Scientists determine evolutionary relationships among organisms using various methods, including comparative anatomy, molecular biology, and fossil analysis.

To explore evolutionary relationships, one widely used method is comparative anatomy, which involves examining the physical structures of different organisms. This includes analyzing bones, muscles, and other bodily components. For instance, the similar bone structure in the limbs of humans, cats, whales, and bats suggests that these species share a common ancestor. These similarities are referred to as homologous structures. In contrast, analogous structures—such as the wings of bats and birds—serve similar functions despite having different structural forms, indicating that these organisms evolved independently.

Another important method is molecular biology, where scientists compare the DNA, RNA, and proteins of various organisms. The greater the similarity among these molecules, the more closely related the organisms are likely to be. For example, humans share approximately $98\%$ of their DNA with chimpanzees, highlighting a close evolutionary relationship. This molecular approach can also help estimate the time of divergence for different species from a common ancestor, based on the rate at which mutations accumulate over time.

Fossil analysis plays a crucial role as well. Fossils provide a historical record of past life forms and evolutionary events. By examining the characteristics of fossils and the geological layers in which they reside, scientists can reconstruct the evolutionary history of organisms. A notable example is the discovery of transitional fossils like Archaeopteryx, which displays features of both dinosaurs and birds, providing compelling evidence for the evolution of birds from theropod dinosaurs.

Lastly, biogeography—the study of the distribution of species and ecosystems across geographic space and through geological time—also contributes to understanding evolutionary relationships. Organisms in different regions may evolve similarly due to analogous environmental pressures, a phenomenon known as convergent evolution. Conversely, organisms within the same region may evolve divergently due to differing environmental conditions, referred to as divergent evolution. By analyzing these distribution patterns, scientists can infer the evolutionary relationships among various organisms.