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Origin of Life
Pages 261-272
Modern Biology
Holt, Rinehart, Winston
Biogenesis
• Biogenesis: all living things come from other
living things.
• Spontaneous generation: living things come
from non-living things (false statement)
Redi, Spallanzani and Pasteur
• Redi set up jars with meat. The control
uncovered, the experimental covered. Maggots
appeared only in the control group.
• Spallanzani used boiled broth, with his control
group being uncovered and experimental group
covered. His control group became cloudy.
• Pastuer used a curve-necked flask and then a
flask with the neck removed. The flask with no
neck became cloudy.
• All disproved spontaneous generation.
Earth’s History
• Estimated age of the Earth 4.6 billion years, 700,00
times as long as the period of recorded history.
• Through carbon-14 dating (radioactive dating) and
studying layers of the Earth’s crust, the age was
determined.
• Isotopes of carbon decay with a half-life. When an
organism dies, traces of carbon-14 in the body start to
decay with respect to the normal carbon in the body
carbon-12.
• Carbon-14 has a 5715 year half-life, so after this time,
half of it is decayed in a body.
Formation of Organic compounds
• Oparin(scientist) thought that early atmosphere
of the Earth consisted of ammonia(NH3),
Hydrogen gas(H2), water vapor(H2O) and
compounds made of carbon and hydrogen like
methane(CH4). Later was the formation of
carbon dioxide(CO2) and nitrogen gas(N2).
• These gases may have come together to form
amino acids and macromolecules, the basic
building blocks of all life.
• Space debris may have also contributed to the
formation of organic compounds.
From Molecule to Cell-like Structure
• Simple organic chemicals may have started all life
on Earth.
• Ex:Microshperes are composed of many protein
molecules that are organized as a membrane.
• Ex: Coacervates are composed of amino acids and
sugars.
• Membrane bound structures may have existed on
early Earth and may have had enclosed
replicating molecules of RNA. Their descendants
may have been the first cells.
The Origin of Heredity
• DNA serves as a template for RNA which serves
as a template for specific proteins.
• Since RNA may assume different shapes, some
RNA molecules may behave like proteins and
catalyze chemical reactions.
• A ribosome can act as an enzyme and may have
the ability to replicate itself.
• Since RNA plays a vital role in the replication of
DNA, self-replicating RNA would have provided
the heredity information in cells. So RNA was
critical for the beginning of life.
The First Prokaryotes
• The first cells were prokaryotes, simple cells
with no nuclear membrane. These would have
been anaerobic (without oxygen) and
heterotrophs (self feeders). They would
eventually have to evolve into heterotrophs
(feeds on others).
Chemosynthesis
• Chemosynthesis: CO2 serves as a carbon
source for the assembly of organic molecules.
Energy is obtained from the oxidation of
various inorganic substances, such as sulfur.
• Some of the earliest bacteria, archaebacteria
lived under harsh conditions and used
chemosynthesis for energy.
Photosynthesis
• Oxygen was destructive to early cell function.
• When oxygen bonds, it is not as destructive. So aerobic
respiration (with oxygen) occurred to prevent the
destruction of organic compounds by oxygen.
• Photosynthetic organisms started to evolve 3.5 billion
years ago. The are related to the modern
cyanobacteria.
• It took billions of years for oxygen to reach today’s
level. With sunlight splitting the oxygen molecule and
O3 being formed in the upper atmosphere, ozone (O3)
absorbed ultraviolet light, thus making an atmosphere
that could sustain life.
The First Eukaryotes
• Eukaryotes are larger and have their DNA
enclosed in a nucleus with a membrane.
• Small aerobic prokaryotes started living in other
larger prokaryotes which led to endosymbiosis.
• The smaller aerobic prokaryotes eventually gave
rise to modern mitochondria and small
photosynthetic prokaryotes gave rise to
chloroplasts. These two carry their own DNA and
replicate independently from the rest of the cell.