Evidences of Organic Evolution

Hello everyone, In previous posts we talk about the journey of evolution from non-cellular life to prokaryotes and after that eukaryotes and also we would see the various theories which try to explain this journey, and also the evidences of the theory which explain this journey in best way, which was Modern theory given by Oparin and Haldane.Now this part end i.e., how the first life originated on earth now we will discuss evolution of life forms on earth i.e., the changes in flora and fauna that have occurred over millions of years on earth.

When we think about evolution the first question arise in our mind is, What is Evolution? 

The word evolution means to unfold or unroll or to reveal hidden potentialities. Evolution simply means an orderly change from one condition to another. The term evolution was introduced by Herbert Spencer and It can be best explained by Darwin's Concept of "Descent with modification" which we will discuss in our Upcoming posts.

Our today's topic is evidences of organic evolution. There are many evidences for organic evolution which we will discuss in this post,

(1) Palaeontological evidences:

Study of fossils is called palaeontology. According to Charles Lyell, "Fossils are impression or remains of hard parts of life-forms found in rocks". Mostly fossils are found in sedimentary rocks. Sedimentary rocks are the rocks which form sediments and a cross-section of earth's crust indicates the arrangement of theses sediments one over the other during the long history of earth. Different aged rock sediments contains fossils of different life forms who probably died during the formation of the particular sediment. Generally, fossils found in older rocks are of simpler types and found in newer rocks are of complex type because the study of fossils in different sedimentary layers indicates the geological period in which they existed, and the study also shows that life-forms varied over time and certain life forms are restricted to certain geological time spans.The geological history of earth closely correlates with the biological history of earth. By fossils we can study the evolutionary pedigree of animals like horse, elephants and man e.t.c.

There are different types of fossils:

1. Unaltered fossils: Fossils which are preserved in their original or  intact form in ice, amber, etc. e.g. Wooly mammoths found frozen ice (25000 years before extinct fossils were found from Siberian region ) and insect and plant parts in amber.
2. Petrified fossils: Replacement of organic or soft parts of dead organisms by mineral deposits is called petrification. Here only the hard parts like bones, teeth, shells, and woods etc. get preserved, which are called petrified fossils. These are the most common type of fossils.
3. Mould fossils : Only an impression of the external structure of body is preserved in wet soil and no body parts is recovered of dead organisms. 
4. Cast fossils : Sometimes mineral are filled in the mould, resulting in cast fossils.
5. Print fossils : Fossilized impressions of foot, wings, leaves, stem etc.
6. Coprolites : Preserved faeces or excreta of organisms. 

By calculating the age of fossils we determine that how much time ago this organism were sedimented in the rocks and the age of fossils were calculated by calculating age of rocks from various technique which was radioactive technique, Electron Spin Resonance (this is the modern and most accurate technique), e.t.c.

(2) Evidences from comparative morphology and anatomy:

Similarities and differences are found among organisms of today and those that existed years ago. Such similarities can be interpreted to understand whether common ancestors were shared or not.

These similarities are of two types: (a) Homology    (b) Analogy

(A) Homology: The organs which have common origin, embryonic development and same                                            fundamental structure but perform similar or different functions are called                                            Homologous organs and this phenomenon is called Homology.

Some examples: 

• Forelimbs of mammals: Whales, Bats, Cheetah, they all have similar structure of forelimbs but perform different functions as swimming, flying, running respectively.
• Mouth parts of insects.
• Vertebrate hearts or brains.
• Testes in male and ovaries in female.
• Potato and ginger: Both are modified shoot
• Radish and carrot: Both are modified roots
• Thorns of Bougainvillea and tendril of Cucurbita both are modification of axillary bud.

                           

Homology indicates common ancestry and based on divergent evolution. When the same structures develop along different directions due to adaptations to different needs, this is called as divergent evolution.

(B) Analogy : The organs which have different origin and fundamental structures but perform similar functions are called Analogous organs and this phenomenon is called as analogy.

Some examples: 

• Wings of butterfly and birds, They are not anatomical similar structures though they perform similar functions i.e. used for flying.
• Eye of the Octopus and of mammals.
• Flippers of Penguins and Dolphins.
• Sweet potato (root modification) and potato (stem modification).
• Sting of Bee and Scorpion.
• Chloragogen cells of earthworm and liver of vertebrates.

Analogy doesn't indicates common ancestry and it is based on convergent evolution where different group of organisms have similar adaptive features due to similar habitat or towards the same function, hence analogous structures are a result of Convergent evolution. When different structures evolve for the same function due to the similar habitat, this is called Convergent evolution.

(3) Evidences from vestigial organs: 

The organs which are present in reduced form and do not perform any function in the body but are functional in related animals are called vestigial organs. They are remnants of organs which were complete and functional in their ancestors.

Examples:-

• Nictitating membrane.
• Muscles of pinna (auricular muscles).
• Vermiform appendix (Caecum).
• Coccyx.
• Canine teeth.
• Third molars (wisdom teeth).
• Body hair.
• Nipples in males.
• Segmented muscles of abdomen. These all are vestigial organs of human.

(4) Evidences from Atavism (Reversion):-

Sometimes in some individuals such characters suddenly appears which were supposed to be present in their ancestors but were lost during the course of evolution, this phenomenon is known as atavism or reversion. Atavism proves that animals developing atavistic structures have evolved from such ancestors in which these structures were fully developed.

Example:-

• Tail in new born baby.
• Extra long and pointed canine teeth.
• Functional auricular / pinna muscles.
• Long and thick body hair.
• Extra nipples in female.
• Cervical fistula (pharyngeal gills slits).

(5) Evidences from connecting links:-

Some organisms possess characters of two separate groups called as connecting links, which proves that members of higher groups have evolved from the lower group.

Examples:- 

• Virus - Between living and non-living
• Euglena - Between plants and animals
• Proterospongia - Between protozoa and porifera
• Neopilina - Between annelida and mollusca
• Peripatus - Between annelida and arthropoda
• Balanoglossus - Between non chordates and chordates
• Chimera - Between cartilaginous and bony fishes
• Protopterus(Lung fish) - Between fishes and amphibians
• Archaeopteryx - Between reptiles and amphibians

(6) Evidences from biochemestry:-

Similarities in proteins and genes performing a given function among diverse organisms give clues to common ancestry. Composition and structure of protoplasm, enzymes, hormones, DNA, blood in chordates is also almost same. It shows that organisms shared ancestors in recent or distant past.

(7) Evidences from biogeographical distribution:-

The study of geographical distribution of animals and plant species in different parts of earth is called as biogeography. It is believed that millions of years ago all the continents were present in the form of a single land mass called Pangea. Later due to continental drift these land masses got separated from each other by the seas. As these continents had different climates so plants and animals evolved there were of different varieties.

In prehistoric time Australia was a part of Asian continent. After the evolution of prototherians from reptiles Australia got separated from mainland of Asia. Later on eutherian mammals evolved in Asia which were carnivores in nature and they destroyed prototherians and marsupials from Asia but pouched mammals (marsupials) of Australia survived because of lack of competition from any other mammal. Today Eutherians are also found in Australia, because some of them evolved there and some were later transported by man.

The whole world was divided into six biogeographical regions called realms which was given by Alfred Russel Wallace on the basis of flora and fauna. These realms are:

1. Nearctic - North America
2. Neotropical - South America
3. Ethiopian - Africa
4. Palearctic - Europe
5. Oriental - Asia
6. Australian - Australia