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TAMU BIOL 112 - test 1

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Lecture: chapter 25: the history of life on Earth 1/17/12 Lost Worlds Past organisms were very different from now Fossil record shows macroevolutionary changes over large time scales including: The emergence of terrestrial vertebrates The origin of photosynthesis  Long-term impacts of mass extinctions  Conditions on early Earth made the origin of life possible Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages: Abiotic synthesis of small organic molecules Joining of small molecules into macromolecules. (carbohydrates, proteins, lipids, nucleic acids) Packaging of molecules into “protobionts”. Origin of self-replicating molecules. ( DNA/RNA) Synthesis of organic compounds Earth was formed about 4.6 billion years ago, with the rest of the solar system Earth’s early atmosphere contained water vapor and chemicals released by volcanic eruptions (Nitrogen, Nitrogen Oxides, CO2, Methane, Ammonia, Hydrogen, Hydrogen Sulfide.) A.I Oparin and J.B.S Haldane hypothesized that the early atmosphere was a reducing environment. Stanley Miller and Harold Urey conduced lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible. (1953) Instead of forming in the atmosphere, the 1st compounds may have been synthesis near submerged volcanoes and deep sea vents. Amino acids have also been found in meteorites.Lecture: Chapter 25 11/19/12 Abiotic synthesis of macromolecules and protobionts Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock. Replication and metabolism are key properties of life. Protobions are aggregates of abiotically produced molecules surrounded by a membrane or membrane like structure. Protobions exhibit simple reproduction and maintain and internal chemical environment. Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds For example, small membrane-bounded droplets called liposomes can form when lipids or other molecules are added to water. Self-replicating RNA and the Dawn of Natural selection The first genetic material was probably RNA, not DNA. RNA molecules called ribozymes have been found to catalyze many different reactions For example, ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA. Early protobions with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection The early genetic material might have formed and “RNA world”. The fossil record documents the history of life The fossil record reveals changes in the history of life on earth. Sedimentary rocks are deposited into layers called strata and are richest source of fossils. Few individuals have fossilized, and even fewer have been discovered. The fossil record is biased in favor of species that: Existed for a long time. Were abundant and widespread. Had hard parts. How rocks and fossils are dated Sedimentary strata reveal the relative ages of fossils. The absolute ages of fossils can be determined by radiometric dating. A “parent” isotope decays to a “daughter” isotope at a constant rate. Each isotope has a known half-life, the time required for half the parent isotope to decay. Carbon 14- 2 more neutrons, 5,000-6,000 year half-life, max: 75,000 yrs old. For older fossils, some isotopes can be used to date sedimentary rock layers above and below the fossil. The magnetism of rocks can provide dating information. Reversals of the magnetic poles leave their record on rocks throughout the world. Key events in life’s history include the origins of single-celled and multi-celled organisms and the colonization of land. The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons. The Phanerozoic is divided into 3 eras: the Paleozoic, Mesozoic, and Cenozoic.  Major boundaries between geological divisions correspond to extinction events in the fossil record. The first single-celled organisms  The oldest known fossils are stromatolites, rock-like structures composed of many layers o bacteria and sediment.  Stromatolites date back 3.5 billion yrs ago Prokaryotes were earth’s sole inhabitants from 3.5 to about 2.1 billion yrs ago. Photosynthesis and the oxygen revolution Most atmospheric oxygen (O2) is of biological origin O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations. The source of O2 was likely bacteria similar to modern cyanobacteria.  By about 2.7 billion yrs ago, O2 began accumulating in the atmosphere and rusting iron-rich terrestrialrocks This “oxygen revolution” from 2.7 to 2.2 billion yrs ago Posed a challenge for life Provided opportunity to gain energy from light Allowed organisms to exploit new ecosystems The first eukaryotes The oldest fossils of eukaryotic cells date back to 2.1 billion yrs ago. The hypothesis on endosymbiosis proposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cells. An endosymbiont is a cell that lives within a host cell. The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites. Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events. Key evidence supporting an endosymbiotic origin of mitochondria and plastids: Similarities in inner membrane structures and functions Division is similar in these organelles and some prokaryotes These organelles transcribe and translate their own DNA Their ribosomes are more similar to prokaryotic then eukaryotic ribosomes. The origin of muiltcellularity The evolution of eukaryotic cells allowed for a greater range of unicellular forms A second wave of diversification occurred when muiltcellularity evolved and gave rise to algae, plants,fungi, and animals Comparisons of DNA sequences date the common ancestor of multicellular* * The Cambrian explosion The Cambrian explosion refers to the sudden appearance of fossils


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