EARLY EARTH AND THE ORIGIN OF LIFE. A recipe for the spontaneous origin of life. Sources of NEW Life. Life arose spontaneously?

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1 EARLY EARTH AND THE ORIGIN OF LIFE Sources of NEW Life 1. Some god, group of gods, or all powerful being is responsible 2. Life came from some place else in the universe 3. Life arose spontaneously from non-living chemicals Looking Outside Earth for Clues About the Origin of Life Life may have infected Earth from some other planet -This hypothesis is termed Panspermia The possibility that life is not restricted to Earth is becoming more accessible to scientific testing A recipe for the spontaneous origin of life Raw materials + Suitable environment + Energy sources Pasteur and biogenesis of microorganisms Life arose spontaneously? Redi s work with spoiled meat & Pasteur s work on sterilization showed that spontaneous generation does not occur. 1

2 In the 1920s, two scientists,russian A. I. Oparin and British J. B. S. Haldane independently proposed that organic molecules could have formed on the early Earth The early Earth likely had a reducing atmosphere - no O 2 gas Modern atmosphere is rich in O 2, which oxidizes and disrupts chemical bonds The possible composition of Earth s early atmosphere H 2 O vapor and compounds released from volcanic eruptions, including N 2 and its oxides, CO 2, CH 4, NH 3, H 2, and H 2 S As the Earth cooled, water vapor condensed into oceans, and most of the hydrogen escaped into space Types of rocks formed liquid water Liquid water temperature Conditions on early Earth made the Many energy sources existed on the origin of life possible early Earth Chemical conditions Physical conditions Intense volcanic activity Lightning UV radiation Stage 1 Stage 2 Stage 3 Stage 4 Abiotic synthesis of monomers Formation of polymers Packaging of polymers into protobionts Self-replication Conditions on Early Earth First organisms emerged between 3.8 and 2.5 BYA Early atmosphere had CO 2, N 2, H 2 O and H 2 -Reducing atmosphere In 1953, Miller and Urey did an experiment that reproduced this early atmosphere -Used electrodes to simulate lightning -Small organic molecules were generated in their apparatus In 1953, graduate student Stanley Miller tested the Oparin-Haldane hypothesis Miller set up an airtight apparatus with gases circulating past an electrical discharge, to simulate conditions on the early Earth 2

3 Origins of Life The Earth formed as a hot mass of molten rock about 4.5 billion years ago (BYA) -As it cooled, chemically-rich oceans were formed from water condensation Life arose spontaneously from these early waters After a week, Miller s setup produced abundant amino acids and other organic molecules Similar experiments used other atmospheres and other energy sources, with similar results Miller-Urey experiments demonstrate that Stage 1, abiotic synthesis of organic molecules, was possible on the early Earth An alternative hypothesis Submerged volcanoes and deep-sea hydrothermal vents may have provided the chemical resources for the first life Stanley Miller s experiments showed that the abiotic synthesis of organic molecules is possible Stage 2: Abiotic Synthesis of Polymers The formation of polymers Monomers could have combined to form organic polymers Same energy sources Clay as substratum for polymerization? Amino acids polymerized into proteins Nucleotides into RNA Monomers 1 Formation of short RNA polymers: simple genes 3

4 Stage 3: Packaging of polymers into protobionts (pre-cells) Polymers could have aggregated into complex, organized, cell-like structures What characteristics do cells and protobionts share? Structural organization Simple reproduction Simple metabolism Simple homeostasis Protobionts Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure 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 organic molecules are added to water LE 26-4 Glucose-phosphate 20 µm Life requires the maintenance of a complex, stable, internal environment Glucose-phosphate Phosphorylase Life requires accurate self replication What provides these in modern cells? Phosphate Starch Maltose Amylase Membranes Which came first? Simple reproduction Simple metabolism Maltose Enzymes Genes LE 26-5 Stage 4: Self-replication and regulation RNA may have served both as the first genetic material and as the first enzymes The first genes may have been short strands of RNA that replicated without protein support RNA catalysts or ribozymes (which exist today in some organisms) may have assisted in this process. RNA world! Nucleotides 3 Template 3 Ribozyme (RNA molecule) Complementary RNA copy 5 5 4

5 A variety of protobionts would have existed on the early Earth Some of these protobionts contained selfreplicating RNA molecules How could natural selection have acted on these protobionts? Overview: Changing Life on a Changing Earth Life is a continuum extending from the earliest organisms to the variety of species that exist today Geological events change the course of evolution Conversely, life changes the planet that it inhabits Earth formed 4.6 billion years ago The first living things would have been simple By 3.5 billion years ago, bacteria appear in the fossil record. Shortly after, photosynthetic bacteria formed sandy stromatolite mats By 2.1 bya, O 2 level in atmosphere approaches today s level 5