A short story Leathem Mehaffey, III, Fall 201946 47 Kenorland: The First Supercontinent* • It is 2.7 Billion Years ago. • A vast continent sits almost alone in a world-girdling sea. The interior is mountainous due to the collisions of the cratons forming it. The interior is arid, so far from the shores. • A dim sun (80% of today’s luminosity) shines in the sky. The Moon appears 2 to 3 times larger than today. • The day is short, only 18 hours or so. • There is no life on the land beyond bacteria (Archaean), nothing green, just rocks. • The atmosphere is poisonous and noxious: mostly CO2, methane and sulfur compounds. There is very little oxygen. • With little or no oxygen there is no ozone, and the surface is bombarded with unfiltered ultraviolet light. 48 *Some authors postulate one earlier and smaller continent called Ur, but it would not have contained today’s land masses. Photosynthesis Arises • The first cells to carry out photosynthesis did not use oxygen in energy-producing reactions. • Early photosynthesis probably used H2S, as it was abundant and requires less energy to oxidize than water • 6CO2+12H2S+hν→C6H12O6+6H2O+12S • This method predominated for millions of years • Many organisms today are anaerobic, and even find oxygen toxic. • They use other sources for energy such as hydrogen sulfide • But eventually some organisms developed a second photosystem that could use water (H2O) instead of H2S, producing oxygen rather than sulfur as the end product and gleaning more energy. 49 Earliest life: blue-green algae 50 The First Oxygen producers: Bluegreen Algae* CO2+H2O+hν→ Cn(H2O)n+O2 Note: these are the ONLY organisms ever to invent oxygenic photosynthesis!! Some estimates place their origin at 3.5BYA! 51 *”algae” is a misnomer. The preferred term is “cyanobacteria”. Some earliest “fossils”: Stromatolites • Stromatolites are formed by blue-green algae and sediment. Sediment sticks to the surface of the living algae, forming a crust. In turn, the algae grows through the sediment to form a new layer of living matter. A new film of sediment sticks to the algae and so on. The resulting structure looks like a mound. In cross section, each layer can be seen. • Stromatolites are the most abundant fossils known from the Precambrian. They are less common in the Paleozoic, perhaps because snail-like predators began to graze on the algae. 52 A Paradox: If oxygen is so good, why do we take antioxidants? • Oxygen is highly reactive, especially in atomic (O-) form. It can wreck cells and mutate DNA, causing cell death or cancer. • All cells have biochemical mechanisms for eliminating free radical oxygen. • So how could oxygenic photosynthesis arise, unless those first cells already had protective mechanisms? How could those mechanisms arise before oxygenic photosynthesis? • UV light splits atmospheric water into oxygen and hydrogen; the lighter hydrogen escapes the earth, leaving behind the oxygen. • Before the ozone layer formed this reaction probably produced enough oxygen to “inoculate” the developing cells, providing a selective force. 53 The Great Oxidation Event • The Hadean Era began with the formation of the Earth, 4.5BYA. • It is considered to have ended with the laying down of the first rock records, marking the beginning of the Archean Era, 4.2BYA • The Archean Era began with the Late Heavy Bombardment and ended with the rise of oxygen in the atmosphere (<2%). • This marks the beginning of the Proterozoic Era. • Photosynthesis probably began early in the Archaean, soon after the first life, but oxygenic photosynthesis took another billion or so years to arise. • But it was to change the face of the Earth. 54 55 Oxygen and genera fractional origination, rate Fi Evolution five-point-centered moving average of those data Did fluctuating O2 or CO2 ratio RCO2 of historical pCO2 to recent pCO2 levels drive speciation? Was it CO2 or was it O2? Note that high CO2 usually correlates with low O2. Documenting a significant relationship between macroevolutionary origination rates and Phanerozoic pCO2 levels. James L. Cornette, Bruce S. Lieberman, and Robert H. Goldstein. Proc Natl Acad Sci U S A. 2002 Jun 11; 99(12): 7832–7835 56 Kenorland and the Great Oxygenation Event • Kenorland, the first “Supercontinent”, formed from colliding cratons about 2.7 GYA. • For the first time the Earth had shallow inland seas and coastal waters in quantity. • The high CO2 in the atmosphere led to substantial mineral erosion, flooding these inland seas and coastal plains with minerals. • The stage was set for organisms to photosynthesize using the copious CO2, minerals and water and to produce O2 as a byproduct. • By about 2.5GYA, mineral deposits showed clear signs of free oxygen in the atmosphere. • Around 2.4GYA, Kenorland began to break up, leaving even more shallow seas between pieces, rich with floating mats of blue-green algae producing oxygen. 57 The Snowball Earth hypothesis proposes that Earth's surface became entirely or nearly entirely frozen at 58 least once, and possibly two more times. A number of unanswered questions remain, including whether the Earth was a full snowball, or a "slushball" with a thin equatorial band of open (or seasonally open) water. Causes of Snowball Earth • Weaker sun (70-80% of today’s sun’s output) • Drop in CO2: • Biological • Photosynthesis sequesters CO2 into organic tissue • Oxygen production removes methane from the atmosphere* • Erosion • Produces minerals which absorb CO2 to form carbonates • Dumps sediments onto shallow seas and shorelines, burying organic matter and locking it away from the CO2 cycle • Continental Drift: • Clustering of continental masses disrupts ocean patterns • Barren equatorial continents reflect sunlight *Conversely the earliest snowball earth could have been partially caused by excess methane causing a screening pink haze in the atmosphere. 59 • Vulcanism • Continental drift causes subduction zones leading to volcanic activity. • Volcanoes release large amounts of CO2 and methane into the atmosphere • Biological: large glaciation diminishes photosynthesis Recovery • Diminished sequestration of CO2 by photosynthesis • from Shifts balance of CO2 cycle toward breakdown of organic matter with subsequent release of CO2 Snowball • Release of methane from clatherates and permafrost • Two scenarios here: • Deep-earth production of methane from hydrogen and Earth calcium carbonate producing clatherates upon contact with the cold ocean water, accumulates during ice ages.* • Biological production by methanogens, diminishes during ice ages. *Mendeleev posited that methane and other hydrocarbons are produced under pressure deep in the earth from hydrogen and carbonate. Russian geologists today thus claim petroleum and natural gas are renewable resources. 60 Other Ice Ages • Typical Ice Ages last about 80,000 years and are interspersed with milder Interglacial periods of around 20,000 years. • Typically ice ages form slowly and dissipate quickly. • Since the last Snowball Earth there have been three notable glaciations: • Andean-Saharan Ice Age: 460-430 MYA • Late Paleozoic Ice Age: 360-260 MYA • Followed by a rapid (40 million year) transition to a hot and dry desert planet. • Quaternary Ice Age: • Began 2.58 MYA and continues today. • We live in and interglacial period which started about 11,000 years ago. 61 Breakup of Kenorland and the first Snowball Earth • Kenorland begins to break up 2.6GYA. With water invading in the rift valleys, a massive spike in rainfall begins. With the high CO2 content in the atmosphere the rain is acidic. The rocks are heavily eroded, which in turn absorbs much CO2. • As CO2 falls the greenhouse effect dwindles. The weaker sun cannot keep the ice from forming at the poles. • The forming ice reflects more sunlight, and a downward spiral begins until the entire planet is covered from pole to pole with ice a mile thick, with temperatures hovering around -50oF. It will remain that way for many millions of years. • There will be several more “Snowball Earth” episodes to follow. 62 Icehouse/Hothouse Earth: The Cryogenian Ice Age, 750-580MYA • For 170 Million Years the Earth was locked in a vicious cycle of Icehouse (Snowball or “Slushball” Earth) and hothouse Earth. • During Icehouse phases glaciers extended into the tropics • At least two such phases are generally accepted: • Sturtian from 717 to 660 MYA • Marinoan from 640 to 635 MYA • During Hothouse phases tropics extended to the polar regions and the equatorial regions were uninhabitable. • CO2 concentrations were as much as 20 times today’s levels. • There was no ice anywhere on the planet. • Sea surface temperatures were near 70oC (158oF) 63 Milancović Cycles Eccentricity (ca 100,000-year cycle) influences overall insolation. Precession (26,000-year cycle) and Tilt (41,000-year cycle) both influence the extremes of summer and winter in the northern and southern hemispheres. 64 Hail Columbia: the second supercontinent forms. • Formed about 2.1-1.8GYA from the remnants of Kenorland. • Also called Nena, Nuna or Hudsonland. • 12,900 km (8,000 mi) from North to South at its broadest part, and 4800 km (3,000 mi) wide. • Incorporated almost all of the present continents and all of the earth’s continental crust. • Surrounded by a world-girdling sea (unnamed). • Barren, arid and hot. 65 Enter the Eukaryotes (ca 2.7GYA) 66 Lynn Margulis Eukaryotes: A Matter of Endosymbiosis • Lynn Margulis originated the theory by observing that certain eukaryotic organelles (mitochondria and chloroplasts) are unique among organelles in that they have their own membrane and their own DNA and reproduce independently of the host cell. 67 Prokaryotes and Eukaryotes 68.