Transcript Chapter 17
Chapter 17 Origins of Life Origins of Life • Astrobiology is study of the origin, evolution, distribution, & destiny of life in the universe. • It attempts to answer 3 fundamental questions: – How did (does) life begin and develop? – Does life exist elsewhere in the universe? – What is life's future on Earth and beyond? Advances in the biological sciences, space exploration, and astronomy make it possible for us to realistically attempt to answer them. Here’s what we “know”: • Free oxygen probably was not present in the early atmosphere. – Compounds in early rocks do not contain oxygen…… significant amounts of oxygen would not have formed until the process of photosynthesis was under way. • Some of the ideas about the origin of life cannot be stated in the form of hypotheses or tests; – therefore, science is limited in it’s ability to investigate such ideas. • Mineral crystals have been proposed as an early information molecule because they have a definite repeating structure. • RNA is a more likely candidate for life formation than DNA because DNA is more complex than RNA and requires proteins for its replication. – RNA can cataylze its own splicing. • The conditions on early Earth were probably similar to those on Venus and Mars today, – that is, containing high levels of CO2, water vapor, and N2 and small amounts of H2. • It’s easier to understand the formation of the organic compounds necessary for life than the formation of the first cells. • Chemical evolution can provide a mechanism by which life could have originated from increasingly more complex chemicals in the primitive seas. – Darwinian evolution gives us a way by which organisms can change over time, but that could not start until chemical evolution resulted in the first life. • And then there is the question of energy sources………. – Early energy sources included lightning, UV radiation, and heat from the radioactive decay within earth. – These energy sources drove the chemical reactions that produced the first life. – Those organisms most efficient at using the energy had an advantage over less efficient organisms. Heterotroph Hypothesis • Proposed that the first living cells were heterotrophs – Therefore they had to obtain organic nutrients from the environment • In the 1920’s, Oparin and Haldane independently proposed this theory – Early atmosphere had methane, ammonia, hydrogen, and water vapor in it. – An energy source (radioactivity, lightening, cosmic radiation, and/or heat from volcanoes) reacted with those gases and formed organic compounds. • This hypothesis requires 3 major steps: – A nonbiological processes had to supply some organic molecules. – Some process had to trigger the small molecules to form polymers like nucleic acids and proteins – Some other processes had to organize the polymers that could replicate itself. Urey & Miller’s Experiment- 1952 • Can organic compounds be generated under conditions similar to those that existed on primeval earth? Conclusions: The organic building blocks of life (amino acids and all the building blocks for nucleic acids) formed in the liquid. Clip Early Organisms • Early atmosphere lacked oxygen gas – the first organisms must have been anaerobic (probably methanogens) • There is some evidence of photoautotrophs dating back to 4 billion years ago. • Oxygen gas was probably insignificant until about 2 billion years ago. – Aerobic respiration may have evolved as a way to produce large amounts of ATP from food sources. Endosymbiotic Hypothesis • The first microfossils of eukaryotes are about 1.4 billion years old. • This hypothesis proposes that eukaryotes originated from a symbiotic relationship between large anaerobic prokaryotes and smaller aerobic or photosynthetic prokaryotes animation • The question of whether life was created by a supernatural being cannot be investigated by the methods of science because no controlled experiments can be set up and run.