CBSE Class 10 Science 2017 · Paper 31/2/C · Question 18

Formation: usually, when an organism dies, its body is decomposed by microorganisms and lost. Sometimes, however, the body or some of its parts (especially hard parts such as bones, teeth and shells) is buried quickly in mud, silt or sand - for example at the bottom of a lake or a sea - where it cannot decompose completely. Layers of sediment settle over it and, over millions of years, harden into rock; minerals slowly replace the original material, so the remains are preserved in the rock as a fossil. An organism may also leave its impression, for example an insect caught in hot mud that later hardens. All such preserved traces of organisms are fossils. Role in the study of evolution (any one): fossils provide evidence of evolution by showing organisms that lived in the past and how they differed from present-day ones. Dated by the depth of their rock layer (deeper layers are older) or by isotope ratios, they show the order in which different groups appeared - for example, fossils of feathered dinosaurs show that birds are very closely related to reptiles.