Understanding why living organisms resemble each other has fascinated human beings for millennia, long before evolution provided a unifying concept for biology.
The term "homology" was originally coined in the 19th century by Richard Owen, a British comparative anatomist. Owen noticed the similarities between certain structures in different organisms, such as the similarities of forelimbs in vertebrates. He thought that some kind of abstract ideal plan accounted for the similarity of structure among groups of animals.
Charles Darwin later suggested that such similarities could arise from common ancestry. For example, the arrangement and structure of the limbs of all vertebrates are similar; these similarities come from the common ancestry of vertebrates.
Because the term homology was coined in a pre-evolutionary era, its meaning is sometimes confused and confusing. Today, most scientists prefer the term "synapomorphy," from Greek words meaning "shared shape or form."
A synapomorphy is a trait or feature that is shared by all of the descendants of a common ancestor, but not shared with other groups. The trait is considered to be "newly derived" in the lineage.
In an evolutionary framework, similarity in structure results either from common ancestry or from convergent evolution. All bats share a common wing shape because they share a common ancestor; that similarity is a homology. The bones in the bat wing have the same arrangement as the bones in your hand, and the bones at the top of the wing are very much like your arm bones. These similarities are homologies, because bats and humans share a common ancestor.
Birds have the same arm bones as humans and bats, but the bones are arranged in their wings very differently than the bones in bat wings. Bats fly using skin stretched across their finger bones, while birds fused those bones together and use feathers to generate lift. So even though bat wings and bird wings are similar in some ways, we can be sure that the wing shape is not homologous. This similarity is call convergence. Convergence results from shared evolutionary pressures and constraints, which require structures to be similar in order for the organism to perform similar functions. As the figure above shows, other vertebrates have evolved wings which have entirely different bone arrangements from birds or bats.
How do scientists tell the difference between homology (or synapomorphy) and convergence? Scientists test a claim of homology by making a prediction based on that claim, and then seeing whether the evidence supports that claim or refutes it. If repeated tests continue to confirm a claim of homology, scientists can be increasingly confident that they are right.
To test a claim of homology, scientists use a wide range of traits to trace the family tree of the organisms they are interested in. Then, they look at the trait they think might be homologous. If the trait is shared by two species because they have a common ancestor, then it should be found in the descendants of that common ancestor, and not in other branches. By comparing lots of species, and by tracing the tree of life using lots of different sorts of measurements, it is possible to test whether structures are homologous.