I. Chemical conditions of the early Earth that could have fostered the origin of lifeBIOL 1020 – CHAPTER 26 LECTURE NOTESChapter 26: The Tree of LifeI. Chemical conditions of the early Earth that could have fostered the origin of lifeA. the Earth is about 4.6 billion years old (time of the first likely solid surface)1. supported by radioisotope dating of- oldest known Earth minerals (date to 4.4 billion years ago, or 4.4 bya)- oldest known rocks on Earth (4.1 bya)- oldest known meteorites (4.6 bya; for the age of the solar system)2. some models go out to 6 billion years, it is hard make a rule for a definitive starting point of planet formationB. Earth’s early atmosphere (when life first appears in the fossil record) most likely consisted of CO2, H2O, CO, H2, N2, and small amounts of NH3, H2S, and CH4 – note the lack of O2, which is a major constituent of today’s atmosphereC. Four requirements for the current chemical evolution model were likely met in the early Earth1. little or no free oxygen2. abundant energy sources (volcanism, thunderstorms, and bombardment with particles and radiation from space were alllikely present as energy sources; especially important is more UV radiation than today)- the sun was hotter, producing more UV light- the Earth had no ozone layer to filter out most of the UV light coming in3. chemical building blocks of water, dissolved mineral ions, and atmospheric gases4. time (there was plenty of time before the first traces of life from 3.8 bya])D. attempts to mimic the early Earth’s atmosphere and chemical profile have led to production of organic molecules from simpler materials after energy is added1. 1920s – Oparin and Haldane independently proposed that organic molecules could form spontaneously from simpler raw materials when sufficient energy is supplied in a reducing (energy-rich, electron-adding) environment2. 1950s – Miller and Urey made a “reducing atmosphere” of H2O, H2, NH3, CH4 in a spark chamber; after sparking, they found that amino acids and other organic compounds had formed- designed to mimic what was thought at the time to have been Earth’s early atmosphere- later experiments with different “reducing atmospheres” that were thought to be better matches to the likely atmosphere of the early Earth produced all 20 amino acids used in proteins, various sugars and lipids, and components of DNA and RNA nucleotides- current models of the Earth’s early atmosphere are that in general the atmosphere was not reducing, but that there were likely many local environments that were reducing – especially near volcanic activity3. organic polymers can form spontaneously from monomer building blocks on some sand, clay, or rock surfacesE. there are several models for exactly where and how life as we know it on Earth began1. prebiotic broth hypothesis – life began from an “organic soup” in the oceans2. bubble hypothesis – a variation on the prebiotic broth, with “oily bubbles” from an organic soup interacting with land surfaces at shallow seas or seashores 3. iron-sulfur world hypothesis – life began from an “organic soup” interacting with mineral surfaces at hydrothermal vents in the ocean floor, with abundant iron and sulfur there impacting the early metabolism that developed4. deep-hot biosphere hypothesis – life began in an “organic soup” deep within the Earth5. exogenesis – Earth was seeded with life from an extraterrestrial sourceII. A model for how the first cells could have originated and functionedA. protobionts have been produced that resemble living cells1. microspheres, a type of protobiont, form spontaneously when liquid water is added to abiotically produced polypeptides2. microspheres can grow, divide, and maintain internal chemistry different from their surroundings3. microspheres show that some spontaneous production and maintenance of organization is possible, but are incomplete as a model for formation of the first cellsB. genetic reproduction was crucial in the origin of true cells1. RNA likely was first (RNA world hypothesis)- RNA can catalyze a variety of reactions, including some self-catalytic reactions- RNA can also store genetic information- in vitro evolution of RNA has shown that the RNA world hypothesis is feasible – selection can act on self-replicating RNA molecules in vitro2. DNA likely came later and had the selective advantage of greater stabilityIII. First life, however it came to be (or, Enough theory, Dr. Bowling, give me some dates to learn for the test!)A. the first evidence of life in the fossil record are isotopic carbon “fingerprints” in rocks from ~3.8 byaB. the first evidence of cells are microfossils of prokaryotic cells in fossils of stromatolites dated to ~3.5 bya1. stromatolites are rocklike structures made up of layers of bacteria and sediment2. in some areas stromatolites are still being formed todayC. the first cells were most likely anaerobic heterotrophs1. there was likely an abundance of organic molecules available for food early on1 of 3BIOL 1020 – CHAPTER 26 LECTURE NOTES2. later, as organic molecules became scarcer, photosynthetic organisms were favoredD. the first photosynthetic organisms were likely the purple and green sulfur bacteria, which use H2S as a hydrogen donorIV. Life changes the planet: oxygenating Earth’s oceans and atmosphereA. cyanobacteria were likely the first photosynthetic organisms to use H2O as a hydrogen donor, releasing O2 into the environment1. stromatolites from as old as 3.5 bya containing what appear to be fossil cyanobacteria2. many stromatolites with what appear to by fossil cyanobacteria date to about 2.5 bya B. banded iron formations from about 2.5 bya indicate the release of O2 into the oceansC. by 2 bya, O2 levels began to build up in the atmosphereD. the presence of O2 had a profound impact on life on Earth1. O2 is toxic to organisms that don’t have protective mechanisms; many died as O2 levels built up- creates an oxidizing atmosphere, which can destroy precious reduced organic molecules- some anaerobic organisms survive (even today) only in environments with little to no oxygen- some evolved adaptations to the presence of oxygen- some organisms developed means to use O2 in respiration to extract more energy from foods (aerobic respiration)2. the formation of the ozone layer (O3) soon after oxygenation of the atmosphere provided protection from UV radiation and allowed life to expand to regions at