Chapter 26 The Origin and Evolution of
In the Beginning . . .
A. The universe is expanding but long, long ago it
was greatly compressed, then: the Big Bang!
B. About 4.5 billion years ago a cloud of gas and dust
began to cool to form our solar system.
I. Conditions on the Early Earth
A. Origin of the Earth
1. About 4.5 billion years ago remnants of
exploding stars began to condense into planets around
2. The earth was initially very hot, but cooled to
form an outer mantle and partially-molten core.
3. Within 200 million years life had originated on
its surface, but how?
a. What were the prevailing conditions on the
earth at this time?
b. Could large organic molecules have formed
spontaneously and then evolved into the molecular
systems of life?
c. Can we devise experiments to test whether
living systems could have emerged by chemical
B. The First Atmosphere
1. The first atmosphere probably consisted of
gaseous hydrogen, nitrogen, carbon monoxide and carbon
2. Gaseous oxygen and water were not thought to be
3. When the crust cooled the water condensed, rains
began, and pools of chemicals began to form.
C. Synthesis of Organic Compounds
1. Evidence of neighboring bodies in our solar
system indicates that precursors for building
biological molecules were present on the primitive
2. Energy in the form of sunlight, lightning, and
heat from the earths crust was also present.
a. Stanley Miller used a lab apparatus to
demonstrate synthesis of amino acids from hydrogen,
methane, ammonia, and water under abiotic
b. Even if molecules were formed spontaneously,
they would have quickly hydrolyzed unless clay
templates served to hold the molecules together for
II. Emergence of the First Living Cells
A. Origin of Agents of Metabolism
1. During the early history of the earth, enzymes,
ATP, and other molecules could have assembled
spontaneously. Remember the "RNA world" and the
abiotic origins of life due to lack of oxygen on the
2. The participation of these and other entities in
metabolic pathways could have been facilitated by clay
templates that brought them together in the same place
B. Origin of Self-Replicating Systems
1. From accumulated organic compounds emerged
replicating systems consisting of DNA, RNA, and
2. Ribonucleotides may have then stuck to the clay
and eventually replaced clay as a template.
3. An RNA world may have preceded DNAs
dominance as the main informational molecule.
4. How DNA entered the picture is not yet clear,
but we do know that some reactions were more probable
than others&endash;not random.
C. Origin of the First Plasma Membranes
1. The metabolism in living cells cannot occur
without a barrier against the chemical actions on the
2. Proto-cells were probably membrane-bound sacs
containing nucleic acids that served as templates for
3. Sidney Fox heated amino acids to form protein
chains, which when allowed to cool self-assembled into
small spheres that were selectively permeable.
III. Origin of Prokaryotic and then Eukaryotic Cells:
Recal lthe Endosymbiotic Theory of Eukaryotic Cell
A. The Archean eon (3.9 to 2.5 billion years ago) was
the time of macromolecule synthesis plus the origin of
1. The original prokaryote line split into Three
Domains: Archaea, Bacteria, and a line leading to
2. Evolution of the cyclic pathway of
photosynthesis in Bacteria (Cyanobacteria) tapped a
renewable source of energy&endash;sunlight; large
accumulations of these cells are seen today as fossils
known as stromatolites.
B. In the Proterozoic eon (2.5 billion to 550 million
years ago), the noncyclic pathway evolved in first in
eubacteria and then later in eukaryotic cells (algae,
fungi); oxygen accumulated, and aerobic respiration
Where Did Organelles Come From?