Origin of Life
Introduction to Biology
Origin of Cellular Life
• The Earth formed about 4.6 billion years ago.
o For about 500 million years, the Earth was continually
bombarded by chunks of rock and ice in the solar system.
• The early atmosphere of Earth contained:
o Water vapor H2O
o Nitrogen N2
o Carbon dioxide CO2
o Methane CH4
o Ammonia NH3
Origin of Cellular Life
• How did life arise from such a harsh environment?
• Two scientists designed a model of what conditions
were like on Earth at this time.
o This is called the Miller-Urey Apparatus
• This apparatus
– The high amount
– Heat and gases
– Water vapor
present in the
Results of Miller-Urey
• Simple compounds including water (H2O), methane
(CH4), ammonia (NH3), and hydrogen (H2) were used to
simulate the atmosphere.
• After 2 weeks, 10-15% of the carbon had been used to
form sugars, amino acids, and parts of nucleic acids.
o These simple organic compounds could have produced the
proteins, lipids, and carbohydrates that make up life today.
The First Cells
• The first life forms on Earth were likely single-celled
• Prokaryotic organisms are single-celled organisms
that do not have a nucleus.
o Their DNA or RNA is usually floating freely inside the cell.
• Prokaryotic cells also do not have any membrane
• Most prokaryotes are microscopic, but what
they lack in size they make up for in numbers.
• There are more in a handful of fertile soil than
the number of people who ever lived.
• Prokaryotes thrive almost everywhere,
including places too acidic, too salty, too
cold, or too hot for most other organisms
• They have an astonishing genetic diversity
• Prokaryotes are able to get the energy they
need for life from four different sources:
• “Photo” means light.
• “Chemo” means inorganic (non-living) chemicals.
• “Auto” means self.
• “Hetero” means different.
Relationships to Oxygen
• Prokaryotic metabolism varies with respect to
o Obligate aerobes require oxygen
o Facultative anaerobes can survive with or without oxygen
o Obligate anaerobes are poisoned by oxygen
• Archaea are prokaryotic organisms that are
very similar to bacteria in size and structure.
• Bacteria and archaea likely evolved
separately from the original life forms on Earth.
• Key differences between bacteria and archaea:
o Archaea can survive in extremely hot, cold, and salty environments.
o Archaea are not affected by many antibiotics.
o Archaea are obligate anaerobes; they can only survive in oxygen-
• Archaea can be classified by the type of
environment they live in:
o Extreme thermophiles thrive in very hot environments
o Extreme halophiles live in high saline environments
o Acidophiles live in environments with a very low pH (high
amounts of acid).
• Prokaryotes reproduce quickly by binary
fission and can divide every 1–3 hours.
o Binary fission is asexual reproduction and
produces exact clones of the original bacteria.
• Many prokaryotes form endospores, which
can remain viable in harsh conditions for
• Prokaryotes are able to reproduce so quickly that
under the right conditions they grow exponentially.
• Exponential growth is when the population
increases at a faster and faster rate.
• This only occurs if all the needs of the bacteria
culture are met (food, space, etc).
• Bacteria are cultured in petri dishes, which contain
agar, a food source.
• Petri dishes are close to perfect conditions for
bacteria to grow, so they reproduce exponentially.
• Prokaryotes like bacteria are able to evolve
much more quickly than multicellular
o They reproduce quickly.
o Their genome is small, so a single mutation can
change a bacteria drastically.
o Prokaryotes are able to exchange plasmids
with each other through pili.
• Most prokaryotes are unicellular, although
some species form colonies
• Prokaryotic cells have a variety of shapes
• The three most common of which are:
o Coccus – Spherical
o Bacillus – Rod-shaped
o Spirilla – Spiral-shaped
1 µm 2 µm 5 µm
Spherical Rod-shaped Spiral
A typical A thin section through the
rod-shaped bacterium Bacillus
bacterium coagulans (TEM)
• Most prokaryotes have a ring of DNA that is
not surrounded by a membrane.
o No nucleus.
o DNA is kept within a nucleoid region instead.
• Some species of bacteria also have smaller
rings of DNA called plasmids.
o These can be exchanged between invididuals.
• DNA provides instructions to ribosomes, which
produce any proteins the bacteria need.
• Prokaryotic cells do not have membrane-
o Ex: Chloroplast, mitochondria
• However, some prokaryotes have special
membranes that perform some of the same
o Thylakoid membranes enable photosynthesis
o Respiratory membranes enable the use of oxygen to break
down nutrients into ATP.
0.2 µm 1 µm
Aerobic prokaryote Photosynthetic prokaryote
• An important feature of nearly all prokaryotic
cells is their cell wall.
• The cell wall…
o Maintains cell shape
o Provides physical protection
o Prevents the cell from bursting in a hypotonic
• The cell wall of many prokaryotes is covered by a
capsule, a layer of polysaccharides that protect them
from being caught by white blood cells.
• Some prokaryotes have fimbriae and pili, which
allow them to stick to their growing surface or
attach to other prokaryotes to exchange genes..
• Most motile bacteria propel themselves by
• In the environment, many bacteria exhibit the
ability to move toward or away from certain
Video: Prokaryotic Flagella (Salmonella typhimurium)
Cell wall Hook
Importance of Prokaryotes
• Prokaryotes are so important to the biosphere
that if they were to disappear, most other life
would not be able to survive.
• Symbiotic relationships are when two
organisms live close together.
• In mutualism, both symbiotic organisms
o Example: Bacteria that live in deep-sea fish,
• In commensalism, one organism benefits while
neither harming nor helping the other.
o Example: Most of the bacteria on human skin
• Prokaryotes also need
nitrogen to build amino
acids and proteins.
• One common source of
nitrogen is called nitrogen
fixation, where prokaryotes
nitrogen to ammonia.
• Some plants, called
legumes, have nodules in
their roots that contain
bacteria to help them fix
• Prokaryotes help recycle elements needed for
life between living and nonliving parts of the
• Heterotrophic prokaryotes function as
decomposers, breaking down corpses, dead
vegetation, and waste products into smaller
molecules that can be used by other living
• Some bacteria produce light as a result of their
internal chemical reactions.
• Other animals have formed mutualistic relationships
with these bacteria to take advantage of this
• In parasitism, one organism, called a parasite,
benefits at the expense of the host
o Example: The bacteria that causes strep
• Not all prokaryotes are harmful, but some are
human pathogens – they cause disease.
• Prokaryotes cause about half of all human
• Lyme disease is one example.
• Pathogenic prokaryotes typically cause
disease by releasing exotoxins or endotoxins
• Exotoxins are released externally by the
bacteria, often as waste products.
o Example: Botulism, tetanus, anthrax
• Endotoxins are released only when bacteria
die and their cell walls break down
o Example: Meningitis
• Antibiotics are anti-bacterial chemicals that
originally came from mold.
• Each antibiotic works in different ways.
o Penicillin disrupts the bacteria’s ability to produce a cell
wall, causing it to burst due to an influx of water into its
• Bacteria can mutate and evolve quickly, due to
their small size and fast reproduction rate.
• Sometimes, a mutation will result in their ability to
resist the action of antibiotics.
o Over time, this mutation can spread throughout an entire
colony, creating a strain of antibiotic-resistant bacteria.
• Resistant bacteria will not be
affected by the antibiotics in
the same way.