The Origins of Life • The Big Bang occurred about 18 billion years ago, creating the universe. • The Earth formed about 4.6 billion years ago. • Life on Earth probably originated between 3.5 and 4 billion years ago. Evidence of First life Stromatolites • Stromatolites are mounds of banded sediment formed by mobile cyanobacteria. • They are found in the warm waters of Australia and off the coast of Baja, Mexico. Evidence of First Life Stromatolites • Stromatolites have been found that date back to 3.5 billion years ago. • They are strikingly similar to those produced by modern day bacteria. Evidence of First Life Fossils • Fossilized filamentous bacteria has also been found that dates back to 3.4 billion years. • This fossilized prokaryote was found in a 3.4 billion year old stromatolite in Western Australia. Early Earth • When the Earth first formed, it was very different from its current state. • It was filled with reducing gases, like H2, NH3, CH4, CO, and CO2. • Most notably, the atmosphere of early Earth was completely anaerobic. • It lacked a protective ozone layer. Chemical Origins • In the 1920’s, Oparin and Haldane predicted that the conditions of early Earth favored the spontaneous formation of organic compounds. • In 1953, Miller and Urey tested this hypothesis. From Molecules to Cells 1. Small organic monomers formed in reducing atmosphere. 2. Monomers joined to form polymers – binding sites in clay helped concentrate the monomers, while metal ions acted as catalysts. 3. Polymers aggregated to form protobionts. 4. A molecule of heredity originated. Protobionts • Protobionts are aggregates of molecules that have some cell-like qualities such as: – Self-assembly – Response to osmotic forces – Ability to store a membrane potential – Engulfing and splitting First Life • Prokaryotic • RNA as genetic material • Heterotrophic – absorbed organic molecules from surroundings • Anaerobic – oxygen was not present in atmosphere until it was put there by photosynthetic bacteria as a waste product (began accumulating about 2 billion years ago Origin of photosynthesis • Some time after the first living things appeared, cells developed that could perform photosynthesis. • These organisms put oxygen into the air as a waste product. • About 1.5 billion years ago, oxygen still only made up about 1% of the atmosphere • Accumulation of oxygen in the atmosphere is thought to have ended chemical evolution Open for debate • Some scientists believe organic molecules came to Earth from outer space. Meteors have been found containing amino acids and nucleic acids (including several amino acids nonexistent on Earth). • There may have been a simpler hereditary molecule that preceded RNA, or maybe RNA was never genetic material. • Maybe life began near hydrothermal vents (and still forms there?) Taxonomy – the classification of living things • From that first cell, life has diversified into millions of different forms. • These forms are divided into five major kingdoms: Animalia, Plantae, Fungi, Protista, and Monera.
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