The Evolutionary Process

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### Introduction The diversity of living organisms on Earth is a result of the evolutionary process. Understanding evolution requires considering the origin of the universe and life itself. ### Origin of the Earth The Earth is believed to have originated approximately 4.5 billion years ago. #### Nebular Theory The first scientific explanation for the origin of the solar system. It posits that small particles in the universe clumped together due to gravity, forming galaxies, the sun, and other planets. #### Big Bang Theory Considered the modern theory for the origin of the Earth. According to this theory, the universe began as an energy source with immense energy. A giant explosion created large clouds of dust particles, which then clumped together to form galaxies. Our solar system is believed to have formed in the Milky Way galaxy. Initially, Earth was extremely hot with high volcanic activity. As it cooled, dense, less volatile metals formed the core, and later, the crust was created by light silicon rocks. #### Early Earth Atmosphere Various elements at the center of the Earth reacted, forming different gases. The early atmosphere contained gases like Carbon dioxide ($CO_2$), Methane ($CH_4$), and Hydrogen sulphide ($H_2S$). A significant fact is the absence of Oxygen ($O_2$) in the early atmosphere. The extreme heat caused water on the planet to evaporate, condense into clouds, and fall as continuous heavy rain for many years. This mineral-rich rainwater collected in lowlands, forming the oceans. ### Origin of Life on Earth Life on Earth is believed to have originated about 3.5 billion years ago. Several theories attempt to explain this: #### Theory of Special Creation This theory suggests that all living organisms were created by a supernatural power. It lacks scientific evidence and is not accepted by scientists. #### Spontaneous Generation Theory This theory proposed that life originated spontaneously from non-living matter. - **Examples:** - Rats from pieces of clothes - Maggots from rotten meat - Weevils from decayed wood This theory was disproved by experiments conducted by Louis Pasteur. #### Cosmozoic Theory (Panspermia) This theory suggests that living materials might have reached Earth via meteors containing living organisms or spacecraft from other planets. This theory has not been scientifically proven. #### Theory of Biochemical Evolution This theory suggests that early Earth's atmospheric gases reacted, forming ingredients necessary for life. Energy for these reactions came from electric discharges (lightning), volcanic eruptions, and UV radiation from the sun. These materials dissolved in rainwater, collected in oceans, forming what is known as the **primordial soup**. The first living cell, or **pre-cell**, emerged from a biochemical reaction in the primordial soup. This first organism was unicellular, anaerobic, and heterotrophic. Experimental evidence supports the formation of the first living cell from primordial soup. #### Evolution of Life Forms - The first life form was simple unicellular bacteria. - Then, photosynthetic organisms (unicellular algae) emerged, leading to an oxygen-containing atmosphere. - Unicellular organisms evolved into multicellular organisms, with tissues, organs, and systems specializing, creating the animal and plant kingdoms. - **First Multicellular Organisms:** Cnidarians, some annelids, and arthropods. - **Vertebrate Evolution:** Fish were the first vertebrates. Amphibians originated from fish and were the first vertebrates to colonize terrestrial environments. Reptiles evolved from amphibians and were better adapted to terrestrial life. Birds and mammals are believed to have evolved from reptiles. - **Human Evolution:** The evolution of humans began about 12 million years ago, with modern humans originating approximately 5 million years ago. - **Plant Evolution:** Photosynthetic algae created flora in the oceans. Evolutionary less developed plants, then non-flowering plants, and finally flowering plants emerged. ### Evolution Evolution involves continuous changes in living organisms over time in response to environmental changes. #### Evidences for Evolution Various evidences support conclusions about evolution: - **Geographical animal distribution (Biogeography)** - **Comparative anatomy** - **Fossil study (Paleontology)**: Fossil records provide detailed evidence of changes in living organisms over time. #### Fossils Fossils are the preserved remains, parts, or traces (footprints, imprints on shells) of dead organisms found in rocks, ice, peat bogs, volcanic ash, and mud. **Examples of Fossilization:** - **Hard parts:** Skeletons, teeth, or shells. Bone parts decompose, mud seeps in, and under pressure, the bone turns into bony rock. - **Molds and Casts:** Dead animals deposited in mud decay, and the decayed matter is removed as gas, leaving a cavity. This cavity fills with materials like silica, forming a fossil with the original shape. - **Preserved in Resins:** External skeletons of some insects are preserved and fossilized in tree resins. - **Preserved in Ice:** Bodies of extinct animals like mammoths are found preserved in ice in polar regions. - **Trace Fossils:** Footprints of dinosaurs preserved under extreme environmental conditions are also considered fossils. #### Radioactive Carbon Dating Radioactive carbon ($^{14}C$) is an isotope used to determine the age of fossils. #### Evolution of Horse The evolutionary history of horses is well-documented due to extensive fossil records. The ancestor of the modern horse, which lived in North America 54 million years ago, was about 40 cm tall, dog-shaped, and had small legs with three toes on the front. Gradual changes occurred in its locomotion and feeding system over time. ### Importance of Evolution in Biodiversity Competition for limited resources leads to natural selection, where successful organisms thrive and their populations increase. This process contributes significantly to biodiversity. Novel species can evolve from existing ones through **speciation**, further enhancing biodiversity. #### Charles Darwin Considered the **father of evolution**, Charles Darwin proposed the **Natural Selection theory**, a scientifically accepted theory of evolution. ### Summary - **Origin of Earth:** Approximately 4.5 billion years ago. - **Origin of Life:** Approximately 3.5 billion years ago. - **Accepted Theory of Life's Origin:** Result of biochemical reactions. - **First Life Form:** Unicellular organism in the sea, which developed into multicellular organisms. - **Evolution:** Gradual development from simple to complex organisms. - **Evidence:** Fossil records are the primary detailed evidence for evolution. - **Living Fossils:** Organisms that have not undergone significant evolutionary changes over millions of years. - **Horse Evolution:** An organism with complete fossil evidence of its evolution. - **Human Evolution Evidence:** Important evidence found in Sri Lanka.