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Evidence of Common Ancestors

Grade 8
Aug 30, 2022

Key Concepts

  • Evolution
  • Evidence of evolution
  • Biogeographical evidence
  • Evidence from the fossil record
  • Structural evidence
  • Embryological evidence


Evolution can be defined as a slow, continuous, gradual, and irreversible process through which the present-day complex forms of life developed from their simpler pre-existing forms. In simple terms, evolution can be defined as a change over time. 

We have learned about various scientists who have put forward their theories of evolution. Most of these theories state that all organisms that exist today have evolved from a common ancestor. But what is the evidence that backs this theory? The most accepted theory of evolution is given by Darwin, who used fossil evidence and similarities between related living organisms in order to support his theory. 

Methods like DNA testing weren’t available in Darwin’s time but are employed by scientists today to find out more about evolution. 


Evidence for Common Ancestors:  

When Darwin published his book “On the Origin of Species” in 1859, scientists were not able to test all of his ideas.  

However, in the past few years, scientists have discovered a great deal of evidence. This evidence comes from several different sciences like —biogeography, geology, anatomy, chemistry, genetics, and even molecular biology. Evidence from all of these areas supports Darwin’s basic ideas about evolution. 


1. Biogeography: 

Biogeography can be defined as the study of the geographical distribution of animals and plants in different parts of the earth. 

Darwin noticed that there are similarities between species of one biogeographical area. Whereas species living in different regions tend to differ more, even if the species habitat a similar ecological niche. This led to the wondering of how closely related species ended up in entirely different parts of the world. 

Similar species habituating in different parts of the world are due to geological changes, migrations, and convergent evolution.  

Darwin noted two important patterns:  

  1. Closely related organisms living in different environments often show great differences. Darwin’s finches serve as a great example to explain this pattern. Darwin hypothesized that birds on the islands were descended from birds in South America. Natural selection in different environments has led to differences among the closely related population. 
  1. Distantly related organisms living in similar environments are often similar. Darwin compared plants and animals living in similar environments around the world. For example, he found ground-dwelling birds in the grasslands of Europe, Australia, and Africa. These organisms looked similar but were not closely related. Because they lived in similar habitats, natural selection has led to similar adaptations. 

Another piece of evidence of biogeography is the breakup of the supercontinent Pangaea. Groups that evolved after the separation appear uniquely in regions of the earth, like the uniquely diverse flora and fauna of the northern continents that originated from the supercontinent Laurasia and of the southern continents that originated from the supercontinent Gondwana.  


The evolution of unique species on the islands is an example of how evolution and geography intersect with each other. 

Consider the example of the mammals in Australia; these mammals are marsupials. We know that most mammals elsewhere in the world are placental. This is because Australia was isolated by water for millions of years; these species were able to evolve without competition from other mammalian species in the world. 

Australia’s marsupials, Darwin’s finches in the Galapagos, and many Hawaiian island species are all unique to their island environments, but they all have distant ancestors on the mainland. 

2. Fossils and the Fossil Record: 

Fossils are preserved remains imprints, or traces of living organisms from the remote past. The study of fossils is well-known as paleontology. Fossil mainly preserves only a part of the dead organism like the skeleton, bone, teeth, etc. 

These remains give us a record of past changes through vast periods and establish the missing links between evolved species. 

Fossils are primarily found in sedimentary rocks. When layers of sand, clay, silt, or mud are compacted and cemented together, or when minerals are deposited from a solution, sedimentary rock is created. 

Sedimentary rock includes limestone, sandstone, and shale. Fossils are mostly found in limestone than in any other sedimentary rock. 

Formation of sedimentary rocks 

Fossils can include bones, shells, footprints, etc., several types of fossils are shown in the picture below. 

Types of fossils 
Types of fossils 

The age of fossils can be determined by using the following two methods: 

  1. Relative dating: It determines the approximate age of a fossil by comparing it to similar rocks and fossils of known ages. 
  1. Absolute dating: It uses radiometric dating to determine the exact age of a fossil. In radiometric dating, the precise age of a fossil is measured by the decay of isotopes, either within the fossils or, more often, the rocks that are associated with it. Carbon-14 dating is one of the most readily used dating techniques. 

A fossil record is a group of fossils that have been studied and arranged in taxonomic and chronological order. The order in which the events occurred, from first to last, is known as chronological order.  

All fossils that are known and their placements in the rock formations and positions in time make up the fossil record. The fossil record is not complete as most organisms never fossilize, and even the organisms that do fossilize are rarely found by humans. 

Archaeopteryx is a well-known fossil that is often regarded as the missing link between dinosaurs and birds. It was first described in 1861 by Hermann von Meyer (1801-1869), the German paleontologist. 



Fossils help us understand evolutionary relationships between organisms. For instance, scientists have been able to reconstruct a large, branching “family tree” for horses and their now-extinct relatives with the help of fossils. 

3. Structural Evidence: 

In Darwin’s time, researchers discovered that many different animals have similar structures. For example, the front legs of amphibians, birds, and reptiles all have the same basic bones. Darwin suggested that animals with similar structures are related. Their structures are inherited from a common ancestor. 

Homologous structures: parts of organisms that are similar in structure and origin but serve different functions in different organisms.  

A classic example of this is that the forelimbs of whales, humans, and birds look quite different on the outside. This is because they’re adapted to function in different environments. However, if we look at the bone structure of the forelimbs, we’ll find that the organization of the bones is quite similar across species. It’s highly unlikely that such similar structures would have evolved independently in each species and more likely that the basic layout of bones was already present in a common ancestor of whales, humans, and birds. 

Homologous structures are the result of divergent evolution and can indicate how closely two or more species share common ancestors.  

Homologous structures 

Analogous structures: Organs that have a different structure but perform the same function. The wings of birds and insects are examples of analogous structures. They serve similar functions but have different ancestral origins. They resulted from similar environmental conditions that produced similar natural selection outcomes over time but on distantly related organisms. 

Analogous structures are an example of convergent evolution. These structures do not show common descent. 

Analogous structures 

Vestigial organs are another source of evidence that helps establish evolutionary relationships. In the present-day form, these organs are non-functional and rudimentary. But they were very functional in the ancestors of the organism. 

For example, nearly all mammals have pelvic bones that support a pair of legs. Present-day whales have pelvic bones but they do not have leg bones like their ancestors. 

Evolution of whale  

The presence of such structures in one and a related organism, despite the presence of a vestigial structure in the other, suggests that the organisms evolved from a common ancestor. 

Example- nictitating membrane, pinna muscles & vermiform appendix (man). 

4. Embryological Evidence: 

Embryology is the study of embryo development from fertilization to birth. Scientists who study embryos from various animals frequently compare and contrast their developmental patterns. Similar patterns can reveal information about organisms’ evolutionary relationships. You may look nothing like a chicken now. But your embryonic cells grew in similar patterns. 

The early developmental stages of different vertebrate animal species are very similar and show various common features. As the embryos mature, these features become distinct structures in different species.  

For example, the embryos of vertebrate animal species have bulges in their neck region called pharyngeal pouches. They become facial and neck structures. In fish, they develop into structures called gill arches; in humans, one of the pharyngeal pouches develops into a part of the ear. 

Embryos of all vertebrates have pharyngeal pouches 

It is essential to remember that the more closely related species are, the more characteristics they share in the course of development. These shared similarities are best explained by the theory of common ancestors and evolution through natural selection. However, while developmental patterns indicate evolutionary history, they are not a historical journey through evolution. 


  • Evolution can be defined as a slow, continuous, gradual, and irreversible process through which the present-day complex forms of life developed from their simpler pre-existing forms.
  • Evidence of common ancestry comes from several different sciences like —biogeography, geology, anatomy, chemistry, genetics, and even molecular biology.
  • Biogeography can be defined as the study of the geographical distribution of animals and plants in different parts of the earth. It helps explain how closely related species ended up in entirely different parts of the world due to geological changes, migrations, and convergent evolution.
  • Fossils are preserved remains imprints or traces of living organisms from the remote past. Fossils give us a record of past changes through vast periods and establish missing links between evolved species.
  • Structural similarities and differences between organisms help us understand evolutionary relationships between them.
  • Embryological evidence shows that the early stages of closely related organisms are very similar and tend to develop into distinct structures as they grow.


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