When observing stars in the night sky, we are essentially looking back in time due to the vast distances involved.
The universe is incredibly old, approximately 20 billion years, and Earth is just a small part of it.
The Big Bang theory explains the origin of the universe, starting with a massive explosion.
Earth formed about 4.5 billion years ago within the Milky Way galaxy.
Initially, Earth had no atmosphere; gases like water vapor, methane, carbon dioxide, and ammonia covered the surface.
The sun’s UV rays split water into hydrogen and oxygen, with hydrogen escaping into space.
Oxygen combined with ammonia and methane, forming water and CO2, and the ozone layer developed.
As Earth cooled, water vapor fell as rain, filling depressions and forming oceans.
Life appeared around 500 million years after Earth’s formation, roughly four billion years ago.
The theory of panspermia suggests that life might have come from outer space, with early Greek thinkers proposing the transfer of life spores to different planets.
The idea of spontaneous generation, where life arises from decaying matter, was debunked by Louis Pasteur’s experiments.
Oparin and Haldane proposed that the first life forms could have arisen from non-living organic molecules through chemical evolution, driven by high temperatures, volcanic storms, and a reducing atmosphere.
In 1953, Miller recreated these conditions in a laboratory experiment, producing amino acids and other organic compounds.
Similar experiments and meteorite analysis supported the concept of chemical evolution.
The origin of the first self-replicating metabolic capsule of life remains unknown.
The first non-cellular life forms might have emerged around three billion years ago, possibly as giant molecules like RNA, proteins, and polysaccharides.
The first cellular life forms likely appeared around two billion years ago and were likely single-celled organisms.
All early life forms existed in water environments.
The prevailing view is that life slowly evolved from non-living molecules through evolutionary forces, leading to the complex biodiversity observed today.
Evolution of Life Forms – A Theory
Conventional religious literature presents the theory of special creation, which has three key ideas:
All living organisms were created in their current forms.
The diversity of life has remained constant since creation and will remain so in the future.
Earth is approximately 4000 years old.
During the 19th century, these ideas were strongly challenged.
Charles Darwin, based on observations from his voyage on the H.M.S. Beagle, concluded that existing living forms share similarities, not only among themselves but also with life forms that existed millions of years ago.
Many extinct life forms no longer exist.
Extinctions and the emergence of new life forms have occurred throughout Earth’s history.
Darwin proposed the theory of gradual evolution, stating that any population has inherent variations in characteristics.
Individuals with traits better suited to their natural environment (e.g., climate, food, physical factors) tend to produce more offspring.
Reproductive fitness is the key to survival, and those with higher fitness are selected by nature, a concept known as natural selection.
Alfred Wallace, a naturalist in the Malay Archipelago, independently arrived at similar conclusions around the same time.
Over time, apparently new types of organisms emerge.
All existing life forms share similarities and common ancestors, which existed at different periods in Earth’s history, including epochs, periods, and eras.
The geological history of Earth closely aligns with its biological history.
The consensus is that Earth is not thousands of years old, as previously believed, but billions of years old.
Evidences for Evolution
Fossils are preserved remains of life forms found in rocks. Different rock layers contain fossils of life forms from different time periods.
The study of fossils in various sedimentary layers helps determine the geological period in which different life forms existed.
Fossils show that life forms have changed over time, and some species have gone extinct while new ones have emerged.
Paleontological evidence is derived from the study of fossils and provides crucial support for the concept of evolution.
Embryological support for evolution was proposed by Ernst Haeckel based on the presence of common features during the embryonic stage of vertebrates.
Karl Ernst von Baer refuted this proposal, emphasizing that embryos do not pass through the adult stages of other animals.
Comparative anatomy and morphology reveal both similarities and differences among organisms, providing clues about common ancestry.
Homologous structures, such as the similar bone patterns in the forelimbs of mammals, indicate shared ancestry and divergent evolution.
Analogous structures, like the wings of butterflies and birds, perform similar functions but have different anatomical structures, resulting from convergent evolution.
Biochemical similarities in proteins and genes among diverse organisms support the idea of common ancestry.
Artificial selection, as seen in the breeding of plants and animals by humans, demonstrates that new breeds can be created within a short time frame.
Observations of moth populations in England before and after industrialization showed how natural selection operates. Dark-winged moths thrived when tree trunks darkened due to pollution, while white-winged moths survived in lichen-covered environments.
Lichens can serve as pollution indicators because they do not grow in polluted areas.
Selection for resistance to herbicides, pesticides, antibiotics, and drugs in organisms occurs rapidly, indicating the potential for evolution in response to human activities.
Evolution is a stochastic process driven by chance events and mutations, not a predetermined or directed process.
Adaptive Radiation
Adaptive radiation is a phenomenon in evolutionary biology where multiple species evolve from a common ancestor to exploit different ecological niches or habitats.
Charles Darwin observed an example of adaptive radiation during his visit to the Galapagos Islands, particularly in the case of small black birds known as Darwin’s Finches.
In the Galapagos Islands, Darwin noticed that there were several varieties of finches on the same island, each with different beak shapes and adapted to different diets.
These finches evolved from a common ancestor, and their beak variations allowed them to exploit various food sources, including seeds, insects, and vegetation.
Adaptive radiation occurs when species diversify and adapt to different environmental conditions within a specific geographical area.
The process involves the emergence of new species with distinct characteristics, often resulting from natural selection and competition for resources.
Darwin’s finches serve as an excellent example of adaptive radiation, where different species evolved from a common ancestor, radiating out to exploit various habitats and food sources.
Another example of adaptive radiation can be seen in Australian marsupials, where diverse marsupial species evolved from a common ancestor, each adapted to different ecological niches within the Australian continent.
When multiple instances of adaptive radiation occur in isolated geographical areas, resulting in different habitats, it is referred to as convergent evolution.
Convergent evolution can lead to the development of similar features or adaptations in different species, even if they are not closely related.
For example, placental mammals in Australia underwent adaptive radiation, producing varieties of placental mammals that appear similar to corresponding marsupial species (e.g., Placental wolf and Tasmanian wolf marsupial).
Biological Evolution
Biological evolution, driven by natural selection, likely began with the emergence of cellular life forms on Earth.
Darwin’s theory of evolution is centered around the concept of natural selection.
The rate at which new forms appear is tied to the life cycle and lifespan of organisms.
Microbes with fast division rates can multiply rapidly, whereas larger organisms like fish or birds have longer life spans and evolve over millions of years.
Darwin’s theory emphasizes the idea that individuals with characteristics better suited to their environment (fitness) are more likely to survive and reproduce.
In a population of bacteria, for example, variations in their ability to utilize a specific food component can lead to the emergence of new species within days if the medium composition changes.
Fitness is a measure of an organism’s ability to adapt and survive, and it is based on inherited characteristics.
Organisms that are better adapted to hostile environments have a genetic basis for their adaptive ability.
Adaptive traits are inherited, and fitness is the outcome of the ability to adapt and be selected by nature.
Darwinian Theory of Evolution is based on two key concepts: branching descent (common ancestry) and natural selection.
Before Darwin, French naturalist Lamarck proposed an idea that evolution was driven by the use and disuse of organs, such as giraffes elongating their necks to reach leaves on tall trees. However, this idea is no longer accepted.
Evolution can be viewed as either a process or the result of a process. When describing the world, we see it as the outcome of evolution; when describing the story of life on Earth, we consider evolution as a consequence of natural selection.
The work of Thomas Malthus on populations, particularly the concept of limited resources and competition, likely influenced Darwin’s thinking.
Natural selection is based on factual observations, including limited natural resources, stable population sizes with seasonal fluctuations, variations among individuals, and the inheritance of most variations.
Darwin’s brilliant insight was that heritable variations that improve resource utilization in some individuals enable them to reproduce more successfully, leading to changes in population characteristics and the emergence of new forms over many generations.
Darwin’s theory of natural selection revolutionized our understanding of how life on Earth has evolved and continues to shape our understanding of biology and biodiversity.
Mechanism of Evolution
The mechanism behind the origin of variation and the process of speciation has been a topic of scientific investigation.
Charles Darwin’s theory of evolution, although groundbreaking, did not address the specific mechanisms behind variation.
Gregor Mendel’s work on inheritable “factors” influencing phenotype was known during Darwin’s time, but Darwin did not integrate this knowledge into his theory.
In the early 20th century, Hugo de Vries proposed the idea of mutations as a mechanism for evolution based on his studies of evening primrose.
De Vries believed that mutations, which are sudden and large differences arising within a population, were the driving force behind evolution, as opposed to the minor, heritable variations that Darwin discussed.
Mutations are random and occur without a specific direction, while Darwinian variations are small and tend to be directional.
For Darwin, evolution was a gradual process, whereas de Vries believed that mutations could cause speciation in a single step, which he called saltation.
Later studies in population genetics helped clarify the mechanisms of evolution, including the role of mutations, natural selection, genetic drift, and gene flow in shaping the genetic makeup of populations over time.
Hardy-Weinberg Principle
The Hardy-Weinberg principle is a fundamental concept in population genetics that describes the stability of allele frequencies in a population over generations.
This principle states that allele frequencies in a population remain constant from generation to generation, assuming specific conditions are met.
The gene pool, which consists of all the genes and their alleles in a population, remains constant, resulting in genetic equilibrium.
The principle is expressed using algebraic equations. In a diploid population, the frequency of alleles can be represented as p and q, where p represents the frequency of allele A, and q represents the frequency of allele a.
The frequency of individuals with homozygous AA genotype is denoted as p^2, while the frequency of individuals with homozygous aa genotype is q^2. The frequency of heterozygous Aa individuals is 2pq.
The equation p^2 + 2pq + q^2 = 1 represents the Hardy-Weinberg equilibrium, where the sum of all allelic frequencies equals 1.
If observed allele frequencies in a population differ from the expected values under the Hardy-Weinberg equilibrium, this difference indicates the extent of evolutionary change.
Several factors can affect Hardy-Weinberg equilibrium, leading to changes in allele frequencies in a population:
Gene migration or gene flow: The movement of genes between populations can change allele frequencies.
Genetic drift: Random changes in allele frequencies can occur over time, especially in small populations.
Mutation: New alleles can arise due to mutations, leading to changes in allele frequencies.
Genetic recombination: During gametogenesis, the shuffling of alleles can result in different combinations.
Natural selection: Variations that provide better survival and reproduction may lead to changes in allele frequencies, resulting in evolutionary adaptations.
In some cases, genetic drift can be so significant that a new population becomes isolated, and its allele frequencies change substantially from the original population, leading to the founder effect.
Natural selection can result in various outcomes, including stabilizing selection (where more individuals acquire mean character values), directional change (where more individuals acquire values away from the mean), or disruptive selection (where more individuals acquire values at both ends of the distribution curve).
Overall, the Hardy-Weinberg principle and factors affecting genetic equilibrium help us understand the mechanisms of evolution and the changes in allele frequencies within populations over time.
A Brief Account of Evolution
Approximately 2 billion years ago (mya), the first cellular life forms appeared on Earth. The process by which non-cellular aggregates of giant macromolecules evolved into cells with membranous envelopes is not fully understood.
Some of these early cells had the ability to release oxygen (O2), possibly through a process similar to the light reaction in photosynthesis, where water is split using solar energy captured by light-harvesting pigments.
Over time, single-celled organisms evolved into multicellular life forms.
Around 500 mya, invertebrates emerged and became active.
Jawless fish likely evolved around 350 mya.
Seaweeds and a few plants were present around 320 mya.
The first organisms to colonize land were plants, which were already widespread when animals ventured onto land.
Fish with strong fins could move on land and return to water, marking the transition to land life, around 350 mya.
In 1938, a Coelacanth fish was discovered in South Africa, a species previously thought to be extinct. These lobefin fish were ancestors of modern-day frogs and salamanders.
Amphibians, capable of living on both land and in water, evolved from lobefin fish.
These amphibians eventually gave rise to reptiles, which laid thick-shelled eggs that did not dry up in the sun, unlike amphibian eggs.
Reptiles of various sizes dominated Earth for about 200 million years.
Giant ferns (pteridophytes) were present during this time and eventually formed coal deposits.
Some land reptiles returned to the water and evolved into fish-like reptiles, such as Ichthyosaurs.
The most famous land reptiles were the dinosaurs, including the massive Tyrannosaurus rex.
Around 65 mya, dinosaurs suddenly disappeared from the Earth, with various theories proposed, including climatic changes and some evolving into birds.
Small-sized reptiles from that era still exist today.
The first mammals were shrew-like and small-sized.
Mammals were viviparous (giving birth to live offspring) and protected their young inside the mother’s body.
As reptiles declined, mammals became the dominant group on Earth.
In South America, mammals resembling horses, hippos, bears, rabbits, and others existed. When South America joined North America due to continental drift, these animals were replaced by North American fauna.
Due to continental drift, pouch-bearing mammals (marsupials) in Australia survived without competition from other mammals.
Some mammals, such as whales, dolphins, seals, and sea cows, adapted to aquatic life.
The evolution of specific mammals like horses, elephants, and dogs is a complex and fascinating story covered in more advanced classes.
Human evolution is one of the most successful stories, marked by the development of language skills and self-consciousness.
Origin and Evolution of Man
About 15 million years ago (mya), primates known as Dryopithecus and Ramapithecus existed. They were hairy and had a gait similar to gorillas and chimpanzees. Ramapithecus displayed more human-like characteristics, while Dryopithecus was more ape-like.
Fossils from Ethiopia and Tanzania have been discovered, showing hominid features and suggesting that around 3-4 million years ago, human-like primates walked in eastern Africa. These creatures were not very tall, perhaps not exceeding 4 feet, but they walked upright. They are considered the first human-like beings and are categorized as Homo habilis. Their brain capacities ranged between 650-800cc, and they likely did not primarily consume meat.
Australopithecines, living about 2 million years ago in East African grasslands, are believed to have hunted with stone weapons but primarily ate fruit. Some fossils among the discovered bones were distinct, leading to the classification of Homo habilis.
Homo erectus, with a larger brain size of around 900cc, appeared approximately 1.5 million years ago. They are believed to have had a diet that included meat.
Neanderthal humans, with a brain size of 1400cc, inhabited the near east and central Asia between 100,000-40,000 years ago. They used hides for clothing and buried their dead.
Homo sapiens originated in Africa and later migrated across continents, diversifying into distinct races.
During the ice age, which occurred between 75,000-10,000 years ago, modern Homo sapiens emerged.
Prehistoric cave art, dating back approximately 18,000 years, provides insights into the creativity and expression of early humans. Notable cave paintings, created by prehistoric humans, can be found in the Bhimbetka rock shelter in Raisen district, Madhya Pradesh, India.
Agriculture began approximately 10,000 years ago, leading to the establishment of human settlements.
The subsequent course of human history includes the growth and decline of civilizations, marking the development of complex societies and cultures across the globe.
