CHAPTER 16 The Origin and Evolution of Microbial Life: Prokaryotes and Protists

1. CHAPTER 16The Origin and Evolution of Microbial Life: Prokaryotes and Protists Modules 16.1 – 16.6

2. EARLY EARTH AND THE ORIGIN OF LIFE 16.1 Life began on a young Earth • Planet Earth formed some 4.6 billion years ago • Heat generated by the impact of meteorites, radioactive decay, and compaction of gravity thawed earth and turned it into a molten mass. • The more dense material became the core (nickel and iron) and the less dense material the thin crust. • The first atmosphere was most likely H2 gas which quickly escaped into space. • Vents from volcanoes belched gases that now formed a new atmosphere.

3. Volcanic activity, lightning, and UV radiation were intense. • The first seas were created when the earth cooled enough for water vapor to condense. • It was in this environment that it is theorized that life began. • This new early atmosphere probably contained H2O, CO, CO2, N2, and possibly some CH4, but little or no O2 Figure 16.1A

4. Fossilized prokaryotes date back 3.5 billion years from stromatolites from western Australia. • They were photosynthetic autotrophs whose metabolism was too complex to represent the first organism. • Thus, life most likely arose approximately 3.9 billion years ago. Figure 16.1B, D

5. = 500 million years ago Earliest animals; diverse algae Earliest multicellular eukaryotes? • It is hypothesized that life may have developed from nonliving materials as early as 3.9 billion years ago. • The earliest entities may have been aggregates of molecules which would have then evolved into aggregates of polymers with simple metabolism and self replication ability. • How this occurred was hypothesized by the Russian biochemist, A.I. Oparin (1923), and the British geneticist, B.S. Haldane.. Earliest eukaryotes Accumulation of atmospheric O2 from photosyntheticcyanobacteria Billions of years ago Oldest known prokaryotic fossils Origin of life? Figure 16.1C Formation of Earth

6. 16.2 How did life originate? • They hypothesized that small organic molecules must have appeared first • This probably happened when inorganic chemicals were energized by lightning or UV radiation. • The early atmosphere had no oxygen, a strong oxidizing agent that tends to disrupt chemical bonds by attracting electrons from them. • Before the early prokaryotes added oxygen to the air, earth most likely had a reducing atmosphere, donating electrons, bringing simple molecules together to combine into more complex ones. • Oparin and Haldane never tested their hypothesis , but in 1953, Stanley Miller and Harold Urey did at the University of Chicago.

7. The Present Controversy • Question: Was the early Earth atmosphere reducting or oxidizing? In some books, it is said that reducting, in others the opposite. What is the latest scientists' opinion? Answer: • The oxidizing or reducing state of the early atmosphere is not well understood. Fifty years ago it was thought to be reducing, with lots of hydrogen, methane and ammonia. Later studies tended toward mildly oxidizing, with carbon dioxide as the major gas. Recently both carbon dioxide and methane have been suggested as necessary to provide sufficient greenhouse effect to warm the Earth at a time when the Sun was about 30% dimmer than it is today. Thus the latest scientists' opinion is that the early atmosphere was probably neither strongly reducing nor strongly oxidizing,with carbon dioxide and nitrogen the main gases. This opinion may well change, however, as we learn more about the ancient environment of our planet. • David MorrisonNAI Senior Scientist(NASA Astrobiology Institute) 30 May 2006

8. 16.3 Talking About Science: Stanley Miller’s experiments showed that organic materials could have arisen on a lifeless earthhttp://www.accessexcellence.org/WN/NM/miller.html Figure 16.3A

9. atmosphere CH4 Water vapor Electrode lightning NH3 H2 • Simulations of such conditions have produced amino acids, sugars, lipids, and nucleotide bases as well as ATP. Condenser Causing rain Coldwater Cooled watercontainingorganiccompounds H2O sea Sample forchemical analysis contained 20 amino acids, sugars, lipids, nucleotides and even ATP. Figure 16.3B

10. 16.4 The first polymers may have formed on hot rocks or clay • The organic monomers had to form polymers. • These monomers could have polymerized on hot rocks or clay through dehydration synthesis. • This could have produced such polymers as polypeptides and short nucleic acids.

11. Surrounding membranes may have protected some of these polymers and macromolecules as they evolved rudimentary metabolism (protobiont). • Stanley Fox (1912) did research on structures of this nature. • These structures form spontaneously in the lab from solutions of organic molecules. • Microspheres composed of may protein molecules are organized into a membrane like structure. • Coacervates are collections of droplets that are composed of molecules of different types which may include: lipids, amino acids, sugars and nucleic acids. Microspheres and coacervates have a number of lifelike qualities: take up substances from the surroundings, increase in size, form buds, fuse with similar structures. This proves a membrane bound structure did not need genetic information to form. –The Debate is to the nature of the membrane sac!!! Protein or Lipid??????????????

12. Coacervates Membrane RNA Polypeptide Which organic molecules came first??? Another raging debate!!!!!!!!? Figure 16.6B, C

13. 16.5 The first genetic material and enzymes may both have been RNA..The RNA World • The first genes may have been RNA molecules (Thomas Cech-University of Colorado) • These molecules could have catalyzed their own replication in a prebiotic RNA world 2 Assembly of acomplementary RNAchain, the first step inreplication of theoriginal “gene” 1 Monomers Formation of short RNApolymers: simple “genes” Figure 16.5

14. 16.6 Molecular cooperatives enclosed by membranes probably preceded the first real cells-- A Peptide-Nucleic Acid World RNA Self-replicationof RNA • These molecules might have acted as rough templates for the formation of polypeptides • These polypeptides may have in turn assisted RNA replication Some argue the proteins came first …for without enzymes (most of which are proteins), nothing could replicate!!!!! A Protein World Self-replicating RNAacts as template onwhich polypeptideforms. Polypeptide Polypeptide actsas primitiveenzyme thataids RNAreplication. Figure 16.6A

15. The Evolution of prokaryotes • The early protobionts which relied on molecules present in the primitive soup (primitive heterotrophs) were gradually replaced by organisms that could produce their own needed compounds (chemoautotrophs). • The diversification of these autotrophs led to the emergence of true heterotrophs that relied on the autotrophs. • These inhabitants (prokaryotes) were the earth’s sole inhabitants from 3.5 to about 2 billion years ago. • They began to transform the atmosphere as atmospheric oxygen began to appear about 2.7 billion years ago as the result of prokaryotic photosynthetic autotrophs.

16. How Ancient Bacteria Changed the World • Biological and geologic history are closely intertwined • Fossilized mats of prokaryotes 2.5 billion years old mark a time when photosynthetic bacteria were producing O2that made the atmosphere aerobic • These fossilized mats, as you recall, are called stromatolites.