ORIGIN OF LIFE ON EARTHD1

FUNDAMENTAL QUESTIONS THAT HAVE PLAGUED HUMANKIND….

•Where do we come from?

•How did life start on Earth?

•What were our ancestors like millions of years ago?

PROBLEMS FOR STARTING LIFE ON EARTH

• How could the lifeless ball of rock that the planet Earth was 3.5 billion years ago, become home to such lush vegetation and a wide variety of bacteria, fungi, protists, and animals that we see today?

• There are 4 problems which needed to be overcome for the life on Earth to exist.

FOUR PROCESSES NEEDED FOR THE SPONTANEOUS ORIGIN OF LIFE

1. The non-living synthesis of simple organic moleculeslife is based on organic molecules (amino acids, CHOs, lipids, nucleic acids)

early Earth only had inorganic matter (i.e. rock, minerals, gasses- H2, CO2, NH3, CH4, H2O)

organic compounds may have come from inorganic molecules via volcanoes, UV radiation and electric discharge

or they may have been introduced to Earth from space

2. The assembly of these molecules into polymers Polymerization of these simple molecules

occurred to form more complex organic chemicals.

3. ORIGIN OF SELF-REPLICATING MOLECULES (MAKES INHERITANCE POSSIBLE)

• For something to be “alive” it must reproduce on its own.

• Need a self-replicating molecule

• Only self-replicating molecules are able to undergo evolution by natural selection

• DNA is the molecule most used for replication of organisms but it is complex and requires enzymes for its formation.

• Therefore, it is unlikely that DNA developed very early.

• Also, to get the proteins required to form from DNA, RNA is required

4. The packaging of these molecules into membranes with an internal chemistry different from their surroundings Many compounds tend to dissolve in water, and

therefore will depolymerise (it makes it hard to organize small molecules into larger ones)

Closed membrane vesicles form spontaneously from lipids and proteins

The chemical composition inside of these vesicles can be different from their surroundings

Allows for an internal cellular metabolism

FORMATION OF EARTH

Scientific evidence a cloud of dust particles surrounded the Sun to form Earth about 4.5 billion years ago

Early Earth had an atmosphere that probably contained hydrogen, water vapour, methane, ammonia, nitrogen, and hydrogen sulfide (called a reducing atmosphere as there was no molecular oxygen - O2).

Biological monomers must have formed from chemical reactions between these compounds in shallow waters of the oceans (called “primordial soup” or “primeval soup” or “chemical soup”)

SOLUTIONS TO PROBLEM 1 & 2

Possible mechanisms for the formation of organic compounds Abiogenesis Panspermia

Possible Locations with conditions conducive to abiogenesis and polymerization: Volcanoes Deep sea hot water vents Wet/dry conditions Mars

MILLER AND UREY

Conducted experiments in 1953 which simulated the conditions of early Earth.

MILLER AND UREY EXPERIMENTS

Tried to recreate the primordial soup in a glass sphere.

Used H2O, NH3, H2, and CH4 in a sealed loop of glass tubes and flasks.

They kept the system at a warm temperature and exposed the apparatus to UV light

This heat evaporated the water, that was then allowed to cool and condense (the water cycle)

Generated electric sparks to simulate lightning

After one week: 15% of the carbon was in the form of organic

compounds 13 of the naturally occurring amino acids were

detected Sugars were formed Adenine (nitrogenous base) had formed

Problems with abiogenesis: Miller-Urey experiment showed abiogenesis

(creating organic matter from inorganic matter) A limitation to the Miller-Urey primordial soup

theory is that it is difficult to explain how amino acids and nucleotides polymerised in an aqueous environment (which would have promoted hydrolysis)

COULD COMETS HAVE BROUGHT ORGANIC COMPOUNDS TO EARTH?

COMETS MAY HAVE DELIVERED THE GOODS

Panspermia: the hypothesis that life on Earth may have originated by the introduction of organic chemicals or even bacteria from comets

• Comet: small body of rock, dust, and ice that orbits the Sun

• Meterorite: a solid piece of debris, from such sources as asteroids or comets, that originates in outer space and survives its impact with the Earth's surface.

• Geological records show that our planet was bombarded by a shower of comets and asteroids about 4 billion years ago (Late Heavy Bombardment)

• Organic molecules hitchhiking on comets could survive the impact and the impact could help to polymerize certain amino acids into polypeptides)

COULD LIFE EXIST ON A COMET IN THE EXTREME CONDITIONS IN SPACE?•Some bacteria and archaebacteria can survive in extreme environments (Bacterial endospores found in ice cores in Antarctica)

•Cosmic radiation could provide the energy to form complex organic molecules

•By studying spectral lines of distant clouds of cosmic dust particles, astronomers claim to have revealed the presence of glycine, which is the simplest amino acids. This suggest organic molecules can form in space

POSSIBLE LOCATIONS FOR THE ORIGIN OF LIFE

Specific locations must have existed where polymerisation would have been promoted.

Alternating wet/dry, Deep oceans, volcanoes, mars

ALTERNATING WET DRY CONDITIONS

• On earth at a seashore or the flood plains of a river where there is an alternation of wet –dry conditions

• The drying of clay particles could have created catalyzing reactions and formed early organic molecules

DEEP OCEANS

organic molecules could have formed around hydrothermal vents, where hot water spews from below the ocean floor

formed when cracks in the crust of seabed expose sea water to heat from magma below

hot water rises and picks up minerals along the way (looks like black smoke AKA black smokers)

Many communities of organisms currently live around these vents and suggests life can be supported there (chemosynthetic bacteria, archaea, giant tube worms, clams etc.)

VOLCANOES emit water vapour, gases and minerals which

could be used to form organic matter early Earth had many volcanoes, lightning

and was bombarded by UV rays the raw materials plus the conditions

presented by volcanoes could have favoured formation of larger organic compounds

MARS AND EXTRATERRESTRIAL BODIES alternate theory to abiogenesis is that

life formed elsewhere in space organic compounds are common in

outer solar system Earth was still too hot, Mars (smaller

and further) was cooler and allowed for abiogenesis

organic molecules could have been blasted from Mars via asteroid or comet impacts

Little direct evidence, however, meteorites from Mars (possibly containing fossilized bacteria) have been found in Antarctica

D1 ORIGINS OF LIFEPart 2

SOLUTION 2

In the millions of years following the creation of organic compounds in the primordial soup, these compounds became more complex.

Amino acids, monosaccharides, nucleic acids would have undergone polymerization

This process may have occurred in shallow rock pools, particularly those where organic compounds had accumulated by absorption on the surface of clay particles

When clay dries out and is heated, as many as 200 amino acids can spontaneously join together in polypeptide chains.

PRECURSORS TO CELLS- SOLUTION 3

Coacervate Droplets microscopic sphere that forms from lipids in

water form spontaneously due to the hydrophobic

interactions between water and lipids can be selectively permeable not composed of phospholipids but may

incorporate proteins can grow and split

PRECURSORS TO CELLS-SOLUTION 3

Proteinoid Microspheres Related to coacervate droplets After amino acids are heated, mixed with hot

water, and then cooled, small protein globules are formed

catalytic properties can undergo simple division

PRECURSORS TO CELLS-SOLUTION 3

Although they are not living organisms, coacervates and proteinoid microspheres are a significant step toward the formation of cells.

They solve the problem of protecting polymers from their destructive environments.

Could be primitive versions of the first cell membranes

PRECURSORS TO CELLS-SOLUTION 3

Protobionts likely coacervate droplets or proteinoid

microspheres which surrounded polynucleotides (i.e. RNA or DNA).

have a membrane-like structure which can surround organic molecules and maintain an interval environment that is different than the external one

Overtime, true cell membranes evolved and other characteristics of cells developed like; Cellular respiration Asexual reproduction

PROPERTIES OF RNA- SOLUTION 4

Evolution by natural selection can only occur on molecules that exhibit properties of variation and heredity.

So before life was created there needed to be a molecule with variation and heredity – that molecule was probably RNA Variation – in a population of RNA molecules

there are a number of molecules with different base sequences

Heredity – the RNA molecule can produce copies (or slightly modified copies) of itself

Eventually RNA would be replaced by DNA and enzymes

RNA WORLD HYPOTHESIS-SOLUTION 4

RNA World Hypothesis: theory that RNA stored genetic info and acted as a catalyst in a very primitive self-replicating system

Ribozymes Small sequences of RNA that can act as enzymes Can be used to polymerize nucleotides and made

to cleave chemical bonds including peptide bonds

Example: The ribosome – catalyses a peptide bond to create proteins

HOW LIFE CHANGED OUR PLANET

20% of our atmosphere is now oxygen, but early Earth had very little O2.

Early prokaryotes therefore had to rely on other forms of respiring Methanogens- metabolize methane Others fed on other inorganic material like H2S

to obtain energy As bacteria reproduced, food became scarce It is believed that because of this food

shortage, bacteria that contained chlorophyll and could photosynthesize (i.e. related to cyanobacteria) would be selected for.

PHOTOSYNTHESIZING BACTERIA photosynthesis is a significant event in

history of Earth Bacteria could now rely on the sun as the

main energy source, which produced O2 as by-product

However, O2 is toxic to anaerobic bacteria and killed many anaerobic bacteria

Anaerobic bacteria that survived would live in mud or places protected from the new oxygen-rich atmosphere.

The ability of an organism to make its own food gives it a distinct advantage over those that cannot.

As a result, photosynthetic bacteria proliferated and produced more and more oxygen

FROM PROKARYOTES TO EUKARYOTES

Prokaryotes are relatively simple compared to eukaryotes, but many chemical pathways (e.g. Glycolysis) are similar between the two types of cells.

3.8-2 bya, bacteria (prokaryotic cells) were the only organisms on Earth

The first fossils of cells with a nucleus (eukaryotes) is from around 2 bya.

How did prokaryotes develop into eukaryotes?

Endosymbiosis is the most popular theory of how eukaryotic cells formed

ENDOSYMBIOTIC THEORY

Endo = into Symbiosis = interaction between two different

organisms living in close physical association, typically to the advantage of both.

Suggests that chloroplasts and mitochondria (and possibly even the nucleus) are derived from free living prokaryotes being engulfed by larger prokaryotes.

These smaller prokaryotes survived in the cytoplasm and eventually evolved into the organelles.

ENDOSYMBYOSIS

The host cell would provide protection for the smaller prokaryotic cell

The engulfed cell would be beneficial to the host if it was photosynthetic (providing food) for the host or able to metabolize food efficiently and produce energy for the host.

Rather than being digested, the prokaryotes were kept alive inside the host cell in exchange for their services

Explains how membrane bound organelles such as chloroplasts and mitochondria became part of eukaryotic cells.

EVIDENCE FOR ENDOSYMBYOSIS

DNA both contain own DNA that is different from

nuclear DNA but similar to bacterial DNA (i.e. circular)

Double membrane both are surrounded by two membranes inner membrane could belong to original

engulfed prokaryote, outer one could be the vesicle in which it was engulfed

EVIDENCE FOR ENDOSYMBYOSIS

Replication both are self-replicating (do so separately

from nucleus) both divide similarly to bacteria (i.e. binary

fission)

Ribosomes mitochondrial and chloroplast ribosomes are

similar to prokaryotic ribosomes (i.e. 70S vs 80S in eukaryotes

EVIDENCE FOR ENDOSYMBYOSIS

Internal structure and chemistry chloroplast and mitochondria chemistry and

structure very similar to bacteria

Size chloroplasts, mitochondria and bacteria all

similar in size (1-10 m)

PROBLEMS WITH ENDOSYMBIOSIS

Inheritance: The ability to engulf another cell and have it survive in the cytoplasm does not guarantee that the host cell can pass it on to its offspring the genetic code to synthesize the newly acquired organelle

Gene transfer: The genes for making mitochondrial and chloroplast proteins have been transferred to and are controlled by nucleus

Independence: When chloroplasts or mitochondria are removed from a cell, they cannot survive on their own.