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Chapter 26.Origin of Life

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The history tree oflife can bedocumented withevidence as alreadydiscussed.The Origin of Life onEarth is another story

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Origin of Life hypothesis Abiotic synthesis of organic molecules

amino acids & nucleotides Building polymers

joining molecules into polymers proteins & nucleic acids

Protobionts packaging polymers into membrane-

bound droplets maintain internal chemistry

Origin of self-replicating molecules makes inheritance possible

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Conditions on early Earth Reducing atmosphere

electron-adding water vapor, CO2, nitrogen, NOx, CH4,

NH3, H2, H2S Energy source

lightning, UV radiation, volcanic

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Abiotic synthesis Testable hypothesis

1920Oparin & Haldanepropose reducingenvironmenthypothesis

1953Miller & Ureytest thehypothesis

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Stanley Miller

University of Chicago

yieldedamino acids

hydrocarbonsN bases

other organicmolecules

It is unclear whether young Earth’s atmosphere contained enough methane andammonia to be reducing. Growing evidence suggests that the early atmospherewas made up primarily of nitrogen and carbon dioxide and was neither reducingnor oxidizing (electron–removing). Miller–Urey–type experiments using suchatmospheres have not produced organic molecules. Stil l, it is l ikely that small“pockets” of the early atmosphere—perhaps near volcanic openings—werereducing.Instead of forming in the atmosphere, the first organic compounds on Earth mayhave been synthesized near submerged volcanoes and deep–sea vents—weakpoints in Earth’s crust where hot water and minerals gush into the ocean.

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Protobionts Life → reproduction & metabolism

Life is defined partly by two properties: accurate replication andmetabolism. Neither property can exist without the other.Self–replicating molecules and a metabolism–like source of thebuilding blocks must have appeared together. How did that happen?The necessary conditions for life may have been met by protobionts,aggregates of abiotically produced molecules surrounded by amembrane or membrane–like structure. Protobionts exhibit some ofthe properties associated with life, including simple reproduction andmetabolism, as well as the maintenance of an internal chemicalenvironment different from that of their surroundings.Laboratory experiments demonstrate that protobionts could haveformed spontaneously from abiotically produced organiccompounds. For example, small membrane–bounded dropletscalled liposomes can form when lipids or other organic moleculesare added to water.

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RNA world RNA is likely first genetic material

multi-functional molecule codes information enzyme functions

ribozymes replication

regulatory molecule transport molecule

Dawn of natural selection

Why RNA?RNA molecules are important catalysts in modern cells. Modern cellsuse RNA catalysts, called ribozymes, to remove introns from RNA.Ribozymes also help catalyze the synthesis of new RNA, notablyrRNA, tRNA, and mRNA. Thus, RNA is autocatalytic, and in theprebiotic world, before there were enzymes (proteins) or DNA, RNAmolecules may have been fully capable of ribozyme-catalyzedreplication.

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Key Events in Origin of Life Key events in

evolutionaryhistory of life onEarth life originated

3.5–4.0 bya

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Prokaryotes Prokaryotes dominated life

on Earth from 3.5–2.0 bya

3.5 billion year old fossil of bacteria modern bacteria

Electron Transport SystemsThe chemiosmotic mechanism of ATP synthesis, in which a complexset of membrane–bound proteins pass electrons to reducibleelectron acceptors with the generation of ATP from ADP, is commonto all three domains of life—Bacteria, Archaea, and Eukarya. There isstrong evidence that this electron transport mechanism actuallyoriginated in organisms that lived before the last common ancestorof all present–day life. The earliest of these electron transportsystems likely evolved before there was any free oxygen in theenvironment and before the appearance of photosynthesis; theorganisms that used it would have required a plentiful supply ofenergy–rich compounds such as molecular hydrogen, methane, andhydrogen sulfide. A great challenge facing scientists studying theorigin of life is to determine the steps by which this electron transportmechanism originated, and how important early versions of it mighthave been in the emergence of the first cells.So considerable metabolic diversity among prokaryotes living invarious environments had already evolved more than 3 billion yearsago. Most subsequent evolution has been more structural thanmetabolic.

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Stromatolitesfossilized mats ofprokaryotesresemble modernmicrobial colonies

Lynn Margulis

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Oxygen atmosphere Oxygen begins to accumulate 2.7 bya

photosynthetic bacteria cyanobacteria (blue-green algae)

reducing → oxidizing atmosphere corrosive

banded iron in rocks = rusting

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Endosymbiosis Evolution of eukaryotes

membrane bound organelles = 2.1 bya

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Cambrian explosion 543 mya

within 10–20 million years most of the majorphyla of animals appear in fossil record

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Diversity of life & periods of mass extinction

Diversity of life and periods of mass extinction. The fossil record of terrestrialand marine organisms reveals a general increase in the diversity of organismsover time (red line and right vertical axis). Mass extinctions, represented bypeaks in the extinction rate (blue line and left vertical axis) interrupted thebuildup of diversity. The extinction rate is the estimated percentage of extanttaxonomic families that died out in each period of geologic time.The fossil record chronicles a number of occasions w hen globalenvironmental changes w ere so rapid and disruptive that a majority of speciesw ere sw ept aw ay Such mass extinctions are know n primarily from thedecimation of hard–bodied animals that lived in shallow seas, the organismsfor w hich the fossil record is most complete. Tw o mass extinctions—thePermian and the Cretaceous—have received the most attention. The Permianmass extinction, w hich defines the boundary betw een the Paleozoic andMesozoic eras, claimed about 96% of marine animal species. Terrestrial lifew as also affected. For example, 8 out of 27 orders of insects w ere w ipedout. This mass extinction occurred in less than 5 million years, possibly muchless—an instant in the context of geologic time. The Cretaceous massextinction of 65 million years ago, w hich marks the boundary betw een theMesozoic and Cenozoic eras, doomed more than half of all marine speciesand exterminated many families of terrestrial plants and animals, includingmost of the dinosaurs.

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The Chicxulub impact crater in theCaribbean Sea near the YucatanPeninsula of Mexico indicates anasteroid or comet struck the earth andchanged conditions 65 million years ago

Trauma for Earth and its Cretaceous life.One clue to a possible cause of the Cretaceous mass extinction is athin layer of clay enriched in iridium that separates sediments fromthe Mesozoic and Cenozoic eras. Iridium is an element very rare onEarth but common in many of the meteorites and other extraterrestrialobjects that occasionally fall to Earth. Walter and Luis Alvarez andtheir colleagues at the University of California proposed that this clayis fallout from a huge cloud of debris that billowed into theatmosphere when an asteroid or a large comet collided with Earth.This cloud would have blocked sunlight and severely disturbed theglobal climate for several months.Where did the asteroid or comet hit? Research has focused on theChicxulub crater. The 65–million–year–old Chicxulub impact crater islocated in the Caribbean Sea near the Yucatán Peninsula of Mexico.The horseshoe shape of the crater and the pattern of debris insedimentary rocks indicate that an asteroid or comet struck at a lowangle from the southeast. This artist’s interpretation represents theimpact and its immediate effect—a cloud of hot vapor and debris thatcould have killed most of the plants and animals in North Americawithin hours. About 180 km in diameter, the crater is the right size tohave been caused by an object with a diameter of 10 km.

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Classification Linnaeus used structural similarity 1969

Robert Whittaker devised 5 Kingdomsystem Monera Protista Plantae Fungi Animalia

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Re-Classifying Life New groupings

molecular data challenges 5 Kingdoms Monera is too diverse

2 distinct lineages of prokaryotes Protists are too diverse

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3 Domain system Super Kingdoms

Bacteria Archaea

extremophiles = live in extreme environments Eukarya

eukaryotes

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A New Tree of Life