CBSE Class 10 Science 2017 · Paper 31/2/2 · Question 12

Three methods: (1) Homologous organs: organs with the same basic structure and origin but different functions, such as the forelimbs of frogs, lizards, birds and humans, show that the organisms share a common ancestor; the more such features two organisms share, the more closely related they are. Analogous organs (same function, different structure and origin, such as the wings of a bat and of a bird) must be told apart, since they do not show close relationship. (2) Fossils: preserved remains or impressions of past organisms. Fossils in deeper layers of rock are usually older, and their age can also be found from the ratios of isotopes in them; arranged by age, fossils show which forms came earlier and how features changed. (3) Molecular phylogeny: comparing the DNA of organisms. Changes in DNA build up over generations, so the fewer the differences between the DNA of two organisms, the more closely related they are.