CBSE Class 10 Science 2020 · Paper 31(B) · Question 28

(a) (i) Homologous organs have the same basic structure (a common origin) but perform different functions. Example: the forelimbs of amphibians, reptiles, birds and mammals, such as a lizard's forelimb (walking), a bird's wing (flying) and a human arm (grasping). (ii) Analogous organs have different basic structures (different origins) but perform similar functions. Example: the wings of a bat (skin stretched between the fingers) and the wings of a bird (feathers along the arm), both used for flying; the wings of an insect and a bird are another example. (iii) Fossils are the preserved traces (remains or impressions) of organisms that lived in the past. Example: fossils of an ammonite, a trilobite or the dinosaur Rajasaurus. (b) Method 1, relative dating: on digging into the earth, fossils found closer to the surface are more recent than those found in deeper layers. Method 2, isotope (carbon) dating: the ratio of different isotopes of the same element in the fossil, such as carbon-14 to carbon-12, falls at a known rate after death, so measuring it gives the age of the fossil. Any other correct example earns credit.