Download Honors Biology Lesson Notes 1 Unit 11

Honors Biology
Lesson Notes 1
Unit 11 - Biological Evolution
Lesson Objectives: You will:
1. describe the atmosphere and energy conditions thought to exist early in Earth's history.
2. explain the results of experiments designed to test whether simple organic compounds might have
formed in the early atmosphere.
3. explain the steps that might have led to the evolution of the first cells according to the heterotroph
hypothesis.
I. Formation of the Universe and Solar System
A. Big Bang Theory - all matter was originally concentrated in one mass that blew apart about 18
billion years ago.
B. The smaller atoms, hydrogen and helium, which make up about 99% of all matter, fused to produce
the heavier elements found in our solar system.
C. Stars were borne out of huge masses of interstellar gases several light years across.
D. Our sun was formed about 6 billion years ago.
E. About 4.6 billion years ago the planets of our solar system formed by the condensing of peripheral
gases and matter around our sun.
II. Formation of Organic Compounds
A. Fossils of photosynthetic bacteria have been found that date back 3.5 billion years, but how did
simple life arise in the first place?
B. Frankly, there are many theories and we really don't know. However, there is some direct and indirect
evidence that gives us an idea of how life began.
C. We know that the young earth was a violent place. UV radiation and lighting bombarded the surface.
The early atmosphere consisted mainly of gases released by volcanic activity hydrogen, nitrogen gas,
carbon monoxide and carbon dioxide. Hydrogen sulfide, methane and ammonia also appeared to be
present in small quantities. Water vapor was also likely abundant in the atmosphere due to much
higher temperatures and some have suggested that it rained for millions of years as the earth cooled.
D. Oxygen which now makes up 21% of our atmosphere was not present. This means that the
atmosphere was reducing (favored the buildup of more complex organic compounds). It is doubtful
that the processes which led to the formation of the earliest organic molecules could take place in our
current oxidizing atmosphere (favors the breakdown of more complex organic compounds).
E. In the 1920's Oparin and Haldane postulated that the reducing atmosphere coupled with vast
amounts of free energy (i.e. useable energy) from volcanic activity, lightning, radioactive minerals,
and tremendous amounts of energy from the sun (there was no ozone layer back then) enhanced
reactions which joined simple molecules together to form the first organic molecules.
F. This hypothesis was tested in 1950's by Miller and Urey who duplicated these early earth conditions
in the lab. They constructed an artificial system which contained an "atmosphere" and "ocean". They
introduced hydrogen, methane, ammonia and water into the system and turned on an electric spark as
a supply of energy.
G. In less than one week amino acids and other small organic molecules formed.
H. Other scientists repeated their work, utilizing updated primeval atmospheric models, eventually
producing: other amino acids, ATP, glucose and other sugars, lipids and the bases which form RNA
and DNA, and adenine the key component of ATP and NAD.
I. Over the course of hundreds of millions of years these compounds would be washed by rains into the
oceans where they were built up in large concentrations forming a "primordial soup".
C. Pace
III. Origin of Life
A. Abiotic synthesis (not from life) of organic monomers (simple organic molecules) - many
experiments demonstrated that it is possible to get many different types of organic monomers by
starting with a primitive atmospheric model and adding energy.
1. This includes the work of Stanley Miller in 1953.
2. You may have read about abiotic synthesis of living organisms is also known as spontaneous
generation which was once widely believed. Work done by Redi, van Leeuwanhoek,
Spallanzani, Pasteur, and Tyndall from the late 17th century to the mid-ninetenth century
essentially disproved this concept. However, they were working under modern atmospheric
conditions.
B. Synthesis of polymers - recall that polymers are chains of monomers (i.e. monosaccharides, amino
acids, and nucleotides) synthesized by condensation or dehydration synthesis reactions (i.e.
eliminating a water molecule.
1. Experiments have demonstrated that polymers can be made by dripping organic monomers onto
hot sand, rocks or clay. Clays and other minerals attract amino acids and repeated wetting and
drying out promotes condensation reactions. The mineral pyrite may have played a major role in
abiotic polymerization because it provides a charged surface and electrons freed during its
formation could support bonding between abiotically synthesized organic molecules.
2. Sidney Fox found that heating a dry mixture of amino acids to about 60oC produced proteinoids,
protein-like molecules of roughly 100 amino acids.
3. Molecules such as chlorophyll and the cytochromes involved in cellular respiration may have
arisen from formaldehyde, a component of cosmic clouds.
C. The Formation of Protobionts
1. Protobionts - Aggregates of abiotically produced molecules able to maintain an internal
environment different from their surroundings and exhibiting some life properties such as
metabolism, excitability, and self replication (yet not able to precisely reproduce). Probable
antecedents of first true cells. Sometimes called proto-cells or pre-cells.
2. Proteinoids placed in water form aggregates called proteinoid microspheres which exhibit some
cell-like attributes. They have a protein bilayer that forms a boundary between the interior and
external environment. This boundary layer is differentially permeable and allows primitive
nucleotide-like molecules to enter. They can contain lipid-like areas, watery areas, and boundarylayer areas that provide distinct places for specific chemical reactions. When they grow to
unstable size, they split to form daughter microspheres.
3. Liposomes can form spontaneously when phospholipids form a bilayered membrane similar to
those of living cells.
4. Coacervates (colloidal drops of polypeptides, nucleic acids, and polysaccharides) self-assemble
under the right conditions.
D. Chemical Evolution (Selection)
1. Chemical selection is similar to natural selection, but acts on nonliving systems.
2. Early protobionts probably enjoyed an abundance of sustaining organic monomers and polymers
in their environment. But, as raw materials became scarce, protobionts with more stable
combinations were favored.
3. Stable protobionts would likely have catalysts that would allow for primitive metabolic activities.
Some of these catalysts may have been inorganic, while others may have been organic. Any
protobiont that developed a useful catalytic pathway would have a selective advantage over other
competing protobionts.
4. It is thought that complex, multistep metabolic pathways evolved backwards from useful, rapidly
diminishing end products to less directly useful raw materials.
C. Pace
E. Origin of self replication - it appears that one of the first polymers was RNA and like now it can selfreplicate in the presence of nucleotides.
1. Simple self-replicating systems of RNA, enzymes and coenzymes have been created in the
laboratory. Applying heat to nucleotides causes them to assemble into short strands of RNA.
Adding zinc as a catalyst (Zinc is part of the RNA Polymerase enzyme) allows the formation of
longer strands of RNA and for the formation of RNA complimentary to existing strands of RNA,
much like DNA.
2. However, RNA is a relatively unstable molecule and the strands are too fragile to become very
long. Perhaps the more stable, double stranded DNA eventually replaced RNA as the carrier of
the genetic information.
3. Whether proteins or RNA came first is an intriguing question. Some feel that they coevolved
eventually forming a relatively complex feedback cycle involving many primitive “genes” and
“enzymes”.
F. Formation of plasma membranes - cells are separated from their environment by a plasma membrane.
Recall that the plamsa membrane is a lipid bilayer and that such layers will spontaneously form
when lipids are placed in water.
1. Proto-cells, may have developed plasma membranes similar to those of living cells early in the
chemical evolutionary process. Needless to say, plasma membranes became extremely important
structures as proto-cells evolved into the first versions of true-cells.
G. What do most scientist's agree on? A summary follows.
1. There was production of simple organic compounds.
2. Larger, more complex molecules were synthesized from simpler ones.
3. Concentrations of many complex molecules became surrounded by membranes.
4. A means of obtaining energy for life functions developed.
5. A reliable means of reproduction evolved.
IV. Evidence of Early Life
A. The earliest evidence of life occurs as 3.8 billion year old organic carbon deposits in Greenland.
B. Stromatolites, banded domes of sediment formed by bacteria, have been dated at about 3.5 billion
years old.
C. Also, 3.5 billion year old spherical and filamentous prokaryotic fossils have been found in western
Australia and south Africa.
D. At first energy requirements were met by living off the abundance of organic matter in the seas.
1. This is called the Heterotroph Hypothesis. The idea that heterotrophs came first because their
was an abundance of raw materials available in the environment to support primitive living
systems.
E. However, over millions of years available nutrients would become scarce and competition for food
would become intense. Those most likely to survive, primitive autotrophs, could make their own
food.
F. The most successful organisms were those that developed ability to make direct use of the suns
energy, i.e. photosynthetic. An early form of photosynthesis (cyclic photophosphorylation) appeared
between 3.5 - 3.2 billion years ago.
G. By about 2.5 billion years ago the most common form of photosynthesis, which included noncyclic
photophosphorylation, evolved.
1. This was a major milestone in the evolution of the earth and its organisms because the byproduct,
oxygen, began to accumulate in the atmosphere.
2. Evolution of photosynthesis is well documented in the geological record. Iron deposits began to
oxidize (rust).
H. Recall the early atmosphere lacked oxygen. This oxygen was important for two reasons:
1. Some of the oxygen was converted to ozone (O3) which filters out harmful solar radiation.
C. Pace
I.
J.
K.
L.
M.
2. Oxygen build up allowed for the evolution of aerobic respiration. Recall how much more
energy aerobic respiration produces compared to anaerobic. This allowed for larger and more
complex organism to evolve.
It took at least a billion years for atmospheric oxygen levels to reach the current 21% and atmospheric
oxygen levels appear to fluctuate over time. For example, we know that oxygen levels in the
atmosphere has been high enough (greater than 21%) in the past to support gigantic forms of
arthropods, especially the insects.
The first Eukaryotes appeared about 1.4 billion years ago. It seems certain that the eukaryotic cell
arose via endosymbiosis.
1. Recall the endosymbiont theory - prokaryotic organisms began symbiotic relationships in which
they lived together within a common cell.
2. Recall that chloroplasts and mitochondria have two membranes, their own DNA and ribosomes,
and they can self-replicate.
3. The nucleus probably arose by an infolding of the plasma membrane to protect the genetic
material.
By about 900 million years ago there are abundant fossils of various types of marine algae.
By 700 million years ago simple multicellular marine organisms had evolved all over the earth.
The invasion of land by plants took place about 450 million years ago. This probably took place in
intertidal zones. Once land plants evolved they provided the food necessary for higher animals to
evolve.
V. Plate Tectonics
A. Plate tectonics (continental drift) has had a profound effect on the evolution of life.
1. According to plate tectonic theory, the plates of the earth’s thin crust are constantly moving apart
at ridges and colliding at trenches.
2. Basically, the land masses on earth have been drifting around almost since they were first formed
4 billion years ago.
B. At one time all the continents formed one large land mass called Pangea.
C. Later there were two major land masses:
1. Gondwanaland - the current southern hemisphere continents plus India (which was part of
eastern Africa).
2. Laurasia - the current northern hemisphere continents.
VI. Biological Evidence for Continental Drift
A. Fossil Glossopteris (seed ferns) date back about 250 million years ago. Their distribution was: South
America, Antarctica, South Africa, Australia and India. This is a classic Gondwana distribution.
B. There are three extant genera of lungfishes, one each in Australia, Africa and South America. This is
also a classic Gondwana distribution.
C. Lack of native placental mammals in Australia - Australia has no native placental mammals.
1. They are replaced by marsupials - opossums, kangaroos, koalas and other mammals that complete
their development in a marsupium (pouch).
2. Marsupials arose several million years before placental mammals. Later on placental mammals
arose in Laurasia and spread southward into Gondwanaland. However, Australia had already
drifted away from Gondwanaland before placental mammals got there.
C. Pace
These notes are revised from lecture notes written in 1997 by Chuck Pace for his A.P. Biology students that were
themselves based on lecture notes originally published on the World Wide Web at
<http://arnica.csustan.edu/biol1010/Evolution/evolution.htm> by Dr. Steven J. Wolf of California State University
Stanislaus.
C. Pace