14-3: The First
Life Forms
The Origin of Heredity
► DNA
RNA
Proteins
► Why is RNA so important in
the process?
 Its’ STRUCTURE
►Takes
on a variety of shapes
►Questions + observations led
to the speculation that some
RNA molecules might actually
behave like proteins and
catalyze reactions
The Origin of Heredity
► The
Roles of RNA
 1980s - Thomas Cech found that a type of RNA
in some unicellular eukaryotes is able to act like
an enzyme
►Ribozyme
- acts as an enzyme, catalyzes chemical
reactions, + can self-replicate
 Hypothesis made that indicated life may have
started with self-replicating molecules of
RNA
 Model known as “RNA WORLD”
►RNA
has heredity, responds to natural selection, and
will evolve
The Origin of Heredity
 Two other hypotheses about RNA or other
simple self-replicating systems could have
evolved into modern cellular life:
Minerals formed template so organic
molecules could form polymers
2. Self-replicating RNA molecules started
to evolve inside microspheres or
coacervates
1.
The First Cells
► Description
based on inferences…
 No O2 in atmosphere
 Oldest fossils – CELLS – same size + shape of some
living prokaryotes
► The
first organisms were small, anaerobic,
prokaryotic heterotrophs
►Depended
on organic molecules as food
►Competed against one another
►Environmental pressure to evolve into
autotrophs
 Did not depend on photosynthesis
Chemosynthesis
► Archaea
 Unicellular organisms that can survive in harsh
environmental conditions
► Hydrothermal
vents, sulfurous hot springs, + rims of volcanos
 Thought to have populated Earth 4 billion years ago
 Were autotrophic but not by photosynthesis
►Chemosynthesis
 CO2 serves as source to assembly organic molecules (carbs)
 Energy obtained from inorganics – sulfur
Photosynthesis + Aerobic Respiration
 Some life forms became photosynthetic
3 billion years ago
►Discovered fossils in Australia
 3.5 billion year old stromalite fossils
►Related to modern day cyanobacteria
 Group of photosynthetic unicellular
prokaryotes
 Oxygen was damaging to many
unicellular organisms
 It could destroy some coenzymes
essential to cell function
 Certain organisms would bond to
oxygen, not allowing it to be harmful
► Early
function may have been to prevent the
destruction of essential org. comps. by
oxygen
► This was a first step towards aerobic
respiration
Photosynthesis + Aerobic Respiration
► Oxygen
eventually reached the
upper layer of the atmosphere
more than one billion years later
► Sunlight can split O2 to form a
highly reactive single O atom
 These combine to form ozone, O3
 Ozone is poisonous to both plants
and animals
 Absorbs UV radiation from sun to
prevent DNA mutations
►Without
the ozone layer, life
could not exist on land
The First Eukaryotes
►
Eukaryotic differences:
 Cells are larger
 DNA is organized in chromosomes
 Have membrane-bound organelles
►
Lynn Margulis proposed that eukaryotes further evolved from a
mutually beneficial relationship btwn the primitive eukaryote +
prokaryote it engulfed
 Known as THEORY OF ENDOSYMBIOSIS
►
►
First Invasion:
 2 billion years ago, a small aerobic prokaryote invaded
and live inside a large anaerobic eukaryote
 small aerobic prokaryote = modern mitochondria
Second Invasion:
 Photosynthetic cyanobacteria into large anaerobic
eukaryote
►Gave rise to chloroplasts
The First Eukaryotes
► Invasions
(cont.)
 Evidence to support
eukaryotic evolution?
►Chloroplasts
+
mitochondria replicate
independently
►Chloroplasts +
mitochondria contain their
own DNA (circular)
►Both are doublemembrane bound
structures
A look @ Endosymbiosis…