Homologs Are Descended from a Common Ancestor

I. The Molecular Design of Life
7. Exploring Evolution
Evolutionary relationships are manifest in protein sequences. The close kinship between human beings and
chimpanzees, hinted at by the mutual interest shown by Jane Goodall and a chimpanzee in the photograph, is revealed in
the amino acid sequences of myoglobin. The human sequence (red) differs from the chimpanzee sequence (blue) in only
one amino acid in a protein chain of 153 residues. [(Left) Kennan Ward/Corbis.]
I. The Molecular Design of Life
7. Exploring Evolution
7.1. Homologs Are Descended from a Common Ancestor
The exploration of biochemical evolution consists largely of an attempt to determine how proteins, other molecules, and
biochemical pathways have been transformed through time. The most fundamental relationship between two entities is
homology; two molecules are said to be homologous if they have been derived from a common ancestor. Homologous
molecules, or homologs, can be divided into two classes (Figure 7.3). Paralogs are homologs that are present within one
species. Paralogs often differ in their detailed biochemical functions. Orthologs are homologs that are present within
different species and have very similar or identical functions. Understanding the homology between molecules can
reveal the evolutionary history of the molecules as well as information about their function; if a newly sequenced protein
is homologous to an already characterized protein, we have a strong indication of the new protein's biochemical function.
How can we tell whether two human proteins are paralogs or whether a yeast protein is the ortholog of a human protein?
As will be discussed in Section 7.2, homology is often manifested by significant similarity in nucleotide or amino acid
sequence and almost always manifested in three-dimensional structure.