Origin of Life 3 How could a system as complex as a living cell get started?



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Origin of Life 3

  • How could a system as complex as a living cell get started?

  • Where did the spark of life come from?

  • Spontaneous generation is a time-honored theory on the origin of life

  • Spontaneous generation holds that life is an innate property of organic matter

  • Idea originated with Epicurus, made popular by Lucretius

  • Belgian chemist Jan Baptista von Helmont in the 17th Century provided some sure-fire recipes

  • Take two pieces of moist sod, place grassy sides together, put into the sun, within a few hours you’d get a large number of small eels

  • Open jars stuffed with wheat kernels and dirty underwear would produce live mice in 3 weeks!

  • Revived in the 18th Century by Joseph Needham - gravy yields microorganisms

  • Refuted by Pasteur (boiling), Francesco Redi (covered meat)

  • In a sense, the ancients may have been correct…

  • Life is turning out to be a spontaneous property of the physical and chemical nature of the universe

  • Searching for the origin of life is a daunting prospect

  • How could living things evolve from a mixture of organic compounds?

  • The structure and diversity of life is extremely complex

  • Despite the superficial complexity, there are many common threads in the tapestry of life

  • There are thousands of different proteins, but all are built from the same group of 20 amino acids

  • Amino acids themselves are variations on a simple chemical scheme

  • The genetic code is incredibly complex

  • But the entire code is based on an “alphabet” of only four letters (ACGT)

  • Throw in a small number of sugar and fat molecules and you have the essential building blocks of life

  • But what are the odds of forming complex biomolecules through a random combination of chemicals?

  • Thanks to a stubborn grad student named Stanley Miller, back in 1953, we know that those odds are much higher than you might think!

  • Although the Millers wanted their two sons to become doctors or lawyers, both Stanley and his older brother Donald became chemists

  • When asked later on why he chose chemistry, Stanley told his brother, whom he idolized, “I followed you”

  • When Stanley Miller was a young graduate student at the University of Chicago, he approached Harold Urey with an idea for a thesis project

  • Miller was interested in prebiotic synthesis, the formation of biomolecules that must have preceded the origin of life – the building blocks

  • Miller asked Urey for permission to try an experiment based on Urey’s theory that Earth’s early atmosphere was a reducing atmosphere, with a chemistry dominated by Hydrogen

  • Urey thought that would take too long, and didn’t want Miller getting in over his head

  • Urey suggested a much safer alternative, measuring the amount of Thallium in meteorites – safe, but boring…

  • Any sane graduate student would have jumped on Urey’s offer – a paved four-lane highway to the PhD, with a Nobel laureate at the wheel!

  • Fortunately, Miller stuck to his guns, and finally convinced Urey to let him give it a try

  • Miller mixed water with methane, ammonia, and hydrogen, and passed electric sparks through the mixture to imitate lightning strikes

  • Within two days, amino acids were forming in the mixture – Miller was delighted!

  • His brother Donald says “He was three feet off the floor”

  • Miller wrote up his findings, added his mentor’s name, and proudly presented his results to Urey

  • Urey asked Miller to remove his name from the manuscript, graciously telling him: “I’ve already got my Nobel Prize”

  • In the world of academia, that’s a prince…

  • Miller-Urey experiments, as they came to be called, have since created:

  • All 20 amino acids found in living cells

  • All five nucleotide bases in DNA and RNA (ACGT and U), and…

  • Fatty acids, which are the precursors of cell membranes

  • Many have since abandoned Miller’s model in favor of a CO2/Neutral model

  • Neutral models base their chemistry on carbon dioxide and nitrogen and water, a more probable starting point for Earth’s atmosphere, and one that relies on heavy volcanic activity

  • Miller also ran a series of experiments using clouds of vapors to model a gas cloud from an erupting volcano

  • NASA funded a study in 2008 that reanalyzed several of Miller’s old samples, including his volcanic samples, using modern equipment

  • Samples from Miller’s volcano model yielded an astonishing collection of 22 amino acids

  • Although Earth’s primordial atmosphere was most likely neutral, not reducing, erupting volcanoes vent large amounts of hydrogen and methane in addition to sulfur oxides and CO2

  • And there is no reason to assume life began on the surface, where it would have been limited by atmospheric chemistry

  • The recently discovered deep sea vents on the ocean floor also discharge large amounts of methane and ammonia – life’s building blocks may have first been created at the bottom of the sea

  • Many possible sources of energy for prebiotic evolution:

  • UV - ultraviolet radiation - no protective ozone layer around early Earth

  • Lightning - probably more frequent, powerful storms as the Earth gradually cooled (may be too violent)

  • Heat - molten mantle etc.

  • Radiation - radioactive decay of elements

  • If Miller-Urey results have an Achilles' heel, it is fatty acids

  • Precursor molecules of cell membranes are formed in very low yields

  • Molecules are “branchy”, not “pointy” like the ones used in living cells

  • Miller’s work builds on an earlier and long neglected Russian astronomer, A.I. Oparin

  • Oparin’s was published in 1938

  • Very far ahead of his time…

  • Oparin first proposed in the 1920’s that life on Earth was preceded by a long period of prebiotic evolution

  • British biologist J.B.S. Haldane came independently to the same conclusion

  • Oparin-Haldane hypothesis - simple organic compounds were changed by heat and solar radiation into more complex organic compounds

  • Oparin and Haldane were the first to consider the contents of the primordial soup

  • Haldane may have coined the phrase, referring to the hypothetical cradle of life as “a hot thin soup”

  • Darwin was the first to propose this idea, in a letter to Hooker dated 2/1/1871

But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts - light, heat, electricity etc. all present, that a protein compound was chemically formed, ready to undergo still more complex changes.”

  • Oparin proposed that once the organic compounds were sufficiently complex, they separated into colloidal organic droplets

  • Colloid is a thick mixture of tiny particles suspended in fluid (like mayonnaise)

  • Oparin called these colloidal organic droplets coacervates – a small ball of organic matter formed by the repulsion of water and certain chemicals

  • Artificially created coacervates could accumulate and concentrate organic compounds

  • Like living cells, they let some compounds get inside

  • But once inside, larger compounds were trapped

  • Result was an ever increasing accumulation and complexity of chemical reactions inside

  • Oparin later modified his theory to acknowledge the importance of cell membranes in the early evolution of life

  • His coacervates were colloidal spheres of protein wrapped in droplets of fat molecules (precursors of cell membranes)

  • Idea was criticized as too artificial

  • Oparin had used modern materials like gum arabic to create his “primitive” cells

  • But Oparin’s theory has been recently revived by Sidney Fox and Freeman Dyson

  • Coacervates were a kind of protocell (hypothetical precursor of early cells)

  • Coacervates were like living cells in another important respect

  • When coacervates became large enough, Oparin observed that they divided into smaller spheres,

  • Enzymes could convert sugar to starch inside a coacervate

  • Larger starch molecules would accumulate inside the coacervate

  • When the droplet became too swollen, it would spontaneously divide into smaller coacervates

  • The Oparin-Haldane hypothesis provides strong support for Darwin’s “warm little pond”

  • But what kind of pond, fresh water or salt?

  • Was the ocean the cradle of life?

  • Life depends on the formation of polymers

  • Polymers are long molecular chains, formed by attaching identical subunits called monomers

  • Simple sugars (monomers), for example, are strung together by cells into a more complex molecule, starch (polymer)

  • But polymer formation requires the removal of a molecule of water (condensation)

  • How could this happen in the water??

  • It would be like trying to squeeze water out of a sponge while submerging it in the sink…

  • Most theories on the origin of life envision some primeval tidal pool, thick with organic slime

  • As these tidal pools dried up, they would concentrate the organic compounds

  • But tidal pools are also full of salt

  • As tidal pools dried, the salt would also concentrate

  • Salt would encrust or trap the organic compounds, keeping them from reacting with one another

  • Not a problem in fresh water…

  • So we’re back to Darwin’s warm little pond”

  • But maybe the “warm little pond” was actually cold!

  • Improbable as it seems, ice and snow could have been the birthplace of life on Earth

  • As ice freezes, it squeezes out impurities like organic compounds, concentrating them

  • Water at or near freezing, for example, will squeeze out any hydrogen cyanide in the liquid

  • HCN is a critical molecular step in the formation of many important biomolecules

  • Concentrates up to 50 fold when frozen out of water

  • Colder temperatures also preserve molecules, though they slow down the pace of chemical reactions

  • Organic compounds are delicate, though many are stable for millions of years

  • Cold temps greatly extend their half life (how long it takes for half to disappear)

  • This could even have worked with frozen sea water

  • As ice ages, salt trapped within gradually migrates further down

  • Leaves fresher ice at the top

  • Inuits often rely on this “rotten ice” for drinking water

  • Fresh water from frozen sea ice would also concentrate organic compounds, preserve them indefinitely

  • We don’t really know the temperature of the early Earth

  • 4 bya the sun was much cooler, about 75% of present heat

  • But Earth may have been warm due to the greenhouse effect (methane, carbon dioxide)

  • What if water were not involved at all?

  • J.D. Bernal proposed that water was not necessary in prebiotic evolution

  • According to Bernal, life originated “on the rocks”

  • Maybe rocks and minerals provided a safe harbor for early chemical evolution

  • Calm surface of a warm pool would suffice, but be subject to higher UV, lightning, other disturbances

  • Many rocks, like volcanic rocks and feldspar, have numerous small pores that could have both concentrated and protected early biomolecules

  • Some minerals, like clay, can even promote polymerization (joining) of molecules

  • Solves a problem faced by Miller-Urey models based on water

  • Water is a great medium to make the basic building blocks

  • But water tends to dissolve the more complex proteins that those building blocks can form

  • Clay minerals have the ability to capture and hold proteins and similar molecules

  • Clay or mineral crystals might have provided the matrix on which organic compounds could concentrate, react

  • Clay could thus act as a chemical catalyst, holding molecules in place to react

  • Of all the hypotheses on the origin of life, the most likely seems to be the Oparin-Haldane hypothesis

  • Supported by the Miller-Urey experiments

  • If Miller and Urey are correct, also helps explain why new life has never originated again since the first event

  • Present atmosphere is high in oxygen, inhibits prebiotic synthesis

  • Microorganisms would also gobble up any prebiotic molecules that did form (food)

  • Miller and Urey proved that it was possible to readily synthesize most of life’s essential molecules in a laboratory environment

  • But how do you determine if those chemicals and those conditions actually existed in ancient Earth environments?

  • Miller-type experiments assume that the primordial atmosphere was dominated by hydrogen (strongly reducing)

  • But what if Miller and Urey are wrong in this key assumption?

  • There is little doubt that there was essentially no free oxygen in the early atmosphere

  • But some researchers have assumed that the atmosphere was chemically neutral, neither strongly oxidizing or reducing

  • The CO2 / neutral hypothesis is based on observations of modern volcanoes

  • Volcanoes emit enormous clouds of gas, most of which is CO2

  • Widespread volcanism would have flooded the atmosphere with carbon dioxide

  • Neutral model experiments start with a different mixture to represent the hypothetical early atmosphere

  • Neutral models start with CO2, N2, H2, H2O

  • Melvin Calvin (Nobel prize in Chemistry), one of Miller’s chief critics

  • Calvin’s neutral model mixed CO2, N2 H2, H2O

  • Got formaldehyde, formic acid, other organic molecules

  • Did not get amino acids, proteins, or nucleotides because he did not provide a source of nitrogen (most neutral models include N2)

  • Miller’s model and the neutral model use different starting mixtures

  • No surprise that they yield very different results

  • If you go heavy on the CO2, you get low yields of organic compounds, and limited variety

  • Heavy on CH4, you get very high yields, and a wide variety of complex compounds

  • In Urey’s day, prevailing opinion was reducing atmosphere

  • Majority today hypothesize more neutral, high in N2 and CO2

  • Not all – Tian et al. 2005 Science – early atmosphere was high in Hydrogen

  • The answer may ultimately depend on temperature

  • Which carbon compounds are formed (CH4 versus CO2) depends on environmental temperatures

  • Colder temps, CH4 is favored over CO2

  • Volcanoes produce more CO2 than CH4

  • But volcanoes are not the only source of atmospheric gases

  • Deep sea vents also emit staggering amounts of gas, same ultimate source as volcanoes (molten mantle of the Earth)

  • Deep-sea thermal vents are places where new crust is bubbling up from within

  • Result is sea-floor spreading, continental drift

  • Because temps are lower on the bottom of the sea floor, vent gas is high in CH4

  • Vent environment is high in iron and sulfur

  • Strongly reducing, perfect for prebiotic chemistry

  • But there is a big problem with a vent origin

  • Temps reach 3500C near vents

  • Would denature proteins

  • DNA only lasts a few seconds, amino acids a few minutes

  • So what was the real “starting mixture” for the origin of life on Earth?

  • Answer depends on which source of carbon dominated in the formation of the early atmosphere, volcanoes or vents

  • Indirect evidence comes from thermophilic bacteria (heat loving)

  • They are among the most ancient form of life (still abundant today)

  • Implies that first organisms might have evolved in thermal vents or similar habitats

  • But we are not limited to organic compounds formed on the Earth itself

  • Constant rain of small organic particles from outer space

  • Cosmic dust clouds are vast synthetic factories for an astonishing array of organic compounds

  • Early bombardment of Earth by comets would have added great quantities of water

  • Such comets may have been the source of most of the Earth’s water

  • Comets would also have added large amounts of organic material

  • Countless meteors would also have added their share of organic material to the primordial soup

  • Strong support for Miller-Urey’s results come from a recent meteor impact

  • A gigantic meteor slammed into Victoria, South Australia, on Sept. 28, 1969

  • Murchison meteorite broke up on entry, scattered over 7 km2

  • It was the first major impact since the search for prebiotic compounds began

  • Analysis of the fragments showed that Mother Nature had been conducting her own Miller-Urey experiment, deep inside an asteroid…

  • Found same compounds in the same proportions as Miller-Urey, including several amino acids

  • Most common component was a complex oily organic goo, precisely like the goo that coated the inside of Miller’s apparatus!

  • Many possible sources for the building blocks of life

  • Primordial soup (oceans etc.)

  • Deep-sea vents

  • Comets and meteors

  • Many possible sources for the building blocks of life

  • Lightning strikes in air/water

  • UV radiation reactions in atmosphere

  • Underground (hydrocarbons formed deep down by heat and pressure)

  • Some scientists have gone as far as claiming that not only life’s ingredients, but life itself originated in outer space

  • Panspermia is the theory that life was seeded on Earth and other planets from outer space

  • Idea originated with Greek philosopher Anaxagoras

  • Similar ideas are found in the ancient Hindu vedas

  • Word “panspermia” was coined by Tyndall (who discovered why the sky is blue)

  • Original meaning was that the air on Earth was full of microscopic organisms riding on tiny particles of dust

  • Francis Crick (co-discoverer of DNA) and Leslie Orgel carried it much further

  • Directed panspermia - life was intentionally seeded on Earth and other planets by an intelligent race of aliens, or by their robotic probes

  • Idea was popularized by the astronomer Fred Hoyle in The Black Cloud

  • Hoyle claimed that interstellar dust was teeming with bacteria, seeding life everywhere throughout the cosmos

  • Summary of Hypotheses:

  • Spontaneous generation hypothesis (from antiquity)

  • Oparin-Haldane hypothesis

  • Miller-Urey models (strongly reducing)

  • Vent model (strongly reducing, high temp and pressure)

  • Neutral models (Calvin etc.)

  • Panspermia hypothesis

  • Clay matrix hypothesis (Bernal etc.)

  • Oparin/Haldane hypothesis – possible conditions for the “primordial soup”

  • fresh water or salt water

  • warm water or cold water

  • CO2 favored (volcanic sources)

  • CH4 favored (vent sources)


Directory: ~bfleury -> historyoflife -> lectures
~bfleury -> In our last lecture, we looked at the ways that trade, travel, technology and agriculture can provide new habitats and new dispersal routes for microbes
lectures -> Dinosaurs 3 Several morphological features suggest dinosaurs relied on visual displays for communication, like modern animals
lectures -> Cells to Organisms 3 Eukaryotic cell was a giant leap forward in the early history of life
lectures -> Id is nothing new latest attempt to put a modern face on some very old ideas is old wine in new bottles…
lectures -> The mystery of life itself… We each spend our own lifetimes trying to answer that question in our own way
lectures -> The Discovery of Time
lectures -> The Discovery of Time We have made great progress in discovering the history of life on Earth
lectures -> Dinosaur Renaissance Most young boys (and many young girls!) play with dinosaurs


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