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Cells come from pre-existing cells
The first cells arose from non-living materials
• Endosymbiosis
Cell Division & The Origin of Cells
 All cells are formed by the division of
pre-existing cells.
 You are made of trillions of cells.
 Any cell that is produced by your body is the product
of cell division from another cell.
 You originally started life as a single cell, a zygote, that
underwent numerous cell divisions to produce the
trillions of cells that comprise you today.
 Even that single celled zygote came from
other cells – the combination your father’s
sperm cell and your mother’s egg cell.
 We can trace the origins of all the cells in our parents
back to the zygotes from which they developed, to our
human ancestors before them, to human’s pre-existing
ancestral species, all the way back to the earliest cells
on Earth.
 We, and all other living things on this planet, are
descendants from the “first cell”.
 But if all cells come from pre-existing cells, where did
the first cell come from?
 How did life start on this planet?
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
life on Earth to exist.
Requirements to start life
1.
Production of Simple Organic Compounds
2. The assembly of organic compounds into polymers
3. Development of a mechanism for inheritance.
4. Formation of membranes
1. Production of Simple Organic
Compounds
• Life as we know it is based on organic compounds
(compounds containing carbon and hydrogen), such
as amino acids (the building blocks of proteins)
• But early Earth only had inorganic matter: rocks,
minerals, gases, water….
• Abiogenesis had to occur.
• abiogenesis: the creation of organic matter from
inorganic matter)
• It is believed that organic molecules were formed in
the shallow waters of the oceans as the products of
chemical reactions between compounds in the
atmosphere and the water.
Miller and Urey
 Scientists Stanley Miller
and Harold Urey
performed a groundbreaking experiment in 1953
 They recreated the conditions of early Earth and
proved that organic compounds could be synthesized
from inorganic compounds.
N2 = nitrogen gas
H2 = hydrogen gas
CO2 = carbon dioxide gas
H2O = water
NH3 = ammonia gas
* CH4 = methane gas
The Experimental Design
• The apparatus included an “oceanic compartment” and an
“atmospheric compartment”
• The H2O in the oceanic compartment was heated to
evaporate and cooled to condense – thereby recreating the
H2O cycle.
• Since early Earth did not have an ozone layer, they kept the
system at a warm temperature and exposed it to UV
radiation
• Generated electric sparks to simulate lightning
The Results
• After 1 week:
• 15% of the carbon was now found in organic form!
• 13 of the 20 amino acids had formed inside the primordial
soup!
• Sugars had formed!
• The nitrogenous base adenine (a component of DNA and
ATP) had formed!!!
2. Assembly of these molecules
into polymers
• Organisms are organized!
• Simple organic molecules would have needed to
undergo a process of polymerization to form the larger
more complex organic chemicals required by cells.
Deep-Sea Vents
 Organic molecules could
have first formed around
hydrothermal vents –
places where hot water
emanated from beneath
the ocean floor.
 Form when cracks in the
crust of the seabed expose
sea water to rocks below
which are heated by
magma
 As the hot water rises it picks up countless minerals
along the way.
 Hydrothermal vents are sometimes referred to as black
smokers because the water coming out of them
contains so many dark minerals it looks like smoke.
 The chemicals and source of energy in this
environment could be suitable for the formation of
biological polymers.
3. The development of a mechanism for
inheritance
• Today, most organisms use DNA as its molecule for
heredity.
• To replicate DNA and pass it on to the next generation,
enzymes are required
• However, enzymes cannot be made without DNA.
• Therefore, it is unlikely that DNA was the early molecule
for heredity
Ribozymes
 However, the small sequences of the molecule RNA
can act as enzymes and replicate itself.
 These are called ribozymes
 Thus, RNA may be the early molecule for hereditary.
4. Formation of Membranes
• Water is important to life but tends to depolymerize (break
down)molecules
• Many compounds dissolve in water, making it difficult to
organize into polymers
• The formation of closed membranes is likely an early and
important event in the origin of cellular life
• It allows for the development of an internal chemistry
different from the external environment
Coacervates
• Coacervate - a microscopic sphere that forms from
lipids in water.
• Forms spontaneously due to the hydrophobic forces
between the water and lipid molecules.
• Can maintain an internal chemical environment
different from the surrounding environments.
• Coacervates can be selectively permeable
Coacervates
Coacervates (lipids)
 Although they are not living organisms, coacervates
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
 PROTOBIONTS – the first precursors to cells, were
likely coacervate droplets which included
polynucleotides (DNA or RNA)
 (remember our cell membranes are lipid based)
 Overtime, true cell membranes evolved and other
characteristics of cells developed.
 Cellular respiration
 Asexual reproduction
Where did all the oxygen come
from?
 1/5 of the air you are breathing right now is oxygen.
 However, there was none at all present 4 billion years
ago.
 The earliest life forms on Earth were bacteria and they
lived in an environment with an atmosphere of mostly
CO2
 Thus, early life forms were anaerobic cells (did not
require oxygen)
• These single-celled organisms would consume organic
molecules (i.e. simple sugars) that were forming from
chemical reactions on Earth
• The more they reproduced, the more food that was
consumed.
 After million of years, their population would have
reached such large numbers that food began to be
scarce.
 In this food shortage, bacteria that could make their
own food would have an advantage.
 ~3.5 billion years ago, bacteria (that is believed to be
related to today’s cyanobacteria)developed the ability
to photosynthesize.
 Must have contained a form of chlorophyll
 Development of photosynthesis was one of the most
significant evens in the history of Earth
 Gives bacteria a source of energy (sunlight) to survive
 Created a mass pollution of the atmosphere
 Pollution of oxygen!!!
 Oxygen gas is toxic to the kinds of bacteria which
preceded photosynthetic ones, so this pollution would
have eventually killed off large populations of
anaerobes.
 Anaerobic bacteria that survived would live in mud of
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
Evidence of Endosymbiosis
1.
Chloroplasts and
mitochondria are surrounded
by a double membrane (like
the cell membrane)
1.
Mitochondria and bacteria (a
prokaryote) have a similar
size
Evidence of Endosymbiosis
3. Mitochondrial and bacterial ribosomes are very
similar in size and shape.
4. Mitochondria and chloroplasts have their own
DNA – which is circular like bacteria.
5. Mitochondria divide in a process similar to binary
fission like bacteria
Problems with Endosymbiosis
 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
 When chloroplasts or mitochondria are removed from
a cell, they cannot survive on their own.