Chapter 17 The History of Life 1 How Did Life Begin People used to believe that new organisms sprang up all of the time through spontaneous generation Microorganisms from broth Maggots from meat Mice from sweaty clothes and wheat Experiments disproved spontaneous generation 17 1 2 3 Fig E1 1 Louis Pasteur Pasteur also performed expermients that refuted spontaneous generation 17 1 Fig 17 1 4 How Did Life Begin The first living things must have come from nonliving materials We know that organic molecules can form spontaneously under low oxygen conditions This has been experimentally supported 17 1 5 17 1 Fig 17 2 6 How Did Life Begin The first living things must have come from nonliving material Some RNA molecules called ribozymes can catalyze reactions Membrane like vesicles may have enclosed ribozymes forming protocells But there is no real evidence as to how life began 17 1 7 Early Earth Earth is 4 5 billion years old Early Earth was thought to be very hot with a lot of volcanic activity and electrical storms Early earths atmosphere lacked oxygen gas 17 2 8 Fig 17 4 Life Began During the Precambrian Era The oldest fossil organisms found so far are about 3 5 billion years old 17 2 9 Early Life Some organisms evolved the ability to capture energy from sunlight while using the earths most abundant source of hydrogen water H2O Photosthesis increased the amount of oxygen in the atmosphere 6CO2 6H2O light energy C6H12O6 6O2 17 2 10 Early Life Newly liberated oxygen gas was quickly consumed by reactions with other molecules One common reactive atom in the earths crust was iron There are large iron oxide rust deposits in rocks from this period 17 2 11 Early Life Once most of the iron had turned to rust oxygen began to accumulate in the atmosphere Chemical analysis of rocks suggests significant amounts of oxygen first appeared in the atmosphere about 2 3 billion years ago Oxygen is potentially dangerous to living things Many anaerobic organisms are killed by oxygen It reacts with and breaks down organic molecules 17 2 12 Early Life Aerobic metabolism evolved in response to the increased oxygen levels Provides defense against the chemical action of oxygen Also channels oxygen to make useful energy Cells that could utilize aerobic respiration had a significant advantage over cells that do not 17 2 13 The Evolution of Eukaryotes Some organisms acquired membrane enclosed organelles Eukaryotes could have evolved through a combination of two processes The endosymbiont hypothesis Inward folding of the plasma membrane 17 2 14 The Evolution of Eukaryotes Inward folding of the plasma membrane Membrane infolding of near DNA could have formed precursors to nuclei Infoldings could also have formed ER and Golgi structures 17 2 15 The Evolution of Eukaryotes The endosymbiont hypothesis Mitochondria and chloroplasts may have arisen from engulfed photosynthetic bacteria How could this happen Mitochondria a predatory anaerobic cell could have captured an aerobic bacterium for food but failed to digest Chloroplast some of these predatory cells could also have captured photosynthetic bacterium 16 17 2 The endosymbiont hypothesis 17 2 Fig 17 5 17 Evidence for the endosymbiont hypothesis There are many distinctive similarities between eukaryotic organelles and living bacteria Mitochondria and chloroplasts each contain their own DNA The presence of living intermediates Ex Paramecium harbor a photosynthetic algae 17 2 18 Evidence for the endosymbiont hypothesis The presence of living intermediates Ex Paramecium harbor Photosynthetic algae 17 2 19 Fig 17 6 The Evolution of Eukaryotes Summary 17 2 20 Evolution of multicellular organisms Predation provided selective pressure that favored large size Large cells are harder to engulf and digest Larger organisms are usually faster But there is a limitation on cell size Back to chapter 5 page 91 17 3 21 The ability to exchange materials across the plasma membrane limits cell size A large cell will have A relatively small Surface area through Which to obtain Nutrients and Eliminate waste 17 3 Fig 5 16 22 Evolution of multicellular organisms There are only two ways an organism larger than about one millimeter in diameter can survive 1 Possess a low metabolic rate Requires less oxygen Produce little carbon dioxide 2 Be multicellular Consist of many small cells packaged into a larger unified body The oldest multicellular fossils are about 1 2 billion years old 23 17 3 How did life invade land There were many obstacles to overcome in order for an organism to leave the sea and inhabit dry land Benefits of life in the water include Protection from gravity Ready access to water Sperm and or eggs can swim or drift to one another 17 4 24 How did life invade land Inhabiting land was very advantageous esp for plants Unobstructed sunlight water absorbs light Abundance of nutrients lacking in the sea Particularly nitrogen and phosphorus There were no organisms on land that ate plants Soon after plants invaded land animals followed 17 4 Fig 17 9 25 What role has extinction played in the history of life Evolutionary history has been marked by periodic mass extinctions the relatively sudden disappearance of a wide variety of species over a large part of the earth Mass extinctions have separated many series of evolutionary dynasties Where one dominant group rules the land of sea for a period of time 17 5 26 Mass Extinctions Indicated by asterisks 17 5 Fig 17 11 27 What Can Cause Mass Extinctions Climate change One cause of climate change is the shifting positions of continents continental drift Catastrophic events Massive volcanic eruptions Impact of a huge meteorite 17 5 28 Continental drift is caused by plate tectonics 17 5 Source NASA 29 Continental drift is caused by plate tectonics 17 5 Fig 17 12a 30 Continental drift is caused by plate tectonics 17 5 Fig 17 12d 31 Continental drift is caused by plate tectonics The effects of plate tectonics continues to be seen today Ex the Atlantic Ocean widens by a few centimeters each year 17 5 32 Was the extinction of the dinosaurs caused by the impact of a huge meteorite 65 million years ago there was a giant meteorite strike and a mass extinction event Chicxulub crater 100 mile wide crater buried beneath the Yucatan Peninsula of Mexico Impact site of a giant meteorite 6 miles in diameter Did the impact cause the extinction 17 5 33
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