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Astro Chapter 15.docx

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Astronomy 1021
Stacey Hallman

Astronomy Chapter 15: Life on Other Worlds THE NATURE OF LIFE • Life is a process by which an organism extracts energy from the surroundings, maintains itself, and modifies the surroundings to foster its own survival and reproduction • All living things on Earth, share certain characteristics in how they perform the process of life The Physical Basis of Life • The physical basis of life on Earth is the element carbon • They can form long, complex, stable chains that are capable of storing and transmitting information Information Storage and Duplication • Almost every action performed by a living cell is carried out by chemicals it manufactures • The “DNA: The code of life” identifies three major points: 1. The chemical recipes of life are stored in each cell as information on DNA molecules that resemble a ladder with rugs that are composed of chemical bases, providing instructions to guide chemical reactions within the cell 2. DNAinstructions normally are expressed by being copied into a messenger molecule called RNAthat causes molecular units called proteins. Proteins serve as the cell’s basic structural molecules or as enzymes that control chemical reactions 3. The instructions stored in DNAare genetic information passed along to offspring. The DNAmolecule reproduces itself when a cell divides so that each new cell contains a copy of the original information Modifying the Information • If the information stored in DNAcould not change, then life would quickly go extinct • The process by which life adjusts itself to its changing environment is called biological evolution • When an organism reproduces, its offspring received a copy of its DNA • Sometimes external effects such as radiation alter the DNAduring the parent organism’s lifetime, and sometimes mistakes occur in the copying process • Offspring born with random alternations to their DNAare called mutants o Amutation can actually help an organism survive • Differing rates of survival and reproduction are examples of natural selection • Over time, the beneficial variation becomes more common and a species can evolve until the entire population shares the trait • Natural selection adapts species to their changing environments by selecting, from the huge array of random variations, those that would most benefit the survival of the species • Evolution is not random • Natural selection is not random LIFE IN THE UNIVERSE • The key to understanding the origin of life lies in the process of evolution • This means that life on Earth could have begun very simply, even in as simple a form as carbon chain molecules able to copy themselves The Origin of Life on Earth • The oldest fossils are all the remains of sea creatures, and this indicates that life began in the sea • Individual organisms were microscopic from WesternAustralia that are more than 3 billion years old contain features that experts identify as fossil stromatolites, which are remains of colonies of single-celled organisms • Simple organisms lived in Earth’s oceans 3.4 billion years ago • The Miller experiment consisted of a sterile, sealed glass container holding water, hydrogen, ammonia, and methane o An electric arc inside the apparatus created sparks to simulate the effects of lightning in Earth’s early atmosphere o They found that the interaction between the electric arc and the simulated atmosphere had produced many organic molecules from the raw material of the experiment, including such important building blocks of life as amino acids o Earth’s early atmosphere probably consisted of carbon dioxide, nitrogen, and water vapour instead of the mix of hydrogen, ammonia, and methane assumed by Miller and Urey o The Miller experiment is important because it shows that complex organic molecules form naturally in a wide variety of circumstances • Lightning, sunlight, and hot lava pouring into the oceans are just some of the energy sources that can naturally rearrange simple common molecules that make life possible • If you could travel back in time, you would expect to find Earth’s first oceans filled with a rich mixture of organic compounds called the primoridial soup • Amino acids can link together proteins by joining ends and releasing a water molecule • Because some molecules are more stable than others, and some bond together more easily than others, scientists hypothesize that a process of chemical evolution eventually concentrated the various smaller molecules into the most stable larger forms • Eventually, a molecule formed that could copy itself • At that point, the chemical evolution of molecules became the biological evolution of living things • Radio astronomers have found a wide variety of organic molecules in the interstellar medium, and similar compounds have been found inside meteorites, such as the “Murchison” • The first reproducing molecules could have been formed by natural processes • The first cells must have been simple single-celled organisms much like modern bacteria o These kinds of cells are preserved in stromatolites, mineral formations formed by layers of photosynthetic bacteria and shallow ocean sediments • Over the course of eons, the natural processes of evolution gave rise to stunningly complex multicellular life forms with their own widely differing ways of life Geologic Time • Life has existed on Earth for at least 3.4 billion years • There is no evidence of anything more than simple organisms until about 540 million years ago • This sudden increase in complexity is known as the Cambrian explosion, and it marks the beginning of the Cambrian period • Life in a year: st o Jan 1 – world began o March/earlyApril – signs of life o November – complex organisms of the Cambrian explosion o November 28 – life on land th o December 12 – dinosaurs walking the continents o December 26 – dinosaurs extinct, mammals and birds on the rise o December 31 – first humanoids, making the first stone tools • Tremendous amounts of time were needed for the first simple living things to evolve in the oceans Life in our Solar System • Liquid water seems to be a requirement of carbon-based life, necessary for both vital chemical reactions and as a medium to transport nutrients and wastes • Water is a cosmically abundant substance with properties such as heat capacity that set it apart from other common molecules that are liquid at the temperatures of planetary surfaces • The moon and Mercury are airless, and water would boil away into space immediately • Venus has traces of water vapor in its atmosphere, but its is too hot for liquid water to exist on the surface • The Jovian planets have deep atmospheres, and at a certain level water condenses into liquid droplets
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