Theories of Origin of Life on Earth and Miler Urey Experiment Paper

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Week 3: Origin of Life
This week, much of your material focuses on cells and organelles, but part of your readings focuses
on where the first living organisms came from. In this week’s discussion, we will be talking about
hypotheses
surrounding
the
origin
of
life
on
earth.
Choose one of the “theories” (they are actually hypotheses, not theories, but I didn’t write
the article) from the slideshow above – all of them are mentioned at some point in From
Soup to Cells. (Be sure to check to see what other leaders have posted about first – we want
as much variety as possible). Please research that hypothesis using outside sources. In your
initial response, please describe the hypothesis in your own words. What lines of evidence
do scientists seem to think make the hypothesis seem possible? What are some of the
weaknesses of the hypothesis?
Readings:
7 Theories on the Origin of Life: https://www.livescience.com/13363-7-theories-originlife.html Discussion Leaders: (Acampora, Arimura, and Brown)
Chapter 3: https://openstax.org/books/concepts-biology/pages/3-introduction
18.1 Beginnings: The Big Bang
• Earth formed more than 4 billion years ago
Conditions on Early Earth
• Atmosphere: hydrogen, nitrogen, carbon
monoxide, carbon dioxide
• No oxygen
▫ No rust in geologic record until much later
• No liquid water
▫ Too hot – evaporation
The Source of Life’s Building Blocks
• All living things are made from organic subunits: simple
sugars, amino acids, fatty acids, and nucleotides
• Where did the subunits of the first life come from? There are
several possibilities:
1. Lightning-fueled atmospheric reactions
2. Reactions at deep-sea hydrothermal vents
3. Meteorites from space
Lightning fueled
• In 1953, Stanley Miller and
Harold Urey showed that
reactions in Earth’s early
atmosphere could have
produced building blocks for
the first life
• Mix of water, hydrogen (H2),
methane (CH4), and ammonia
(NH3)
• Sparks simulated lightning
• Amino acids formed
Reactions at Hydrothermal vents
• Reactions in the hot, mineralrich water near deep-sea
hydrothermal vents also
produce organic building
blocks
• Experiments combining hot
water with carbon monoxide
(CO) potassium cyanide (KCN)
and metal ions formed amino
acids
• hydrothermal vent
• Rocky, underwater opening
where mineral-rich water
heated by geothermal energy
streams out
Delivery From Space
• The presence of
amino acids, sugars,
and nucleotide bases
in meteorites that fell
to Earth suggests that
such molecules may
have formed in
interstellar clouds of
ice, dust, and gases
and been delivered to
Earth by meteorites
• Known as Panspermia
From Polymers to Cells
• Similarities in structure, metabolism, and
replication among all life indicate descent from a
common cellular ancestor
• Experiments demonstrate how traits and
processes seen in all living cells could have
begun with physical and chemical reactions
among nonliving collections of molecules
Steps on the road of life
Origin of Metabolism
• Proteins that speed metabolic reactions might
have first formed when amino acids stuck to
clay, then bonded under the heat of the sun
• Or, metabolism may have begun in rocks near
deep-sea hydrothermal vents when iron sulfide
in the rocks donated electrons to dissolved
carbon monoxide
Origin of the Cell membrane
• Membrane-like structures and vesicles form
when proteins or lipids are mixed with water
• They serve as a model for protocells, which
may have preceded cells
• Membranous sacs that contain interacting organic
molecules; hypothesized to have formed prior to
the earliest life forms
Laboratory Produced Protocells
• One type consists of a bilayer membrane of fatty acids
that holds strands of RNA
• Ribonucleotides diffuse into the protocell and become
incorporated into complementary strands of RNA
• Vesicle enlarges by incorporating additional fatty acids
• Another type consists of RNA-coated clay surrounded by
fatty acids and alcohols
Field Testing
• No vesicle-like
structures formed when
David Deamer poured a
mix of small organic
molecules and
phosphates into a hot
acidic pool in Russia
Origin of the Genome
• Protein synthesis depends on DNA, which is
built by proteins; how did this cycle begin?
• An RNA world, a time in which RNA was the
genetic material, may have preceded DNA-based
systems
• Switch from RNA to DNA would have made the
genome more stable
Life’s Early Evolution
• Fossils and molecular comparisons
among living species inform us
about the history of life on Earth
• The first cells evolved when oxygen
levels in the atmosphere and seas
were low, so they probably were
anaerobic
• An oxygen-releasing, noncyclic
pathway of photosynthesis evolved
in one bacterial lineage,
cyanobacteria
• Over time, oxygen released by
cyanobacteria changed Earth’s
atmosphere
Increasing Oxygen
1. Oxygen interferes with self-assembly of
complex organic compounds – prevented
evolution of new life from nonliving molecules
2. Presence of oxygen gave organisms that
thrived in aerobic conditions an advantage
3. Formation of an ozone layer in the upper
atmosphere protected Earth’s surface from
high levels of solar ultraviolet (UV) radiation
Earliest Eukaryotes
• Possible oldest eukaryote (2.1 billion years old); an early alga;
and fossils of red alga (1.2 billion years old)
Evolution of Organelles
• Little Mito Case Study
Infolding of Membranes
Endosymbiosis
Cell Structure
4.1 What is a Cell?
• Smallest unit that shows properties of life
• Eukaryotic cells
▫ Divided into various functional compartments,
including a nucleus
• Prokaryotic cells
▫ Smaller and simpler
▫ None have a nucleus
All cells are similar in three ways:
• Plasma membrane
▫
▫
▫
▫
Cells outer membrane
Separates metabolic activities from outside the cell
Allows water, CO2, and O2 to pass into the cell freely
Other substance cross only with assistance of membrane proteins
• DNA containing region
▫ Eukaryotic – nucleus (double membrane sac)
▫ Prokaryotic – nucleoid (concentrated region of cytoplasm)
• Cytoplasm
▫ Semifluid mixture of water, ions, sugars, and proteins
▫ Cell compartments and other components are suspended in
cytoplasm
Constraints on Cell Size
• Surface-to-volume ratio limits cell size
• A relationship in which the volume of an object
increases with the cube of the diameter, but surface
area increases with the square of the diameter
Implications of cell size
• If the cell gets too big, inward flow of nutrients and
outward flow of wastes across the membrane will
not be fast enough
• Big, round cells would have trouble moving
substances efficiently through cytoplasm
• Many cells are long and thin, or have folds on the
surface to increase surface area.
History of Cell Discovery
• 1665: Antoni van Leeuwenhoek first observed “many
very small animalcules”
• Robert Hooke magnified a piece of thinly sliced cork and
named the tiny compartments he observed “cellae”
• 1820s: Robert Brown was first to identify a cell nucleus
• Matthias Schleiden, hypothesized that a plant cell is an
independent living unit even when it is part of a plant
• Schleiden and Theodor Schwann concluded that the
tissues of animals as well as plants are composed of cells
Cell Theory
• Four generalizations constitute the cell theory:
1. Every living organism consists of one or more
cells
2. A cell is the smallest unit of life, individually alive
even as part of a multicelled organism
3. All living cells come from division of preexisting
cells
4. Cells contain hereditary material, which they pass
to their offspring during division
Membrane Structure and Function
• A cell membrane functions as a selectively
permeable barrier that separates an internal
environment from an external one
Basic Cell
• At its most basic, a cell
is a lipid bilayer bubble
filled with fluid
Membrane Proteins
• Proteins associated with a membrane carry out
most membrane functions
Bacteria and Archaeans
• Single-celled bacteria and archaeans are the
smallest and most diverse forms of life:
▫ The cytoplasm contains ribosomes and plasmids
▫ A single, circular chromosome is located in a
nucleoid
▫ Many have a cell wall, flagella or pili
Eukaryotic Cells
• All protists, fungi,
plants, and animals
are eukaryotes
• Eukaryotic cells
start out life with
membraneenclosed organelles,
including a nucleus
Animal vs. Plant Cells
The Nucleus
• The nucleus contains the cell’s
genetic material (DNA)
• In the nucleus, ribosome
subunits are assembled in dense
regions called nucleoli
• The nuclear membrane controls
passage of certain molecules
between the nucleus and the
cytoplasm
• The outer bilayer of the double
membrane is continuous with
the membrane of the ER
The Endomembrane system
4.9 Mitochondria and Chloroplasts
• Mitochondria
▫ Break down organic compounds by aerobic
respiration (oxygen-requiring)
▫ Produce ATP
• Chloroplasts
▫ Produce sugars by photosynthesis
Cytoskeleton
• Dynamic framework of protein filaments that support,
organize, and move eukaryotic cells and their internal
structures
▫ Microtubules
 Long holow cylinders consisting of subunits of tubulin
 Rapidly assemble when needed
 Ex. Microtubules separate chromosomes during cell division
▫ Microfilaments
 Fibers that consist of subunits of actin
 Strengthen and change shape of eukaryotic cells
 Make up cell cortex, which reinforces the cell membrane
▫ Intermediate filaments (in most)
 Strengthen and maintain cell and tissue structures
Components of the Cytoskeleton

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Cell Structure

amino acids

Miler Urey Experiment

complex molecules

hydrogen deficient

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