The pentadactyl limb is an an homologous structure. It has evolved to
perform various functions for different organisms.
If widely separated mammals, for example, share a common ancestor then it
is expected that despite adaptive radiation, [1] they would have some basic
structural characteristics in common. The fewer the similarities, the
greater length of time since the divergence from the common ancestor. The
greater the similarities, the more recent the divergence.
Similar structures that may have evolved to have a different function in
different organisms are described as homologous. The pentadactyl limb
possessed by most terrestrial vertebrates (amphibians, reptiles, birds and
mammals) is an example. Despite variations, the fundamental structure is
the same.
In some instances comparing embryological development can give insights
into evolutionary relationships. Gill slits are structures through which
aquatic vertebrates expel water after gaseous exchange at the surface of
gills. Gill slits are present in the early embryos. It is interesting to
find that non-functional gill slits are evident in the early embryos of
terrestrial vertebrates (birds, reptiles, mammals) but not later. Human
embryos have a primitive tail in the early stages but it is absent later.
These observations suggest that the vertebrates had a common ancestor and
are therefore related by evolutionary descent. In recent years, the basis
for this evidence has come into question.
