In this blog, we are going to learn about evolution and learn how did humans evolve. The extremely large numbers and types of living organisms are present on this earth and interestingly the majority of them have not even been identified.

Now the question arises:-

1) How and from where did such a wide variety of organisms came to existence on this earth?
2) How did human beings evolve?

What is evolution?

The constant process of gradual change occurring in the organisms since the origin of life that has given rise to an enormous variety of complex organisms on this earth is called evolution. The concept of organic evolution was first given by Charles Darwin.

A plethora of organisms is present on Earth. Diversity in life forms is a measure of the health of ecosystems. Different life forms may possess certain similarities among themselves. The existence of similarities among organisms allows us to group them together, thus enabling their study as a group.

Characteristics of an organism

The placement of organisms into groups and sub-groups on the basis of similarities and dissimilarities between them reveals their evolutionary relationship. The similarities or differences in the various characteristics form the basis for the classification of organisms. But what actually are the characteristics of an organism?

The details of the appearance or behavior of an organism that distinguishes it from other organisms are called characteristics of that organism.

The characteristics may be:

�    Ancestral (or basic) characteristics
�    Derived characteristics

The ancestral or basic characteristics are those that are shared by most members of a population and thus, form their basic body design.

Evolution and Classification: Basic Characteristics

There are some basic characteristics that are shared by most organisms. For example, cells are the basic unit of life in all organisms. But we can classify different organisms based on the basic characteristic of the cells. Cells can be classified based on the nucleus. On the basis of this, organisms are broadly classified into two groups namely:

�    Eukaryotes which are organisms with a well-defined nucleus, and�
�    Prokaryotes do not possess a well-defined nucleus.

The eukaryotes can be further classified into-

�    Unicellular organisms
�    Multicellular organisms

With the specialization of cell types and tissues, organisms can further be classified on the basis of other basic characteristics. Their classification into plants and animals is based on their ability to perform photosynthesis. Furthermore, the non-photosynthetic multi-cellular organisms can be classified on the basis of whether the skeleton is present inside or around the body.

Derived characteristics

On the other hand, the derived characteristics develop due to evolutionary changes in the ancestral characteristics. The evolutionary changes or derived characteristics lead to further classification of organisms into smaller sub-groups.

By identifying the hierarchies of characteristics between species, the evolutionary relationships of the species around us can be worked out.

Now let us discuss how evolution occurred?

During reproduction, variations arise either because of errors in DNA copying or as a result of sexual reproduction. Let us understand the process of evolution with the help of few examples. Let us imagine that a group of twelve red beetles live on the green leaves of some bushes. These beetles have the natural tendency to increase in numbers by sexual reproduction and consequently generate variations.

Let us also consider that crows are the natural predators of these beetles. The crows regulate the population of beetles by eating them and, therefore, fewer beetles are available for sexual reproduction. Now let us assume that beetles develop color variation during reproduction, Consequently, one beetle develops green body color instead of normal red color. This beetle can inherit this variation in color onto its progeny so that all its offsprings are green in color.

As a result, the Crows cannot locate green-colored beetles on the green leaves of bushes but continue to eat red beetles which are easily located on green leaves by them. As a result, the number of green beetles in the beetle population increases in each generation.

Another color variation arises in beetles during sexual reproduction and in the progeny, one beetle develops blue body color instead of the normal red. This beetle can also pass this color variation to its progeny so that all its offspring are blue-colored beetles. Crows can detect both red and blue colored beetles easily on green leaves of bushes and eat them regularly.  

As the population of the beetles expands, initially there are few blue-colored beetles among the majority of red-colored beetles. At this point, an elephant comes by and stamps on the bushes where the beetles live. Consequently, most of the beetles get killed. By chance, most of them survived beetles are now blue-colored. This population again slowly expands and will contain mostly blue-colored beetles over a period of time.

Here color variation in beetles gave no survival advantage to them in the existing environment as crows could easily detect and eat both red and blue beetles on green leaves. However, a simple accident (stamping by an elephant foot) killed the majority of the red-colored beetles from the population.

Survival of more blue beetles in the population changed the color characteristic from normal red to blue over a period of time. Had the beetle population been very large, the accident would not have caused such major havoc. Let us consider one more situation.

As the beetle population starts expanding by sexual reproduction, a plant disease spreads, resulting in a decrease in the amount of leaf material in the bushes. Expanding population of beetles is now poorly nourished. Consequently, the average weight of adult beetles decreases compared to what it used to be when leaves in the bushes were in plenty.  

After a few years time and a few beetle generations of such scarcity, the plant disease is eliminated. Now more leaves of bushes are available as food for the beetles and again the beetles would gain weight.

While considering this situation we find that there are two kinds of traits in every organism:

Inherited traits: These traits are controlled by specific genes and are passed on from one generation to another. Any alteration in the DNA will be passed on, through germ cells to the progeny resulting in variations in them.

Acquired traits: Certain traits are acquired by organisms in their lifetime.  For instance, a decrease in the bodyweight of beetles due to starvation is an acquired trait by the beetles during their lifetime.  It involves changes in the non-reproductive tissues caused by environmental factors. It will not bring any change in the DNA. 

Therefore, even if some of the generations of beetles are low in weight because of starvation, this trait cannot be inherited by the progeny over generations. So, we see that variation occurs because of Sexual reproduction natural selection

Genetic drift

We can say that in small populations accidents can change the frequency of some genes in a population, even if these give no survival advantage to the possessors. The elimination of the genes of certain traits from the small population when a section of the species population dies of natural calamity or migrates to another region is called genetic drift.

It alters the gene frequency of the remaining population. In this situation, natural selection is exerted by the crows. More the crows are there, more red-colored beetles would be eaten up and more will be the proportion of green beetles in the population.

In other words, natural selection is directing the evolution of the beetle population. selection is the phenomenon wherein nature, in the wild, selects traits favorable to the species in its environment.

Read More: Evidence of Evolution: Morphological, Anatomical & Embryological