CBSE Class 10 Science 2017 · Paper 531/3 · Question 12

(a) Homologous organs have the same basic structure and origin, even though they may perform different functions. Example: the forelimbs of a human (grasping), a bat (flying) and a cat (walking) are built from the same set of bones (upper-arm bone, forearm bones, wrist bones and hand bones). The same design modified for different uses shows that these animals descended from a common ancestor. (b) Analogous organs have different basic structures and origins but perform the same function, so they look alike. Example: the wings of a bird (a feathered forelimb with bones) and of an insect (a thin membrane with no bones) are both used for flying; so are the wings of a bird and a bat (a feathery covering along the arm against a skin fold between elongated fingers). They show that organisms that are not closely related, adapting to the same function, evolve similar-looking organs from different structures, so a similar appearance alone does not show a common ancestry. (c) Fossils are the preserved remains or impressions of organisms that lived in the past. Example: Archaeopteryx, with feathers and wings like a bird but teeth, clawed fingers and a long bony tail like a reptile, suggests that birds evolved from reptiles. Fossils, dated from the depth of their rock layer or from isotope ratios, show that organisms have changed over time.