CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 20

(i) Homologous organs have the same basic structure (and origin) but perform different functions. Example: the forelimbs of a lizard, a bird, a whale and a human are built on the same plan of bones (one upper-arm bone, two forearm bones, wrist, palm and finger bones) but are used for different jobs: walking, flying, swimming and grasping. The common design shows that these animals inherited it from a common ancestor and that it was modified during evolution. (ii) Fossils are the preserved remains or impressions of organisms of the remote past; fossils in deeper rock layers are older, so they show how organisms changed with time. Example: fossils of some dinosaurs show feathers, although these dinosaurs could not fly; feathers could have first given insulation in cold weather and later become useful for flight. The fossil Archaeopteryx has a bird's feathers and wings with a reptile's teeth and long bony tail. So birds have evolved from reptiles.