History of life on Earth
Billions of years ago before life on Earth began
the Earth’s atmosphere was very different from
what it is like today.
Experiments have shown that organic molecules
could have formed under those different
Stanley Miller and Harold Urey
conducted experiments trying
to recreate the conditions of
They succeeded in creating
Miller and Urey concluded
that organic compounds could
have formed on pre-biotic
For life to exist organic molecules are not
A method of storing and transmitting
information to enable the living structures to
function is needed. Nucleic acids can perform
There are two types of nuclei acid:
• DNA – Deoxyribose Nucleic Acid
• RNA – Ribose Nucleic Acid
DNA and Evolution
All living things use DNA to store and transmit
Some viruses use RNA as their genetic material (but
viruses are not considered living)
DNA is the fundamental chemical of living things.
The structure of DNA determines the characteristics
of the organism made possible by the different
arrangements of its building blocks – nucleotides.
DNA is universal to all living things.
Two main roles of DNA
The fact that all living things share this common
genetic code is strong evidence that all life evolved
from a common ancestor.
1. To pass on the hereditary characteristics from
one generation to another
2. To act as a code for the production of other vital
molecules in living things, particularly proteins.
If life had many separate beginnings, we would
expect different mechanisms for storing and
transmitting genetic information.
However, all living organisms use the same
genetic code, the same 20 amino acids in their
proteins and the same cellular processes.
But not all organisms have the same DNA, so
changes to genetic information must have
occurred over the billions of years since life first
These changes to DNA are called Mutations.
DNA has diversified over billions of years leading
to the wide range of different organisms on the
DNA has diversified
Mutations have led to a diversity of DNA which has
produced diversity of life.
The first organisms (prokaryotes) had circular
Eukaryote DNA has since broken up into linear sequences.
Now there is more DNA in cells due to increased number
of genes and non-coding (junk) DNA.
Eukaryote genes contain introns and exons.
Exons are translated into proteins
Introns are transcribed but removed from mRNA before
Prokaryotes don’t have introns.
Mouse gene for dihydrofolate reductase has
31000 base pairs of which 558 base pairs code
for the amino acids required.
Using information from studies of:
• comparative anatomy
• comparative embryology
• fossil studies
Scientists have been able to piece together
possible evolutionary pathways.
This has provided strong evidence that
organisms evolved from a common ancestor.
DNA is the genetic code responsible for the features
and inheritable characteristics of all living things.
Charles Darwin first proposed the theory of
evolution claiming that all living things have evolved
from common ancestors.
He established evolutionary relationships on the
basis of structural similarities. Such structures are
E.g. limbs of different vertebrates
We call this comparative anatomy
Modifications to the forelimb of
A well documented example of comparative anatomy is
the pentadactyl limb of many vertebrates.
Similar bone structure has evolved for different functions.
Another method of
links is through
Comparing the early
It is now possible with modern technology to
analyse and compare the DNA and protein
molecules from different organisms.
If different species produce proteins what are
very similar in their amino acid sequences, it is
logical to infer that their DNA is very similar,
inherited from a common ancestor.
Scientists have been able to construct evolutionary
trees based on DNA analysis of different organisms.
Phylogenetic trees show the evolutionary
relationships in a variety of organisms based on
studying the amino acid sequence of a particular
Two proteins studied extensively are:
• Cytochrome C
If 2 species have evolved from a common
ancestor and their separation was recent, it is
likely that there will not have been enough time
for many new mutations to have taken place.
Their DNA sequences should therefore be very
They occur close together on a phylogenetic
However, if 2 species have evolved from a
common ancestor and they have been
separated for a much longer time, it is likely
many more mutations have occurred in each
species. Their DNA sequences should therefore
be very different.
They occur further apart on a phylogenetic tree.
A phylogenetic tree indicates the evolutionary
relationship of organisms based on studying the
amino acid sequence on a particular protein
called cytochrome c.
Cytochrome c is a protein that is necessary for
respiration in all living organisms. It can vary
from one species to another. The more similar,
the closer the evolutionary relationship.
The sequences for humans and chimpanzees
match all of the 104 amino acid positions.
Haemoglobin is another protein molecule used
for amino- acid sequencing.
To compare 2 species the same segment of DNA
from each species must be analysed.
Two techniques are:
• DNA Sequencing
• DNA Hybridisation
• Involves determining the base sequence of a
segment of DNA.
• Very precise method.
• Costly and time consuming.
• Only used for small segments.
• Involves heating DNA from 2 species to get
• Upon cooling DNA strands recombine.
• The degree of bonding between 1 species
DNA and another gives a measure of how
closely related the 2 species are.
A permanent change in the DNA sequence is called a
Mutation rates can be increased by certain environmental
• Mutagenic chemicals
Factors that cause increased mutation rates are called
While natural mutations do occur spontaneously at
a low rate others occur by physical or chemical
factors or agents called mutagens. These mutations
are often passed from one generation to another
They occur during DNA replication or
recombination and are called spontaneous
All alter genes in control of cell division leading to
the development of cancerous cells
Many forms of cancer are the direct result of
Chemical agents that cause or increase the
incidence of cancer are called carcinogens.
In nature, mutations are random events that can be
caused by mutagens.
The vast majority of mutations are harmful but in
some circumstances they can provide a source of
variation which nature can select from.
This process is called Natural Selection.
Natural Selection works on the changes within
individuals of a species brought about by
One nucleotide is
substituted for another i.e.
wrong base is placed into
position on the DNA,
leading to a different amino
acid placed in the protein
Eg sickle cell anemia
Base insertions or deletions
Adding or deleting nucleotides on the mRNA will
alter the reading of the nucleotide during
translation – “frameshift” mutations
Bigger mutations involve changes to
Eg down syndrome
Part of one chromosome is moved to another
Segment of one chromosome may be flipped
upside down from its normal position
Parts of chromosomes appear twice
Inheritable Human Diseases
Sickle cell anaemia
A fatal blood condition located on chromosome
11, coding for the haemoglobin protein which
has an altered base leading to single amino acid
The haemoglobin formed is abnormal gives rise
to red blood cells that do not carry oxygen
Sickled Blood Cells
Distorted red blood cells often block blood
Amino acid glutamic acid is replaced by valine
Genetic disorder where those afflicted bleed
excessively because of an abnormal gene which
prevents blood clotting
A carrier for the disease may not actually suffer
from the disease but can pass on the mutant
gene to their offspring.