INTRODUCTION
The
theory of evolution explains the diversity of life on earth today by the
descent of each and every species from common ancestors. The study of evolution
should lead us back to a first form of life from which all others have been
descended. What was this first form? We do not know, and the best we can do is
to make intelligent guesses about its characteristic. Where did it come from?
We do not know the answer to this question either although every thoughtful
person has probably wondered about it at one time or another.
THEORIES OF THE ORIGIN OF LIFE
Among
the first attempt at answering this question in our civilization were the
stories of creation that are found in the bible other cultures, too, have their
stories of the creation of life. These stories share two features in common.
First, they were created long before man had gained any knowledge of the
physical, chemical, and biological principles that are the basis of life.
Second, they invoke divine intervention in the creation of life and thus fall outside
the scope of scientific inquiry.
THE COSMOZOA THEORY
This
theory of the origin of life does fall within the scope of scientific inquiry.
It explains the presence of life on our earth by assuming it was brought here
from elsewhere in the universe.
THE THEORY OF SPONTANEOUS GENERATION
As
move was learned about biology, however, men began to doubt the possibility of
spontaneously generation in 1668 the Italian physician Francesco Redi performed
an experiment to show that maggots do not arise spontaneously in decaying meat
but are produced from the eggs of flies. Although the spontaneous generation of
large forms of life began to be doubted after this. Van leenwenhock’s discovery
of microorganisms reopened the question.
OPARIN’S THEORY
While
conceding that life does not arise spontaneously now, oparin felt that it might
well have arisen spontaneously under the conditions that existed earlier in the
history of the earth.
THE DAWN OF LIFE
No
one knows exactly when life first appeared on the earth. We would not expect
the first forms of life to leave fossil remains for us. However, fossils that
resemble algae have been found in rocks over three billion years Old. Nourishment
for the first form of life was no problem. Surrounded by the same soup of organic
molecules from which it arose, it had only to use these molecules by that the
first living organism secured its energy from these organic molecules.
THE EVOLUTION OF THE PROTISTS
Geology
tells us very little about the evolution of protists only those with hard parts
e.g. (foraminifera, din toms) leave satisfactory remains. The best we can do is
to work intelligent geneses about their evolutionary relationships on the basis
of the study of modern forms .however, the photosynthesizing organisms which h must
have evolved from it may possible be still represented today by the green
photosynthetic bacteria. These organisms live on the surface of ocean mud and,
like the purple sulfur bacteria. Photosynthesis by these organisms would have
liberated oxygen into the atmosphere and thus established the conditions for
the evolution of aerobic, respiring organisms. The blue-green algae were the
descendants of the green sulfur bacteria first to use water in photosynthesis.
The
red algae, too, may have been among the first photo synthesizers to release
oxygen into atmosphere. Although present members of the phylum are considerably
more complex than the blue-green algae. The presence of oxygen in the
atmosphere opened the way for the evolution of the aerobic, heterophic
bacteria. Many of these move by means of a simple flagellum which resembles one
of the central fibrils of the multistrandel cilia. At the same time,
chemoautotrophic bacteria, such as the colorless sulfur bacteria.
EVOLUTION TRENDS IN PLANTS
THE FIRST VASCULAR PLANTS
Although
the anatomy of the first plant is not entirely dear, we are confident that they
belonged to phylum tracheophyta and were more or less adapted to life on land.
By the Devonian period, four distinct groups had appeared, each of which has
left some descendant right up to the present time. These groups were the
psilopsida, lycopsida sphenopsida and pteropsida. The psilopsida like their
modern descendant psilotum, had no root or leaves. They did, however, have both
underground stems (rhizomes) and erect stems and these also produced the
sponrangia. The photosynthesis pigment were surely chlorophyII a and b. from a
study of psilotum, we deduce that they product only one kind of spore, and
these developed into tiny gameto phytes which in turn, produced both antheridia
and archegonia fertilization was by swimming sperm and hence these plants must
have been restricted to habitats which were quite wet, at least some of the
time. Both the lycopside and sphenoswids had roots and leaves, each containing
xylem and philoem tissue connected with
that of the stem. The leaves were simple and small, all their vascular tissue
occurring in just a single, unbranchedvein.
Some
of the lycopsids produced not one but two kinds of spores.
The first pterosids, the ferms, also contributed a large array of species to the flora of the Devonian landscape. Unlike the members of the other subphyla veins. Like most of our temperate climate ferms of today, they were homosporous, that is, only one kind of spore was produced each fern spore, develop into a prothallus that bears both male and female sex organs. Fertilization requires moisture in which the climate sperm can swim to the egg. Fossil remains from the Devonian indicate that some early fern like plant were heterosphorous, that is, they seed ferns as these plants are called, were among the earliest of the gymnosperm, another class in the subphylum pteropsida.
The first pterosids, the ferms, also contributed a large array of species to the flora of the Devonian landscape. Unlike the members of the other subphyla veins. Like most of our temperate climate ferms of today, they were homosporous, that is, only one kind of spore was produced each fern spore, develop into a prothallus that bears both male and female sex organs. Fertilization requires moisture in which the climate sperm can swim to the egg. Fossil remains from the Devonian indicate that some early fern like plant were heterosphorous, that is, they seed ferns as these plants are called, were among the earliest of the gymnosperm, another class in the subphylum pteropsida.
THE MOSSES AND LIVERWORTS
The
nonvascular plants have sometimes been considered the ancestors of the vascular
plants. Their simplicity of structure, lack of vascular tissue and restriction
to dump location do suggest that they are intermediate forms between the algae
and the vascular system and woody tissue and the necessity for surface water in
which the sperm can swim from antheridia to archegonia have limited the
evolutionary potentialities of these organisms.
THE ANGIOSPERMS
The
angiosperms are found in practically every habitat. Although they incorporate a
variety of features that enable them to live even in arid location, some have
returned to an aquatic existence. The possession of roots permits the
extraction of moisture and minerals from beneath the surface of the land. Roots
also save to anchor the plant against the wind. The presence of a cambium
capable of producing woody tissue provides for the support, high in the air of
need be, of leaves and flowers. Xylem and ploemenable water, foods and hormones
to be translocated long distance in the plant quickly. A waxy inticle on leaves
and herbaceous stems and the lork on woody stems prevent rapid loss of water
from the plant by evaporation. The angiosperms share with the gymnosperms the
characteristics of retaining the female gametophytes within the megasporagium
in the gymnosperms and some angiosperms, the polen grains are simply brown from
plant by the wind. Many other angiosperms attract insect or other animals by
power and thus exploits the animals
power of locomotion to aid cross- pollination.
REFERENCES
Oparin A.I., The origin of life,
Dover publication, Inc., New-York, 1953.
Wald G., the origin of life”
scientific American, Reprint No. 47, August, 1954
Brown, H., the Age of the solar
system scientific American, Reprint No. 102, April. 1957.
Clark
J.D. “ Early Man in African”, scientific American, Re print No. 820, July, 1958
Broom, R., “The Ape-Men”, scientific
American, Reprint No. 832, November, 1949.
Howells, W., Mankind in the Making, 2nd
editor, Editions, New-York 1961.
Roomer, A. S. the vertebrate story
University of Chicago press, Chicago, 111, 1959.
Napier, J. the Evolution of the
Hand, scientific American, Reprint No. 140, December, 1962