Evidences for evolution: fossils, embryos and molecules
Rock layers hold fossils of different ages, and all living things share similar genes and proteins. Both point to common ancestry and change over time.
What is biological evolution?
Evolution is the slow change in living things over many generations, by which new kinds of organisms arise from older ones. Since we cannot watch millions of years, we look for evidence: fossils, the structure of bodies, embryos, and molecules such as DNA and proteins. All of them point the same way: today's organisms share common ancestors.
Paleontological evidence (fossils)
- Rocks form as sediments settle in layers. A cut through the Earth's crust shows one layer lying on another; lower layers are older.
- Each layer holds fossils of organisms that probably died when that layer formed.
- Some fossils look like modern organisms; others belong to extinct groups such as dinosaurs.
- Life forms varied over time, and some are found only in certain time spans. So new forms arose at different times in the Earth's history.
- The age of a fossil is found by radioactive dating, which uses the steady decay of radioactive elements.
Embryological evidence: Haeckel's idea and von Baer's correction
Gill slits work as breathing organs only in fish; in other adult vertebrates they are not found. Your textbook spells the name 'Heckel'; the usual spelling is Haeckel.
Molecular evidence
Very different organisms use similar proteins and genes to do the same job. All living things use DNA (or RNA) with the same genetic code, and use ATP for energy. The more closely two organisms are related, the more alike their DNA and proteins are; for example, human and chimpanzee proteins are very similar. These biochemical similarities point to the same shared ancestry as similar body structures do.
The evidences at a glance
| Evidence | What we look at | What it shows |
|---|---|---|
| Paleontology | Fossils in rock layers of known age | Life forms changed over time; many are extinct |
| Comparative anatomy | Homologous and analogous organs | Divergent and convergent evolution |
| Embryology | Features shared by vertebrate embryos | Common features in early development (Haeckel's idea was later disapproved) |
| Molecular | Genes, proteins, genetic code | Common ancestry; degree of relationship |
India has rich fossil sites, such as the dinosaur fossil park at Balasinor in Gujarat and the Shivalik hills, where fossils of early apes were found. DNA comparison, the molecular evidence, is also used in Indian labs to trace how crops like rice were domesticated.
2 marks: “How do fossils found in different rock layers support evolution?” Lower layers are older. Each layer has fossils of different life forms, some extinct. This shows life forms changed over time and new forms arose at different times.
2 marks: “What was Haeckel's embryological evidence for evolution, and why was it disapproved?” All vertebrate embryos show vestigial gill slits behind the head. Von Baer showed embryos never pass through the adult stages of other animals.
3 marks: “Explain how molecular similarities support common ancestry, with one example.” Similar genes and proteins do the same job in diverse organisms; same genetic code and DNA in all; closely related organisms such as humans and chimpanzees have very similar proteins.
F-E-M: Fossils show time, Embryos show early likeness, Molecules show kinship.