|Homologous Organs||Organs which are similar in basic design but serve different purposes in different organisms.|
|Analogous Organs||Organs which serve the same purpose in different organisms but have similar basic design.|
|Fossil||Preserved remains of organisms from distant past.|
The modern system of classification is based on evolutionary relationship. Due to this, it is also known as phylogenetic classification. The kingdom is the highest taxa, while the species is the lowest taxa. Members of a species have a higher number of common characters, than members of a kingdom. For example, all human beings belong to the species Homo sapiens. Human beings can interbreed, irrespective of their race or skin colour. All human beings come under the class mammalia, as do the monkeys, elephants and cows. Apparently, each species of the class mammalian is quite different yet they have certain common characters, like hairs on the body and mammary glands in females. Similarly, all animals are eukaryotes and cell wall is absent in their cells. The degree of similarity or dissimilarity shows that all animals have evolved from a common ancestor.
Homologous Organs: Organs which have common design but serve different functions in different animals are called homologous organs. For example, the forelimbs of all tetrapods are composed of humerus, radio-ulna, tarsals and metatarsals. Yet, the forelimbs of frogs are adapted to a jumping movement, those of birds are used for flying and those of humans are used for handling tools. This shows that frogs, birds and humans have evolved from a common ancestor.
Analogous Organs: Organs which have different design but serve a common function in different animals are called analogous organs. Wings of birds and wings of bat are good examples of a pair of analogous organs. Wings of birds are composed of all the bones of forelimb and are covered with feathers. Wings of bats are mainly composed of the digital bones and a thin membrane covering the structure. Yet wings in both the organisms are used for flying.
Fossils: The preserved remains of animals or plants or other organisms from the distant past are called fossils. The term distant is a key term in this definition. Scientists usually take 10,000 years as the minimum age for the remains to be categorized as fossil. Many fossils have been discovered till date. These fossils tell us about many extinct animals and also give insights into how the evolution could have taken place.
Evolution of complex organs and thus of complex organisms has happened in stages. Let us take the example of evolution of eyes. Planaria is the first animal which shows eye like structure. The dark spots on planaria are light sensitive spots but a planaria cannot distinguish between two different objects. Eyes of insects are compound eyes which are made up of thousands of optical surfaces. Eyes of higher animals are simple eyes which are composed of a single lens. Most of the animals cannot differentiate among colours. Depth perception is also weak in many animals. Human eyes are the most advanced; because humans can recognize colours and have very good depth perception.
Most of the tetrapods have to use all the four limbs for locomotion. Some apes can walk up to smaller distances by using just the hind limbs. Humans have finally evolved the bipedal walking.
Evolution does not mean progress in every case. This can be proved by example of bacteria. Bacteria are the simplest and one of the oldest organisms on the earth. Their simple body design does not make them weak from any angle. Bacteria are known to survive some of the harshest climates, like craters of volcanoes and sulfur springs. Many animals have certain features which hamper even their routine activities. For example; the branch-like horns of antelope are a handicap for them. When an antelope runs for its life; there are times when its horns get entangled in branches or bushes. This results in the death of the antelope. Colourful feathers of a male peacock are very good when it comes to attract a female. But because of its conspicuous feathers, it can be easily spotted by a predator. Because of its bulky feather it cannot fly away to safety.
The modern humans are called Homo sapiens. Many scientific investigations have shown that the modern humans evolved in Africa. They migrated towards north, in due course of time and settled near what is known as the Mediterranean Sea. When the ice age ended, melting of ice resulted in the in water level. The humans migrated in different directions from that area. One branch went to the western Asia, then to the Indian Peninsula and finally to Australia. From the Indian Peninsula, branch migrated towards China and subsequently to the North America. From North America, the humans migrated to the South America. From the Mediterranean Sea, the second branch migrated towards Europe; where they are believed to replace the Neanderthals.
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