Darwin and other nineteenth-century biologists found compelling evidence for biological evolution in the comparative study of living organisms, in their geographical distribution, and in the fossil remains of extinct organisms. In the Origin of Species, Darwin dedicates five chapters to the evidence for evolution: two chapters to the geological record, or, as we are more likely to say nowadays, to paleontology; two chapters to biogeography; and one chapter to comparative anatomy and embryology. Since Darwin’s time, the evidence from these sources has become stronger and more comprehensive, while biological disciplines that have emerged recently – genetics, biochemistry, ecology, animal behavior (ethology), neurobiology, and especially molecular biology – have supplied powerful additional evidence and detailed confirmation.

Darwin surely would have been pleased by the enormous accumulation of paleontological evidence, including the discovery of fossils of organisms intermediate between major groups, such as Archaeopteryx, intermediate between reptiles (dinosaurs) and birds, and Tiktaalik, intermediate between fish and tetrapods (Ahlberg and Clack 2006) and the numerous fossils and diverse species of hominins, intermediate between apes and Homo sapiens (e.g., Dalton 2006; T. D. White et al. 2006; Cela-Conde and Ayala 2007). But there are good reasons to believe that Darwin would have been most pleased and most impressed with the overwhelming evidence for evolution and precise information about evolutionary history provided by molecular biology, a source of evidence and document of history that Darwin could not have even imagined.