Common descent, an idea central to Darwin's theory of evolution, is well supported by multiple chains of evidence from the fields of genetics, biogeography and comparative anatomy. The fossil record also provides both direct and indirect evidence of descent with modification from common ancestors, though this line of evidence was generally unavailable to Darwin in the mid-19th century.
Closely related plants and animals tend to be grouped geographically. Most of the birds in the Galapagos Islands, whatever their ecological niche, are recognizably finches. Honeycreepers occupy much the same position for the Hawaiian Islands. Penguins live exclusively in the southern hemisphere. The simplest explanation for this kind of distribution is that the modern species all derive from a recent common ancestor that migrated into the area they now occupy.
Direct comparison of genes contributes to the picture of common descent. Analysis of gene-influenced protein structure done by researchers at Brandeis University found that the cleanest explanation for the variance between modern eukaryotes, bacteria and archaea is that they are all descended from a universal common ancestor that lived several billion years ago.
The common descent model is also supported by its utility as a predictive tool. The close kinship between humans and other primates is the presupposition that underlies much animal testing. The more distant, but still relatively recent, common ancestry between all mammals is the reason experiments on rats can be loosely extrapolated to human physiology.