Fossils - Tracing Evolutionary Relationship

Over thousands and millions of years, a bewildering variety of species has evolved on the earth. Among these, some are known and some are unknown to us. The life forms around us range from, tiny ants to tall trees, colorless worms/insects to brightly colored flowers and birds. Understanding and tracing all of them in a fraction of the time, one by one is not a wise choice. We need to group them based on certain basic criteria. Let us have a glance at evolution and classification, tracing of evolutionary relationships and formation of fossils in detail.

Evolution and Classification

Evolution is the successive modification in inherited traits over a huge span of time, usually over generations. Evolution has created a huge biodiversity. Classification is the way to handle these diversifying organisms.

Living organisms were classified based on characteristics like their similarities and dissimilarities. Characteristics are the appearance/form and behavior/function of something. These characteristics decide which organism will belong to which group. For example, a dog has limbs but a snake does not. A dog and a snake could move but a plant cannot. These are characteristics of different organisms. These behaviors classify them into different groups.

Hierarchy of Classification

Hierarchy of classification begins with the cell. The cell is the fundamental unit of every organism. This characteristic is shared by all living organisms. The body design, the level of organization and the development of organs come later. As we move towards the last, the number of organisms sharing common characteristics becomes lesser. Thus, we could conclude that more the number of common characteristics shared between two species, more closely related they are. And more recently they have had a common ancestor. This is the same as your relationship with your brother and cousin. Characteristics you share with your brother are more than that with your cousin. Also, the common ancestor of you and your brother is your parents but the common ancestor of you and your cousin is your grandparents.

Tracing Evolutionary Relationships

Characteristics assist us to trace the evolutionary relationship between organisms. Evolutionary relationships help us to trace who we are more close to and who is our common ancestor.

A group of organisms is similar enough to be thought of together by certain characteristics.

For instance, mammals and reptiles. All of them have four limbs and the basic structure of their organs is similar even though they use it for different functions. Such organs are called homologous organs. This does not mean that they share a common ancestor.

Some organisms appear to be similar but basic designs are different.

For instance, wings of bats and birds. Both of these animals use wings for the same function but they have different basic structures. Such organs are called analogous organs.

Thus, the homologous and analogous characteristics help to trace an evolutionary relationship between different species.

Tracing Evolutionary Relationships


We use living species to trace evolutionary relationships. But organisms which once lived and do not exist anymore also have a story to tell about evolution.  They can be used for tracing the evolutionary relationship between species. Fossils are one of the examples. They are the preserved traces of living organisms.

Generally, when living organisms die, their body will degrade and decompose eventually.  Our environment will not decompose them all at once or completely. It will leave some evidence for us. When a body is trapped under mud, as the mud hardens, the impression of the body will be left in the mud. This leads to the formation of fossils. Fossils of dinosaurs are a prominent example.


We could estimate the age of fossils in two ways. The first method is more relative. In this method, we assume that the fossils which we find at top layers are more recent ones while fossils at deeper layers are older. In the second method, the ratios of different isotopes of the same element in the fossil are estimated.

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The process of fossilisation includes: