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

No Life 0.5-1 billion years Life?

How did we get here?

4.5 billion years ago ~3.5 billion years ago

Inorganic Organic Cellular organization

ProgenoteUniversal 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

1. Spontaneous formation of organic molecules

2. “RNA” world

3. 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 life1. Contains information (nucleotide bases)

2. Catalytic (RNA can do some chemical reactions)

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

self-replicates Proteins take over catalysis

DNA becomeslong term storage

and major coding molecule

Packaging evolves - RNA codes and catalyses

RNA

Rybozymes

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

Order of the Information Macromolecules

DNAProteinsRNA

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

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)

The RNA World

self-replicates Proteins take over catalysis

DNA becomeslong term storage

and major coding molecule

Packaging evolves - RNA codes and catalyses

RNA

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

Cells

Inorganic Organic Cellular organization

ProgenoteUniversal Ancestor

Origin of Cells

Organic molecules have tendency to aggregatemembrane-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

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.

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.

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

OxygenEarliest 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.

OxygenEvidence 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