CBSE Class 10 Science 2019 · Paper 31/5/1 · Question 21

(a)(i) Homologous organs have the same basic structure (and origin) but perform different functions, e.g. the forelimbs of a human (grasping) and of a lizard (walking): both have one upper arm bone, two forearm bones, wrist bones and finger bones. The common plan shows that these animals descended from a common ancestor; the differences arose by adaptation. (ii) Analogous organs have different basic structures (and origin) but perform the same function, e.g. the wings of an insect (a thin membrane, no bones) and the wings of a bat (a forelimb with skin stretched between the fingers). They show that unrelated organisms develop similar organs when they adapt to the same way of life - the similarity comes from adaptation, not from a common ancestor. (iii) Fossils are the remains or impressions of organisms that lived in the past, preserved in rocks. They show what extinct organisms were like and how organisms changed over time, e.g. Archaeopteryx, with feathers and wings like a bird but teeth and a long bony tail like a reptile, links reptiles and birds. (b) Method 1 - relative dating: fossils found in layers closer to the surface are more recent than those found in deeper layers, because newer layers settle on top of older ones. Method 2 - isotope (carbon) dating: the ratio of different isotopes of the same element, such as carbon-14 to carbon-12, in the fossil is measured; carbon-14 decays at a known rate after death, so the ratio gives the age of the fossil.