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Unformatted text preview: Chapter 25: the History of Life on Earth 25.1: Conditions on Early Earth made the origin of life possible- Macroevolution: the pattern of evolution over large time scales o Antarctica was once surrounded by tropical waters and was covered in forests- The earliest evidence of life on Earth comes from fossils of microorganisms that are about 3.5 billion years old- Scientists hypothesize that chemical and physical processes here on early Earth, aided by the emerging force of natural selection could have produce very simple cells in four main stages o The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nucleotides o The joining of these small molecules into macromolecules, including proteins and nucleic acids o The packaging of these molecules into “protobionts”, droplets with membranes that maintained an internal chemistry different from that of their surroundings o The origin of self-replicating molecules that eventually made inheritance possible Synthesis of Organic Compounds on Early Earth- There is evidence that suggests that the Earth formed about 4.6 billion years ago- At first, it would have been uninhabitable because of the debris bombarding the Earth, creating heat and preventing oceans from forming- Once this phase ended about 3.9 billion years ago, the first atmosphere was thick with water vapor and there were various compounds released by volcanic eruptions o As Earth cooled, water vapor condensed to form oceans o Earth’s early atmosphere is thought to have been a reducing (electron adding) environment, in which organic compounds could have formed from simple molecules The energy for this could have come from lightning and intense UV radiation The early oceans could have been solutions of organic molecules from which life arose o This has been recreated in lab with promising results by Miller and Urey However, it is unclear whether the early atmosphere have enough methane and ammonia to be reducing • Growing evidence suggests that the atmosphere was made up of nitrogen and carbon dioxide which are neither reducing nor oxidizing Instead, early organisms could have formed near volcanic openings which were reducing and were rich in sulfur and iron compounds • This has also been recreated in lab with promising results- Abiotic synthesis of organic molecules is possible o Meteorites that have fallen from space have contained more than 80 amino acids, some in large amounts Abiotic Synthesis of Macromolecules- The presence of small organic molecules is not sufficient for the emergence of life- By dripping solutions of amino acid on hot sand, clay, or rock, researchers have been able to produce amino acid polymers without the help of enzymes or Protobionts- Two key properties of life are accurate replication and metabolism, neither of which can exist without the other- Miller-Urey experiments have not been able to produce nucleotides, although they have yielded some of the nitrogenous bases of DNA and RNA...
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This note was uploaded on 04/21/2009 for the course BISC 120Lg taught by Professor 11:00-01:50pm during the Fall '06 term at USC.
- Fall '06