Short essay on evidences for evolution

1. Homologous Organs Provide Evidence for Evolution:

If we look at the way in which living organisms are made, we can often see quite striking similarities in their construction. One of these is the presence of homologous organs. Those organs which have the same basic structure (or same basic design) but different functions are called homologous organs. The homologous organs of different animals provide evidence for evolution. This will become clear from the following examples.

There are many organs in different groups of animals (or plants) which all seem to be built from the same basic design but are used for many different purposes. These are called homologous organs. For example, the forelimbs of a man, a lizard (reptile), a frog (amphibian), a bird and a bat (mammal) seem to be built from the same basic design of bones but they perform different functions.

The forelimbs of a human (man), a lizard, a frog, a bird and a bat have the same basic design of bones. They are homologous organs, forelimbs of a human (man) are used for grasping; the forelimbs of a lizard are used for running; the forelimbs of a frog are used to prop up the front end of its body when at rest, and also act as shock absorber when the frog lands back on the ground after a leap whereas the forelimbs of a bird and a bat are modified for flying. Since the forelimbs of a human, a lizard, a frog, a bird and a bat have similar structures (or design) but perform different functions, they are the homologous organs.

Heredity and Evolution:

The presence of homologous forelimbs in humans (man), a lizard, a frog, a bird and a bat indicate that all these forelimbs have evolved from a common ancestral animal which had a ‘basic design’ limb. In other words, it tells us that a human, a lizard, a frog, a bird and a man, all have evolved from a common ancestor.

Thus, the presence of homologous organs in different animals provides evidence for evolution by telling us that they are derived from the same ancestor who had the ‘basic design’ of the organ on which all the homologous organs are based. Please note that the wings of a butterfly (which is an insect) and the wings of a bat cannot be considered to be homologous organs because they have different basic designs (though they are used for the same purpose of flying).

2. Analogous Organs Provide Evidence for Evolution :

Those organs which have different basic structure (or different basic design) but have similar appearance and perform similar functions are called analogous organs. The analogous organs provide the evidence for evolution. This point will become clear from the following discussion.

There are many organs in different groups of animals which seem to be built from different basic structure but appear to be similar in shape and perform similar functions. These are called analogous organs.

For example, the wings of an insect and a bird have different structures (the insects have a fold of membranes as wings which are associated with a few muscles whereas a skeleton, flesh and feathers support bird’s wings) but they perform the same function of flying. Since the wings of insects and birds have different structures (or different designs) but perform similar functions, they are analogous organs.

Now, since the analogous organs have different basic design, so they do not indicate a common ancestor for the organism. The analogous organs provide evidence for the evolution in another way. The presence of analogous organs indicates that even the organisms having organs with different structures can adapt to perform similar functions for their survival under hostile environmental conditions.

Thus, the presence of analogous organs in different animals provide evidence for evolution by telling us that though they are not derived from common ancestors, they can still evolve to perform similar functions to survive, flourish and keep on evolving in the prevailing environment. The analogous organs actually provide a mechanism for evolution.

3. Fossils Provide Evidence for Evolution:

The remains (or impressions) of dead animals or plants that lived in the remote past are known as fossils. The fossils provide evidence for evolution. For example, a fossil bird called Archaeopteryx looks like a bird but it has many other features which are found in reptiles. This is because Archaeopteryx has feather wings like those of birds but teeth and tail like those of reptiles. Archaeopteryx is, therefore, a connecting link between the reptiles and birds, and hence suggests that the birds have evolved from the reptiles. Thus, fossils provide the evidence that the present animals (and plants) have originated from the previously existing ones through the process of continues evolution.

We will now describe how fossils are formed. Usually, when organisms (plants or animals) die, their bodies will decompose by the action of micro-organisms in the presence of oxygen, moisture, etc.

Sometimes, however, the conditions in the environment are such (like absence of oxygen or moisture, etc), which do not let the body of the organism to decompose completely. It is such body (or body part) of an organism which we get as fossil on digging the earth.

In many cases the soft parts of the organisms get decomposed and what we get as a fossil is a skeleton of hard parts (like bones, etc). Even the soft parts of the plants and animals (which usually decompose quickly) are sometimes preserved as fossils in the form of their impressions inside the rocks.

For example, if a dead leaf gets caught in mud, it will not decompose quickly. The mud around the leaf will set around it as a mould, gradually harden to form a rock and retain the impression of the whole leaf.

This forms a leaf fossil which can be dug out from the earth a long time later. The fossil of a dead insect caught in mud is also formed in a similar way to leaf fossil. All such preserved impressions of the body parts of the once living organisms are also called fossils.

Fossils are obtained by digging into the earth. The age of fossils can be estimated in two ways: by the relative method, and by the carbon dating method. The relative method works like this: When we dig into the earth, we find fossils at different depths.

The fossils which we find in layers closer to the surface of the earth are more recent; the fossils which are found in deeper layers are older; whereas the fossils found in the deepest layers of earth are the oldest ones. Fossils which we find today were once living objects.

All the living objects contain some carbon- 14 atoms which are radioactive. When a living object dies and forms fossil, its carbon-14 radioactivity goes on decreasing gradually. In the carbon dating method, the age of fossils is found by comparing the carbon-14 radioactivity left in fossils with the carbon-14 radioactivity present in living objects today.

There are various kinds of fossils. Some of the important fossils which have been studied are those of ammonite, trilobite and dinosaur. Ammonites were the invertebrate animals (molluscs) with a flat, coiled, spiral shell which lived in the sea. The estimation of the age of ammonite fossils has told us that they are about 180 million years old.

This means that ammonites lived in the sea about 180 million years ago. Another invertebrate animal fossil which has been studied is that of trilobite. Trilobites were marine arthropods which were common between 400 to 600 million years ago. Dinosaurs are extinct carnivorous or herbivorous reptiles (The word ‘dinosaur’ means ‘terrible lizard’).

The estimation of the age of dinosaur fossils have told us that they first appeared on earth about 250 million years ago and became extinct about 65 million years ago. It is clear from t he above discussion that we can even study about those species which are extinct (no longer exist), by studying their fossils which are found during the digging of earth.