Unit 3:  The origin and evolution of life                          1   2   3   4   5   6 

3. Evidences of the evolution

3.1. Palaeontological evidences

Palaeontology is the science which studies the ancient life by analysing the fossils.

Fossils are mineralised remains of an organism. These remains are not only parts of the body that can be conserved, but also any mark that these living beings could be produced (tracks, impressions, excrements, etc.).

The fossil’s study allows to scientists to determinate the evolutionary story of extinct organisms. This offers evidences in favour of the change of the species with the pass of time (evolution).

In addition, they inform us that this process has been very slow.

Fossil record shows a tendency in livings beings to increase their complexity and their diversity (number of different species)

The simplest and more primitive forms of life are located in the most ancient rocks, while the more complex ones are located in the most modern rocks. Some groups of organisms appear in little number during a period, reach a big expansion during other posterior, and finally disappear (extinction).

The fossils’ study allows us:

- To deduce the moment of the apparition and extinction of a specie.

- To reconstruct the evolutionary story of a specie (phylogenetic serial).

- To establish relative relationships among species (phylogenetic trees)

The intermediate forms or transitional fossils (also known as “missing links”) allow us to analyse the transition between one and other species and it is especially important in the study of the connexions between large groups of living beings.

An example of this is Archaeopteryx. It is a primitive bird with feathers, but with teeth into the beak and claws as a reptile. This sharing of characteristics indicates that these groups are related.

Interactive: Fossil evidence (NOVA PBS)

Interactive: Tiktaalik (Chicago University)

A phylogenetic tree is the graphic representation of these evolutionary relative relationships among species.

Interactive: Tree of life (BBC)

3.2. Anatomical evidences

They are based on the Comparative Anatomy. This branch of science allows us to deduce evolve changes and relative relationships among species by comparing the structure and function of the same organ in them.

The found similarities can be analogies and homologies. 

a) Homologous organs

Homologous organs have the same embryologic origin and as consequence the same internal structure. However, their function and shape can be very different due to adaptation to different environments.

They are the result of a divergent evolution from a common ancestor and indicate a relative relationship among the studied species. It is also known as adaptive radiation.

An example of homologous structure is the limb of tetrapods. The wing of a bird, the fin of a whale, the leg of a horse or the arm of a human, although have different functions (fly, swim, run, catch), have the same bones.

b) Analogous organs

Analogous organs have different embryologic origin and different internal structure. They are similar in their external shape and in their function, because they have adapted to similar environments.

They are the result of a convergent evolution from different ancestors. They do not indicate relative relationship among the studied species.

An example of analogous organ is the wing. A bird and an insect have wings, but these wings are very different among them, although both allow fly. They have different internal structure: insect wing is a membrane support by chitin veins while bird wing has an internal osseous structure that supports feathers.

c) Vestigial organs

There is a third type of organs, the vestigial or rudimentary organs. These organs do not have function now, but they had it in the ancestors of the studied species. This is the case of whale that has hip bones as other vertebrates, although it does not have legs. In humans are vestigial organs the appendix or the wisdom tooth.

3.3. Biogeographical evidences

The geographical distribution of species also gives data about their evolution process.

     - Continents which was joined in the past (e.g. South America and Africa) have equal fossil

       records. Their actual living species are different although similar.

   This indicates the existence of common ancestors. When continents separated the populations

   keep isolated and change in different ways with the time.

    - Isolated oceanic archipelagos have fauna and flora very different by comparing

      with the continent.

   Many little species can colonise this type of islands. They keep isolated with a large amount

   of resources, many ecological niches to conquer and without predators. These conditions

   favour the diversification of these species. The isolation of these islands also explains the lack

   of complete groups of animals or plants. For example, in Hawaii there are not amphibians,

   because anyone got colonise it.

3.4. Embryological evidences

They are based in the comparative study of embryological development of animals (ontogeny)

Embryos show along their development some primitive characteristics which later disappear. It seems repeat the evolve story of the specie (phylogeny). For example, all vertebrate’s embryos have gill clefts that keep in fish and amphibians, but disappear in the other groups as they develop. This phenomenon provoked that Ernest Haeckel, in 1866, formulated that “ontogeny recapitulates phylogeny” (biogenetic law) Today, we know it is false.

However, the study of embryologic development allow us recognise easily homologies among embryos than among adults. The existence of similar structure indicates a common ancestor. The embryos of different species are more similar in the earliest phases of their development. As they grow the number of differences increase. These differences are more evident between the farthest related species.

3.5. Biochemical evidences

Molecular biology gives the most convincing arguments in favour of evolution. The existence of the same chemical compounds, in all living beings indicates their common origin. This is very evident in the case of proteins (all of them made only by 20 amino acids) and nucleic acids (all of them made only by 4 nucleotides).

The comparative study of protein and nucleic acids sequences among different species allow us to establish relative relationships. The more similar are two nucleotides or amino acids chains, the higher the evolutionary closeness between the studied species is.

For example, the human and the chimpanzee haemoglobin have identical sequence, while the gorilla sequence is different in four amino acids. It means that the relate relationship between humans and chimpanzees is closer than the relation of both with gorillas.

3.1. What does it mean that archaeopteryx is an intermediate fossil?

3.2. Which evolutionary trends can be deduced by observing the fossil register

       of the horse?

3.3. What is a homology? What relationship has it with the concept of divergent

       evolution?

3.4. Are analogous organs useful to deduce evolutionary relationships?

       And rudimentary organs?

3.5. How can the huge biological diversity that exists in isolated archipelagos

       be explained?

3.6. In the diphtheria treatment is use the horse serum rich in antibodies against

       the illness. What does it indicate from the evolutionary point of view?