PinkMonkey Online Study Guide-Biology
11.1 (A) Common Origin of Living Organisms
There are numerous varieties of living organisms. But
they show some basic similarities which suggest that a common early form
of life gave rise to millions of species through the process of evolution.
There is a great deal of evidence that proves that there is a common origin
of all organisms.
Figure 11.1 Connecting links
(i) The study of comparative anatomy supports
the claim of a common origin of organisms. Homologous organs are
formed on the same basic plan though they may be modified variously to
perform different functions. For example the forelimbs of a bird, bat,
whale, horse and man are different in grass appearance and function but
have the same basic pattern of skeleton. They must have a common ancestral
structure which gave rise to different modifications . Homology
is also seen in the structure of eye, brain, joint appendages of arthropods,
etc. The basic similarity of vertebrate forelimb structures indicates
inheritance from a common ancestor. It is thus evidence for evolution.
Occurance of vestigeal organs such as vermiform appendix
in man, vestigeal hindlimbs and pelvic girdle in snakes, reduced nictitating
membrane in mammalian eye, etc. suggests that these organs were well developed
in the ancestors.
There are certain animals with intermediate characters
between two major groups of animals. They are called connecting links.
For example, lung fishes show common features of both fishes and
amphibians, Seymouria (extinct) connect amphibia and reptiles.
Archaeopteryx (extinct) connects reptiles and birds, and Prototheria
(egg laying mammals) connects reptiles and mammals(fig.11.1). All
these examples suggest common ancestors of the organisms.
(ii) Embryological studies also suggest the common
origin of the organisms. The early stages of development of the embryos
of fish, salamander, tortoise, hen and man show peculiar similarity. Earlier
stages are so similar that they cannot be differentiated. Only later stages
show species-specific differences. The similarity of early embryos in
closely related animals is an evidence of evolution.
(iii) Fossils are the remains of organisms that lived in the past. A study of fossil record helps to build a historical sequence of biological evolution of complex organisms from simple ancestors.
Fossil evidence shows that the horse has undergone considerable
evolutionary change over a period of 60 million years (Figure 11.2). Adapting
to life on the open plains, the horse evolved from a dog-sized, 5-toed
browser to a large 1- toed grazer. The modern horse evolved from Eohippus
through 14 distinct stages extending through the age of mammals and
the age of man. Few important representative stages are Eohippus (Eocene),
Mesohippus (Oligocene). Merychippus (Miocene), Pliohippus
(Pliocene), and Equus (Pleistocence and recent epochs).
Important structural changes during the evolution of horse are :
1. Increase in size from 11" (Eohippus) to
about 60" (Equus).
2. Elongation of the 3rd digit, side toes are lost,
and only the middle toe and splints of second and fourth digits remain
in horses now. (3) Elongation of the head and neck so as that it can
reach the ground. (4) The front teeth are modified as chisel-like cropping
structure, molars elongated and became adapted for grinding.
Thus, evolution of the horse, elephant, camel, etc.
show that much can be learned from fossil records.
Equus Merychippus Mesohippus Eohippus
(Recent) (Miocene) (Oligocene) (Eocene)
One toe Three toes Three toes Four toes
Two splints Splint of 5
Figure 11.2 Evolution of horse
(iv) A comparative study of physiology and biochemistry
also supports the common origin for different organisms. The protoplasm
of all organisms is more or less same in composition. The chromosomes
of all organisms also show similarity in their chemical nature (i.e. nucleic
acids and histones). Enzymes, hormones, the nitrogenous waste materials,
hemoglobin, composition of blood plasma, types of antibodies and antigens
etc. also demonstrate the relationship.