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

(a) Fossils are the preserved remains or traces of organisms that lived in the remote past, such as bones and shells, or impressions of leaves and footprints preserved in rock. (b) (i) Relative method: when we dig into the earth, the fossils found closer to the surface are more recent than those found in deeper layers, because the layers of rock were laid down one above the other over time. (ii) Dating fossils by isotopes: by detecting the ratios of different isotopes of the same element in the fossil material, for example carbon-14 to carbon-12. Carbon-14 decays at a fixed rate after the organism dies, so the less of it that remains, the older the fossil.