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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.

Table of Contents

11.0 - Introduction
11.1 Darwin's Theory of Natural Selection

Chapter 12





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