1) Life originated on Earth around 4 billion years ago from simple organic molecules formed in the planet's primordial oceans and atmosphere under intense bombardment from solar energy and lightning. 2) The first self-replicating organisms were likely RNA molecules or prokaryotes that appeared around 3.5 billion years ago, with oxygenic photosynthesis emerging around 2.7 billion years ago and filling the atmosphere with oxygen. 3) Eukaryotic cells developed around 2 billion years ago through endosymbiotic events where ancient bacteria were engulfed but not digested by other cells, eventually evolving into organelles like mitochondria and chloroplasts.

1. Origin and Evolution of
Life
BI 201 Natural History of Guam
Class Presentation 23

2.  Origin of Life
 Environmental conditions of primitive Earth
• Earth was “born” about 4.5 Bybp
• The planets probably formed from debris cast off
from our Sun following its condensation from
interstellar gas and dust of a nebula about 5–7
Bybp
• At first, Earth was semi-molten, but gradually
cooled enough to form a solid rock crust
 The oldest rocks are 3.964 Bybp—from Canada

3. • Decay of radioactive elements of Earth caused
partial melting of the planet’s interior, with denser
materials making up the core
• Differential temperatures of the core and crust set
convection currents in motion in the mantle

4. M
a
g
m
a
o
c
e
a
n
Core
Solid
Continental Crust
Convecting
mantle
Mid-ocean
ridge
Ocean
island
L
s
i
t
h
o
pheric mantle
Atmosphere-
magma ocean
equilibrium
Modern atmosphere
Impact
degassing
Hydrodynamic
gas loss
Degassing
M
assive early atmosphere
Undegassed
material?
<4.44 billion years ago Today

5. • As Earth cooled, there was a period of intense
volcanic activity that released gases into the
primitive atmosphere, primarily
 ammonia = NH3
 carbon monoxide = CO
 hydrogen sulfide = H2S
 methane = CH4
 nitrogen = N2
 water vapor = H2O

6. • As the rate of radioactive decay slowed, water
vapor condensed and fell through the atmosphere,
dissolving gases and forming a “poisonous” rain
• Run-off of rainwater from the land carried
dissolved minerals into a shallow proto-ocean
• The collective materials in the proto-ocean formed
a “hot, thin primordial soup”

7. • Earth’s surface was bombarded by solar
energy, especially ultraviolet (UV) radiation
(N.B., the ozone layer was not formed until ca.
600 Mybp)
• Intense electrical storms generated lightning
that assaulted the surface
• Together, the UV and lightning provided energy
necessary to arrange simple molecules of CH4,
NH3, PO4 salts, and H2O into the basic building
blocks of organic compounds, and then into
complex macromolecules

8. • This chemical evolution of organic molecules is
called abiogenesis
Life’s First Steps
Water and
Volcanic Gases
Activated
AceticAcid PyruvicAcid Alanine
Peptide and
Water
Carbon
Monoxide
Ammonia
Carbon Oxygen Nitrogen Sulfur Hydrogen
A blueprint describing how biomolecules may have evolved from inorganic chemicals spewing from the seafloor;
with a jump-start from sulfide minerals (not shown), each step incorporates raw materials readily a vailable at
hydrothermal vents to fashion a more complex molecule.

9.  Life began with the first self-replicating
“organism”
• The earliest organism may have been no more
than a ribozyme
 A ribozyme is an RNA molecule capable of enzymatic
actions, e.g., the self-splicing introns of some RNAs,
which can excise themselves from the molecule without
the help of protein enzymes

11.  These first self-reproducing organisms
appeared about 3.5 Bybp
 They were prokaryotes and anaerobic
heterotrophs that fed on organic molecules in
the primordial soup
• [anaerobic, without oxygen]
• [heterotrophs, consume organics from other
sources]

12.  Early life forms were continuously bombarded
by UV radiation, causing mutations that
produced new life forms
 As new life forms evolved, the predatory
mode appeared
• Consumption of prey organisms required
catabolism of their organics through the process of
respiration

13. • Anaerobic respiration released CO2 into the
atmosphere
C6H12O6 → energy + C2H3OH + CO2
glucose ethanol carbon
dioxide
• In the atmosphere, UV radiation cleaved CO2 to
form ozone
3CO2 → O3 + 3CO
carbon ozone carbon
dioxide monoxide
UV

14. • The ozone accumulated into the ozone layer in the
stratosphere, and blocked most of the UV radiation
from penetrating to surface
• Formation of the ozone layer enabled development
of terrestrial life forms
 Other organisms developed the ability to
utilize energy released from breaking
chemical bonds to manufacture their own
organic compounds in the process called
chemosynthesis

15.  About 2.7 Bybp, still other organisms
developed the ability to utilize energy from
sunlight to manufacture their own organic
compounds in the process called
photosynthesis
 Photosynthesis released a new gas into the
atmosphere, oxygen
CO2 + H2O → C6H12O6 + O2
carbon water glucose oxygen
dioxide

16. • At first, all of the oxygen released by
photosynthesis reacted with iron in the
lithosphere
• Gradually, free elements reached oxidized
states, and O2 began to accumulate in the
atmosphere
• Because oxygen interferes with anaerobic
respiration, another great change in life
occurred, the development of aerobic
respiration
C6H12O6 + O2 → energy + CO2+H2O

17. • The oxygen-rich atmosphere and aerobic
respiration are thought to have led to the evolution
of eukaryotes
Time (Bypb)
2.8 2.0 1.0 0
Atmospheric
pressures
of
oxygen
(atmospheres)
0.0001
0.001
0.01
0.1
1.0
The first spike in oxygen levels was a toxic challenge to life and may
have shattered a life-sustaining greenhouse. [Adapted from Kerr, 1999].

18.  Serial Endosymbiosis Theory
 In the 1970s, scientists hypothesized that
eukaryotes arose as free-living bacteria in
symbiosis with some archaebacterial host that
became the nucleus-containing cytoplasm of
the resulting eukaryote
 They further hypothesized that organelles
were modified from prokaryotic
endosymbionts during eukaryote evolution

19.  Evidence in support of SET
• Some organelles possess their own DNA outside
the nucleus of the cell
• DNA in organelles is in the form of a convoluted
loop

20. • For each organelle, there is an analog that that has
been identified among living Eubacteria
 Bacteria giving rise to mitochondria were aerobic
respirers (and predatory?)
• similar to modern α purple bacteria
 Bacteria giving rise to plastids were oxygenic
phototrophic bacteria
• similar to cyanobacteria, and probably were ingested
but not digested or digestible
 Bacteria giving rise to undulipodia (e.g., cilia, sperm tail,
etc., but NOT true flagella of bacteria)
• similar to spirochetes

21. MULTICELLULAR UNICELLULAR
EUKARYOTES
PROKARYOTES COMMON
ANCESTRAL FORM
OXYGEN-FREE
ATMOSPHERE
OXYGEN-RICH
ATMOSPHERE
PROTISTS
ARCHAE-
BACTERIA
EU-
BACTERIA
PLANTS
FUNGI
ANIMALS
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
BILLIONS
OF
YEARS
AGO
Evolutionary tree depicts major events in the history of life. Some scientists argue
that the last common ancestor of all living beings existed a little more than two billion
years ago. [Modified from de Duve, 1996].
PRIMITIVE
PHAGOCYTE
ENDOSYMBIONTS