Download Option D Evolution - A - Origin of Life

Option D Evolution
SL and HL
A. Origin of Life on Earth
Four Processes needed for life to begin:
i. The non-living synthesis of simple organic molecules
ii. The assembly of simple organic molecules into polymers.
iii. The origin of self-replication molecules that made inheritance possible
iv. The packaging of these molecules into membranes with an internal chemistry
differ from their surroundings.
I. Organic Molecule Formation
i. The non-living synthesis of simple organic molecules
3 theories:
1. Formation organic molecules by energy sources (UV or electrical)
Miller and Urey Experiment:
*One issue is the mix of gases might not have reflected the atmosphere of early Earth
Table 1: Organic molecules that have been produced from inorganic
molecules in various experiments.
Precursor molecule
Formaldehyde
CH2O
Macromolecule of Life
Ribose, glycerol
Carbon monoxide +
hydrogen
CO + H2
Hydrogen cyanide
HCN
Cyanamide
H2NCN
Fatty acids Hydrogen cyanide
Purines (adenine, guanine)
Cyanamide
Peptides and Phospholipids
2. Delivery by extraterrestrial objects (comets and/or asteroids)
-Comets and asteroids have organic molecules
-UV light can help create organic molecules
-Organic molecules have been detected in interstellar clouds.
3. Organic synthesis driven by impact shocks (asteroids).
-The impact of comets and/or asteroids on the Earth could have
caused organic molecule formation.
-
However, if the first genetic material was RNA then this is not an issue. RNA is
self-replicating and can also act as a catalyst. Some reactions in ribosomes are still
catalyzed by rRNA. Additionally, RNA is used to synthesize proteins. This offers a way
around the genes require proteins and enzymes require genes.
-
RNA has also been shown to catalyze the formation of more RNA. It can also
bind amino acids together. RNA can also be transcribed into DNA (using reverse
transcriptase)
-
Recent studies have shown that certain types of clay might have helped the first
cells to be formed. Clay seems to speed along formation of proto-cell membranes and to
keep RNA stable (RNA is generally unstable for long periods of time).
-
Remember that a prokaryotic cell does not have membrane bound organelles. So the above
scenario would provide the basics for a prokaryotic cell to form.
II. Organic molecules to Protobionts
Protobionts - aggregate of abiotically produced organic molecules surrounded by a
membrane or a membrane-like structure.
ii. The assembly of simple organic molecules into polymers
iii. The origin of self-replication molecules that made inheritance possible
i. “Genes First Model”
This model suggests that genes came first, then the metabolism
(chemical reactions need for life). RNA is preferred over DNA as the first genetic
material for a number of reasons:
1. RNA can store genetic information in its sequence of bases.
2. Single stranded RNA exhibits catalytic activity. RNA in the ribosome acts to
catalyze protein synthesis in present day cells.
3. Laboratory studies have demonstrated that RNAs with no catalytic activity can
evolve to structures with various catalytic functions. (see Ribozymes)
4. RNA has also been shown to catalyze the formation of more RNA. It can also
bind amino acids together. RNA can also be transcribed into DNA (using reverse
transcriptase)
Ribozymes
Messenger RNA in eukaryotic organisms contains sequences that are transcribed
but are not part of a gene. The initial mRNA transcript needs to be processed to
remove these intervening sequences, called introns, before the mRNA leaves the
nucleus. The splicing together of the functional pieces of mRNA (the exons) is
usually accomplished with protein enzymes. In the 1980's special sequences of
RNA were found to be "autocatalytic." (self-catalyzing)
ii. “Metabolism First Model”
This model suggests that a primitive metabolism formed first which could provide an
environment for the later start of RNA replication.
Ideas:
-hydrothermal vents could have acted like a cell membrane, trapping organic
molecules inside, which could have reacted making complex molecules.
-single amino acids can catalyze the creation of sugars from simple starting
materials.
iii. Possible Membranes or Membrane-like structures
iv. The packaging of these molecules into membranes with an internal
chemistry differ from their surroundings.
a. Coacervates - spherical arrangements of lipid molecules making up a which is
held together by hydrophobic forces.
b. Microspheres- protein protocells that are small spherical units that form
membrane like structures
c. Clay Crystals – clay are a collection of crstyls which could have formed
“cages” around organic molecules. Some types of clay can catalyze chemical
reactions needed for life.
d. Ice – Ice can have small packets of liquid water which could isolate organic
molecules.
iv. Possible Locations:
Deep-sea hydrothermal vents - life may have began at the interface where chemically rich
fluids, heated by some mechanisms like tidal forces of surrounding moons or planets,
emerge from below the sea floor. Chemical energy is derived from the reduced gases by
the redox reactions, such as hydrogen-sulfide and hydrogen coming out from the vent in
contact with a suitable oxidant, such as carbon dioxide
Volcanoes – Similar to hydrothermal vents
Deep Hot Biosphere Model- states that life originated deep underground. Supports state
that a trickle of food from a deep, unreachable, source is needed for survival because life
arising in a puddle of organic material is likely to consume all of its food and become
extinct. Methane and iron would have been the sources of nutrients.
Exogenesis - primitive life may have originally formed extraterrestrially, either in space
or on a nearby planet (Mars).
Panspermia - is the hypothesis that "seeds" of life exist already all over the Universe, that
life on Earth may have originated through these "seeds", and that they may deliver or
have delivered life to other habitable bodies.
Clay theory - states that complex organic molecules arose gradually on a pre-existing,
non-organic replication platform — silicate crystals in solution.
B. Prokaryotic to Eukaryotic Organisms
-
Early prokaryotic bacteria had a major effect on the Earth’s environment.
Cyanobacteria (also called blue-green algae) release large amounts of oxygen
into the atmosphere.
This killed off a lot of other organisms; oxygen was a poison at that time.
Other organisms moved away from the oxygen.
It also forced organisms to adapt
The origin of Eukaryotic Cells
-Evolved between 1.5-2.5 billion years ago, about the same time oxygen levels
started to increase
-Remember that Eukaryotic cells have membrane bound organelles. The idea is
that eukaryotic cells evolved from prokaryotic cells. But then where did the
membrane-bound organelles come from? Remember that prokaryotic cells do
not have membrane bound organelles.
Endosymbiotic Theory
-States that mitochondria and chloroplasts evolved as independent prokaryotic
cells, which were taken into a heterotrophic cell by endocytosis. Instead of being digested
the cells were kept alive.
In general, mitochondria and bacteria are basically the same size. This cannot be said about
the other eukaryotic organelles.

Mitochondria have a set of double membranes, as do most bacteria; and the lipid
composition of mitochondrial membranes shows no similarities with the eukaryotic cell
cytoplasm. If mitochondria evolved inside of proto-eukaryote, their membrane
composition would be expected to be of the same material. Instead, it appears that
mitochondrial membranes more closely resemble bacterial membranes in terms of lipid
composition (Margulis, 1981, p. 217).

The inner-membrane infoldings in the mitochondria lend more credence to the symbiosis
theory. According to Margulis, the infoldings (cristae) "are adaptations that increase the
surface area of oxidative enzymes, evolutionary analogues to the mesosomal membranes
of many prokaryotes" (Margulis, 1981, p. 208). In addition, the cristae keep the various
enzymes segregated according to use, as do bacteria.

Mitochondrial ribosomal RNA sequences bear much more in common with bacteria than
with ribosomes in the eukaryotic cytoplasm. For example, certain types of transfer RNA
has been found to exist only in mitochondria and bacteria (Dyer and Obar, 1985, p. 78).

Not only do mitochondria possess their own DNA, but it is circular, as is bacterial DNA;
and DNA synthesis is continuous as opposed to that of eukaryotic nuclear DNA.
Furthermore, the ratio of guanine-cytosine base pairs in mitochondrial DNA is
proportionately higher, as with bacteria, than in eukaryotic nuclear DNA (Margulis,
1981, p. 206).

Mitochondrial division resembles bacterial reproduction, according to Margulis. She
writes, "Genetic recombination in (mitochondria) is far more reminiscent of phage and
bacterial sexuality than it is of eukaryotic nuclear sexual behavior" (Margulis, 1981, p.
218).
C. Evolution
- Evolution is the cumulative change in the heritable
characteristics of a population.
i. Conditions for Evolution
a.
Variation
-variation exists in a population of organisms.
-variation is a result of sexual reproduction.
b. Inheritance
-DNA is the molecule responsible for inheritance. Only changes in the DNA are
affected by inheritance.
c. Selection
-Most species produce more offspring than necessary. This results in a
competition for survival. THIS IS ONLY A SMALL PART OF EVOLUTION!
(SEE DIAGRAM BELOW)
d. Time
-Changes accumulate over a large amount of time.