Section 3 The Origin of Life on Earth Section 3 The Origin of Life on Earth What Do You See? Learning Outcomes Think About It In this section, you will In the last section, you learned that evidence for life on Earth • Considera hypothesis about dates back 3.5 billion years. Early life forms were cyanobacteria. how life may have originated These are simple, single-celled organisms. Now you will examine on Earth. scientists’ ideas and theories about how these formed. • Examinemodels of the development of compounds • How do you think life began on Earth? necessary for life on Earth. • Where did life form? • Examineevidence of the oldest forms of life on Earth. • How did early life affect Earth’s other spheres? • Inferhow ancient organisms Record your ideas about these questions in your Geo log. Be lived by comparing them to modern ones. prepared to discuss your responses with your small group and the class. Investigate In this Investigate, you will look at some of the thinking that has led to current scientifi c hypotheses for the origin of the biosphere. Part A: The Formation of Life’s Chemical Building Blocks 1. Read the following passage about a famous scientific investigation into the origin of life. 893 EarthComm EC_Natl_SE_C8.indd 893 7/15/11 12:12:46 PM Chapter 8 Earth System Evolution The Inhospitable Young Earth part contained another round fl ask. It had a mixture of gases: hydrogen (H ), When Earth formed about 4.6 billion years 2 ammonia (NH3), methane (CH4), and ago, there was no life on it. The young the circulating water vapor. An electrical Earth’s atmosphere was toxic. There were discharge (spark) was added to the mix constant and violent volcanic eruptions. of gases. This took place for a number of Earth in the early Hadean was much hotter days. The energy from the discharge caused than today. Air temperatures were, on the gases to interact. The products of those average, as high as 300°C. That is three reactions passed through a condenser. times the boiling point of water! The This caused the water vapor to transform atmosphere was formed by gases emitted back to a liquid. The resulting compounds from volcanoes. They consisted mainly of dissolved in the liquid water. They collected hydrogen, methane, ammonia, and water in a fl ask at the bottom. When the water in vapor. Clouds shrouded Earth’s surface. It the bottom fl ask was analyzed, it was rich would have been quite dark. There was in amino acids. These are small organic little oxygen. There was no ozone layer to molecules. Amino acids are needed to make protect Earth from ultraviolet radiation. As protein, which is one of the groups of a result, the Sun’s energy baked the surface compounds that make up living things. of Earth. Over hundreds of millions of years, Earth’s surface began to cool. Water vapor in the atmosphere began to condense. Storm clouds developed. Lightning strikes were frequent. Water fell as rain onto a cooler surface and collected in low-lying areas. Bodies of water were formed. By about 4 billion years ago, the global ocean had formed. One hypothesis about how life began on Earth is called the chemosynthesis hypothesis. It suggests that life began from simple molecules. These assembled into what eventually became living organisms. The conditions on Earth allowed this to happen. In 1953, scientists Stanley Miller and Harold Urey tested this hypothesis. They created a model of the Earth system early in its history. A drawing of Miller and Urey’s model. Miller and Urey’s model contained a 2. Miller and Urey showed that certain number of different parts to explore molecules that make up living things organic chemical processes. The bottom could form spontaneously when the part of the apparatus contained a round conditions of Earth’s early atmosphere fl ask. It was fi lled with water. The water were recreated in the laboratory. was heated to force water vapor to Identify the parts and processes of the circulate through the apparatus. The top Earth system that are in the model. 894 EarthComm EC_Natl_SE_C8.indd 894 7/15/11 12:12:47 PM Section 3 The Origin of Life on Earth a) What does the flask with heated water represent? b) What does the flask with the mixture of gases represent? c) What do the electrodes and their electrical discharges represent? d) What does the condenser represent? e) What does the cooled water containing organic compounds Coacervates represent? 1. You will need to use a table to record Part B: The Role of Coacervates your data and observations. in Understanding How Life May Have Formed a) Make a table similar to the one In this part of the Investigate, you will below in your Geo log. make and observe coacervates. They 2. Work in groups of three or four. are bead-like forms surrounded with Make sure that you are wearing membrane-like structures. At one time, safety goggles. many scientists thought that coacervates 3. Pour 5 mL of 1 percent gelatin solution were the precursors to living things. This (protein source) into a test tube. is generally not accepted now. However, this part of the Investigate will help you 4. Add 5 mL of 1 percent gum-arabic to understand how structures are able to solution (carbohydrate source). self-assemble in a primordial “soup” of various chemical compounds. Mixture Sketch of coacervates Drops Formed at pH of HCl Appearance added pH Seen at magnification (clear, cloudy) in microscope 0 1 2 3 4 5 6 895 EarthComm EC_Natl_SE_C8.indd 895 7/15/11 12:12:47 PM Chapter 8 Earth System Evolution 5. Seal the tube with a stopper. Gently 12. Observe a sample of the liquid under mix the solutions by swirling the tube. the microscope. Wash your hands with Do not shake. soap and water when you fi nish the 6. Remove the stopper. Use a dropper to investigation. remove a drop of the liquid. Test its pH 13. Consider the following: using pH paper. a) What conditions caused the a) Record whether the liquid in the formation of coacervates? Describe tube is clear or cloudy. If the mixture how they changed under different is clear, there are no coacervates, conditions. and you should continue to the next b) What conditions in ancient oceans step. If the mixture is cloudy, there may have caused coacervates are coacervates, and you should to form? continue with Step 8. Part C: Examining Evidence of 7. Add 1 drop of dilute hydrochloric acid Early Life (HCl) to the test-tube mixture. Seal the tube. Mix the solutions gently by In this part of the Investigate, you will swirling. Do not shake. examine photos of stromatolites. These are 8. Remove the stopper. Use a dropper to rings that can be seen in carbonate rock. remove a drop of the liquid and test These rings were made by living organisms. its pH. They are among the oldest evidence for life on Earth. a) Record whether the liquid in the tube is clear or cloudy. If the 1. Go to the EarthComm Web site or use mixture is clear, there are still no the photographs shown. Examine the coacervates and you should repeat images of stromatolites of various ages Step 6. If the mixture is cloudy, from different parts of the world. there are coacervates and you should a) What similar features do you notice continue with Step 9. among all the stromatolite samples? 9. Place 1–2 drops of the mixture onto a b) How do you think these formations microscope slide and add a coverslip. were created? Place the slide under a microscope and look for coacervates under low magnification and then under high magnification. a) Sketch your observations. 10. After sketching coacervates, add another drop of acid to the mix in the test tube. Seal the tube and mix the solutions gently. Do not shake. 11. Keep adding drops of acid until the cloudiness disappears and the mix becomes relatively clear again. When This particular stromatolite is the oldest this happens, check the pH. known evidence for life on Earth. Warrawoona Series, North Pole, Western a) Record the pH in your Geo log. Australia; 3.5 billion years old 896 EarthComm EC_Natl_SE_C8.indd 896 7/15/11 12:12:47 PM Section 3 The Origin of Life on Earth Libby Supergroup, Nash Fork Shear Zone, Libby Flats Medicine Bow Range, Wyoming 2.58 billion years old Chocolay Group, Chocolay Hills, Northern Michigan 2.2 to 2.4 billion years old Cochabamba, Bolivia, South America 2.3 billion years old Irregully Formation, Bangemall Basin, near Ashburton River, Kooline Station, Western Australia 1.2 billion years old Gunflint Formation, Ontario, Canada 1.9 billion years old 897 EarthComm EC_Natl_SE_C8.indd 897 7/15/11 12:12:47 PM Chapter 8 Earth System Evolution 2. Examine the following pictures of Part D: Comparing Ancient and magnified sections from a stromatolite, Modern Life Forms 850 million years old, located in When scientists observed these ancient central Australia. The objects in the organisms, they realized that they had pictures are the remains of microscopic striking similarities to present-day organisms. The organic matter of the organisms from the kingdom Bacteria. organisms has been changed, but their forms are well preserved. They resemble organisms from the phylum Cyanobacteria. 1. Compare the first microscopic organism from the Bitter Springs Formation with magnified picture of the present-day cyanobacteria, known as “Oscillatoria” on the next page. a) In what ways are these ancient and modern forms of life similar? 2.