AST103 chapter 6 notes

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1 Chapter 6 The Origin and Evolution of Life on Earth From chapter 3 the Cosmic Calendar 6.1 When Did Life Begin? Evidence: Stromatolites Colonies of photosynthetic cyanobacteria living on outer layers of sedimentary rocks Ancient (3.5 byr old) stromatolites exhibit almost identical layered structure to modern-day stromatolites Inconclusive evidence: nonbiological sedimentation layering may mimic stromatolites 1

2 When Did Life Begin? More evidence: Microfossils Individualized fossilized cells Hard to find because: Age vs. geological processes Identifying origin: biological or mineral? When Did Life Begin? More evidence: Microfossils 3.5B yr old Australian rock shows cells Could this form naturally from minerals? Older sites: sedimentary rock too altered to be useful Younger sites: at least two more (3.2B - 3.5B yr old) Better still, sites dating between 2.7B 3.0B yr old) Why? Hydrocarbon molecules When Did Life Begin? Isotopic evidence found in ancient rocks Carbon: two stable isotopes: 12 C/ 13 C Normal abundance in nonbiological samples: 89:1 Living tissue and fossils always show lower ratio (less 13 C) because 12 C is slightly easier to metabolize Some rocks older than 3.85B yr show low 13 C abundance Does that imply life?? 2

3 Where Did Life Begin? On land? Not likely because: Solar UV radiation: protection today by ozone (O 3 ) But no atmospheric oxygen in the early Earth In water: no problem, UV absorbed effectively Where Did Life Begin? Shallow ponds First evidence: YES! Recent evidence, however: Lacked chemical energy needed to spawn life Exposed to lethal UV radiation from Sun Where Did Life Begin? Deep-sea hot vents Advantages: More chemical energy Protection from UV Deep-sea hot vents better protected against bombardment DNA evidence shows early thermophiles 3

4 6.2 The Origin of Life No atmospheric oxygen at first O 2 is a byproduct of photosynthesis Could not have been present before life began! Good thing, since O 2 destroys many organic compounds First life arose out of chemical reactions on early Earth How? One answer is the Miller-Urey Experiment Miller-Urey experiment Amino-acid soup can form from methane (CH 4 ) and ammonia (NH 3 ) with electric energy (lightning) Current thinking: early atmosphere was dominated by CO 2, N 2, CH 4 Shallow ponds close to surface UV sterilization Some question as to how much H 2 was in atmosphere Would have affected primordial soup mix Sources of amino acids 1. Chemical reactions near the ocean surface 2. Deep-sea vents: abundant chemical energy & protected from UV 3. Extraterrestrial: amino acids are abundant in meteorites 4. Heat and pressure of impacts 4

5 Chemistry to Biology Organic soup of amino acids Has to be the initial step, however the first amino acids formed But how did chemistry evolve into biology? Where did DNA come from? Two Problems: DNA needs enzymes to replicate Enzymes need DNA to exist Intermediate step: Ribonucleic acid (RNA) Simpler structure than DNA DNA-based life evolved from an earlier RNA world From Chemistry to Biology in 5 Easy Steps From Chemistry to Biology in 5 Easy Steps 5

6 From Chemistry to Biology in 5 Easy Steps From Chemistry to Biology in 5 Easy Steps From Chemistry to Biology in 5 Easy Steps 6

7 Could life have migrated to Earth? Against: No atmosphere or water Solar and stellar radiation For: Fact: amino acids found in meteorites Could life have migrated to Earth? 1996: ALH84001 Martian meteorite found in Antarctica may contain fossilized microorganisms. Further examination needed. 6.3 Evolution of Life Early microbes Rudimentary metabolism: at least a few enzymes Anaerobic: there was no oxygen Chemoautotrophs: obtained energy from inorganic chemicals Thermophiles favored: Abundant energy & chemicals (H, S, Fe) Simple creatures faster mutation and evolution Supported by fossil evidence Thermophiles produce some of the bright colors of Grand Prismatic Spring, Yellowstone National Park 7

8 Rise of Oxygen Early atmosphere: no oxygen Anaerobic life first evolved Photosynthesis: Key step to harnessing sunlight Water-based photosynthesis oxygen (slowly!) Rise of O 2 began ~2 Byr ago Evidence for O 2 at today s levels: only 200 million years ago Some anaerobic microbes went extinct, others evolved Eventually, evolved to thrive in O Rise of Oxygen Where did O 2 come from? Answer: Cyanobacteria Exist today (blue-green algae) Earliest fossils (2.7 byrs) resemble modern cyanobacteria Release oxygen in photosynthesis O 2 is highly reactive: would disappear from the atmosphere in a few million years To maintain oxygen, it therefore needs to be replenished constantly Today: living creatures consume most of the O 2 Cambrian Explosion All known phyla (plus a few extinct ones) appeared within ~40 myr, beginning 542 myr ago Phyla: groups of organisms with a certain degree of evolutionary relatedness Why? Sudden rise in oxygen Significant increase in genetic complexity Climate: emerging from snowball Earth episodes No efficient predators 8

9 Hard to know when land colonization occurred First: probably Microbes Easiest to find water and UV protection on land Algae DNA: plants evolved from algae Algae plants in small pools during periods of dryness? Larger organisms Remained in the oceans longer, particularly animals Plant colonization of land began ~ 475 myr ago Carboniferous period Animals followed plants to land within 75 myr Large forests ~ 360 myr ago coal 6.4 Impact & Explosions Once upon a time, 65 million years ago, dinosaurs were at the top of the food chain. Life was good! Cretaceous-Tertiary (K-T) Event But then all of a sudden, they disappear as a species instantaneously Why? Asteroid impact!! Luis and Walter Alvarez: boundary zone rich in iridium (Ir), like in meteorites High K T iridium the same around the world High abundances of shocked quartz: formed at high temperature and pressure Spherical rock droplets: molten rock solidifies in air Soot (some sites): widespread fires 200 km crater in Yucatán peninsula: 10 km meteorite 9

10 Iridium K T Event 10

11 K T Event Mass extinction ~10 8 hydrogen bombs Tidal wave up most of low-lying North America Forest fires worldwide harsh winter plants die lack of food Acid rain killed life in the oceans too 99% of all living died, 75% of all species became extinct Other Mass Extinctions Multiple mass extinctions At least 5 big ones Many smaller ones Event like K T every ~100 myr Old seafloor craters would be gone by now Nearby supernova explosions also every ~100 myr Magnetic reversals every few myr remove cosmic-ray protection of the magnetosphere Evolution Catastrophes create opportunities, not just disaster May have more effect than gradual evolution Other Mass Extinctions Multiple mass extinctions At least 5 big ones Many smaller ones Event like K T every ~100 myr Old seafloor craters would be gone by now Nearby supernova explosions also every ~100 myr Magnetic reversals every few myr remove cosmic-ray protection of the magnetosphere Evolution Catastrophes create opportunities, not just disaster May have more effect than gradual evolution 11

12 Tunguska! Photo taken in 1927 Massive explosion near the Podkamennaya (Lower Stony) Tunguska River ~ 7:15 a.m., June 30, 1908 Natives: column of bluish light, nearly as bright as the Sun, moving across the sky Tunguska! Photo taken in 1927 The peasants saw a body shining very brightly (too bright for the naked eye) with a bluish-white light... The body was in the form of 'a pipe', i.e. cylindrical. The sky was cloudless, except that low down on the horizon, in the direction in which this glowing body was observed, a small dark cloud was noticed. It was hot and dry and when the shining body approached the ground (which was covered with forest at this point) it seemed to be pulverized, and in its place a loud crash, not like thunder, but as if from the fall of large stones or from gunfire was heard. All the buildings shook and at the same time a forked tongue of flames broke through the cloud. All the inhabitants of the village ran out into the street in panic. The old women wept, everyone thought that the end of the world was approaching. Tunguska! Photo taken in 1927 What was is? Comet? Black hole? UFO? Meteorite/asteroid? 12

13 Tunguska! 2007: Crater found? Russian Meteor Strike: Feb 15, 2013 Russian Meteor Strike: Feb 15,

14 Terrestrial impact craters North American Impacts North American Impacts 1,200 m (4,000 ft) in diameter 170 m deep (570 ft) Rim rises 45 m (150 ft) above the surrounding plains 14

15 Meteors Solid particles entering Earth's atmosphere from interplanetary space Speeds 8-30 miles/sec Height = km (50-80 miles) Heated by friction with atmosphere 3,000 F Average between 5 and 6 per hour, every hour of the night Likely origin from cosmic debris, possibly from collisions between asteroids Typical Meteor Typical meteor = grain of sand to pea diameter Golf ball = bolide or fireball Quantity = 100 tons of meteoric debris per day Sounds like a lot until you spread it out over the entire planet Meteor showers Annual events associated with specific comets Some meteor showers are better than others The trick is knowing when to look Major Annual Meteor Showers 15

16 Continuing Impact Threat Impact objects Meteor: small (<1 cm), ~ 25 million per day, burn in atmosphere Fireball: medium (10 cm 1 m), explode in the atmosphere Meteorite: large (> few m), vaporizes solid rock, leaving a crater Tunguska meteorite (1908): <30 m, energy of several atomic bombs, sound heard round the globe Future Probability declines rapidly with size Currently able to detect threat, but not divert it What are the odds?? Near-Earth Asteroids (NEAs) Asteroids that closely approach the Earth About 250 known Estimates have >2,000 total 16

17 Meteors Solid particles entering Earth's atmosphere from interplanetary space Speeds 8-30 miles/sec Height = km (50-80 miles) Heated by friction with atmosphere 3,000 F Average between 5 and 6 per hour, every hour of the night Likely origin from cosmic debris, possibly from collisions between asteroids Typical Meteor Typical meteor = grain of sand to pea diameter Golf ball = bolide or fireball Quantity = 100 tons of meteoric debris per day Sounds like a lot until you spread it out over the entire planet Meteor showers Annual events associated with specific comets Some meteor showers are better than others The trick is knowing when to look Major Annual Meteor Showers 17

18 Near-Earth Asteroids (NEAs) Eros Ganymed 32.0 Toutatis Toutatis Phaethon 5.0 Mithra 3.0 Castalia 1989 JA 2.0 Geographos PC1 1.9 Apollo 1.6 Icarus UB WT Braille 1994 PM AN DV km x11km 4.6x x x x2.2 AKA: Potentially Hazardous Asteroids (PHAs) 6.5 Human Evolution 18