring.

A Brief Account Of Evolution

The history of life on Earth spans billions of years, marked by major evolutionary transitions and the rise and fall of diverse life forms. This section provides a brief overview of the timeline of evolution.

Major Milestones in the History of Life:

• Origin of Universe: $\approx$ 20 billion years ago.
• Origin of Earth: $\approx$ 4.5 billion years ago.
• Origin of Life (Chemical Evolution): $\approx$ 4 billion years ago. Formation of first simple organic molecules.
• First Non-cellular Life: $\approx$ 3.8-3.9 billion years ago. Protobionts, RNA World.
• First Cellular Life (Prokaryotes): $\approx$ 3.5 billion years ago. Simple bacteria, anaerobic heterotrophs, later anaerobic photosynthesizers.
• Evolution of Oxygenic Photosynthesis (Cyanobacteria): $\approx$ 2.5-3 billion years ago. Release of oxygen into the atmosphere.
• Evolution of Eukaryotic Cells: $\approx$ 1.5-2 billion years ago. Likely through endosymbiosis (mitochondria and chloroplasts originating from prokaryotes living inside larger host cells).
• Evolution of Multicellularity: $\approx$ 1 billion years ago. Simple multicellular organisms evolve from unicellular eukaryotes.
• First Animals (Invertebrates): $\approx$ 600-800 million years ago. Simple invertebrates (sponges, jellyfish). Followed by the Cambrian explosion (around 541 million years ago) - rapid diversification of animal phyla.
• Colonisation of Land: $\approx$ 400-500 million years ago. Plants colonise land, followed by animals (arthropods, vertebrates).
• Evolution of Vertebrates: Fishes (first vertebrates) $\approx$ 500 million years ago. Amphibians (first vertebrates on land) $\approx$ 350-400 million years ago. Reptiles (first fully terrestrial vertebrates) $\approx$ 300-350 million years ago. Dinosaurs dominated during the Mesozoic era.
• Evolution of Birds: From reptilian ancestors $\approx$ 150 million years ago.
• Evolution of Mammals: From reptilian ancestors, existed alongside dinosaurs, diversified after dinosaur extinction (around 66 million years ago).
• Evolution of Humans: From ape-like ancestors $\approx$ 5-7 million years ago.

Geological Time Scale and Major Events:

The history of Earth and life is divided into major eras:

• Precambrian Eon: From Earth's formation to $\approx$ 541 million years ago. Includes the origin of life, prokaryotes, eukaryotes, and early multicellular life.
• Phanerozoic Eon: From $\approx$ 541 million years ago to present. Characterised by abundant complex life and major evolutionary radiations.
  • Paleozoic Era ($\approx$ 541-252 million years ago): Cambrian explosion, colonisation of land, evolution of fishes, amphibians, reptiles.
  • Mesozoic Era ($\approx$ 252-66 million years ago): Age of Reptiles (dinosaurs), origin of mammals and birds, origin of flowering plants.
  • Cenozoic Era ($\approx$ 66 million years ago to present): Age of Mammals and Birds, diversification of flowering plants, evolution of primates and humans.

*(Image shows a simplified geological time scale (vertical or horizontal) with major eons/eras and annotations indicating key events like origin of life, first prokaryotes, eukaryotes, multicellularity, colonisation of land, first vertebrates, age of dinosaurs, age of mammals, human evolution)*

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Evolution is a continuous process, constantly shaping the diversity of life on Earth.

Origin And Evolution Of Man

The evolution of humans is a fascinating story of descent from ape-like ancestors through a series of intermediate forms over millions of years. Humans ($Homo \: sapiens$) belong to the order Primates.

Ancestral Forms and Key Evolutionary Trends:

• Primates originated in the Cenozoic era.
• Humans and chimpanzees are closely related, sharing a common ancestor relatively recently in evolutionary history (estimated 5-7 million years ago).
• Early human ancestors (hominins) evolved key traits that distinguished them from apes:
  • Bipedalism: Walking upright on two legs. This freed the hands.
  • Increase in cranial capacity and brain size: Associated with increased intelligence and cognitive abilities.
  • Tool use: Development and use of tools.
  • Language: Evolution of complex communication.
  • Changes in jaw and teeth structure, facial structure.

Major Hominin Fossils (Timeline is approximate and debated):

• Dryopithecus and Ramapithecus: Lived about 15 million years ago. Considered ape-like and human-like, respectively.
• Australopithecus: Lived in East Africa about 2 million years ago. Were essentially ape-like but walked upright. Their brain capacity was around $400-600 \text{ cm}^3$. They hunted with stone weapons and ate fruits.
• ***Homo habilis***: Lived about 2 million years ago. Considered the 'handy man', possibly the first true human. Brain capacity around $650-800 \text{ cm}^3$. Did not eat meat.
• ***Homo erectus***: Lived about 1.5 million years ago. Brain capacity around $900 \text{ cm}^3$. Ate meat. Possibly used fire.
• ***Homo neanderthalensis***: Lived in East and Central Asia, around 100,000-40,000 years ago. Brain capacity around $1400 \text{ cm}^3$ (similar to modern humans). Used hides to protect their bodies and buried their dead.
• ***Homo sapiens***: Arose in Africa. Brain capacity around $1400 \text{ cm}^3$. Cro-Magnon man appeared about 35,000 years ago, representing early *Homo sapiens*. Modern *Homo sapiens* arose around 10,000 years ago. Spread to different parts of the world. Developed agriculture and settled communities.

Hominin	Approximate Time Period (Million Years Ago)	Key Characteristics (Approximate Brain Capacity)
Dryopithecus/Ramapithecus	$\approx$ 15 mya	Ape-like/Human-like
Australopithecus	$\approx$ 2 mya	Walked upright, $400-600 \text{ cm}^3$ brain
*Homo habilis*	$\approx$ 2 mya	First humans(?), $650-800 \text{ cm}^3$ brain
*Homo erectus*	$\approx$ 1.5 mya	Ate meat, $900 \text{ cm}^3$ brain
*Homo neanderthalensis*	$\approx$ 0.1 - 0.04 mya (100,000 - 40,000 years ago)	$1400 \text{ cm}^3$ brain, buried dead
*Homo sapiens*	Arose $\approx$ 0.01 mya (10,000 years ago) to present (earlier forms $\approx$ 35,000 years ago)	$1400 \text{ cm}^3$ brain, modern human

*(Note: The exact dates and relationships between these forms are subject to ongoing research and debate based on new fossil discoveries and genetic analysis.)*

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The evolutionary journey of humans is a relatively recent event in the vast history of life on Earth, marked by significant changes in morphology, behaviour, and intelligence, leading to the emergence of modern humans.