CBSE Class 10 Science 2017 · Paper 31/1/1 · Question 15

(a) Homologous organs have the same basic structure and origin, even though they may perform different functions. Example: the forelimbs of a frog, a lizard, a bird and a human being; the human arm (grasping) and the bird's wing (flying) both have one upper-arm bone, two forearm bones, and wrist and hand bones. This common basic design shows that these animals have descended from a common ancestor, whose limb has been modified for different uses: evidence of evolution. (b) Analogous organs have different basic structures and origins but perform the same function, so they look alike. Example: the wings of a bird and of an insect such as a butterfly are both used for flying, but a bird's wing is a feathered forelimb with bones, while an insect's wing is a membrane with no bones. They show that organs which do not come from a common ancestral structure, in organisms only distantly related, can evolve to look alike as adaptations to the same function, so similarity of shape alone does not show a common ancestry. (c) Fossils are the preserved remains or traces of organisms that lived long ago; their age can be judged from the depth of the rock layer or found from isotope ratios. Arranged by age, they show that living forms have changed over time, and some show features of two groups. Example: Archaeopteryx had feathers and wings like a bird, but teeth, clawed fingers and a long bony tail like a reptile, which suggests that birds have evolved from reptiles.