INTRODUCTION – EVOLUTION AND MAN
Evolution is the process that brought us to where we
are today.
It started about 3500 million years ago, when the first living thing,
probably a single complex organic molecule in the form of a long chain,
began to reproduce itself. It did this by latching onto simpler molecules
dissolved in the water around it, until it built up a mirror image of
itself. The two parts then split apart to become two identical complex
molecules. Each of these had the same power of attracting simpler molecules
and building up a mirror image – similar to the way in which viruses
reproduce themselves.
The building up and the splitting took place untold millions of times.
Inevitably on occasion the mirror image so produced was not accurate.
As a result the new molecule had slightly different properties from the
old, and may not have been so efficient at reproducing itself. In this
case the changed molecule - the mutation - stopped reproducing and died
out.
However, the occasional mutation arose that actually helped the molecule
to reproduce itself. The mirror images – the off spring – of this mutation
then survived. This is the basis of the process that we call evolution.
After millions of beneficial chance mutations the single molecule became
more and more complex, if complexity ensured a more efficient reproductive
process. The molecule changed from a virus-like entity to a living cell,
in which the reproductive molecule or molecules were encased and protected
by an outer membrane. This resembled one of our modern bacteria.
The chemical reactions that enabled early molecules to reproduce themselves
may have been insufficient to power the reproduction of more advanced
creatures, and other energy sources developed that allowed the absorption
of energy from sunlight and the use of this energy to build up raw materials
for reproduction. The first single-celled plants had evolved.
Other mutated cells did not use the sun’s energy. Instead they digested
the cells that did, and so used the energy already stored. These were
the first animals.
Eventually creatures evolved that consisted of more than just a single
cell. This came about either by cells reproducing themselves and then
failing to split, or by several cells coming together. Whichever it was,
if the multi-celled creature were more efficient, then it survived and
reproduced in its multi-cellular form.
With the increasing complexity, the different cells in a single creature
evolved to have different functions. Some cells were involved in sense,
helping the creature to find food or light. Other cells were involved
in locomotion, in moving the whole creature towards its food or its light
source. Others were involved in digestion, others in reproduction, and
so on.
The different masses of cells are what we call tissues, and the structures
that they form, each with a different function, are called organs. An
entire creature (made up from molecules that make cells, that make tissues,
that make organs) is called an organism.
At an early stage the pathways of evolution began to branch, and different
types of organism developed. Wherever there was a food source that could
be exploited, evolution produced an organism able to exploit it. Such
a process is called adaptive radiation, and we can see it at work today.
Many species of finch live in the Galapagos Islands, off the west coast
of South America. These all evolved from one type of seed-eating ground
finch that came over from the mainland, and spread to all the islands,
each with different habitats and food sources. The finches on each island
evolved to take advantage of their particular habitat. As a result there
are now many species of finch on the islands, including heavy-beaked
forms that eat seeds, short-beaked forms that eat buds and fruit and
long-beaked forms that eat insects.
Environments are not stable; they change for one reason or another. When
this happens, a creature evolved to live in a particular way in a certain
environment becomes extinct. For example, if all the insects on the Galapagos
Islands died out, then the long-beaked finches would become extinct:
a process known as natural selection. If the insects became extinct,
their places would be taken by another creature, and some other bird
would evolve to eat that.
Evolution produces specific shapes of animals to live in particular environments.
Grass is tough to eat, so an animal that eats grass needs strong teeth
and a specialized digestive system. Grasslands are wide open areas in
which danger can be seen coming from a long way away, and there are no
hiding places. A grass-eating animal, therefore, tends to have long running
legs, as well as strong teeth, and a long face so that its eyes are above
the level of the grass while its head is down eating. This gives us the
shape of the antelope – the typical grass-eating animal of Africa.
However, the grasslands of Australia have evolved a quite unrelated grass-eating
animal – the kangaroo. There seems little resemblance between this and
the antelope of Africa. It does, however, have the same long face with
similar grass-grinding teeth; and the legs are long and built for speed,
albeit in a bounding rather than a running gait. This development of
similar features in unrelated animals in response to similar environmental
conditions is what is known as convergent evolution. It accounts for
the similarities between seals and sealions, aardvarks and anteaters,
ants and termites, vultures and condors.
A similar phenomenon is parallel evolution. In this, two branches of
the same family tree develop along similar lines independently of one
another. For example, the kit fox of North America and the fennec fox
of Africa are both small, with a sandy pelt and large ears. The ears
act as cooling vanes and prevent each animal overheating in its desert
environment, and the pelt is camouflage. Both are descended from a more
conventional fox-like animal, but each has evolved separately to live
in different deserts.
The different colours and patterns in animals can also be attributed
to evolutionary processes. Animal patterns may camouflage them: on the
other hand they may, like the skunk, have striking colours that warn
a would-be attacker that the owner is poisonous. Some animals mimic others,
as when a harmless king snake develops the spectacular pattern of the
poisonous coral snake, and consequently turns away potential enemies.
All these have developed because the animals concerned have benefited
from them, have survived and have gone on to reproduce.
Throughout the world and throughout time, animals and plants have changed
in response to the changes in the environment.
One species has broken with this tradition. Within the last million years
or so the human species Homo sapiens evolved. It has come all the way
from molecules to its present form in 3500 million years by the workings
of evolution. Now, within the last few millennia, intelligence has developed,
and with it cultures and civilizations. The species has spread not by
changing to adapt to the environments it found but by changing the environments
to suit itself. Instead of developing furry pelts and layers of insulating
fat to adapt to cold conditions, it manufactures artificial coverings
and uses available energy supplies to generate heat for the body. Instead
of evolving heat radiating structures such as big ears to adapt to hot
conditions, it manufactures refrigeration and air-conditioning systems,
again using available energy supplies. Instead of developing speed and
killing strategies that allow it to hunt a particular food, it builds
machines to do it. By using its intelligence it can exploit all food
supplies in all environments without having to change itself.
Medical science eliminates much of the effects of natural selection:
no longer does an individual not particularly well adapted to the environment
die out before being able to reproduce.
Under natural conditions not all offspring of a species survive, and
this is reflected in the birth-rate. Thanks to medical science, more
offspring of Homo sapiens survive than ever could before, but this has
not been reflected in a corresponding drop in the birth-rate. As a result
the populations of Homo sapiens are growing without the refining and
modifying processes of natural selection.
Evolution as we know it for Homo sapiens has stopped. However, this does
not mean that the process of change has necessarily stopped.
As science develops, the reproductive molecules - the genes - that exist
within every cell of the human body are becoming better and better understood.
When Homo sapiens finally appreciates which parts control the development
of which features, then the possibility exists for modifying the process.
A stage will be reached when one gene can be suppressed, another encouraged,
with yet another created from new. A human being with particular features,
following a particular preconceived plan, may be born from modified sperm
cells and ova. Without the natural processes of modification, this unnatural
process is the only way of developing the species into new forms to face
the problems that await it in the future: problems generated by overpopulation,
over-use of natural resources and pollution.
Genetic engineering
The mechanics of genetic engineering are already complex,
yet in their current state they are primitive compared to what will undoubtedly
be possible within a few decades.
The reproductive molecules that lie at the nucleus of each cell of a
living organism are in the form of long structures called chromosomes.
These chromosomes are made up of the chemical substance DNA. Its shape
is best imagined as a long ladder that has been twisted along its length.
Each rung of this ladder consists of two compounds, called bases, locked
together. There are only four different kinds of bases: thymine, cytosine,
adenine and guanine, referred to as T, C, AS and
G. A T always unites
with an A, and а С always with a G. The sequence of these base pairs
along the twisted ladder of the chromosome is almost infinitely variable
– there are something like 6,000,000,000 bases in a full set of human
chromosomes.
A chromosome is often described as a page in an instruction manual. Each
base pair, or rung in the ladder, represents a letter of the alphabet,
and the arrangement along the ladder gives ‘words’ and ‘sentences’. Each
understandable instruction so formed gives a gene. The genes in a single
cell produce the total information needed for the growth of the entire
organism.
When an organism grows and develops, it does so by multiplication of
cells. Each cell splits into two complete cells. When this happens, each
chromosome in the cell actually splits down the middle. The uprights
of the twisted ladder pull away from one another as the rungs split into
two along the joins between the bases. What happens then is that these
two half-ladders build up two complete ladders by attracting free bases
made up from the chemicals drifting in the cell. As a result, when the
cell splits into two each new cell carries exactly the same set of gene
instructions.
The exception to this process is in sexual reproduction. Reproductive
cells carry half the normal number of chromosomes. Two half-cells unite
during fertilization to produce one cell with the full number. This new
cell is a unique mix of genes, half from the mother and half from the
father. This cell then divides in the usual manner until the entire organism
is built up, following the instructions now carried in every cell.The big
mystery now is this: how do the genes - the pattern of base pairs along
a chromosome - actually work? How do they control the construction of
an organism?
The idea behind genetic engineering is to manipulate natural processes.
In some way genetic instructions along the chromosomes in a cell have to
be identified then changed so that as the organism grows, it is to a new
set of instructions. Since all the materials involved (cells, chromosomes,
molecules) are microscopic, a whole new technology has to be applied.
Viruses can do it. Viruses essentially consist of a mass of their own DNA
encased in an envelope. When they infect a cell they attach themselves
to the cell’s wall and inject their DNA through it. In the cell’s interior
the invading DNA breaks down the cell’s chromosomes and rebuilds the material
into copies of itself.
For genetic engineers to do the same, they would first of all have to break
in through the cell wall, then break down the DNA of the nucleus and reassemble
it in the desired way. Alternatively, they could cut out segments of the
DNA strand, segments that correspond to particular genes, and replace them
with DNA segments already prepared. This would be done by chemicals that
have specific biochemical reactions – enzymes – some of which have been
found to have the ability to cut DNA strands.
The greatest experimental successes so far have been with bacteria. These
single-celled creatures have cell walls that can be softened by chemical
solutions so that new DNA can be placed inside. The double helix of the
original chromosome can be chopped up using enzymes, and new DNA can be
inserted. The broken ends of the DNA strands have one side longer than
the other, exposing a sequence of bases. If the introduced DNA segment
has matching bases exposed at its end the two DNA pieces will unite, Т to A, and С to G, and produce a complete chromosome. This technique is
known as gene-splicing.
Before any of this can be attempted, however, the whole gene pattern has
to be mapped. At the moment only about 100 human genes have been identified
and interpreted; but, since genetics has only been in existence for a century,
and the structure of the chromosome has only been known for about four
decades, and scientific advance in this area is increasing exponentially,
what was speculation about genetic engineering is quickly becoming fact.
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