Origins of Life-2 Unit 1 Part 1

Origins of Life -2 Objectives Describe the chemical and physical conditions of the pre-biotic Earth Discuss several theories of the origins of life on Earth. Describe how the success of prokaryotic life has changed the chemical conditions of this planet. Discuss how our understanding of the origins of life on Earth is helping us look for life (or the evidence of past life) elsewhere in the universe (discuss examples Mars, Titan, etc.).

Earth The earth is 4.5 to 4.7 Billion Years Old

Major landmarks in biological evolution Major landmarks in biological evolution. The positions of the stages on the time scale are approximate. Note how the oxygenation of the atmosphere due to cyanobacterial metabolism was a gradual process, occurring over a period of about 2 billion years. Also note that for the bulk of Earth’s history, only microbial life forms existed. Although microfossil evidence is lacking, microchemical evidence for eukaryotic cells goes back as far as 2.7 billion years ago.

? Life No Life How did we get here? 0.5-1 billion years 4.5 billion years ago ~3.5 billion years ago

Cellular organization Inorganic Organic Cellular organization Progenote Universal Ancestor Pre-life Earth was a mixture of chemicals Elements of life H, O, C, N, S, P, (and a little P, Fe, Na, and K)

Theories of the Origins of Life Spontaneous formation of organic molecules “RNA” world Solid-phase “Iron-Sulfur” world

Theories about the origin of life on Earth Spontaneous Formation of Organic Molecules (Oparin and Haldane 1920s) the combination of reduced gasses and electricity (or UV radiation or nuclear radiation) caused the formation of more complicated organic compounds

Spontaneous formation evidence Urey & Miller experiments (1950s) Created amino acids, sugars, fatty acids, thioesters, and nucleotide bases in the lab Amino acids and other organic molecules are found in meteorites (shows that it can happen without human intervention) Detected in space (spectral analysis)

Theories “RNA world” RNA has two properties that make it an attractive first polymer of life Contains information (nucleotide bases) Catalytic (RNA can do some chemical reactions)

and major coding molecule The RNA World The current “most accepted “theory of life evolving hypothesizes an RNA world RNA in the early world would have functioned as a self replicating molecule, eventually developing a number of minimal catalytic properties RNA self-replicates Proteins take over catalysis Packaging evolves - RNA codes and catalyses DNA becomes long term storage and major coding molecule

Rybozymes The first ribozyme discovered was a piece of mRNA that could self-edit – cut out the introns.

Order of the Information Macromolecules DNA RNA Proteins RNA required for protein synthesis (mRNA, tRNA, rRNA) DNA is a modification of RNA Where did the first RNA come from?

Ribosymes The first RNA polymers may have formed on clay templates (clay molecules have very regular structures)

Origin of Life Stages of RNA World: RNA World -Most accepted theory -In early world as self-replicating molecule -Later developing other catalytic properties Stages of RNA World: 1-Formation of nucleotides (spontaneous formation?) 2-Formation of RNA molecules (on clay template?) 3-RNA replication RNA molecules that evolve catalytic activities (protein synthesis) had a selective advantage 4-The link between sequence of RNA and sequence of proteins (translation)

and major coding molecule The RNA World RNA self-replicates Proteins take over catalysis Packaging evolves - RNA codes and catalyses DNA becomes long term storage and major coding molecule

Theories Solid-phase “Iron-Sulfur” world – life formed on solid surfaces The reaction between Fe+2 and HS- (or H2S) can be coupled to CO2 fixation This type of reaction can occur in high temperature, high pressure environments - hydrothermal vents

Iron-sulfur world Evidence Iron and sulfur are important catalytic elements in biology (oxidation/reduction reactions) Laboratory demonstrations of the production of pyruvate under “prebiotic” conditions

Abiotic reactions leading to organic compounds – occur under high temp/high pressure conditions Inorganic reactions can catalyze the formation of organic molecules

Cells Inorganic Organic Cellular organization Progenote Universal Ancestor Once macromolecules formed – how did cells form?

Origin of Cells Organic molecules have tendency to aggregate membrane-like structures form easily and even happen in laboratory experiments Microspheres- " coacervates“- form spontenously and can replicate by pinching off new spheres Proteinous microspheres with catalytic and self- replicating properties could be formed in the lab (Fox 1965, Protobionts, Progenotes)

Coacervate growth and division Collection of aggregated polymers Grow by adding new polymers Form a semi-permeable membrane When they get too big they divide Demonstrates how polymers aggregate and act like membranes

Progenote or Protobiont Proteinaceous microspheres – contain proteins and lipids but no nucleic acids Self replicating Some ability to catalyze reactions “pre-cells” Progenotes would eventually pick up RNA and DNA, develop enzymatic capabilities and membrane organization = primitive cell

Origin of Life

Where did life start? Hydrothermal vents RNA lineage puts hyperthermophiles nearest to the universal ancestor (see fig 2.3 where are the hyperthermophiles?) Rich in reduced compounds Fe+2, H2S, H2 etc. Protected from sterilizing cosmic radiation

Where did life start? “Panspermia” – extraterrestrial – life landed here on a meteorite Life started earlier than previously thought (when the earth was less than a billion years old) Organic material is found in meteorites Life exists on earth in habitats similar to those found (or once existed) on Mars, Europa, Titan

Magnetite crystals Evidence for life on other planets?

The first organism must have employed a simple strategy to obtain energy. Primitive metabolism was anaerobic, and likely chemolithotrophic, exploiting the abundant sources of FeS and H2S present. One possibility is FeS + H2S ==> FeS2 + H2 The resulting H2 could have been used to drive a primitive ATPase with S0 as a potential electron acceptor Earliest metabolism Brock pg 428 Fermentations, and anaerobic respiration probably appeared later along with anoxygenic photosynthesis followed by oxygenic photosynthesis. The latter led to development of an oxic environment, and to great bursts of biological evolution.

Oxygen Earliest Evidences: oldest fossils Oldest photosynthetic microbes 3.5-3.2 B.Y. - Bacterium-like - Unicellular - Evidence for breakdown products of photosynthesis Cyanobacteria, 3.5 B.Y. Stomatolites, 3.5- 0.7 B.Y. Evolution of oxygen

Oxygen Evidence for O2 production: Banded Iron Formations (BIF) BIF found in ocean sediments red bands are high in Fe2O3 and Fe3O4 (red bands)- forms when reduced iron reacts with O2

Oxygen

Oxygen Only known source of O2 in the atmosphere is oxygen producing photosynthesis Photosynthesis produced O2 in the oceans that combined with the Fe producing iron oxides-sinking down to ocean floor producing BIF BIFs occur in geological rock formations dating back to 3.2-2 B.Y.

Life on Earth

Oxygen BIFs occur 3.2-2 B.Y. ago, suddenly disappear. Red beds, terrestrial formations similar to BIF, but much lower in iron concentrations Red beds, indication of presence of O2 in the atmosphere Red beds are present in terrestrial sediments in last 2 B.Y. Formation of red beds beginning 2B.Y. ago, after all reduced Fe in the oceans had been oxidized

Life on Earth Oxygen sinks: Volcanic gases scavenge O2 Aerobic respiration uses O2 Weathering of rocks containing reduced elements such as carbon, sulfur, and iron Above sinks has not changed since Archean time But BIFs sink finally became saturated 2 B.Y. ago Massive iron-rich ores that are used today to make steel are legacy of photosynthetic microbes Summary: massive change in chemistry of the Earth, mediated entirely by bacteria

Why did the availability of oxygen drive evolution (or why are eukaryotes aerobic)? Hint – see the Nealson 1999 paper.

Major landmarks in biological evolution Major landmarks in biological evolution. The positions of the stages on the time scale are approximate. Note how the oxygenation of the atmosphere due to cyanobacterial metabolism was a gradual process, occurring over a period of about 2 billion years. Also note that for the bulk of Earth’s history, only microbial life forms existed. Although microfossil evidence is lacking, microchemical evidence for eukaryotic cells goes back as far as 2.7 billion years ago.