CBSE Class 10 Science 2017 · Paper 31/2/3 · Question 15

Fossils are the preserved remains or traces of organisms that lived in the remote past, such as bones, shells, or impressions of leaves or bodies left in rocks. They act as evidence for evolution in these ways: (1) Rock layers form one above another, so in undisturbed rock the fossils in deeper layers are older than those near the surface, and their age can also be found by fossil-dating (the ratios of isotopes of an element in the fossil). Arranged by age, fossils show that organisms very different from today's lived in the past, and that, on the whole, simpler forms appeared first and more complex forms later: life has changed gradually over time. (2) Some fossils have features of two groups and link them. Archaeopteryx had feathers and wings like a bird but teeth and a long bony tail like a reptile, and fossils show that some dinosaurs had feathers — evidence that birds evolved from reptiles.