ntologists are able to classify species according to their similarity to ancestral predecessors, and thereby determine which species might be related to one another. Determination of the age of each fossil will concurrently indicate the rate of evolution, as well as precisely which ancestors preceded one another and consequently which characteristics are retained or selected against. Generally this holds true: probable ancestors do occur earlier in the fossil record, prokaryotes precede eukaryotes in the fossil record. There are however, significant "missing links" throughout the fossil record resulting from species that were, perhaps, never fossilized - never!theless it is relatively co mpatible with the theory of evolution13.It can be postulated that organisms evolving from the same ancestor will tend to have similar structural characteristics. New species will have modified versions of preexisting structures as per their respective habitats (environmental situations). Certainly these varying species will demonstrate clear differentiation in important structural functions, however an underlying similarity will be noted in all. In this case the similarity is said to be homologous, that is, structure origin is identical for all descended species, but very different in appearance. This can be exemplified in the pectoral appendages of terrestrial vertebrates: Initial impression would be that of disparate structure, however in all such vertebrates four distinct structural regions have been defined: the region nearest the body (humerus connecting to the pectoral girdle, the middle region (two bones, radius and ulna are present), a third region - the "hand" - of several bones (carpal and metacarpal, and !region of digits or "finger s". Current species might also exhibit similar organ functions, but are not descended from the same ancestor and therefore different in structure. Such organisms are said to be analogous and can be exemplified in ...
