Every living thing descended from a common ancestor over millions of years. This is the central fact that the theory of evolution explains. House sparrows and tree sparrows only need to go back relatively few generations to find their common ancestor, while trout and tigers must search much closer to the roots of the tree for theirs.
Darwin sketched this idea of common ancestry along with the words "I think" in one of his notebooks. His seminal book "On The Origin of Species" laid out multiple lines of evidence for the relatedness of life and the role of natural selection in creating new species.
Precisely how species are related to one another has been the subject of much research since Darwin's time. The "tree of life" has been continuously refined as new methods became available. In recent years the field of genetics has provided irrefutable proof of common ancestry, confirming and correcting previous models. Numerous threads in this series have looked at some of this genetic evidence, including ERVs, pseudogenes and the genetic sequences of ubiquitous proteins.
Before genetic evidence was available comparisons between species focussed on physical similarities. Not all of these are immediately obvious. Superficially bats look more like birds than they resemble dolphins but a closer examination of their anatomy reveals a different story.
On the surface a bat wing does appear to have anything in common with a dolphin flipper or human arm but when we look at the underlying anatomy they are clearly "homologous".
This basic structure of our arms and legs is shared by an astonishing number of creatures. The relative size of individual bones vary, some may be missing or fused to other bones, but the pattern is clear. Even more amazingly when we look back into the fossil record this same pattern of bones can be seen in the fossils of extinct lobe-finned fish more than 350 million years ago, before our ancestors emerged from the oceans.
The reason for the similarity of these ancient structures is explained beautifully by recent progress in evolutionary developmental biology (Evo Devo). The genes that control the development of each part of our limbs is now understood in precise detail. The emergence of embryonic limb buds and the growth of the various parts of modern limbs are controlled by signalling proteins coded by genes that formed the fins of Eusthenopteron in the Devonian Period.
Natural selection is brilliant at finding ways to adapt ancient structures. The results can be found in homologous structures in diverse modern species and in the fossil record.