O

RIGIN OF ON

E

ARTH

L

IFE

D1

F

UNDAMENTAL 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?

P

ROBLEMS FOR STARTING LIFE ON

E

ARTH • 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.

F

OUR PROCESSES NEEDED FOR THE SPONTANEOUS ORIGIN OF LIFE 

1. The non-living synthesis of simple organic molecules

 life is based on organic molecules (amino acids, CHOs, lipids, nucleic acids)  early Earth only had inorganic matter (i.e. rock, minerals, gasses- H 2 , CO 2 , NH 3 , CH 4 , H 2 O)  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. O RIGIN 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

F

ORMATION OF

E

ARTH    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 - O 2 ).

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”)

S

OLUTIONS TO

P

ROBLEM

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

M

ILLER AND

U

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

M

ILLER AND

U

REY

E

XPERIMENTS    Tried to recreate the primordial soup in a glass sphere.

Used H 2 O, NH 3 , H 2 , and CH glass tubes and flasks.

4 in a sealed loop of 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)

C

OULD COMETS HAVE BROUGHT ORGANIC COMPOUNDS TO

E

ARTH

?

C

OMETS

G

OODS

M

AY

H

AVE

D

ELIVERED

T

HE  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)

C

OULD 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

P

OSSIBLE

L

IFE

L

OCATIONS FOR THE

O

RIGIN OF  Specific locations must have existed where polymerisation would have been promoted.

 Alternating wet/dry, Deep oceans, volcanoes, mars

A

LTERNATING

W

ET

D

RY

C

ONDITIONS • 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

D

EEP

O

CEANS     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.)

V

OLCANOES    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

M

ARS AND

E

XTRATERRESTRIAL

B

ODIES      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 O RIGINS OF L IFE Part 2

S

OLUTION

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.

P

RECURSORS TO

C

ELLS

- S

OLUTION

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

P

RECURSORS TO

C

ELLS

-S

OLUTION

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

P

RECURSORS TO

C

ELLS

-S

OLUTION

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

P

RECURSORS TO

C

ELLS

-S

OLUTION

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

P

ROPERTIES OF

RNA- S

OLUTION

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 W

ORLD

H

YPOTHESIS

-S

OLUTION

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

H

OW LIFE CHANGED OUR

P

LANET   20% of our atmosphere is now oxygen, but early Earth had very little O 2 .

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.

P

HOTOSYNTHESIZING

B

ACTERIA     photosynthesis is a significant event in history of Earth Bacteria could now rely on the sun as the main energy source, which produced O 2 as by-product However, O 2 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

F

ROM

P

ROKARYOTES TO

E

UKARYOTES   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

E

NDOSYMBIOTIC

T

HEORY  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.

E

NDOSYMBYOSIS   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.

E

VIDENCE FOR

E

NDOSYMBYOSIS

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

E

VIDENCE FOR

E

NDOSYMBYOSIS

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

E

VIDENCE FOR

E

NDOSYMBYOSIS

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)

P

ROBLEMS WITH

E

NDOSYMBIOSIS    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.